Victor Poussardin

Michael Paris

William Wilson

Arezki Tagnit-Hamou

Deneele

Calcined palygorskite and smectite bearing marlstones as 1 supplementary cementitious materials 2

Keywords: Palygorskite, marlstone, SCMs, MAS NMR, calcined clay, dolomite 43 44 45 46 47 48 49 50 51 52

Highlights 11  Palygorskite-bearing marlstone is suitable for a use as a SCM 12  Palygorskite-bearing marlstone exhibits a higher dihydroxylation ( 27 Al MAS NMR), 13 self-reactivity in water and compressive strength in cementitious media than smectite-14 bearing marlstone. 15  Pouzzolanic reactivity was significantly higher for systems with calcined palygorskite 16 than calcined smectite only, as shown both by portlandite consumption (XRD) and C-17 (A)-S-H formation ( 29 Si MAS NMR).

INTRODUCTION 53 54

In order to comply with the Paris climate agreements, and to limit the global temperature 55 increase below 1.5°C compared to pre-industrial levels, the most polluting industrial sectors 56 need to reduce significantly their greenhouse gas emissions (may their CO2 emissions). It is 57 estimated that 5 to 8% of global anthropogenic CO2 emissions comes from the cement industry 58 [START_REF] Huntzinger | A life-cycle assessment of Portland cement 437 manufacturing: comparing the traditional process with alternative technologies[END_REF]. In this context, cement producers are seeking to reduce their carbon footprint.

59

The use of Supplementary Cementitious Materials (SCMs) to replace part of the clinker is now 60 seen as one of the major solutions to reduce the environmental footprint of the cement industry 61 [START_REF] Scrivener | Eco-efficient cements: Potential economically 440 viable solutions for a low-CO2 cement-based materials industry[END_REF]. Blast furnace slag [START_REF] Escalante | Reactivity of blast-furnace slag in Portland 443 cement blends hydrated under different conditions[END_REF][START_REF] Yazıcı | Mechanical properties of reactive powder 446 concrete containing high volumes of ground granulated blast furnace slag[END_REF] and power plant fly ash [START_REF] Sakai | Hydration of fly ash cement[END_REF][START_REF] Yao | A comprehensive review on the applications of coal fly 451 ash[END_REF][START_REF] Hu | Compressive strength, pore structure and chloride 453 transport properties of alkali-activated slag/fly ash mortars[END_REF] are the main examples of SCMs 62 used today that reduce the environmental footprint of cement. However, blast furnace slag 63 resources remain limited, preventing large-scale deployment of this technology, and fly ash 64 reserves from coal combustion will continue to decline in the coming years as the energy 65 transition legitimately limits their availability [START_REF] Scrivener | Eco-efficient cements: Potential economically 440 viable solutions for a low-CO2 cement-based materials industry[END_REF]. 66 Among alternative SCMs, calcined clays are gaining in popularity. The resources of clays 67 potentially viable as SCMs after calcination are abundant and well distributed around the world, 68 especially in developing countries where the demand for cement is constantly growing [START_REF] Scrivener | Eco-efficient cements: Potential economically 440 viable solutions for a low-CO2 cement-based materials industry[END_REF]. The 69 numerous studies carried out on the subject have shown that metakaolin (from the calcination 70 of kaolin) has the highest reactivity in cementitious media [START_REF] Alujas | Pozzolanic reactivity of low grade kaolinitic 456 clays: Influence of calcination temperature and impact of calcination products on OPC hydration[END_REF][START_REF] Almenares | Industrial calcination of kaolinitic clays 459 to make reactive pozzolans[END_REF][START_REF] El-Diadamony | Hydration and characteristics 462 of metakaolin pozzolanic cement pastes[END_REF][START_REF] Zhao | Microstructure of cement paste incorporating high volume of 465 low-grade metakaolin[END_REF], whereas calcined smectites 71 [START_REF] Brown | The thermal reactions of montmorillonite 468 studied by high-resolution solide-state 29Si and 27[END_REF][START_REF] Garg | Thermal Activation of a Pure Montmorillonite Clay and Its 470 Reactivity in Cementitious Systems[END_REF][START_REF] Kaminskas | Smectite clay waste as an additive for 473 Portland cement[END_REF] and illites [START_REF] Fernandez | The origin of the pozzolanic activity of 476 calcined clay minerals: A comparison between kaolinite, illite and montmorillonite[END_REF][START_REF] Sc | The effect of zinc oxide additions 479 on the performance of calcined sodium montmorillonite and illite shale supplementary cementitious 480 materials[END_REF][START_REF] Garg | Pozzolanic reactivity of a calcined interstratified illite/smectite 483 (70/30) clay[END_REF] are much less reactive. These results led to a growing interest in the 72 use of calcined kaolins (rock composed of kaolinite, quartz and minor accessory minerals) as 73 SCMs, and to the development of Limestone Calcined Clay Cements (LC3) [START_REF] Díaz | Limestone calcined clay cement as a 486 low-carbon solution to meet expanding cement demand in emerging economies[END_REF][START_REF] Scrivener | Calcined clay limestone cements (LC3). 489[END_REF].

74

Only few studies have looked at other types of clays than kaolinites, smectites and illites [20, 75 21], mainly because of industrial competition (which leads to higher prices) and lower 76 availability. Furthermore, the majority of studies focused on relatively "pure" samples, which are mainly composed of clay minerals, quartz and other accessory minor minerals. Yet it is 78 common to find clay minerals associated with other compounds, especially carbonates. If the 79 proportion of carbonates is high, this mixture of clay minerals and carbonates is named 80 marlstone.

81

Marlstones are often neglected by manufacturers of fired clay materials such as bricks because 82 of their high content of calcium carbonate. They are considered as waste by many industries 83 and supplies are increasing [START_REF] Justnes | CALCINED MARL AND CLAY AS 496 MINERAL ADDITION FOR MORE SUSTAINABLE CONCRETE STRUCTURES CALCINED[END_REF]. However, these materials contain a significant proportion of 84 clay minerals, which can become pozzolans upon calcination. The use of marlstones as SCMs 85 could therefore make it possible to recover mining wastes (and/or overburdens) while reducing 86 the environmental footprint of cement production. Numerous studies looked at the use of 87 marlstones as SCMs [START_REF] Danner | Characterisation of calcined raw clays suitable as 500 supplementary cementitious materials[END_REF][START_REF] Bullerjahn | Novel SCM produced by the co-503 calcination of aluminosilicates with dolomite[END_REF][START_REF] Mohammed | Valorization of pozzolanicity of Algerian clay: 506 Optimization of the heat treatment and mechanical characteristics of the involved cement mortars[END_REF][START_REF] Danner | Calcareous smectite clay as a pozzolanic alternative to 509 kaolin[END_REF][START_REF] Bahhou | Using Calcined Marls as Non-Common 512 Supplementary Cementitious Materials-A Critical Review[END_REF] , but the great variability of this type of material limits the 88 possibility of global analyses. For each type of marlstones considered as a potential SCM, it is 89 imperative to understand the evolution of each phase (clay minerals and carbonates) composing 90 the marlstones during its calcination, as well as its influence on the reactivity and the final 91 mechanical performances of calcined marlstone-cement blends.

92

This study focuses on the use as SCMs of two marlstones considered as waste by the mining 93 industry. The first objective is to determine whether these materials can be used as SCMs. The The materials studied (MS1 and MS2) are two marlstones considered as waste by the mining 104 industry. The samples were received as loose blocks and mechanically crushed before analysis.

105

Table 1 shows the results of the chemical analysis (performed by X-Ray Fluorescence (XRF).

106

The proportions of each element are expressed in weight percent. All the acquisitions were performed with 1 H decoupling. An aqueous solution of Al(NO3)3 was 163 used to referenced 27 Al spectra while 29 Si spectra were referenced against TMS (Tetramethylsilane). The Dmfit sotftware [START_REF] Massiot | Modelling one-and two-dimensional solid-state 532 NMR spectra: Modelling 1D and 2D solid-state NMR spectra[END_REF] the calcium from the decarbonated dolomite and the silicon from calcined clay phases [START_REF] Xie | The thermochemical activity of dolomite occurred in 542 dolomite-palygorskite[END_REF]. silicon atoms and which can be associated with silicon in the quartz structure [START_REF] Lippmaa | Structural studies of silicates by solid-state 553 high-resolution silicon-29 NMR[END_REF]. For MS1, 269 the total signal between -84 and -100 ppm is composed of Q 3 silicon atoms of smectite (-93 270 ppm) [START_REF] Brown | The thermal reactions of montmorillonite 468 studied by high-resolution solide-state 29Si and 27[END_REF], Q 3 (1Al) silicon atoms of biotite (-86 ppm) [START_REF] Mackenzie | The thermal reactions of 556 muscovite studied by high-resolution solid-state 29-Si and 27-Al NMR[END_REF], Q 2 silicon atoms of palygorskite (-84 ppm) [START_REF] Kuang | Dehydration and rehydration of palygorskite and the 559 influence of water on the nanopores[END_REF] and two Q 3 silicon sites of palygorskite at -98 and -92 ppm which correspond to 272 SiO4 at the edges and at the centre of the ribbons of tetrahedra, respectively [START_REF] Barron | Solid state 29Si NMR examination of the 2:1 ribbon 562 magnesium silicates, sepiolite and palygorskite[END_REF]. For MS2, the for MS2, which is explained by the condensation of the Q 3 into Q 4 silica [START_REF] Garg | Pozzolanic reactivity of a calcined interstratified illite/smectite 483 (70/30) clay[END_REF]. By comparing

282

MS1 and MS2 it is evident that the C2S formation is facilitated in MS1 compared to MS2 (a 283 higher relative proportion is observed). This could be explained by the occurrence of 284 palygorskite as it is the only main difference between both materials. However, this difference 285 of C2S formation between MS1 and MS2 is not observable by XRD, meaning that the main part 286 of this C2S is amorphous.

287

Overall, the results of XRD and MAS NMR indicates that 800°C is probably the best 288 calcination temperature for both MS1 and MS2. Moreover, MS1 seems to be more sensitive to 289 the calcination, which leads to higher dehydroxylation of the clay phases and to higher relative

Self-reactivity of calcined marlstones in water 296

The calcination study of MS1 and MS2 revealed the formation of different reactive phases such 297 as lime, periclase and C2S as well as the loss of crystallinity of the clay phases. MS1 and MS2

298

were thus hydrated in water in order to study their reactivity, the contribution of each phases, 299 their evolution and the possible presence of synergies. After 7 days of hydration, for both MS1 and MS2, diffraction peaks characteristic of portlandite, 306 calcite, hydrotalcite and brucite appear on the diffractograms. The formation of portlandite 307 comes from the hydration of lime and calcite comes from its carbonation. The magnesium from 308 the periclase is incorporated into two phases: brucite and hydrotalcite.

309

The formation of brucite can be problematic in cements. The hydration of periclase into brucite 310 is a phenomenon that has relatively long kinetic compared to the hardening kinetic of cement.

311

This post-hardening swelling can lead to cracking problems and therefore a loss of mechanical 312 performance if there is not enough space available in the matrix [START_REF] Cherney | Cement Growth Failure Mechanism in Porcelain Suspension 569 Insulators[END_REF]. Studies are currently 313 underway to determine whether neoformed brucite leads to cracking in this system. However,

314
there is no post-hardening swelling phenomenon associated with hydrotalcite (the second 315 magnesian phase neoformed in our system) formation in the existing literature and the possible 316 existence of synergies with the cement phases could enhance the formation of hydrotalcite 317 instead of brucite in calcined marlstone-cement system.

318

The amount of neoformed brucite and portlandite is higher for MS2 than for MS1 (larger area 319 under the peaks), therefore it is difficult to compare the evolution of these two phases between 320 MS1 and MS2. With the increasing hydration time, the intensity of the peaks characteristic of 321 portlandite decrease for MS1 and MS2. However, after 180 days of hydration the portlandite 322 diffraction peaks are no longer detectable in MS1. This total consumption of portlandite in MS1 323 could be a first indication of a possible stronger pozzolanic reactivity compared to MS2.

324

However, this apparent consumption of portlandite may also be due to its carbonation into 325 calcite as we observe an increase in the intensity of the peak associated with calcite with 326 increasing hydration time.

327 Fig. 5. shows the evolution of the 29 Si MAS NMR spectra of MS1 (Fig. 5.A) and MS2 (Fig.

402

The better mechanical performance of M-MS1 confirms the previous results assessed by 27 Al 403 and 29 Si MAS NMR. The occurrence of palygorskite in MS1 leads to a higher degree of 404 dehydroxylation at 800°C in comparison to MS2 (Fig. 2.). This enhances the pozzolanic 405 reactivity of 800°C-MS1 in comparison to 800°C-MS2 (Fig. 6.) and results in higher 406 compressive strengths at 7 and 28 days (Fig. 7.). These results suggest that the palygorskite has 407 a higher pozzolanic activity after calcination at 800°C than smectite. To a lesser extent, the

  94second objective is to study the physico-chemical changes taking place during the calcination 95 and the reaction of these new pozzolans and to correlate this information with the final 96 mechanical performances. The last objective is to determine how palygorskite (an 97 unconventional clay contained in one of the two materials) contributes to the pozzolanic

157 29 Si

 29 MAS NMR spectra were acquired in a 7 mm MAS probe using a Bruker NEO 300 MHz 158 spectrometer with the following parameters : MAS frequency of 5 kHz, excitation pulse length 159 of π/2 and repetition time of 10s. The chosen 10s repetition time is too short to obtain a 160 quantitative signal from the quartz, but as quartz is unreactive shorter repetition time can be 161 used to save spectrometer time.

  162

Fig. 1 .

 1 Fig.1. displays the evolution of MS1 (Fig.1.A) and MS2 (Fig.1.B) X-ray diffractograms as a

  197

Fig. 1 .

 1 Fig. 1. Evolution of the X-ray diffractograms of MS1 (A) and MS2 (B) as a function of the

  for both MS1 and MS2 the 001 peak of smectite (Sm) is shifted from 6.2°(2θ) to 202 8.75°(2θ). This shift is explained by the dehydration of the smectite during the heat treatment 203 (the removal of the interlayer water leads to a decrease in d001 basal spacing)[START_REF] Bala | Dehydration transformation in Ca-537 montmorillonite[END_REF][START_REF] Morodome | Swelling Behavior of Na-and Ca-Montmorillonite up to 539 150°C by in situ X-ray Diffraction Experiments[END_REF].204Concerning MS1, the characteristic peaks of palygorskite disappears at 600°C, reflecting its 205 loss of crystallinity. 206 207 At 800°C, for both MS1 and MS2 the dolomite (CaMg(CO3)2) decarbonates into lime (CaO) 208 and periclase (MgO) and a peak associated with dicalcium silicate (C2S) appears. However, the 209 low intensity of the C2S peaks (possibly due to low crystallinity of the phase) hinders the 210 determination of the type of polymorph. The fact that this C2S is formed at the same temperature 211 as the lime supports the hypothesis that it results from a recombination phenomenon between 212

219 220Fig. 2 .

 2192 Fig.2. displays the evolution of the27 Al MAS NMR spectra of MS1 (Fig.2.A) and MS2 (Fig.

  227

2 .Fig. 2 .Fig. 3 .

 223 Fig. 2. Evolution of the 27 Al MAS NMR spectra of MS1 (A) and MS2 (B) as function of the

  262

3 . 264 Fig. 3 .

 32643 Fig. 3. Evolution of the 29 Si MAS NMR spectra of MS1 (A) and MS2 (B) as function of the

273

  total signal is only composed of Q 3 silicon atoms of smectite and Q 3 (1Al) silicon atoms of 274 biotite and smectite. For both MS1 and MS2, the increase of the calcination temperature leads 275 to a broadening of the resonances associated with the Q 3 of the clay phases. This broadening is 276 explained by a distribution of the environments of the silicon atoms and indicate a structural 277 loss of the clay phases upon calcination [46]. At 800°C a new resonance appears at -71 ppm in 278 MS1 and MS2 spectra. This new resonance corresponds to Q 0 silicon atoms and can be 279 associated with the silicon contained in the C2S structure [47], in accordance with XRD results. 280 With increasing the calcination temperature there is a low-frequency shift of the Q 3 resonances 281

290

  amount of C2S. The major difference between MS1 and MS2 being the occurrence of 291 palygorskite in MS1, it seems clear that smectite and palygorskite have different responses to

300Fig. 4 .Fig. 4 .

 44 Fig.[START_REF] Yazıcı | Mechanical properties of reactive powder 446 concrete containing high volumes of ground granulated blast furnace slag[END_REF]. displays the evolution of the X-ray diffractograms of MS1 (Fig.4.A) and MS2 (Fig.4.B)

  328

  5.B) calcined at 800°C and hydrated in water during 7, 14, 28 and 180 days.

  329

Fig. 5 .

 5 Fig. 5. Evolution of the 29 Si MAS NMR spectra of MS1 (A) and MS2 (B) calcined at 800°C

Fig. 6 .

 6 Fig. 6. Relative proportions of silicon-containing phases of MS1 (A) and MS2 (B) calcined at

391 3 . 3 .Fig. 7 .Fig. 7 .

 3377 Fig. 7. displays the compressive strength of M-Ref, M-MS1 and M-MS2 after 7 and 28 days of

408 1 .

 1 higher mechanical performance of M-MS1 compared to M-MS2 could also be attributed to the 409 greater amount of C2S neoformed during the calcination (facilitated by the occurrence of 410 the use of two calcined marlstones as SCMs in terms of calcination, self-414 reactivity in water and mechanical performance in cementitious blends. The two marlstones 415 differ mainly by the presence of palygorskite (MS1) or not (MS2). Based on the results 416 presented, the following conclusions can be drawn: 417 The calcination of MS1 and MS2 leads to the dehydroxylation of the clay phases associated 418 with the formation of lime, periclase and C2S. A calcination temperature of 800°C resulted in 419 the highest expected reactivity for both MS1 and MS2. The degree of dehydroxylation and the

Table 1 .

 1 Chemical analysis of MS1 and MS2. 108

	107
		Element	O	Ca	Si	Mg	Al	Fe	K	P	Na	Ti	V
	MS1	wt. %	55.5	14.2	15.6	6.0	4.7	1.5	1.0	0.6	0.2	0.2	0.1
	MS2	wt. %	56.0	16.4	16.2	5.5	3.8	1.2	0.2	0.2	0.2	0.2	0
	109
	110	MS1 and MS2 have a very close chemical composition, especially regarding the proportions of
	111	silicon, calcium, magnesium and aluminium. These similarities in the chemistry of MS1 and
	112	MS2 are not surprising as both materials originate from the same sedimentary deposit.
	113	X-ray diffraction analysis (XRD) was performed to highlight the mineralogical composition
	114	MS1 and MS2 and the different phases were quantified using the Rietveld refinement technique
	115	(see supporting information). Table 2 shows the results of the quantitative phases analysis for
		MS1 and MS2.

116

Table 2 .

 2 wt.% of the crystalline phases of MS1 and MS2.

	117
		Phase	Dolomite	Palygorskite	Smectite	Biotite	Hydroxylapatite	Quartz
	MS1	wt.%	54	17	16	2	3	8
	MS2	wt.%	59		27	1	2	11

. Hydration of calcined marlstones 130

	131	The calcination and hydration of MS1 have already been the subject of two previous studies
	132	[29, 30] which demonstrated the formation of several reactive phases after calcination, notably
	133	CaO. As MS2 has a very close mineralogy to MS1, it is very likely to have the same reactive
		phases formed. Therefore, the pozzolanic activity of calcined MS1 and MS2 could not be

134

measured using the classical pozzolanic activity tests (Chapelle test

[START_REF] Ferraz | Pozzolanic activity of metakaolins by the 523 French standard of the modified Chapelle test: A direct methology[END_REF]

and R 3

[START_REF] Avet | Development of a new rapid, relevant and 526 reliable (R3) test method to evaluate the pozzolanic reactivity of calcined kaolinitic clays[END_REF]

), which 135 requires the material tested not to contain free calcium. It was therefore decided to characterise 136 the reactivity of these calcined materials by carrying out hydration tests.

137

For both MS1 and MS2, 1g of 800°C calcined material was manually mixed with water 138 according to the chosen water to binder ratios (w/b = 0.8; 1; 2 and 4). The mixtures were left to 139 react in sealed conditions for 7, 14, 28 and 180 days. At the end of each time period, the 140 hydration was stopped by freezing at -24°C during 24h and freeze-drying during 48h under vacuum. The dried materials were crushed and analysed by X-ray diffraction (XRD) and solid 142 state nuclear magnetic resonance (MAS NMR). As the result analysis did not reveal any 143 significant differences between the different w/b, only the hydration results for w/b = 0.8 are 144 reported. 145 146 2.4. X-ray diffraction analysis (XRD) 147 Measurements were made with a Bruker D8 diffractometer using a Bragg-Brentano geometry 148 with a copper anode tube X-ray source (40 kV/40 mA) emitting Cu Kα radiation. The 149 acquisitions of the diffractograms were made between 4° and 60° 2θ with a step size of 0.017° 150 2θ and a measurement time of 1s per step. The Rietveld quantification of MS1 and MS2 was 151 performed using the Profex Rietveld refinement program [33].

152 153 2.5. Solid state nuclear magnetic resonance (MAS NMR) 154 27 Al MAS NMR spectra were acquired in a 2.5 mm MAS probe using a Bruker Avance III 500 155 MHz spectrometer with the following parameters : MAS frequency of 30 kHz, excitation pulse 156 length of π/13, radio frequency field of 11 kHz and repetition time of 1s.

Calcined marlstone-cement blends and compressive strength measurements

  was used to perform the spectral decompositions PC) superplasticizer (0.6% by total weight of binder) was used to obtain a 182 flow equivalent to that of the control mixture (± 5 mm). Table4shows the proportions of the

	167 183
	184	investigated mortars.
	168	The compressive strength was assessed on mortar cubes (50 x 50 x 50 mm 3 ) made with graded
	169 185	standard (supports ASTM C109) sand (sand to cement ratio = 2.75) and a constant water to
	170	binder ratio (w/b = 0.484). Blended cement was obtained by mixing 80 wt.% of general use
		Portland cement (GU) with 20 wt.% of 800°C calcined MS1 (for M-MS1 mortar) or MS2 (for
	177
		Label	d10 (µm)	d50 (µm)	d90 (µm)
		MS1	3.5	19.5	64.1
		800°C-MS1	4.0	24.5	91.6
		MS2	2.4	14.8	53.1
		800°C-MS2	3.6	28.2	66.3
		GU cement	3.5	20.5	59.7

165

(see supporting information). 166 2.6. 171 M-MS2 mortar). The particle size distributions of the GU cement and MS1 and MS2 (before 172 and after calcination) are shown in Table 3. The d-values indicate the maximal particle size 173 diameter that includes 10% (d10), 50% (d50) and 90% (d90) of the particles (volume-weighted 174 basis). 175 Table 3. Particle size distributions of GU cement and marlstones (MS1 and MS2) before and 176 after calcination at 800°C. 178 Both 800°C-MS1 and 800°C-MS2 have a particle size distribution close to that of GU cement, 179 which ensures homogeneous blends. Control mortars cubes (labelled M-Ref) were prepared using 100% GU cement. A 181 polycarboxylate (

Table 4 .

 4 Mix proportions of mortars

	186
	Label	Cement (g)	Pozzolan (g)	Sand (g)	Water (g)
	M-Ref	500	0	1375	242
	M-MS1	400	100	1375	242
	M-MS2	400	100	1375	242

187

Mortars were mixed and specimens were molded according to the standard procedure ASTM 188 C109

[START_REF]C01 Committee Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 535 2-in. or [50-mm] Cube Specimens)[END_REF]

. After molding, the specimens were placed in plastic bags for 20-24h until 189 demolding. The demolded mortar cubes were stored in saturated lime water until testing. At 7 190 and 28 days the compressive strength was assessed according to the ASTM C109 loading 191 procedure

[START_REF]C01 Committee Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 535 2-in. or [50-mm] Cube Specimens)[END_REF]

.

  From 7 to 180 days of hydration the relative proportion of silicon in Q 3 -Q 4 configuration associated with 383 the calcined clay phases decreases from 67% to 39%. In parallel, the relative proportion Q 1 ,

	384
	385	Q 2 (1Al) and Q 2 silicon associated with the C-(A)-S-H increases from 14% to 43%. The
	386	consumption of the calcined clay phases, associated with the formation of C-A-S-H and the
	387	consumption of portlandite during hydration confirms the pozzolanic reactivity of MS1 and
	388	MS2 calcined at 800°C. However, this pozzolanic reactivity is more important for MS1 (which
		contains palygorskite) than for MS2 (	𝑄 𝑛 (𝐶𝐴𝑆𝐻) 𝑄 3	= 1,93 for MS1 and	𝑄 𝑛 (𝐶𝐴𝑆𝐻) 𝑄 3 +𝑄 4 = 1,13 for MS2
	379			1 , Q 2 (1Al) and Q 2 silicon associated
		with the C-(A)-S-H increases from 30% to 58%.

380

For MS2, after 7 days of hydration, the relative proportion of silicon in Qn configuration 381 associated with C-A-S-H is only 14%, which is much lower than for MS1 (30%). 389 after 180 days of hydration). Compressive strengths were measured on mortars incorporating 390 the two marlstones calcined at 800°C to confirm their different reactivity.

be associated with akermanite which is not yet crystallised enough to be detectable by XRD at 348 800°C. Finally, several studies have highlighted the formation of gehlenite during the 349 calcination of marlstones [START_REF] Shoval | Mineralogical changes upon heating calcitic and dolomitic marl rocks[END_REF][START_REF] Hughes | Roman cements -Belite cements 581 calcined at low temperature[END_REF] and this signal at -73 ppm could therefore be associated with 350 this phase.


Keywords: privacy, anonymization, sniffer, hashing, IoT

Public Wi-Fi (IEEE 802.11) networks are an abundant data source that may serve different applications such as epidemic tracking and prevention, disaster response, crowdsensing, or ubiquitous urban services. Nevertheless, collecting and exploiting such data brings many privacy liabilities, considering that each transmitted frame has the MAC address (a unique device identifier) of the corresponding personal device, also considered sensitive information. Literature has shown that the MAC randomization performed by manufacturers of phones is not enough to protect devices' identification. Data obfuscation is a promising solution to avoid storing advertised identifiers of devices and prevent attackers from acquiring sensitive data. Obfuscating such identifiers while also being able to differentiate frames sent by different devices poses a significant challenge for frame capturing by low-resource IoT devices in real-time. This paper illustrates the impact of on-the-fly hashing as an obfuscating measure to protect people's privacy. Since no popular off-the-shelf sniffer (using wireshark or tcpdump) allows for on-the-fly hashing, we build upon the scapy library a custommade sniffer capable of hashing the required data needed to protect user privacy. We demonstrate the viability of this privacypreserving IoT sniffer on a Raspberry Pi platform.

I. INTRODUCTION

Passive frame sniffing has become an effective source of information given the widespread adoption of mobile devices with IEEE 802.11 capabilities, from smartphones to IoT (Internet of Things) devices. The passively collected data has multiple applications such as predicting user-mobility, contact tracing, and opportunistic networks [START_REF] Kwon | Poster abstract: Passive activity classification of smart homes through wireless packet sniffing[END_REF], [START_REF] Das | Leveraging ambient sensing for the estimation of curiosity-driven human crowd[END_REF], [START_REF] Huang | Pedestrian flow estimation through passive WiFi sensing[END_REF], [START_REF] Koh | Multiple-perspective clustering of passive Wi-Fi sensing trajectory data[END_REF], [START_REF] Manjappa | Estimating crowd distribution using smart bulbs[END_REF], [START_REF] Sheikh | Sniffers and Social Engineering[END_REF], [START_REF] Vattapparamban | Indoor occupancy tracking in smart buildings using passive sniffing of probe requests[END_REF]. Recently, passive sniffing has become easier through low-power microcomputer units, which enable entire operating systems to run on embedded devices, while allowing rapid development with tools initially made for desktop environments. Such sniffing flexibility enables developers to propose scalable solutions that allow multiple sniffers to acquire data from large regions and have a good sense of the medium being monitored.

Nevertheless, passive sniffing development is limited by data-protecting legislation that requires such devices to create a sniffing solution with obfuscation techniques to protect the user's privacy. The legislation exists due to the demonstration of various kinds of attacks from sniffed wireless packets in the literature [START_REF] Bravenec | Exploration of user privacy in 802.11 probe requests with MAC address randomization using temporal pattern analysis[END_REF], [START_REF] Longo | Pairing wi-fi and bluetooth mac addresses through passive packet capture[END_REF], [START_REF] Nazerian | Passive iot device fingerprinting using wifi[END_REF].

Sniffers are vulnerable to a variety of attacks which can be broadly classified into compromising (i) confidentiality, (ii) data protection, (iii) integrity, iv) availability and (v) access control [START_REF] Bekri | Intelligent IoT systems: security issues, attacks, and countermeasures[END_REF], [START_REF] Sinha | Information security threats and attacks with conceivable counteraction[END_REF]. We focus on the first three types of privacy breaches in this work, as they are the most devastating from the user's perspective.

We introduce a promising and particularly challenging objective to circumvent the breaches mentioned above: on-thefly obfuscation. A sniffing process interfaces with the wireless network interface card (NIC) for frame acquisition. For each captured frame, the sniffing process anonymizes and saves it in the disk storage of the sniffer. Hence, on-the-fly obfuscation brings the benefits of reducing consequences of possible attacks to the sniffer. We illustrate the collection process for the sniffed public Wi-Fi packets in Figure 1. First, the user data is protected at the end of the capturing process inside the sniffer (represented by a Raspberry Pi in the figure). Second, any sensitive data leakage happening during the transfer from the sniffer RAM to its disk will never compromise the rest of the data previously stored at the disk.

In opposition to the aforementioned on-the-fly sniffing proposal, a regular data-network packet capturing and analyser tool, such as tcpdump [START_REF]Tcpdump[END_REF] directly store captured data in its raw format at the sniffer disk, without any previous data protection. Posterior protection, as for hashing anonymization, has to be performed separately at the sniffer hard disk after the raw data storage. In case of a malicious attack, this poststorage protection processing clearly brings vulnerability to passive sniffing strategies and to collected suers data.

However, on-the-fly obfuscation poses a limitation to constrained devices: The more complex the obfuscation technique is, the more frames passive sniffers might lose when their CPU time is dedicated to frames' anonimisation processing than reading. This limitation affects the collected data resolution and quality. To measure and understand related limitations, we We analyze the performance of on-the-fly obfuscation techniques using a Raspberry Pi 4 model B (Pi) device, refered hereafter as privacy-preserving sniffer. The objective is to assess this single-board computer's capacity to capture, process, and obfuscate public Wi-Fi probe-requests. Indeed, public Wi-Fi probe-requests are considered sensible data and useful in tracking applications. In this context, we use as benchmark, the previously mentioned state-of-the-art packet capturing tool, the tcpdump, which does the packet sniffing without any obfuscation. Then, we compare the performance of a privacypreserving sniffer with the benchmark one under different modes of obfuscation.

The main contributions of this paper are:

• We introduce the on-the-fly obfuscation on constrained sniffing devices as an alternative for more secure handling of sniffed privacy-sensitive data. • We perform a thorough analysis of the impact of each privacy algorithm on the Pi's ability to process packets when imposing the on-the-fly obfuscation. The analysis provides a discussion on the features and trade-offs of such sniffers in resource-constrained devices. • We provide in-depth insights on the resource utilization of Pis during the entire capturing process of a privacypreserving sniffer. • Upon publication of this work, we intend to release the on-the-fly sniffer tool as an open-source solution to onthe-fly obfuscation of public Wi-Fi frames. This paper is organized as follows. Section II briefly introduces the scanning process that generates the data to be obfuscated. Section III discusses the methodology to measure the performance of a Raspberry Pi when hashing. Section IV discusses the results and corresponding insights. Section V presents related work on packet sniffers and user privacy. Section VI concludes the paper and presents future directions.

II. SCANNING IN WIFI

The IEEE 802.11 protocol [13, p. 821] defines scanning as a process for discovering nearby access points by host devices. The process can be active or passive. In active scanning, Fig. 2: Format of a MAC management frame header with privacy concerning fields highlighted in red with sizes in bytes. Optional fields are represented with dotted lines. devices transmit frames asking for the identity, connection, and compatibility information from all nearby Access Points (AP). In passive scanning, a device just eavesdrops for beacon frames transmitted by the AP.

Both methods use management frames which contain three MAC address fields that could compromise user privacy, as seen in Figure 2. Some of those frames may also carry information about the Service Set Identifier (SSID) related to a particular access point (AP). The rest of the frame is composed of numerous frame body elements which hold information about multiple aspects of the protocol, the device, and even arbitrary data that is specific to the device's manufacturer.

The passive scanning consists of an AP transmitting at fixed intervals, a beacon frame that contains its MAC address, SSID, and other relevant information about this particular AP. This information could be used to estimate the sniffer's location based on the surrounding SSIDs and prior knowledge of their geographical locations. This beacon frame is sniffed by applications that keep a database of geographical locations of APs. Even with significant relevance, this data is associated with fixed APs most of the time and doesn't disclose any information about users present in the same region.

In this paper, we focus on active scanning frames since those frames contain the potential relevant user-location tracking information and also sensitive data.

A. Active scans

The active scanning procedure consists of a user's device emitting a probe request frame, asking all nearby APs to respond with probe responses with their respective MAC addresses and SSID values. The frame could be a wildcard probe request which requests the response of all of the APs nearby or contains the specific SSID and the MAC address of a particular AP known to the user to limit the search of available connections.

B. Privacy threats

A user's device usually sends its true MAC address in the source address field of the probe request and often sends along with the known SSIDs. Some devices send randomised MAC addresses, but even then, they could be tracked [START_REF] Tan | Efficient association of wi-fi probe requests under mac address randomization[END_REF]. The SSIDs can be associated with places regularly attended by the user (her house, workplace, and so on). This reveals that it is not only the user device that can be identified from the probe request but also the previous networks it has been connected to. Suppose publicly available databases containing the geographical location of publicly available SSIDs are utilized. In that case, it is possible to infer geographical information about a user just by passively sniffing probe requests, posing a significant privacy leak. [START_REF] Di Luzio | Mind your probes: De-anonymization of large crowds through smartphone WiFi probe requests[END_REF] extracts geographical information from probe requests to match people in a crowd with their places of origin.

Given the high occurrence that such frames could reach in some areas [START_REF] Hu | Is there WiFi yet? How aggressive WiFi probe requests deteriorate energy and throughput[END_REF] and also the relevance of the information it carries, it is necessary to create a secure method for obfuscation of those values without compromising the ability to distinguish between frames sent by different users.

As explained in Section I, the introduced on-the-fly sniffer addresses the issue of compromising user-sensitive data by obfuscating it before dumping it in the sniffer's local memory. We consider the MAC address and one SSID field when analyzing the impact of obfuscation in on-the-fly processing.

III. REALISING A PRIVACY-PRESERVING SNIFFER

In this section, we first detail the proposed privacypreserving sniffer. Next, we describe the construction of the benchmark sniffer. We then present the experimental setup and describe the different configurations used for our experiments.

A. Privacy preserving sniffer

The privacy-oriented on-the-fly sniffer is a module capable of being efficiently deployed over a Pi. We present its design and workflow in Figure 3. The sniffer stays in a loop and, for each received packet, checks if the packet is a probe request using a filter and then, if valid, applies the obfuscation to the desired fields. Hashing is the obfuscation technique we choose in this paper.

We develop the sniffer on top of scapy [START_REF] Scapy | [END_REF], an opensource tool for creating sniffers, injectors, and general tasks of packet manipulation. We choose scapy as it provides a ready-made interface to interact with the sniffer functionality of network cards at a high level. It allows us to introduce specific functionalities using packet contents each time a new packet is received.

In the sniffer, for each received frame, we hash received address fields and truncate its value to the same length as the original MAC address (48 bits) before saving it to the disk (hence on-the-fly). The truncation after hashing MAC addresses is a way to purposefully introduce collisions and therefore mitigate the issues pointed out in [START_REF] Demir | The pitfalls of hashing for privacy[END_REF]. We hash and truncate SSIDs to 48 bits as well.

Once we complete the obfuscation, the sniffer saves the current frame in a PCAP file format to the buffer. In this work, we consider two different hashing algorithms (SHA256 [START_REF]Secure Hash Standard -SHS: Federal Information Processing Standards Publication[END_REF] and MD5 [START_REF] Rivest | RFC1321: The MD5 Message-Digest Algorithm[END_REF]) while varying the frame fields to obfuscate. The two algorithms were selected for their efficiency in running in a lightweight system like the Pi.

Though, in the sniffer's design, we let the user have the flexibility of specifying any hash algorithm and the set of packet fields. Moreover, the sniffer's current implementation allows further optimization in the future using techniques like multi-threading and the periodically use hash-tables to save the obfuscated MAC addresses. to avoid repeated calculations

B. Benchmark sniffer

We consider the performance-benchmark sniffer as the sniffer utilizing the state-of-the-art packet capturing tool: tcpdump. It allows sniffing on a selected interface and dumps the captured data in the same PCAP format on Pi's disk. However, there is no anonymization of frame fields in the sniffed data. Moreover, this sniffer has limited functionalities regarding frame-filter configurations, trace file options, and truncation of the captured frame while storing sniffed frames.

Although, due to its implementation in C language and the use of the libpcap [START_REF]Tcpdump[END_REF] to provide an Application Programming Interface (API) to a selected NIC for sniffing, it is expected to perform better than our proposed on-thefly obfuscation sniffer. However, this comes at the cost of neglecting user privacy, which makes it a perfect choice as a benchmark sniffer to investigate the degree of privacy vs. the performance trade-off.

C. Experimental setup

We illustrate the experimental setup in Figure 4. We use two Raspberry Pi Model 4B single-board computers for the experiments, equipped with external TP-Link TL-WN722N v3.2 wireless interfaces. We place the sniffers in an enclosed metal container with only a single open passage for power and Ethernet cables. One Raspberry Pi acts as a transmitter to send Wi-Fi frames, while the other Pi acts as a receiver that captures frames in monitor mode and stores them. We control the Pis using another machine with built-in scripts to configure each device and control all aspects of the test procedure. Also, a USB extension links another wireless interface inside the metal box to a desktop computer outside. The desktop sends the respective commands to the Pis and sniffs the environment inside the box. While running the experiments, we deploy another Python process run in parallel to collect the resource usage information at the receiver node. The process saves the resource information every 3 seconds and it does so throughout the entire capture for each test configuration.

We develop a custom transmitter module written in C language utilising libpcap [START_REF]Tcpdump[END_REF]. The transmitter reads from a PCAP capture file and uses the timestamp information of each contained frame to time the frame injection into the NIC of choice. Its function is to replicate the environment captured by the PCAP files by sending the frames within with the same delay between them as they were received. We evaluate the performance of the transmitter in Section IV-A.

We synthetically generate the PCAP files selected for frame transmission for our experiments. We use scapy and its packet creation and manipulation functionalities to realize the PCAP generation module. This module creates probe request capture files with flexible inter-frame delays, the size of the SSID elements, and the size of frame payloads.

D. Experimental scenarios

Each experiment consists of transmitting a specific synthetically generated capture file while running a sniffer with a specific set of configurations. The output of an experiment is a PCAP capture file from the receiver, a resource graph with CPU and memory utilisation, CPU temperature, and, frame transmission information to calculate the average throughputs in kbps and frames per second. The receiver's capture file is used to measure the receiver's throughput by slicing it in 10 seconds intervals and averaging the results for each slice to obtain statistical results for the whole experiment.

In total, we transmit 10 separate capture files that were created synthetically using the file creator mentioned in Section III-C. All of the files only contain probe requests with arbitrary values for all of the fields without any time delay between frames, to potentially saturate the channel. The frame body elements are variable, and it is not possible to determine a fixed probe request size. Two test groups are considered: One with the smallest probe request size possible and the other with the largest one.

To emulate a larger-sized probe request, we append extra bytes after the SSID element, limited to a maximum of 1024 extra bytes. In this paper, we denote these extra bytes in probe requests as "payload." For each experiment group with and without payload, we vary the size of SSID between 0, 8, 16, 24, and 32 bytes. Finally, the number of transmitted frames is chosen considering the corresponding frame size (determined by the SSID and the payload), to homogenise all the test duration to around 3 minutes (cf. Section IV-A).

We list all experimental configurations in Table I. For each configuration, we define an identification (ID) to label experimental results (A -F). For all the configurations, we set both sniffers to channel 1 and Berkeley Packet Filters (BPF) to capture only probe requests. For the on-the-fly sniffer, we configure it to capture and store directly without hashing as well as after hashing with MD5 and SHA256. When hashing is enabled, we test two hashing possibilities: hashing addresses only and hashing addresses and the SSID value.

Also, we execute a third sniffer on the desktop, which confirms that packets sent by the transmitter were actually being transmitted over the medium and not being dropped by the operating system of the Pi.

IV. RESULTS

In this section, we first show the results regarding the behaviour of the transmitter module. Next, we compare the throughput of the privacy-preserving and the benchmark sniffer. Then, we show the impact of the hashing in the on-the-fly sniffer. Finally, we illustrate the overall resource utilisation by sniffers.

For the box plots in this section (Figures 5 through 9), we limit the whiskers by 1.5 times the interquartile range (except on figure 9, where they limit the maximum and minimum values). Also, orange bars represent median and green triangles average values. When no whiskers or boxes of a column appear, it signifies that the variation was not enough to appear in the plot with the used resolution.

A. Frame transmission

We observe the transmitter throughput with respect to the variation in the size of SSID contained in each frame, in Figure 5. The grey markers represent the transmission with an extra payload appended to the end of each frame and the orange markers represent the transmission without the extra payload. Each pair from those different groups are aligned with the X-axis column that displays their selected size for the SSID field.

Figure 5a shows that the minimum throughput obtained between all transmission types is over 725 kbps from the files without payload whereas the maximum reaches 1 Mbps. The variation in throughput from 1 Mbps is due to imprecision in the compensation method of adjusting the frames per second rate to match the maximum throughput available. The management frames are transmitted at base rates [START_REF]Wireless LAN medium access control (MAC) and physical layer (PHY) specifications[END_REF] and for our selected version (802.11b), the chosen base rate is 1 Mbps. This validates the correctness of our transmitter.

This compensation is demonstrated when comparing the y axis of the graphs in Figure 5b. For both graphs, it is expected that the frames per second rate decrease with larger frames. Since the files without payload have a frame size significantly lower, the rate in frames per second has to be one order of magnitude higher to keep the bit rate relatively constant and the tests at the same duration.

B. Frame reception in a privacy-preserving sniffer

We see a relevant characteristic of the receiver's throughput in Figure 6. In the case without payload, where the throughput is around 2,000 frames per second, tcpdump majorly outperforms the other configurations by at least 12 times more, as seen in Figure 6a. When inspecting the same graph without the tcpdump column (ID A in Table I), it is possible to see that the maximum throughput achieved by the module does not pass 140 frames per second, as seen in Figure 6b.

The graphs in Figure 8 confirm that this limit is not a data rate constraint since that with payload the on-the-fly sniffer can reach very close to the maximum throughput (as seen in Figure 8b), which indicates an underlying problem with packet creation that limits its processing considerably. Although this is a considerably smaller rate than the tcpdump original alternative, it is still larger than the average rates of probe requests per second found in [START_REF] Hu | Is there WiFi yet? How aggressive WiFi probe requests deteriorate energy and throughput[END_REF]. Therefore, this method would still be sufficient in a real-world scenario with the majority of frames being probe requests.

The throughput measured in kbps can be seen in Figure 8. Figure 8a shows the impact of the frame per second limitation on the throughput. Since the limitation is observed when dealing with frames per second, large frames (i.e. the ones with the extra bytes of payload) get processed at the same rate and therefore the apparent throughput appears higher. Both graphs from Figure 8b show the throughput in kbps of all of the configurations with only the payload.

The graph on the left displays the statistical throughput for intervals of 10 seconds while the graph on the right displays the total average of the entire transmission run. The purpose of the graph on the right is to assure stability between tests and it shows that the transmitter side kept stable throughout the different sniffer configurations varying only about 2% between tests. When compared to the receiver side, it's possible to see that without any obfuscation functionality the Raspberry was able to follow the transmission's throughput while keeping the throughput stable. The considerably lower value for average is due to outliers that are not shown at this scale.

C. Impact of hashing on the on-the-fly sniffer

The efficiency of the hashing functions is better analysed in Figure 7. The figure shows the throughput in the same way as the graph on the left of Figure 8b, but now they appear in frames per second and three graphs now illustrate the impact of different SSID lengths, with and without payload.

The three graphs show the following patterns: When no hashing function is applied (ID B), the throughput tends to be higher by median except for Figure 7a when there's no payload. Between different hashing algorithms either with (IDs E and F) or without (IDs C and D) payload, the addresses don't display any significant changes except for Figure 7b where the SHA256 algorithm (selected in IDs D and F) has better results than the MD5 algorithm.

Finally, except for the graph in Figure 7a, losses in frames per second when the hashing was enabled were in the best case scenarios of about 6,7% when not hashing the SSID and 10% when doing so. This comparison indicates that in some cases the on-the-fly sniffer had a noticeable impact on its performance when it applied hashing functions to the frames.

D. Overall resource utilisation

The resource usage all followed the same pattern between SSID values. The CPU usage was 100% for all configurations using the on-the-fly sniffer, as seen in Figure 9a. Noticeably tcpdump had little to no impact on the CPU usage. The higher CPU usage is expected from the on-the-fly sniffer, given the per-packet obfuscation that it performs.

The memory usage varied but never grew over 15 MB, the average size of the synthetically created capture files, as can be seen in Figure 9b. Memory consumed less space without the payload but its consumption noticeably increased the more the SSID size increased. The temperature of the PI was kept constant at around 75º throughout the experiment. The resource plots for one SSID value can be seen in Figure 9.

V. RELATED WORK

In regards to the privacy liabilities involving probe requests, numerous studies point out the issues with the current standard and various ways of exploiting them. In [START_REF] Kumar Mishra | Public wireless packets anonymously hurt you[END_REF], it is illustrated multiple attacks that circumvent MAC address randomisation (a technique done by users devices that consists of attributing a new random address to protect one's real MAC address) by timing attacks and inferences. [START_REF] Di Luzio | Mind your probes: De-anonymization of large crowds through smartphone WiFi probe requests[END_REF] uses the probe requests information to leverage information about crowds regarding their region of origin and plurality, which became an indirect measurement for an election prediction. [START_REF] Greenstein | Can Ferris Bueller still have his day off? Protecting privacy in the wireless era[END_REF] has discussions about scenarios in which possible and more personal attacks can be performed with this sort of data. This paper considers those issues as a necessity to create obfuscation solutions for sniffing.

The hashing method, as pointed out in [START_REF] Demir | The pitfalls of hashing for privacy[END_REF], [START_REF] Vanhoef | Why MAC address randomization is not enough: An analysis of Wi-Fi network discovery mechanisms[END_REF] has its drawbacks in regards to privacy. It's known that the range of MAC addresses is limited by manufacturers and this poses an advantage for a brute force approach in an attempt to deobfuscate the frames. The implementation detail needed to circumvent this issue is considered in our work. As seen in [START_REF] Demir | Analysing the privacy policies of wi-fi trackers[END_REF], those are not the only desired characteristics in a wifi-based tracking application that make them privacy-oriented, and for IoT devices that implies significant more overhead than what is observed in our work.

The availability of probe requests in the real-world scenario is discussed in [START_REF] Hu | Is there WiFi yet? How aggressive WiFi probe requests deteriorate energy and throughput[END_REF]. In their work, sniffing is done in a stadium to measure the number of probe requests made in a crowded environment without any access points nearby. They find that the maximum throughput measured is approximately 62 frames per minute. Our work shows that this number is well within the range of all of the sniffing approaches, and the analysis considered involved idealistic and worst-case scenarios to identify where are the bottlenecks.

There have been efforts in the literature to evaluate the performance aspects of a WiFi tracking system using IoT devices. In [START_REF] Li | A case study of WiFi sniffing performance evaluation[END_REF], factors of the capture process such as the operating channel, the signal strength of nearby access points and the number of devices in the vicinity are all considered to evaluate the performance of a sniffing solution. However, the work lacks consideration with regard to the privacy of collected data.

VI. CONCLUSION

This paper analyses the privacy vs the performance trade-off when using the introduced privacy-preserving sniffer, which uses on-the-fly obfuscation. The traditional sniffer outperforms ours' rate by 12 times, but our approach processes frames at a rate above what was observed in a crowded scenario [START_REF] Hu | Is there WiFi yet? How aggressive WiFi probe requests deteriorate energy and throughput[END_REF]. The obfuscation techniques decreased the sniffers' performance as expected, but the results were only constant when comparing the number of fields to be hashed, and performance differences over the hashing algorithm used were inconclusive. This demonstrates the promise of the on-the-fly sniffer specifically in non-saturated wireless conditions. The traditional approach has an advantage in resource consumption and may impact the decision between the two studied options.

More advanced obfuscation techniques could be explored and its impact on a Raspberry Pi could be analysed. The optimisations mentioned in Section III-A can be investigated in future versions of the on-the-fly sniffer.

Fig. 1 :

 1 Fig. 1: Diagram of the data flow in the sniffing process for a privacy-preserving sniffer. Data is given unprotected by the NIC driver to the sniffing process that secures the data and saves it at the sniffer hard disk.

Fig. 3 :

 3 Fig. 3: Design of the privacy-preserving sniffer.

Fig. 4 :

 4 Fig. 4: Diagram of the setup and wired connections.

  (a) In kbps. (b) In frames per second.

Fig. 5 :

 5 Fig. 5: Transmission average throughput results for all files and separated between files with and without extra payload.

  (a) All configurations. (b) All configurations except the first column (tcpdump without payload).

Fig. 6 :

 6 Fig. 6: Receiver's throughput for SSID of size 0 of different configurations.

  (a) Throughput for SSIDs with 8 bytes. (b) Throughput for SSIDs with 16 bytes. (c) Throughput for SSIDs with 32 bytes.

Fig. 7 :

 7 Fig. 7: Receiver's throughput for all of the on-the-fly sniffer configurations, with the SSID size varying in 8, 16 and 32 bytes.

  (a) All configurations with (P.) and without (No P.) payload. (b) With-payload configurations and with the transmitter's average side-by-side.

Fig. 8 :

 8 Fig. 8: Receiver's throughput for SSID of size 0.

Fig. 9 :

 9 Fig. 9: Receiver's resource monitoring for different configurations with 16-byte long SSID.

TABLE I :

 I List of experimental configuration identifications.

This work was funded by the ANR MITIK project (French National Research Agency (ANR) -PRC AAPG2019), CNPq, Coordenac ¸ão de Aperfeic ¸oamento de Pessoal de Nível Superior Brasil (CAPES), Finance Code 001, FAPERJ, and Fundac ¸ão de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Grant 15/24494-8.


Massimiliano Gerometta
email: massimiliano.gerometta@agrosupdijon.fr

Jeancarlo Renzo

Sandra Domenek
email: sandra.domenek@agroparistech.fr

Jeancarlo R Rocca-Smith

Thomas Karbowiak
email: thomas.karbowiak@u-bourgogne.fr

Physical and Chemical Stability of PLA in Food Packaging

come

Introduction

Introduction

Bioplastics are a new generation of biobased and/or biodegradable polymers [START_REF] Reddy | Biobased plastics and bionanocomposites: current status and future opportunities[END_REF]European Bioplastic Association, 2018b). Conventional plastics (i.e. petroleum-derived and non-biodegradable) have transformed our society and are used in all aspects of our everyday life. The worldwide production of conventional plastics reached 348 million tons in 2017. Although it represents less than 1% of the world production of conventional plastics, the bioplastic market has been continuously growing over the last years, reaching around 2.11 million tons in 2018, and it is estimated to reach 2.62 in 2023 (World Economic Forum, 2016; European Bioplastic Association, 2018a). Food packaging represents one of the main applications of bioplastics. The most popular biopolymers are 30% biobased poly(ethylenterephtalate) (Bio-PET30), biobased polyethylene (Bio-PE) and poly(lactic acid) (PLA) (European Bioplastic Association, 2018a). Among them, only the last one meets both criteria, i.e. being biobased and biodegradable. PLA benefits from an interesting balance between costs, functional properties and processability. The European Bioplastic Association has recently reported that the global production of PLA exceeded 210 thousand tons in 2017, and has forecast that PLA capacity will grow by 50% in 2022. PLA is compliant with European and North American legislation for food contact materials.

The first synthesis of PLA dates back to 1932 when researchers from DuPont [START_REF] Carothers | Studies of polymerization and ring formation. X. The reversible polymerization of six-membered cyclic ester[END_REF] developed a method to polymerize lactide to produce PLA that was later patented by DuPont in 1954. Due to its high costs and ability to hydrolyze in biological environments, it was firstly used only for biomedical applications. The low molar mass of PLA (<100000 g mol -1 ) is now widely used in that field, as stents [START_REF] Soares | Constitutive framework for biodegradable polymers with applications to biodegradable stents[END_REF][START_REF] Erne | The road to bioabsorbable stents: reaching clinical reality?[END_REF], surgical sutures [START_REF] Maharana | Melt-solid polycondensation of lactic acid and its biodegradability[END_REF], and tissue cultures or drug delivery devices [START_REF] Langer | Drug delivery and targeting[END_REF]. In the early 1990s, the ring opening polymerization process was developed (Gruber et al., 1994;Gruber et al., 2001) to produce high molar mass PLA (>100000 g mol -1 ) for different industrial purposes such as fibers and textiles, plasticulture, and food packaging and serviceware [START_REF] Castro-Aguirre | Poly(lactic acid) -mass production, processing, industrial applications, and end of life[END_REF].

Food corporations such as Danone (Germany), Del Monte (USA) and Noble Juice (USA), as well as leading food retailers such as Wal-Mart (USA), SPAR (Austria) and Auchan (France) are already using PLA based packaging in some of their products [START_REF] Castro-Aguirre | Poly(lactic acid) -mass production, processing, industrial applications, and end of life[END_REF]. Various types of foods are currently packed and commercialized with PLA-based materials. These foods are characterized by different physical states, a wide range of water activity (a w ), pH and composition, and moreover different shelf lives. Some examples are potato crisps (amorphous solid, a w = 0.1-0.3), fresh salads and vegetables (biological gel, a w > 0.99), yogurts (gel, a w > 0.99), citrus juices (acidic liquid, a w = 0.97-0.99) and water (liquid, a w > 0.99) (Fig. 1).

Short Overview of PLA Synthesis and Properties

The constitutive unit of PLA is lactic acid, which is derived by lactic fermentation of glucose, obtained from different vegetal sources (Fig. 2). Some examples are corn, sugar cane or sugar beet. Polymers based on lactic acid can be synthesized by different routes: i) direct lactic acid condensation, often using coupling agents to increase molecular weight and, ii) ring opening polymerization (ROP) of lactide. This sometimes leads to confusion in the nomenclature used in the literature. Polymers derived from direct polycondensation should be referred to as poly(lactic acid) and polymers synthesized by ROP of lactide as poly(lactide). Both types of macromolecules are generally named PLA. ROP of lactide is by far the most employed route to obtain high molecular weight polymers used in the packaging industry [START_REF] Ducruet | Characteristics and applications of PLA[END_REF]. This route first consists of a direct polycondensation reaction to produce a low molar mass prepolymer. The prepolymer is then depolymerized in order to obtain lactides (3,6-dimethyl-1,4-dioxane-2,5-dione), being dehydrated cyclic dimers, which are the starting material for ROP into high molar mass polymers. Three forms of lactide can be obtained as a function of the stereochemistry of the feedstock, temperature and catalyst: L,L-lactide, D,D-lactide and D,L-lactide (meso-lactide) [START_REF] Garlotta | A literature review of poly(lactic acid)[END_REF]. The stereocontrol of the macromolecular chain can be achieved by ROP. D-lactic acid perturbs the chain packing of PLA and its quantity has therefore a large impact on the macroscopic properties of the polymer (e.g. glass transition and crystallization behavior). Enantiopure PLLA and PDLA can form a stereocomplex [START_REF] Ikada | Stereocomplex formation between enantiomeric poly(lactides)[END_REF], caused by stereoselective interactions (mostly van der Waals forces) locking the chains into a novel material with different physical properties such as changed crystalline form and higher thermal stability. Many newer technological developments are associated with this finding, and more information can be found in recent reviews, such as Bai et al. (2017).

The obtained PLA polymer can then be processed using the conventional manufacturing techniques like extrusion, injection molding, blow molding, cast film extrusion and thermoforming, in order to achieve different PLA goods such as films, rigid containers, bottles and others. PLA is a transparent polymer, glassy at room temperature, and has satisfying mechanical properties with a tensile strength ≈ 60 MPa, an elongation at break ≈5% and a Young modulus 3-4 GPa (approximate values taken from the review of Domenek et al., 2018). After being exposed to different environmental conditions according to their application, PLA goods can be fully biodegraded or recycled [START_REF] Auras | An overview of polylactides as packaging materials[END_REF][START_REF] Castro-Aguirre | Poly(lactic acid) -mass production, processing, industrial applications, and end of life[END_REF][START_REF] Jamshidian | Poly-Lactic Acid: production, applications, nanocomposites, and release studies[END_REF].

Although there is a general interest to increase the quantity of foods packed in PLA, several factors are currently limiting its application as a food packaging material. PLA displays an intermediate barrier performance to oxygen (O 2 ), carbon dioxide (CO 2 ) and Physical and Chemical Stability of PLA in Food Packaging

U N C O R R E C T E D P R O O F 3 nitrogen (N 2
) compared to medium or high barrier plastics such as poly(ethylene terephthalate) (PET), poly(amide) (PA), poly(vinylidene chloride) (PVDC) or poly(vinyl alcohol) (PVA) [START_REF] Rocca-Smith | Toward sustainable PLA-based multilayer complexes with Improved barrier properties[END_REF]. Its properties are near the benchmark poly(styrene) (PS). PLA has furthermore lower water vapor barrier properties compared to most petroleum-derived polymers. A recent review was dedicated to this subject [START_REF] Sonchaeng | Poly(lactic acid) mass transfer properties[END_REF]. Another factor that reduces the use of PLA is its inherent water sensitivity. Even though PLA sorbs small quantities of water [START_REF] Auras | An overview of polylactides as packaging materials[END_REF][START_REF] Witzke | Introduction to Properties, Engineering, and Propsects of Polylactide polymers[END_REF], it readily reacts with water molecules and is hydrolyzed over time, thus compromising the performance of PLA material (Rocca-Smith et al., 2017b). Other factors which restrict it use in the market are the undesirable crinkling noise that PLA makes when it is handled, its rather low thermal stability and its high price.

Stability Testing of PLA

Containment, protection and preservation represent the basic functions provided by food packaging. They can obviously be accomplished only if the material maintains its integrity in terms of chemical structure and physical state over time. Therefore, the stability of the food packaging material is intimately related to the shelf life of the product and, consequently, to the quality of the food. In the food industry, the food quality term involves a large variety of meanings which depend on the food product category. In the food technology literature, there is thus no single and simple definition of the shelf life, due to its complexity [START_REF] Robertson | Food Packaging: Principles and Practice[END_REF]. In general, it is described as a finite length of time, after production (including in some cases maturation or aging) and packaging, during which the food retains a required level of quality under well-storage conditions [START_REF] Nicoli | Shelf Life Assessment of Food[END_REF]. According to this definition, the shelf life of food represents the failure time at which the level of quality becomes unacceptable to the consumer. Regarding European legislation, shelf life was described for the first time in Directive 13/EC (2000) on food labeling, then repealed by Regulation No 1169 (2011), as the date of minimum durability of foodstuffs. A first definition of shelf life was given in Commission Regulation (EC) 2073 (2005) based on microbiological criteria for foodstuffs. It means either the period preceding the "use by" or the "minimum durability" date as defined in the articles 9 and 10 of the European Directive 2000/13. Concerning the safety of the packaging, Commission Regulation (EC) 10 (2011) defined migration test conditions in order to validate food contact materials. In that case, food simulants are used to mimic:

• hydrophilic foods: ethanol 10% (v/v) food simulant A, acetic acid 3% (w/v) food simulant B, ethanol 20% (v/v) food simulant C.

• Lipophilic foods: ethanol 50% (v/v) food simulant D1, vegetable oil food simulant D2 (used for foods which contain free fats at the surface).

• Dry foods: poly(2,6-diphenyl-p-phenylene oxide), particle size 60-80 mesh, pore size 200 nm, food simulant E.

Dealing with biodegradable packaging, it is of utmost importance to ensure their stability under actual conditions of use in contact with food, in order to preserve their shelf life. The environmental factors such as relative humidity, temperature, and ultraviolet (UV) light play a key role in the stability of biodegradable packaging. These parameters also influence their biodegradation process after use [START_REF] Holm | Shelf life of foods in biobased packaging[END_REF]. To date, neither in the scientific literature nor in the legislation, has any standard procedure been proposed for determining the stability of PLA or other biodegradable packaging during their service lifetime. Nevertheless, it is noteworthy that from an environmental point of view, the same concept applies to composting and biodegradation methods used to analyze the end of life of the material (NF 13432:2000). Concerning industrial applications other than food packaging, in the agronomic field PLA has to comply with the ultimate aerobic biodegradability in soil reported in the standard ISO 17556:2012, while in the medical domain, standard specifications for PLA and poly(lactide-co-glycolide) resin for surgical implants are provided in ASTM F2579-2010 and in ISO 13781:2017.

The inherent water sensitivity of PLA is a serious limitation for products having medium or long shelf life, as reported in different shelf life studies [START_REF] Peelman | Use of biobased materials for modified atmosphere packaging of short and medium shelf-life food products[END_REF][START_REF] Romani | Effect of different new packaging materials on biscuit quality during accelerated storage[END_REF][START_REF] Panseri | Feasibility of biodegradable based packaging used for red meat storage during shelf-life: a pilot study[END_REF]. In order to use PLA as a valuable packaging material for food with a long shelf life, Accelerated Shelf Life Testing (ASLT) has to be designed, as for food, in order to analyze the deterioration process under defined environmental conditions (e.g. temperature and relative humidity) which accelerate by a known factor the reaction rate, reducing the time of analysis [START_REF] Robertson | Food Packaging and Shelf Life: A Practical Guide[END_REF].

This article will thus focus on the importance of using appropriate storage tests to better know the stability of PLA and for later better controlling the changes according to the food application. Based on these considerations, the objective is to review the advances reached in understanding the interactions between water or food simulants and PLA, focusing on the impacts on its chemical and physical stability in its conditions of use as a food contact material.

Chemical Stability of PLA

One of the most important issues about using PLA as a food packaging material is related to its sensitivity to moisture. The hydrolysis phenomenon reduces the molecular weight of PLA, which is considered as the step preceding biodegradation [START_REF] Gorrasi | Effect of PLA grades and morphologies on hydrolytic degradation at composting temperature: assessment of structural modification and kinetic parameters[END_REF]. As reported in the scientific literature, the main studies on the sensitivity of PLA to water have been carried out in the biomedical field and, specifically, in contact with liquid media simulating biological systems [START_REF] Li | Hydrolytic degradation characteristics of aliphatic polyesters derived from lactic and glycolic acids[END_REF][START_REF] Migliaresi | A study on the in vitro degradation of poly (lactic acid)[END_REF]Tsuji et al., 2000b;[START_REF] Tsuji | Hydrolytic degradation[END_REF].

In contrast, only limited information about the stability of PLA is reported in the literature when it is used as a food packaging material, and thus exposed to different environmental conditions in terms of temperature, relative humidity and pH.

Hydrolysis is the main route to PLA degradation. The hydrolysis kinetics of PLA under mild temperature conditions (T < 40 °C) corresponding to a physiological environment, or to the storage conditions of many foods, are of importance for the design of PLA-based materials such as surgical implants or drug delivery devices. Upon immersion in aqueous environments, water only penetrates into the amorphous phase and cannot penetrate the crystallites [START_REF] Sonchaeng | Poly(lactic acid) mass transfer properties[END_REF]. This results in primary degradation of amorphous

U N C O R R E C T E D P R O O F
domains. In the presence of water and a catalyst, ester hydrolysis occurs, leading to a decrease in molecular weight. Once oligomers are small enough to dissolve, mass loss of the PLA is observed.

Main Mechanisms of Degradation in Contact With Liquid Water

The general hydrolysis reaction of the ester groups within the polymer in the presence of water can be either acid-or base-catalyzed [START_REF] Lyu | Kinetics and time-temperature equivalence of polymer degradation[END_REF]. More specifically, when PLA is used as packaging in contact with foods, which are mostly acidic products, the acid-catalyzed hydrolysis mechanism is as detailed in Fig. 3.

The hydrolytic degradation of PLA starts from the surface and its progression is dependent on the ratio of degradation kinetics and dissolution kinetics of oligomers or carboxylic acids. If the degradation products are not leached from the surface, carboxylic acid end groups accumulate in the amorphous phase where they have an autocatalytic effect [START_REF] Li | Biodegradation of aliphatic polyester[END_REF]. In particular, in case of thick objects, the leaching of catalytic molecules from the center is slow compared to the hydrolytic degradation which evolves much faster. The amount of absorbed water, the diffusion coefficient of the chain fragments within the polymer and the solubility of degradation products are thus the main factors that control PLA hydrolysis in contact with aqueous media (Schliecker et

Effect of Relative Humidity

Focusing in more detail on the effect of relative humidity (RH) and the physical state of water, Fig. 4 shows the evolution over time of the mean molar mass ( ) of PLA films (of ∼20 μm thickness), stored at 50 °C, in various environmental conditions mimicking dry to humid foods. The progressive decrease in , from 50% to 100% RH environments, confirms that the higher the RH, the faster the hydrolysis occurs. In parallel to the decrease in the ( ), after 62 days at 50 °C and 100% RH, the glass transition temperature (T g ) of PLA is also Actually, increasing the RH induces an increase in chemical potential of water, an increase of the water uptake by PLA, and, consequently, an acceleration of the hydrolysis reaction [START_REF] Copinet | Effects of ultraviolet light (315 nm), temperature and relative humidity on the degradation of polylactic acid plastic films[END_REF]. Moreover, the degradation products, accumulating in the film under high RH conditions, act as acid catalysts for the hydrolysis of PLA. However, in the case of PLA in direct contact with liquid water, the hydrolysis reaction occurs slower than for the highest RH conditions because the degradation products are solubilized into the medium in contact

Effect of pH

The pH of foods or beverages may also influence the rate of hydrolysis. Rocca-Smith, 2017 investigated this effect using McIlvaine's citric acid-phosphate buffer to control the pH of the liquid phase in contact with PLA films (for food packaging applications) in the range from 3 to 7 (under accelerated conditions at T = 50 °C). The analysis of the molecular weight distribution of PLA clearly indicated that pH had no influence on the rate of hydrolysis (Fig. 5).

Similar results with no effect of pH have also been reported for amorphous polylactide used in biomedical applications [START_REF] Lyu | Kinetics and time-temperature equivalence of polymer degradation[END_REF]. In this work, the pH range was even broader (from HCl water solution at pH 0 up to NaOH water solution at pH = 12) for PLA immersed at 37 °C, for 500 days.

U N C O R R E C T E D P R O O F

Effect of Temperature

The effect of temperature on the hydrolysis rate of PLA immersed in aqueous media is classically described by the Arrhenius model:

where k is the rate constant for deteriorative reaction, A the collision frequency, Ea (J•mol -1 ) is the activation energy, R the gas constant (8.314 J K -1 mol -1 ) and T the temperature in K. The rate at which the hydrolysis takes place obviously depends on the physical state of polymer, and thus on the temperature.

Three temperature ranges can be identified accordingly with different activation energy (Table 1) [START_REF] Tsuji | Hydrolytic degradation[END_REF]. Above the melting temperature of PLA, between 180-250 °C, the activation energy was found to be close to 50 kJ mol -1 [START_REF] Tsuji | A new Strategy for recycling and preparation of poly(l-lactic acid): Hydrolysis in the Melt[END_REF]. The activation energy in the range of temperature between 120-160 °C was reported around 70 kJ mol -1 [START_REF] Tsuji | Comparative study on hydrolytic degradation and monomer recovery of poly(l-lactic acid) in the solid and in the melt[END_REF]. For temperatures below the glass transition temperature (T g ), between 21 and 45 °C, the kinetics of hydrolysis of PLA reached a higher value of activation energy (>80 kJ mol -1 ) [START_REF] Makino | Preparation and in vitro degradation properties of polylactide microcapsules[END_REF]. Hydrolysis thus occurs faster at higher temperatures and has a lower activation energy compared to temperatures below T g . This hydrolysis phenomenon can be described in two steps: a first slow rate, followed by a second accelerated rate. This rate difference is smaller when hydrolysis occurs at high temperature, probably due to the increased chain end mobility [START_REF] Lyu | Kinetics and time-temperature equivalence of polymer degradation[END_REF]. Therefore, according to the temperature range in which the hydrolysis phenomenon is studied, different kinetic behaviors can be observed. The degradation at high temperature above T g follows the Arrhenius model (first order kinetics), while below T g it can more accurately be described by the Williams-Landel-Ferry model [START_REF] Lyu | Kinetics and time-temperature equivalence of polymer degradation[END_REF].

Physical Stability of PLA

When an amorphous or semi-crystalline polymer is stored at temperatures below its T g , slow molecular rearrangements occur in the amorphous region, because only restricted motion as rotation of small groups of atoms and vibration and rotation motion of chemical bonds along the polymer can occur. Over time, the polymer relaxes towards its thermodynamic equilibrium, which causes changes in bulk properties such as volume or enthalpy. This phenomenon is called physical aging [START_REF] Struik | Physical aging in plastics and other glassy materials[END_REF][START_REF] Hutchinson | Physical aging of polymers[END_REF]. It can cause dimensional instability of the polymer, embrittlement over time and changes in gas barrier properties. Above T g , the amorphous phase aging caused by relaxation phenomena is not thought to happen because the thermodynamic equilibrium is installed. In the case of semi-crystalline polymers, three different phases can coexist [START_REF] Wunderlich | Reversible crystallization and the rigid-amorphous phase in semicrystalline macromolecules[END_REF]. Phase coupling of amorphous and crystalline phases can give rise to an amorphous fraction with decreased macromolecular mobility, i.e. the Rigid Amorphous Fraction (RAF), which cannot relax at T g . Except for the crystalline phase, the polymer needs to be described with two fractions in the amorphous phase, the Mobile Amorphous Fraction (MAF) and the RAF. The RAF vitrified at temperatures higher than T g can be subject to physical aging.

PLA has a glass transition temperature (T g ) of approximately 60 °C and is therefore sensitive to quick physical aging at room temperature (i.e. storage below but near T g ). The phenomenon is often investigated by the analysis of the enthalpy recovery peak in a DSC experiment [START_REF] Pan | Enthalpy relaxation and embrittlement of poly(l-lactide) during physical aging[END_REF][START_REF] Monnier | Physical aging in PLA through standard DSC and fast scanning calorimetry investigations[END_REF][START_REF] Dobircau | Molecular mobility and physical ageing of plasticized poly(lactide)[END_REF][START_REF] Righetti | Contribution of the rigid amorphous fraction to physical ageing of semi-crystalline PLLA[END_REF]). An example of the increase in the enthalpy of recovery peak linked to T g is given in Fig. 6 [START_REF] Delpouve | Evidence of two mobile amorphous phases in semicrystalline polylactide observed from calorimetric investigations[END_REF]. This T g is attributed to the MAF. Comparing Fig. 6A (amorphous PLA) and Fig. 6B (maximum crystallized PLA, X c = 42%), a broadening of this recovery peak linked to T g can be observed due to higher heterogeneity in the mobility of the amorphous phase constrained between spherulites. In the case of insufficient crystallization, Fig. 6C (semi-crystalline PLA, X c = 22%) shows that distinct types of mobility can coexist in this amorphous fraction. Both the peak overshoots of relaxation enthalpy linked to T g , consequently to physical aging, and corresponding to the inter-spherulitic amorphous phase and to the intra-spherulitic amorphous phase (inflection point I, Fig. 6C) are observed in that case. The RAF devitrification only occurs between 80 and 130 °C. The RAF contributes to the physical aging mainly at high undercooling (low temperature), while the structural relaxation at low undercooling (high temperature) occurs apparently via cooperative segmental motions.

Table 1

Activation energy of the hydrolysis of PLA according to the characteristic temperatures of the polymer

U N C O R R E C T E D P R O O F U N C O R R E C T E D P R O O F
At high undercooling, only small-scale local motions can take place, and this also occurs in the RAF [START_REF] Righetti | Contribution of the rigid amorphous fraction to physical ageing of semi-crystalline PLLA[END_REF]. Moreover, it has been shown that an increase of D-lactic acid content in the macromolecular structure increases the physical aging [START_REF] Aou | Roles of conformational and configurational defects on the physical aging of amorphous poly(lactic acid)[END_REF].

Recently, fast scanning calorimetry (FSC) was also used to study physical aging of PLA. This technique allows heating up and cooling down polymers as fast as a thousand Kelvin per second, which permits reaching the thermodynamic equilibrium of aging in relatively small time intervals [START_REF] Monnier | Physical aging in PLA through standard DSC and fast scanning calorimetry investigations[END_REF][START_REF] Monnier | Reduced physical aging rates of polylactide in polystyrene/polylactide multilayer films from fast scanning calorimetry[END_REF]. This provides new means for theoretically investigating the aging behavior of PLA as a function of the polymer morphology. Monnier et al., 2018 showed, for example, that constraints imposed by the RAF in the amorphous phase can decelerate the physical aging rate.

From a practical point of view, the physical aging of PLA induces embrittlement [START_REF] Pan | Enthalpy relaxation and embrittlement of poly(l-lactide) during physical aging[END_REF][START_REF] Burgos | Characterization and ageing study of poly(lactic acid) films plasticized with oligomeric lactic acid[END_REF][START_REF] Avolio | Plasticization of poly(lactic acid) through blending with oligomers of lactic acid: effect of the physical aging on properties[END_REF]. [START_REF] Pan | Enthalpy relaxation and embrittlement of poly(l-lactide) during physical aging[END_REF] showed in their pioneering work the decrease of the elongation at break of PLA during aging (Fig. 7). [START_REF] Burgos | Characterization and ageing study of poly(lactic acid) films plasticized with oligomeric lactic acid[END_REF] analyzed the oxygen barrier properties of aged PLA and showed an increase of almost 20% in 90 days of aging at 25 °C and 50% RH. This decrease was even more important in the case of plasticized samples, similar to what is also observed for other types of plasticized materials [START_REF] Ruellan | Palm oil deodorizer distillate as toughening agent in polylactide packaging films[END_REF]. A more detailed analysis on physical aging of plasticized PLA can be found in [START_REF] Ruellan | Plasticization of poly(lactide)[END_REF].

In addition, the physical aging of PLA in contact with water was shown to be faster compared to dry conditions. This effect could be explained by the capacity of water to form clusters inside the polymer matrix, even at very low absorbed quantities. The water clusters bring additional free volume to the structure and could thus be responsible for the acceleration of segmental motion even in the absence of any degradation due to hydrolysis [START_REF] Vyavahare | Analysis of structural rearrangements of poly(lactic acid) in the presence of water[END_REF].

Effect on the Crystalline Phase of PLA

The increase in the percentage of crystallinity (X c ) is a well-known phenomenon in PLA under hydrolytic degradation which occurs also in initially amorphous PLA. Hydrolysis-induced crystallization has been confirmed by different authors, using different techniques such as DSC, WAXS (Wide-Angle X-ray Scattering) and FTIR (Fourier Transform Infrared spectroscopy). The increased chain mobility, induced by the small degradation products and water molecules, favors plasticization and also crystallization of PLA chains [START_REF] Migliaresi | A study on the in vitro degradation of poly (lactic acid)[END_REF]Tsuji et al., 2000a;[START_REF] Tsuji | Hydrolytic degradation[END_REF][START_REF] Harris | Heat and humidity performance of injection molded PLA for durable applications[END_REF][START_REF] Gorrasi | Effect of PLA grades and morphologies on hydrolytic degradation at composting temperature: assessment of structural modification and kinetic parameters[END_REF][START_REF] Kucharczyk | Novel aspects of the degradation process of PLA based bulky samples under conditions of high partial pressure of water vapour[END_REF]. Moreover, this phenomenon is highly influenced by the storage conditions. As an example, there was no significant change in X c (∼23%) for PLA films stored at 50% RH and 50 °C, while increasing the RH conditions up to 100%, or immersing PLA in liquid water, lead to a linear increase of X c with time, up to ∼36% (Rocca-Smith et al., 2017b). This also induced a progressive loss of the transparency of film samples (Rocca-Smith et al., 2017a). As a direct result, the melting temperature (T m ) first increased, due to an increase in the thickness of PLA crystals, favoring their thermostability (and then obviously a decrease for high hydrolysis states when chemical aging occurs). It is also worthy to note that such changes indirectly modify the respective ratios between the crystalline phase, the Rigid Amorphous Fraction (RAF), and the Mobile Amorphous Fraction (MAF) (Rocca-Smith et al., 2017b).

Conclusions and Perspectives

This article reports on the chemical and physical stability of PLA when stored in different environmental conditions mimicking food storage. The physical state of water plays a key role in the kinetics of the hydrolysis of PLA. Hydrolysis occurs faster when increasing relative humidity from 50% to 100%. However, when PLA is stored in direct contact with liquid water, the reaction is slower than for high RH conditions. In this latter case, the degradation products are solubilized in the liquid medium in contact. In contrast, when PLA is stored at high RH conditions, these compounds accumulate in PLA film, acting as catalyzers, and thus accelerating the hydrolysis rate (Fig. 8). Under conditions of use, it is also worth noting that physical and chemical aging of PLA will occur at the same time in most cases.

When PLA is stored at high RH conditions or immersed in liquid water, the shape of the endothermic peak associated with the glass transition becomes broader, due to the hydrolysis and plasticization effects of water that create water-water interactions (clusters) between the carbonyl groups, thus also accelerating the kinetics of physical aging. These storage conditions also lead to an increase in crystallinity (Fig. 8).

The rate of PLA hydrolysis in aqueous media depends on the temperature. For temperatures higher than the T g , the kinetics obeys the Arrhenius model and follow a first-order reaction. Moreover, a lower activation energy is required, due to a higher increase in chain mobility. For temperatures below the T g , the kinetics of hydrolysis can deviate from Arrhenian behavior and display a higher activation energy.

The pH does not affect the hydrolysis rate for PLA films (in the range from 3 to 7). This is key information for the application of PLA films in contact with moist foods whatever their acidity, as a very wide range of moist foods (cooked or fresh fruits and vegetables, cheeses, dairy products, meat and meat-based products, delicatessen, tomato paste, etc.) could be packed in PLA films or trays, provided that the shelf life of the PLA material would be longer than that of the packed product.

From a practical point of view, this overview of PLA stability indicates that PLA film is a very suitable packaging for semi-dry or dry foods, which are characterized by a w ≤ 0.5. In these conditions PLA is chemically stable and no hydrolysis occurs. Nevertheless, physical aging will take place, especially if the storage temperature is close to the T g . When PLA is used to pack high moisture foods or beverages having a w ≈ 1, particular attention has to be paid to the stability. In such cases, shelf life testing of the packaging is required, since the PLA can highly interact with the water molecules coming from the food and therefore be altered. Consequently, the functional properties of the packaging can be modified, and the protection of the food quality can be quickly lost during time. In the case of biodegradable polymers like PLA, it thus becomes necessary to estimate the shelf life of the packaging along with the shelf life of the food according to the application targeted.

Figure 1

 1 Figure 1 Selected examples of commercial applications of PLA as food packaging. Images taken from NatureWorks and brands website.

Figure 2

 2 Figure 2 Production and life cycle of PLA as food packaging.

  reduced to ∼46 °C and broadened (Rocca-Smith et al., 2017a). A similar decreasing trend in T g of PLA was observed in different studies carried out under various environmental conditions (Migliaresi et al., 1994; Gonzalez et al., 1999; Tsuji et al., 2000a; Tsuji et al., 2000b; Copinet et al., 2004).

  (Rocca-Smith et al., 2017b). When PLA is exposed to a humid environment, it can sorb small quantities of water (<1.1 wt% at T = 25 °C and close to saturation)[START_REF] Auras | An overview of polylactides as packaging materials[END_REF][START_REF] Witzke | Introduction to Properties, Engineering, and Propsects of Polylactide polymers[END_REF][START_REF] Holm | The stability of poly(lactic acid) packaging films as influenced by humidity and temperature[END_REF][START_REF] Kucharczyk | Novel aspects of the degradation process of PLA based bulky samples under conditions of high partial pressure of water vapour[END_REF]. It will then react with them, starting the hydrolysis phenomenon[START_REF] Li | Hydrolytic degradation characteristics of aliphatic polyesters derived from lactic and glycolic acids[END_REF][START_REF] Tsuji | Hydrolytic degradation[END_REF]. Whether PLA is in contact with water vapor or liquid water, its water content remains at a very low value until hydrolysis leads to a mean molar mass lower than 25000 g mol -1 , then increasing a lot with the absorbed water quantity(Rocca-Smith et al., 2017b;[START_REF] Lyu | Kinetics and time-temperature equivalence of polymer degradation[END_REF]. Obviously, in contact with liquid aqueous media, PLA dry matter is decreasing once the hydrolysis is sufficiently advanced, due to the solubilization of degradation products which are of small size, highly polar and hydrophilic[START_REF] Li | Further investigations on the hydrolytic degradation of poly (DL-lactide)[END_REF]; Tsuji et al., 2000a; Lyu et al., 2007; Rocca-Smith et al., 2017b).

Figure 3

 3 Figure3Schematics of ester hydrolysis (nucleophilic substitution reaction) which happens for PLA in the presence of water. Acid catalyzed hydrolysis is represented, as the main route of degradation for PLA packaging in contact with foods (which are mostly acidic).

Figure 4

 4 Figure 4 Kinetics of the hydrolysis rate of PLA, following the mean molar mass of PLA films stored at 50% RH, 65% RH, 75% RH, 85% RH, 95% RH, 100% RH or immersed in liquid water, at 50 °C. Error bars are standard deviations. Lines are guide for the eyes. Adapted from Rocca-Smith et al. (2017).

Figure 5

 5 Figure 5 Kinetics of PLA films degradation, reported as the weight average molecular weight ( ), when immersed in aqueous media at different pH, at 50°C.

Figure 6

 6 Figure 6 (A) DSC normalized heat flow obtained on PLA with aging times varying from 0 to 1000 h at 45 °C, at increasing time from left to right. The aging times are 30 min, 1 h, 2 h, 3 h, 5 h, 50 h, 72 h, and 1000 h. The curve corresponding to the zero aging does not exhibit any peak of recovery. (A) Amorphous PLA (B) Maximum crystallized PLA (X c = 42%). (C) Semi crystalline PLA (X c = 22%). Reproduced with permission from Delpouve et al. (2014). Copyright 2013 John Wiley and Sons.

  10

Figure 7 Figure 8

 78 Figure 7 (A) Yield strength, (B) tensile modulus, and (C) fracture strain as a function of log (aging time, t a /h) for PLLA samples annealed at 4, 25, and 40 °C. The dotted lines represent the data of the unaged PLLA specimen. Reproduced from Pan et al. (2007) Copyright © 2007 American Chemical Society with permission.

al., 2003; Auras et al., 2010, Rocca-Smith et al., 2017b).

  The macroscopic outcome of this mechanism is that thick pieces liquefy at first on the inside.

Reference Module in Food Sciences, Volume ■ https://doi.org/10.1016/B978-0-08-100596-5.22471-2 1