Asel Cristiana Nunes
email: nunes@itam.cas.cz

Maria Aguilar Sanchez

Sebastiaan Godts
email: sebastiaan.godts@kikirpa.be

Davide Gulotta
email: dgulotta@getty.edu

Ioannis Ioannou
email: ioannis@ucy.ac.cy

Barbara Lubelli
email: b.lubelli@tudelft.nl

Beatriz Menendez
email: beatriz.menendez@cyu.fr

Noushine Shahidzadeh
email: n.shahidzadeh@uva.nl

Zuzana Slížková
email: slizkova@itam.cas.cz

Magdalini Theodoridou
email: magdalini.theodoridou@newcastle.ac.uk

Asel Maria Aguilar Sanchez

Towards a more effective and reliable salt crystallisation test for porous building materials : Experimental research on salt contamination procedures and methods for3 assessment of the salt distribution

Keywords: salt crystallisation test, porous materials, salt accumulation, salt distribution 48 49 Graphical

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Introduction 53

Salt crystallisation is one of the most recurrent damage processes affecting porous building materials 54 worldwide. The durability of building materials with respect to salt crystallisation is often assessed by 55 performing accelerated weathering tests. According to Alves et al. [START_REF] Alves | Rock Features and Alteration of Stone Materials Used for 726 the Built Environment: A Review of Recent Publications on Ageing Tests[END_REF], salt weathering tests are the most 56 frequent ageing tests performed in the laboratory, reflecting their worldwide importance for conservation 57 and new construction. A recent literature review on this subject [START_REF] Lubelli | Towards a more effective and reliable salt crystallisation test for 730 porous building materials: state of the art[END_REF] reports a significant variation in the 58 weathering procedures used by researchers. Despite the availability of a European standard [START_REF]EN 12370 Natural stone test methods. Determination of resistance to salt crystallisation[END_REF], three 59

RILEM recommendations [START_REF]Recommended tests to measure the deterioration of stone and to assess the effectiveness of 733 treatment methods, Test V.1a-crystallisation test by total immersion (for untreated stone); Test V.1b-734 crystallisation test by total immersion (for treated stone); Test V.2-crystallisation test by partial immersion[END_REF], [START_REF]MS-A.1-determination of the resistance of wallettes against sulphates and 737 chlorides[END_REF], [START_REF]MS-A.2-uni-directional salt crystallisation test for masonry units[END_REF], and a guideline [START_REF]Sanierputzsysteme/ Renovation mortar systems[END_REF], a widely accepted and reliable salt weathering 60 test is still lacking. The use of different test procedures hinders the comparison between the results of 61 different studies. 62

To tackle this problem, in 2016, the RILEM Technical Committee ASC-271 "Accelerated laboratory test for 63 the assessment of the durability of materials with respect to salt crystallisation" (henceforth referred to 64 as TC ASC-271) was launched to develop an effective laboratory weathering test. The test should reliably 65 assess the durability of porous building materials against salt crystallisation within a reasonable period, 66 accelerating the deterioration process without significantly altering its mechanism. Since reliable 67 accelerated test procedures for simulating sea-salt spray are already available, e.g., [START_REF]EN 14147 Natural stone test methods -Determination of resistance to ageing by salt mist[END_REF], [START_REF]Standard Practice for Operating Salt Spray (Fog) Apparatus[END_REF], the work of 68 TC ASC-271 focuses on developing a procedure that simulates salt damage triggered by capillary transport 69 of salt solution towards the evaporative surface of a material. 70 TC ASC-271 proposes a new approach to salt crystallisation tests, which differs from existing weathering 71 procedures in this field. In this approach, which is derived from the field of concrete durability [START_REF] Tuutti | Corrosion of steel in concrete[END_REF], as 72 described in Flatt et al. [START_REF] Flatt | Predicting salt damage in practice: A theoretical insight into laboratory tests[END_REF], the salt crystallisation damage process consists of two phases: (i) the 73 induction phase, in which salts accumulate in the pores of the material (but the degree of pore filling is 74 not high enough to initiate damage); and (ii) the propagation phase, during which 75 dissolution/crystallisation cycles lead to (increasing) damage (Figure 1). 76

Starting from this approach, the new salt crystallisation test, under development by the TC ASC-271, 77 consists of two phases: (1) salt accumulation phase and (2) damage propagation phase. By this approach, 78 not adopted in existing salt crystallization procedures, it is possible to define the risk of decay due to 79 dissolution/crystallization cycles, depending on the salt content (and thus the degree of pore filling) 80 accumulated in the first stage. By separating the two phases, a better understating and forecast of the 81 decay can be achieved. 82

This paper aims to design the procedure to be followed in the accumulation phase of the salt 83 crystallisation test under development by the TC-ASC-271. The second phase of the test, i.e., the damage 84 propagation phase, is not addressed in this paper. The accumulation phase can be considered successful 85 if it enables the accumulation of salts at the material's evaporative surface (because this is the most 86 common situation observed on site) without the occurrence of damage (damage should only start in the 87 propagation phase). 88

For the assessment of the capability of the accumulation procedure to fulfil the requirement mentioned 89 above, it is crucial to measure the salt distribution in the specimen at the end of the accumulation phase. 90

This has been done with several methods (destructive and non-destructive), and their advantages and 91 disadvantages are critically discussed. 92 93 4 94 Figure 1: Schematic representation of durability of a stone, subject to salt damage, adapted from the 95 concept proposed by Tuutti [START_REF] Tuutti | Corrosion of steel in concrete[END_REF] to describe the durability of reinforced concrete (image from [START_REF] Flatt | Predicting salt damage in practice: A theoretical insight into laboratory tests[END_REF] 96 provided by Robert J. Flatt (ETH Zurich). 97

An extensive literature review of different salt crystallization procedures done by the TC ASC-271 is given 98 in [START_REF] Lubelli | Towards a more effective and reliable salt crystallisation test for 730 porous building materials: state of the art[END_REF]. Standardized tests and procedures proposed by researchers can be classified into two main groups 99 [START_REF] Lubelli | Towards a more effective and reliable salt crystallisation test for 730 porous building materials: state of the art[END_REF]: 100  Contamination by capillary absorption or full immersion in a salt solution, followed by drying 101 through one or more evaporative surfaces, e.g., [START_REF]EN 12370 Natural stone test methods. Determination of resistance to salt crystallisation[END_REF], [START_REF]Recommended tests to measure the deterioration of stone and to assess the effectiveness of 733 treatment methods, Test V.1a-crystallisation test by total immersion (for untreated stone); Test V.1b-734 crystallisation test by total immersion (for treated stone); Test V.2-crystallisation test by partial immersion[END_REF], [START_REF]MS-A.1-determination of the resistance of wallettes against sulphates and 737 chlorides[END_REF], [START_REF]MS-A.2-uni-directional salt crystallisation test for masonry units[END_REF], [START_REF] Angeli | Modification of the porous 752 network by salt crystallisation in experimentally weathered sedimentary stones[END_REF], [START_REF] Beck | Evaluation of the compatibility of building limestones from salt crystallisation 755 experiments[END_REF], [START_REF] Shahidzadeh-Bonn | Damage in Porous media due 757 to salt crystallisation[END_REF], [START_REF] Ioannou | Synchrotron radiation energy-dispersive X-ray 759 diffraction analysis of salt distribution in Lépine limestone[END_REF], [START_REF] Derluyn | Saline water evaporation and 761 crystallization-induced deformations in building stone: insights from high-resolution neutron radiography[END_REF], and 102  Contamination by continuous capillary absorption of a salt solution and simultaneous drying, the 103 so-called wick action, e.g., [START_REF] Goudie | Laboratory simulation of "the wick effect" in salt weathering of rock[END_REF], [START_REF] Rodriguez-Navarro | Salt weathering: influence of evaporation rate, supersaturation and 766 crystallisation pattern[END_REF], [START_REF] Benavente | Quantification of salt weathering in porous 769 stones using an experimental continuous partial immersion method[END_REF], [START_REF] Modestou | Tracing the salt crystallisation front in limestone using the DRMS[END_REF]. 104

In the present work, total immersion of the specimen in salt solution was not tested because it was 105 considered not representative of the salt weathering mechanism occurring in natural exposure conditions 106 [START_REF] Benavente | Quantification of salt weathering in porous 776 stones using an experimental continuous partial immersion method[END_REF]. Moreover, full immersion would not be suitable for assessing materials treated with surface 107 treatments or for a combination of materials, such as brick/stone/plaster combinations [START_REF] Lubelli | Towards a more effective and reliable salt crystallisation test for 730 porous building materials: state of the art[END_REF]. Regarding the 108 partial immersion procedure, the contamination can be done repeatedly with salt solution [e.g., 3] or by 109 introducing salts only in the first cycle, followed by rewetting with pure water [e.g., 23]. In the present 110 work, the partial immersion test considered both cases. However, rewetting with pure water was done to 111 mobilize the salts towards the evaporative surface and not to propagate damage. 112

Existing standards and recommendations prescribe the use of single salt solutions [START_REF]EN 12370 Natural stone test methods. Determination of resistance to salt crystallisation[END_REF], [START_REF]Recommended tests to measure the deterioration of stone and to assess the effectiveness of 733 treatment methods, Test V.1a-crystallisation test by total immersion (for untreated stone); Test V.1b-734 crystallisation test by total immersion (for treated stone); Test V.2-crystallisation test by partial immersion[END_REF], [START_REF]MS-A.1-determination of the resistance of wallettes against sulphates and 737 chlorides[END_REF], [START_REF]MS-A.2-uni-directional salt crystallisation test for masonry units[END_REF], usually 113 sodium sulphate or sodium chloride, though, in natural exposure conditions, mixtures of salts prevail. 114 Some researchers choose the composition of the salt mixture based on the analysis of salts present in the 115 building materials on site to reproduce better a specific field condition [START_REF] Lopez-Arce | Durability of stone-repair mortars 782 used in historic buildings from Paris[END_REF], [START_REF] Nunes | Study of nitrate contaminated samples from a historic building with 785 the hygroscopic moisture content method: Contribution of laboratory data to interpret results practical 786 significance[END_REF]. However, a standard 116 test can hardly consider the use of salt mixtures because this would require different thermohygrometric 117 conditions to make it effective in other materials, which would make impossible any direct comparisons. 118

Moreover, the action of single salts can prove more aggressive than salt mixtures [START_REF] Menendez | Effect of mixed vs single brine composition on salt weathering in porous 788 carbonate building stones for different environmental conditions[END_REF], [START_REF] Cardell | Weathering of limestone building material by mixed 791 sulfate solutions. Characterization of stone microstructure, reaction products and decay forms[END_REF]. Hence, the 119 present work considers the use of single salt solutions, following existing standards and 120 recommendations. 121

The salt solution concentration plays a crucial role in the type and severity of damage [START_REF] Ruiz-Agudo | The role of saline solution properties on 794 porous limestone salt weathering by magnesium and sodium sulfates[END_REF]. The use of 122 solutions with a very high salt content is common in salt crystallisation tests but can lead to unrealistic 123 damage types and severity, e.g., [START_REF] Macwilliam | Towards a More Realistic and Effective Use of Sodium Sulfate in Accelerated 797 Ageing of Natural Stone[END_REF]. The KIK-IRPA database encompassing more than six thousand 124 samples collected from buildings in Belgium records salt contents of ca. 1 to 2 wt.% with respect to the 125 dry weight of the sample collected onsite [START_REF] Godts | Common salt mixtures database: a tool to identify research needs[END_REF]. Therefore, in the present study, a realistic amount of salt 126 was introduced in both materials tested (1 wt.% of the dry weight of the stones). 127

The most common methods for the assessment of salt damage in laboratory tests consist of visual and 128 photographic examination and mass loss determination [START_REF]EN 12370 Natural stone test methods. Determination of resistance to salt crystallisation[END_REF], [START_REF]Recommended tests to measure the deterioration of stone and to assess the effectiveness of 733 treatment methods, Test V.1a-crystallisation test by total immersion (for untreated stone); Test V.1b-734 crystallisation test by total immersion (for treated stone); Test V.2-crystallisation test by partial immersion[END_REF], [START_REF]MS-A.1-determination of the resistance of wallettes against sulphates and 737 chlorides[END_REF], [START_REF]MS-A.2-uni-directional salt crystallisation test for masonry units[END_REF]. Mass loss is a standard method 129 for assessing damage in durability experiments and has also been used as a parameter for evaluating salt 130 crystallisation durability estimators by several authors, e.g., [START_REF] Shahidzadeh-Bonn | Damage in Porous media due 757 to salt crystallisation[END_REF], [START_REF] Benavente | Durability estimation of porous building stones 803 from pore structure and strength[END_REF], [START_REF] Desarnaud | Impact of the kinetics of salt crystallisation on 805 stone damage during rewetting/drying and humidity cycling[END_REF]. However, this method does 131 not allow obtaining information on damage and salt distribution inside the specimens. To this scope, other 132 methods and techniques can be used. 133 Ultrasonic pulse velocity (UPV) measurement is a simple non-destructive technique used to evaluate the 134 physicomechanical properties and the extent of weathering of building materials. UPV can provide 135 information about dynamic mechanical properties, porosity, and the presence of discontinuities in the 136 material [START_REF] Benson | Pore fabric shape anisotropy in porous 808 sandstones and its relation to elastic wave velocity and permeability anisotropy under hydrostatic pressure[END_REF]. As a result of the salts filling the pores, an increase of the UPV is recorded [START_REF] Aly | The Effect of Salt 811 Crystallisation on the Mechanical Properties of Limestone: Statistical Correlation between Non-destructive and 812 Destructive Techniques[END_REF], so this 137 technique can be used to assess salt distribution non-destructively. The drilling resistance measurement 138 system (DRMS) and the scratch tool (ST) are micro-destructive techniques that have been used 139 successfully to monitor the salt crystallisation front in salt weathered limestone, both in the laboratory 140 and in situ, by recording increased cutting resistances in areas where pore clogging due to salt 141 crystallisation occurred [START_REF] Modestou | Tracing the salt crystallisation front in limestone using the DRMS[END_REF], [START_REF] Modestou | Micro-destructive mapping of the salt crystallisation front 815 in limestone[END_REF]. The ST uses a diamond cutter to make a scratch (groove) on the surface 142 of the specimen while the cutting force is recorded. The resistance of the material to cutting is related to 143 its physicomechanical properties; in fact, the intrinsic specific energy, which is the energy required for 144 scratching a unit volume of material, is directly proportional to the uniaxial compressive strength of the 145 material [START_REF] Theodoridou | Micro-destructive cutting techniques for the characterization 817 of natural limestone[END_REF]. The DRMS operation is similar to the ST, but instead of linear cutting, the DRMS employs 146 rotational cutting (drilling). 147 Soluble salts are generally hygroscopic, i.e., they can adsorb moisture from the air when the relative 148 humidity (RH) is higher than the RH of equilibrium of the salt. In contrast, most building materials are only 149 weakly hygroscopic. Therefore, by measuring the hygroscopic moisture content (HMC) of a material at 150 high humidity, an indication of the presence of soluble salts can be attained. In the case of single salts, the 151 water activity is known; thus, a fixed relation between the salt content and the HMC at equilibrium can 152 be readily determined at a specific temperature [START_REF] Diaz Goncalves | Evaluating the salt content of salt-contaminated samples on 820 the basis of their hygroscopic behaviour, Part I: fundamentals, scope and accuracy of the method[END_REF], [START_REF] Nasraoui | A comparative study of hygroscopic moisture content, electrical 823 conductivity and ion chromatography for salt assessment in plasters of historical buildings[END_REF]. In the case of a salt mixture, several RH 153 equilibrium values are possible, which shift depending on the ionic composition [START_REF] Price | An expert chemical model for determining the environmental conditions needed to 826 prevent salt damage in porous materials[END_REF]. The HMC method 154 uses powder drilled samples, so it can be combined with the DRMS method, provided that the drilling is 155 performed at different depths within the same drilling hole. 156

The determination of the salt content within building materials in practice usually consists of extracting 157 the soluble salts with water and analysing the ion content of the leached solution. For a semi-quantitative 158 evaluation of the salts, the HMC method can be used, whereas ion chromatography identifies the ions 159 and provides an accurate quantitative analysis [START_REF] Nasraoui | A comparative study of hygroscopic moisture content, electrical 823 conductivity and ion chromatography for salt assessment in plasters of historical buildings[END_REF]. 160 µ-Xay fluorescence mapping is an elemental technique that allows examining small specimen areas and 161 the production of elemental maps, with a spatial resolution on the micrometre scale. It can provide the 162 excitation of at least one measurable characteristic X-ray peak for all periodic table elements with an 163 atomic number higher than 11. Hence, the method can be applied to detect common ions present in 164 soluble salts affecting building materials, e.g., chloride (atomic number: 17) and sulphur (atomic number: 165 16). The direct observation of specimens with SEM enables examining the spatial distribution, the 166 morphological character of the salt crystals, and the size of pores in which the salt preferentially 167 crystallizes. 168

The techniques mentioned above have been used in the present study to evaluate the effectiveness of 169 the designed contamination procedures in accumulating salts close to the evaporative surface. The 170 effectiveness of the methods used is hereby critically discussed. 171

Experimental 172

Contamination procedures 173

Two types of salt contamination procedures have been selected for this study and are described in detail 174 below. A scheme depicting the two procedures is shown in Figure 2. After the accumulation phase tests, 175 the specimens were split into two halves for the analysis described in section 2.2., except for the 176 specimens subjected to UPV measurements. 177

P1) Capillary absorption followed by drying 178

In this procedure, the specimens were first contaminated from the bottom by capillary absorption with a 179 salt solution, followed by drying only through the top surface at 20°C and 10% RH, until 80 wt.% of the 180 solution evaporated (Figure 2.a). Two additional wetting/drying cycles with pure water (with the same 181 amount of water used in the salt contamination step) were carried out to move the salts even closer to 182 the evaporative surface. 183

For both the salt contamination and rewetting steps, the salt solution (or water) was poured into a 184 contamination dish with glass rods. The bottom surface of the specimen was placed in contact with the 185 solution until the top surface was completely wet. After contamination or rewetting, the bottom of the 186 specimen was sealed with parafilm, and the specimen was left to dry only through the top surface. 187

The quantity of solution (or water) used was similar to the capillary moisture content, i.e., the amount of 188 water absorbed by the specimen at satiation (i.e., to wet the top surface), following capillary rise from its 189 bottom. As mentioned, the concentration of the solution was calculated so that, for both stones, a content 190 of 1 wt. % of salt with respect to the dry weight of the specimens was reached. Hence, a salt solution 191 concentration of 6.7 wt.% was used for Migné stone, and 3.8 wt.% was used for Maastricht stone. 192

This procedure was performed with three groups of three specimens of each stone to assess the effect of 193 subsequent wetting/drying cycles on salt accumulation: 194 1) a group of three specimens was subjected only to one drying cycle, following salt contamination; 195 specimens contaminated according to this test are henceforth referred to as P1-1S; 196

2) a group of three specimens was subjected to salt contamination, followed by one rewetting step 197 with water and subsequent drying; specimens contaminated according to this test are henceforth 198 referred to as P1-1W; and 199

3) a group of three specimens was subjected to salt contamination, followed by two rewetting steps 200 with water and subsequent drying; specimens contaminated according to this test are henceforth 201 referred to as P1-2W. 202

At the end of the tests, the specimens were further dried at 60°C in a ventilated oven to constant mass 203 before assessing the salt distribution. 204

P2) Continuous capillary absorption 205

In this procedure, the salt solution was poured into a contamination dish with a geotextile layer on the 206 bottom (ca. 1 mm thick), and the bottom of the specimen was placed in contact with the solution (Figure 207 2.b). The amount of water used in the preparation of the solution was the same as the total amount of 208 water used in procedure P1-2W, i.e., the sum of water used during the initial salt contamination (P1-1S) 209 plus the water used in the two following rewetting cycles to achieve the same amount of salt (1 wt.%) 210 with respect to the dry weight of the specimens. 211 Drying occurred only through the top surface of the specimen at 20°C and 10% RH. Evaporation of the 212 solution from the dish during the test was prevented by covering it with a parafilm sheet. After complete 213 absorption of the salt solution, the bottom of the specimens was sealed, and the specimens were left to 214 dry (20°C and 10% RH) until 80 wt.% of the water absorbed evaporated. Afterwards, the specimens were 215 dried at 60°C to constant mass before assessing the salt distribution. 216 217 NaCl solubility is almost independent of temperature, and NaCl has no hydrate forms at 227 temperatures higher than 0°C. 228 Na2SO4 is generally more damaging than NaCl in laboratory weathering tests. Moreover, these salts are 229 the most commonly used in crystallisation tests [START_REF] Lubelli | Towards a more effective and reliable salt crystallisation test for 730 porous building materials: state of the art[END_REF]. 230

Stone types 231

Migné stone (quarried in France) and Maastricht stone (quarried in The Netherlands) were used in the 232 tests. These limestones have been chosen because of their similar chemical composition (both stones are 233 composed of ca. 95-99% of calcite), their homogeneous structure without macroscopically evident 234 bedding planes, their unimodal pore size distribution, and their significantly different porosity and pore 235 size.

Specimen preparation and testing conditions for both procedures 243

Cylindrical specimens (Ø 50 mm, height 50 mm) were cut from a large stone block in such a way that the 244 bedding planes of the stone laid parallel to the top and bottom surfaces of the specimen. The specimens 245 were sealed along their side surface with parafilm after pre-heating them in an oven at 50°C for 10 min to 246 achieve better adherence. A textile tape was used to glue the end of the parafilm foil and ensure sealing 247 tightness during the entire test. Before contamination, the specimen mass was stabilized at 20°C and 10% 248 RH, which were the conditions used during the contamination and drying phases. Low temperature, RH, 249 and negligible air flow conditions were set to promote the accumulation of salt close to the evaporative 250 surface. 251

For each procedure, a group of three specimens of each stone was contaminated with a salt solution of 252

NaCl and another group with Na2SO4; both salts were ACS reagents 99.0%. The concentration of the salt 253 solution used for the contamination was defined according to the desired salt content in the specimen, 254 which was 1 wt.% of its dry weight. The same salt content was used in both procedures to enable direct 255 comparison of the results. 256

A ventilated climatic chamber (Vötsch VC 4018) was used to provide constant thermo-hygrometric 257 conditions. Throughout the tests, the specimens were placed in the climatic chamber inside plastic boxes 258 (40×34×17cm, three specimens per box), with the opening covered with Japanese silk paper (ca. 15 g/m 2 ) 259 to prevent air flow. Digital thermo-hygrometers (Comet S3621) were placed inside the boxes to monitor 260 the RH, which resulted in being 13 ± 3%. 261 262

Assessment of damage and salt distribution 263

At the end of the accumulation phase, the occurrence of damage and salt efflorescence was visually and 264 photographically recorded. During the accumulation phase, the salt introduced should remain in the 265 specimens, i.e., not crystallise as salt efflorescence, and damage should ideally be absent or of low 266 intensity. Hence, both the amount of salt efflorescence and mass loss were used to assess the suitability 267 of the procedures to be used in the accumulation phase. 268

Several analytical methods (destructive and non-destructive) were used to assess the salt distribution: ion 269 chromatography (IC), hygroscopic moisture content (HMC), micro X-ray fluorescence (µ-XRF), scanning 270 electron microscopy (SEM), drilling resistance measurement system (DRMS), scratch tool (ST), and 271 ultrasonic pulse velocity (UPV). 272

After the accumulation phase tests, the specimens were split into two halves, using a hammer and chisel, 273 for analysis with the mentioned methods, except for the specimens subjected to UPV measurements. 274

Figure 3 shows a scheme summarizing the number and type of specimens and respective analyses 275 performed with each method of assessment of salt distribution. 276

Visual observations and mass variation 280

The development of salt efflorescence and damage was monitored photographically. Since salt 281 efflorescence can hide the underlying damage, photographs were taken before and after removing the 282 efflorescence. The type and severity of damage types, when present, were described according to the 283 ICOMOS glossary [START_REF]Illustrated glossary on stone deterioration patterns[END_REF] and MDCS atlas [START_REF]Monument Diagnosis and Conservation System[END_REF]. 284

The weight of the specimens was recorded before and after salt contamination; based on this, the exact 285 amount of salt introduced in each specimen was calculated. During the drying phase of both procedures, 286 the weight of the specimens was recorded at regular intervals until 80 wt.% of the water had evaporated. 287

The damage was assessed by recording the mass loss. 288

At the end of the test, the salt efflorescence and stone debris, when present, were collected using a soft 289 brush and weighed using a balance (resolution of 10 -4 and accurate to 10 -3 g). The salt efflorescence was 290 separated from the stone debris by dilution in pure water, followed by filtration with filter paper. The 291 weight of the debris was measured, and that of the salt was calculated. Salt that might have accumulated 292 in the pores of the stone debris collected was hence registered as salt efflorescence. 293

Ion chromatography (IC) 294

For the analysis of the salt content (NaCl or Na2SO4), measurements were carried out on drilled powder 295 samples. For each half-core specimen, starting from the evaporative surface, samples were collected up 296 to the following depths (same hole): 0-5, 5-10, 10-15, 15-20, 20-30, 30-40, and 40-50 mm. Additionally, to 297 get more precise information on the salt distribution in the salt-rich outer layer, samples were collected 298 by sanding down a half-cylinder at 1 mm intervals up to 5 mm depth per specimen. 299

The quantity of anions (Cl -and SO4 2 ) and cations (Na + ) of the vacuum filtered extract was analysed by IC 300 (Metrohm). The results in parts per million (ppm -mg/L) were converted to wt.% (relative to the dry 301 sample weight), and the sum of the equimolar contents of Na + and Cl -or SO4 2-was estimated. The salt 302 amount present in each layer was calculated as the percentage of the total salt content present in the 303 sample. The average amount of each ion (in wt.%) in each group of three specimens contaminated with 304 each procedure was calculated in relation to the respective total weight of each group of samples. 305

Hygroscopic Moisture Content (HMC) 306

For the determination of the HMC, the same samples used in the IC analysis were placed in a climate 307 chamber at 20 (±1) °C and 95% (±3) RH until constant weight (41 days). The HMC was calculated according 308 to the equation: 309

𝐻𝑀𝐶 = 100 ⋅ 𝛥𝑚 𝑚 𝑑
(1) 310 in which 𝛥𝑚 represents the mass gain of the sample (g), and 𝑚 𝑑 represents the dry weight of the sample 311 (g). 312

The correlation between the HMC values (wt.%) and the salt content (wt.%) was determined considering 313 a dataset of 292 drilled samples. The salt content considered was limited to equimolar contents derived 314 from the IC analysis of samples containing Na + and Cl -or Na + and SO4 2-. The correlation coefficient indicates 315 how well the data fits the model of regression and is presented as R 2 , calculated according to the following 316 equation: 317

𝑅 2 = ( 𝑛(∑𝑥𝑦)-(∑𝑥)(∑𝑦) √[𝑛∑𝑥 2 -(∑𝑥)] 2 [𝑛∑𝑦 2 -(𝛴𝑦) 2 ] ) 2
(2) 318 in which x corresponds to the value of HMC (wt.%), and y corresponds to the salt content (wt.%). 319

Micro X-ray fluorescence mapping (µ-XRF) 320

Chloride and sulphur μ-XRF maps were produced to check the NaCl and Na2SO4 distribution, respectively. 321

The instrument used for the μ-XRF analysis was an EDAX (Mahwah, NJ, USA) ORBIS μ-XRF spectrometer. 322

The system uses a Silicon Drift Detector (SDD). It focuses the X-rays from a rhodium target anode with a 323 poly-capillary focusing optic, which allows for a beam diameter of ca. 30 μm. The acquisition system is the 324 ORBIS Vision Software supplied by EDAX. The applied acceleration potential and current were 35 kV and 325 500 μA, respectively. At each point, every 100 μm (spacing), a spectrum was acquired for 100 ms (30%-326 40% dead time). The measurements were carried out at atmospheric conditions with a built-in 25-μm 327 thick aluminum filter to eliminate the rhodium Lα radiation, which overlaps with the chlorine Kα X-ray line 328 and increases its limit of detection. 329 A smooth surface is required for this analysis, so prismatic specimens were dry-cut from the half cores 330 along their longitudinal axis. The selected surface of 5x5 cm to be mapped was ground with SiC paper 331 (STRUERS 500) for 15 seconds, at 300 rpm, in a single direction to avoid spreading the salt. 332

Scanning Electron Microscopy (SEM) 333

SEM observations were carried out on the same samples analysed with µ-XRF. Polished sections were 334 produced from 22 cm pieces of stone. The samples were prepared in the same way as those prepared 335 for the µ-XRF analysis but were further embedded under vacuum in low viscosity epoxy resin (EPO-TEK). 336

After 24 h of polymerization, the specimens were grinded with SiC paper (STRUERS) at 500, 1000, 2400, 337 and 300 rpm for 5 min (each step), using isopropanol as coolant. were scratched along the centre of their curved side, moving from the evaporative surface towards the 354 base. A sharp rectangular 10 mm wide diamond cutter with a negative back rake angle of 15°, moving at 355 constant velocity (10 mm/s), was used in all the scratch tests. During scratching, the normal (Fn) and 356 tangential (Ft) components of the total force required to form a groove on the sample were recorded 357 automatically. From these, the total cutting force (F) was estimated as [START_REF] Godts | Common salt mixtures database: a tool to identify research needs[END_REF]: 358

𝐹 = √(𝐹 𝑛 2 + 𝐹 𝑡 2 ) (3

) 359

For the DRMS test, the samples were drilled starting at the evaporative surface of the samples using a 360 device supplied by SINT Technology. The operating conditions during drilling were 600 rpm for the 361 rotational speed and 10 mm/min for the penetration rate. A twist diamond drill bit with a flat point and 362 Ø 5 mm was used; the total penetration depth was 10 mm. During the drilling tests, the DRMS provided 363 a continuous reading of the force required to perforate the test specimen. 364

Ultrasonic pulse velocity (UPV) 365

UPV measurements were performed to assess the salt distribution non-destructively, using a portable 366 instrument (USG 20 Krompholz Geotron Elektronik (DE)) with pointy-ended transducers with a contact 367 section of Ø 3 mm. Only specimens contaminated according to procedure P2 were measured; for this test, 368 three extra cylindrical specimens of each stone were used. Nine measurements were performed in each 369 specimen in planes parallel to the evaporative surface at 5 mm intervals. The measurements were 370 performed before and after salt contamination. 371 372

Results 373

Damage and salt efflorescence 374

A summary of the visual observations, focusing on the qualification of salt efflorescence and damage on 375 the specimens contaminated with both procedures, is shown in Figures 4 and5. The amount of salt 376 efflorescence and mass loss are given in Figures 6 and7, respectively. In the case of procedure P1, the 377 debris of the specimens subjected to further rewetting steps was only collected at the end-step, so the 378 results shown are cumulative. 379 Samples contaminated with NaCl with both procedures developed only salt efflorescence, and no visible 380 damage was detected (Figure 4). The type and amount of salt efflorescence (Figure 6.a) varied according 381 to the procedure followed and type of stone used: a lower amount of NaCl efflorescence developed in the 382 less porous stone (Migne) with procedures P1-1S and P2, but the amount was still slightly higher than for 383 the more porous stone (Maastricht) with procedures P1-1W and P1-2W. The results of mass loss (Figure 384 7.a) can be considered negligible (lower than 0.05 wt.%) and confirm the visual observations. 385

Both types of stones contaminated with Na2SO4 showed damage with all procedures (Figure 5). Minor 386 damage was observed in Migné after Na2SO4 contamination with procedure P1-1S, leading to ca. 1 wt.% 387 of mass loss (Figure 7.b); the amount of salt efflorescence observed was very low and concentrated close 388 to the evaporative surface. After one rewetting, Na2SO4 contaminated Migné specimens (P1-1W) suffered 389 rapid and severe damage (within 24h of drying). For this reason, the planned second rewetting was not 390 performed, and these specimens were not subjected to the analysis of salt distribution. 391

In the case of the more porous stone (Maastricht), samples contaminated with Na2SO4 with procedure 392 P1-1S developed slight granular disintegration and scaling, leading to 0.7  0.4 wt.% mass loss (Figure 7.b). 393

After one rewetting (P1-1W), the amount of salt efflorescence increased significantly; the amount of 394 efflorescence reached almost 90 wt.% of the salt introduced, whereas mass loss was ca. The period for the evaporation of 80% of the water introduced during the contamination varied according 413 to the stone type, salt type, and procedure (Table 2). As expected, samples with higher porosity and pore 414 size dried faster. The drying period with procedure P2 was more prolonged than with procedure P1-1S for 415 both stones contaminated with either salt. This aspect can be attributed to the higher degree of saturation 416 of the porous system of the stones with procedure P2, resulting from the continuous capillary absorption 417 of a solution with two times higher amount of water than procedure P1-1S. 418

Assessment of the salt distribution 422 3.2.1. Ion chromatography (IC) and hygroscopic moisture content (HMC) 423

The salt distribution assessed with IC is presented in Figures 8 and9. Generally, salt accumulated close to 424 the evaporative surface and the salt content decreased with depth. The salt content in the outer 5 mm 425 layer was always above 50 wt.% of the total content, except in the case of Maastricht contaminated with 426

Na2SO4 with procedure P1-1S (Figure 9.b). In this case, the amount of salt present in the outer 5 mm layer 427 was lower than ca. 30 wt.%, but the standard deviation was large. 428 Different results were obtained when the samples were rewetted; the salt content up to 5 mm depth 429 slightly increased or decreased after one rewetting (P1-1W) for Migné and Maastricht contaminated with 430 NaCl, respectively. Subsequent rewetting of Migné stone contaminated with NaCl (P1-2W) led to an 431 increment of efflorescence and salt in the 0-5 mm layer. In Maastricht, the amount of salt efflorescence 432 decreased with subsequent rewetting (ca. 30 wt.%), while the salt content in the 0-5 mm layer increased. 433

Maastricht contaminated with Na2SO4 with P1-1W showed an increment of the salt content in the outer 434 layer (0-5 mm); rewetting twice (P1-2W) led to an increment of efflorescence and a decrease of the salt 435 content in the outer layer. These results are possibly related to the removal of the damaged salt-rich layer 436 after brushing the specimens contaminated with P1-2W. Upon rewetting (P1-1W and P1-2W), salts were 437 transported towards the evaporative surface; consequently, the salt content in the outer 5 mm layer 438 and/or the amount of efflorescence increased. 439 A more detailed analysis was carried out on the first 5 mm of a sample of Maastricht contaminated with 440

Na2SO4 according to procedure P1-1S (Figure 10). The results indicate that 73.5 wt.% of the total salt 441 content was located within the first millimetre of the specimen. Deeper in the stone, the concentration 442 dropped significantly, with 16.8 wt.% between 1 and 2 mm, and 2.6 wt.% between 4 and 5 mm.

Micro X-ray fluorescence mapping (µ-XRF) and scanning electron microscopy (SEM) 473

A summary of the observations from µ-XRF maps and SEM images is presented in Table 3. Figure 12 The results indicate that procedure P1-1S is the most effective in accumulating salt at the evaporative 477 surface of both stones. In general, rewetting reduces the thickness of the salt layer and promotes the 478 redistribution of salts deeper into the samples. The data also indicated that NaCl is more prone to be 479 redistributed within the sample thickness of both stones upon rewetting. Procedure P2 also showed a salt 480 layer at the evaporative surface in both stones, with either salt; NaCl further spread within the sample 481 thickness. 482 SEM gives a more precise assessment of the thickness of the salt layer and salt distribution than the µ-483 XRF maps, although the salt distribution can be quite non-homogeneous. Therefore, a thickness interval 484 is probably more appropriate rather than a single value. The representativeness of the sample observed 485 with SEM can also be an issue; e.g., no salt layer was observed under SEM with Maastricht contaminated 486 with procedure P1-1S with NaCl, but a salt layer up to 1 mm thickness was observed in the µ-XRF maps. 487 The additional SEM analysis of Maastricht contaminated with Na2SO4 with procedure P1-1S showed only 510 partial filling of the pore spaces and a higher concentration of the salt close to the evaporative surface. 511

Images of the fresh fracture showed that the salt crystallizes as porous clusters within the inter-particle 512 spaces (Figure 15). Such a crystallisation pattern was confirmed by the cross-section observation and 513 mapping of the salt distribution (Figure 16). The salt was mostly concentrated up to 400-500 µm from the 514 evaporative surface, showing a rather irregular distribution. The presence of salt was detected at least up 515 to 2 mm depth, as isolated clusters of salt crystals formed along the grain borders of the stone. These 516 observations are in line with the IC results on the same sample (Figure 10) in which ca. 70% of the salt was 517 detected up to 1 mm depth and ca. 20% of salt was detected between 1.5 and 2 mm depth. 518 The scratching and drilling force (F) patterns versus the cutter position (depth of the sample) are provided 530 in Figure 17. From these, evident changes in the microstructure of the salt contaminated samples (i.e., 531 pore clogging resulting in increased cutting resistance areas) can be observed. In particular, in all the 532 patterns derived after salt contamination and drying, localized peaks in material resistance to cutting were 533 recorded very close to the evaporative surface (ca. 0 -2 mm). These peaks, which are not observed in the 534 reference specimens, are prominent both in the ST and the DRMS patterns and suggest pore filling by the 535 salts. 536

The position of the peaks in cutting resistance is nearly identical for each sample, irrespective of the 537 mapping technique used, indicating that both methods are detecting pore clogging due to salt 538 crystallisation at more or less the same location. This is in line with previous applications of the ST and the 539 DRMS [START_REF] Modestou | Tracing the salt crystallisation front in limestone using the DRMS[END_REF], [START_REF] Macwilliam | Towards a More Realistic and Effective Use of Sodium Sulfate in Accelerated 797 Ageing of Natural Stone[END_REF] and confirms the reliability of both techniques and their potential in detecting the position 540 of the salt crystallisation front in limestone. 541

The peak cutting resistance recorded in the case of Migné contaminated with Na2SO4 with procedure P1-542 1S is much higher, compared to the corresponding sample contaminated with NaCl. This is in line with the 543 μ-XRF and SEM results (Table 3), which suggest that Na2SO4 has a tendency to crystallise more locally, 544 closer to the evaporative surface, as opposed to NaCl which shows higher dispersion. The localised 545 crystallisation of Na2SO4 usually leads to more damage [START_REF] Ioannou | Synchrotron radiation energy-dispersive X-ray 759 diffraction analysis of salt distribution in Lépine limestone[END_REF], as was indeed the case in the visual 546 observations hereby reported (Figures 4 and5). In the case of the much weaker and porous Maastricht 547 stone, the peak resistance recorded for the sample contaminated once (P1-1S) with NaCl was again rather 548 low. Upon rewetting with water (P1-1W), the peak resistance was further reduced. In contrast, the 549 thickness of the salt-rich layer (i.e., increased cutting resistance area) appears broader, albeit not as broad 550 22 as in the optimised μ-XRF map, suggesting some redistribution of the salt content with water absorption 551 and possibly some salt escaping the sample as efflorescence; this is in line with the amount of salt 552 efflorescence formed, as well as the IC data (Figure 8). 553

Comparison of procedures 575

A summary of the advantages and disadvantages of the procedures used for salt accumulation (1 wt.% 576 with respect to the total weight of each stone) is presented in Table 4. Procedure P1-1S was the most 577 effective in accumulating salts in a thin layer beneath the evaporative surface of the stones without 578 causing too much damage. Rewetting with procedure P1 led to the damage propagation phase in 579 specimens contaminated with Na2SO4 and redistribution of both salts within the specimens, along with 580 the development of salt efflorescence. Procedure P1-1S with NaCl promoted less salt efflorescence in 581 both stones than with Na2SO4, whereas procedure P2 caused a higher amount of salt efflorescence in both 582 stones with each of the salts. 583

The results of the assessment of the salt distribution showed that in all procedures, most of the salt is 584 concentrated in the first millimetres of the subsurface. Stones contaminated with NaCl according to 585 procedure P1-1S showed higher salt content in the outer layer than stones contaminated with procedure 586 P2. In the case of Na2SO4, P2 induced a higher salt content in the outer layer of both stones. 587 Mass loss was negligible (lower than 0.05 wt.%) in both materials contaminated with each procedure with 588

NaCl. In the case of Na2SO4, slight damage occurred with procedures P1-S and P2. With procedure P1, 589 rewetting caused severe damage in the case of the less porous stone Migné (P1-1W) in the form of 590 bursting, whereas Maastricht showed increasingly higher granular disintegration upon rewetting, leading 591 to the conclusion that rewetting triggers damage propagation. The reason for the severe damage 592 observed in Migné after rewetting (P1-1W) is probably attributed to a high degree of pore filling after salt 593 contamination (P1-1S) and to the development of high stresses in the small pores of this stone after 594 dissolution/recrystallisation of the salt introduced. In the case of Maastricht, it can be assumed that the 595 stress developed was not as high since the pores are larger and the resulting pore filling (also with ca. 1 596 wt.% of salt introduced) is thus lower; consequently, the resulting damage was also less. 597

The salt types considered in this study follow long-term research studies and recommendations, as 598 presented in the introductory section. concentrations used in each stone, are also likely to influence the salt distribution and the resulting pore 617 clogging. Eloukabi et al. [START_REF] Eloukabi | Experimental study of the effect of sodium chloride 833 on drying of porous media: The crusty-patchy efflorescence transition[END_REF] studied the effect of NaCl on the drying of porous media. They found that a 618 salt layer on the evaporative surface of a system with a large pore size does not significantly affect the 619 drying rate and can even accelerate it if the pore sizes are sufficiently large. In contrast, a NaCl layer 620 formed on the evaporative surface of porous media with small pore sizes significantly limited the rate of 621 evaporation. The authors assigned this phenomenon to the fact that, in large pore sized media, the NaCl 622 efflorescence can grow upward, leading to patchy cauliflower efflorescence, as in the case of Maastricht 623 contaminated with NaCl with procedure P1-1S (Figure 4). In contrast, in the case of small pore size media, 624 the salt crystals grow laterally rather than vertically, resulting in a salt crust that may block evaporation, 625 as observed in Migné contaminated with NaCl with procedure P1-1S (Figure 14). This hypothesis is also 626 consistent with the results of the amount of salt efflorescence formed (Figure 6); in general, in the case 627 of Migné, there is lower salt efflorescence forming than in Maastricht, which indicates a higher amount 628 of subflorescence, thus of pore clogging, hence affecting more severely the drying rate. 629

Comparison of methods for the analysis of the salt distribution 630

The salt distribution in the specimens was one of the criteria guiding the choice of the accumulation 631 procedure. Besides, the salt distribution through the depth of the specimen can provide useful 632 information about the risk of damage in salt crystallisation tests: a higher salt accumulation in a thin layer 633 of material close to the evaporative surface implies potentially higher crystallisation pressure and thus a 634 higher risk of damage. Therefore, a reliable measurement of the salt content and distribution is crucial in 635 salt crystallisation tests. In this study, several methods used for assessing the salt distribution have been 636

tested; their advantages and disadvantages, resulting from past experience and from the present 637 research, are summarized in Table 5 and described in detail hereafter. 638

641

HMC is a simple and reliable method for the semi-quantitative analysis of samples contaminated with a 642 single salt and can be used as an alternative or complementary method for more expensive chemical 643 analyses, like IC. However, care should be taken to monitor precisely the RH inside the climatic chamber 644 because a difference of 1% at very high RH, such as 95% RH, can lead to significant differences in the HMC 645 values, especially when dealing with high salt contents and very hygroscopic salts, such as NaCl. Hence, in 646 the case of NaCl, using an RH of 80% may be advantageous. 647 µ-XRF maps allow for the precise distribution of the salt in a representative section of samples (in this 648 study, the entire sample section was mapped). However, it is an expensive and time-consuming type of 649 analysis. SEM allows the most precise observation of the salt distribution in the pores, including 650 information about the preferential location of the salt (e.g., in coarse or finer pores), the crystallisation 651 form (e.g., thenardite or mirabilite), and the salt crystallisation habit. The main drawbacks of using SEM 652 lie with the sample size and its representativeness, and the disturbance of the sample during preparation. 653

The results of DRMS and ST measurements performed in Migné and Maastricht contaminated according 654 to procedure P1 were generally in line with the IC, μ-XRF, and SEM results. This confirms that these two 655 micro-destructive methods can be successfully used to assess the distribution of salts in stone in a reliable 656 way. The DRMS may further be used in-situ, thus providing a useful tool for comparison of laboratory 657 results with practice. 658

The UPV results were in agreement with the results obtained by other techniques in the case of Maastricht 659 stone contaminated with NaCl. In contrast, in the case of Migné stone contaminated with NaCl, the UPV 660 technique was not able to reveal the accumulation of salts in the outer layer, observed by the other 661 investigation techniques. These results suggest that a higher number of specimens should be considered 662 for the UPV test to achieve more reliable results because its accuracy may depend on the stone and salt 663 type; so, more tests are necessary to define the extent of the applicability of this method for detecting 664 the salt distribution. 665 666

Conclusions 667

This study focused on the comparison of two contamination procedures for the accumulation of salts in 668 two limestones, as well as on the comparison of different methods for the analysis of salt distribution. 669 Procedure P1-1S (capillary absorption of a salt solution followed by one drying cycle) proved to be the 670 fastest and most effective in accumulating NaCl and Na2SO4 in a thin layer of material close to the 671 evaporative surface, without resulting in too much salt efflorescence or damage in two types of stone 672 with very different porous structure (Maastricht and Migné limestones). Hence, this procedure has been 673

selected for reproduction using a multiphase numerical model [START_REF] D'altri | submitted to publication in the Construction and 749 Building Materials journal) Towards a more effective and reliable salt crystallisation test for porous building 750 materials[END_REF]. The numerical model confirmed the 674 obtained results in the tested limestones and indicated that the procedure is also valid for other types of 675 porous building materials. Round robin tests with different types of porous building materials are 676 currently ongoing to experimentally validate the test and to continue with the damage propagation phase. 677

All methods used in this study are considered useful for analysing salt distribution. Depending on the aim 678 of the analyses, preferences exist. When non-destructive or minimally destructive techniques are to be 679 preferred, DRMS, ST, and, with further improvement, UPV can offer a solution for the monitoring of salt 680 distribution. If sampling is possible, HMC is a simple technique, not requiring specialized personnel, to 681 obtain a reliable indication of the salt content and distribution. When more precise, quantitative 682 information on the salt content is needed, IC is the most suitable technique. SEM and µ-XRF are most 683 useful to provide information about the distribution of salts in pores, their specific location, and 684 crystallisation form. 685

Figure 2 :

 2 Figure 2: Experimental setup used in the contamination experiments: (a) capillary absorption followed by 218 drying (P1), (b) continuous capillary absorption (P2). 219 2.2. Materials 220 Salt types 221 Two salt types, sodium chloride (NaCl) and sodium sulphate (Na2SO4), have been used. This selection was 222 based on the following criteria: 223  Both salts are commonly found in buildings worldwide and are known to cause severe damage. 224  These salts have different crystallisation behaviour. Na2SO4 has temperature-dependent solubility 225 and multiple polymorphs (hydrate and anhydrous forms) at ambient temperature. In contrast, 226 NaCl solubility is almost independent of temperature, and NaCl has no hydrate forms at 227 temperatures higher than 0°C. 228

9 277Figure 3 :

 93 Figure 3: Summary of the number and type of specimens tested with each procedure and analysis 278 performed. 279

Figure 5 : 14 405Figure 6 :Figure 7 :

 51467 Figure 5: Summary of the macroscopic visual observations on the specimens contaminated with Na2SO4. 402 Scores of severity of damage: low (+) to high (++++). Scores of extension of damage: small (+) to large 403 (++++) surface area. 404

Figure 8 :Figure 9 :Figure 10 :Figure 11 :

 891011 Figure 8: Mean relative percentage of NaCl distribution assessed with IC in Migné (a) and Maastricht (b) 449 after contamination with procedures P1 and P2. The values are the average of 3 specimens; the graphs 450 include the amount of salt efflorescence (efflo). The legend from top to bottom represents the different 451 procedures, which sequentially correspond to the columns from left to right. 452

  gives 474 an example of the µ-XRF maps recorded in Migné and Maastricht contaminated according to procedure 475 P1 with NaCl. Figures 13, 14, 15, and 16 show a few selected SEM microphotographs. 476

Table 3 :

 3 Summary of µ-XRF maps and SEM image observations. 491

Figure 12 :Figure 13 :

 1213 Figure 12: µ-XRF chloride maps of Migné and Maastricht contaminated according to procedure P1. Migné 497 contaminated according to procedure P1-1S: (a) original map, (b) enhanced map by Image J software; the 498 dense red layer on the top of the sample is an image artefact. Maastricht contaminated according to 499 procedure P1-1W: c) original map, d) enhanced map by image J software; the dense green layer on the 500 top of the sample is an image artefact. 501

Figure 14 :

 14 Figure 14: Migné contaminated with NaCl according to procedure P1-1W: (a) SEM image of the ground 508 surface, (b) BSE image of the polished section. 509

Figure 15 :Figure 16 :

 1516 Figure 15: SEM images of the fresh fracture close to the evaporative surface of Maastricht contaminated 520 with Na2SO4 according to procedure P1-1S: (a) BSE image, (b) False colour image after EDS analysis, 521showing the distribution of calcium (yellow), sodium and sulphur (cyan and blue respectively). 522

Figure 17 : 4 .Figure 18 :

 17418 Figure 17: DRMS and average ST patterns for samples contaminated according to procedure P1 554 (coloured lines) and reference samples (black lines). 555 556 557 558

Table 1 :

 1 Table 1 presents the main physical properties of the stones. The open porosity of Migné is ca. 30% 236 and that of Maastricht is ca. 50%. The grain size is homogeneous: ca. 100-150 µm for Maastricht and 1-5 237 µm for Migné. Maastricht has a main pore size of ca. 30 µm, whereas Migné has a main pore size of ca. 1 238 µm. The difference in pore size is reflected in the rate of water absorption by capillarity: Maastricht's 239water absorption coefficient is an order of magnitude higher than that of Migné[START_REF] D'altri | submitted to publication in the Construction and 749 Building Materials journal) Towards a more effective and reliable salt crystallisation test for porous building 750 materials[END_REF]. 240 Properties of Migné and Maastricht limestones[START_REF] D'altri | submitted to publication in the Construction and 749 Building Materials journal) Towards a more effective and reliable salt crystallisation test for porous building 750 materials[END_REF]. 241

	Properties

2.3.6. Drilling resistance measurement system (DRMS) and scratch tool (ST) 349

  344with SiC paper (Struers), and polished with polishing cloth up to 8000 grit. Isopropanol was used during 345 grinding and polishing to avoid salt dissolution. The salt distribution within the stone matrix was mapped 346 up to a 2.5 mm depth from the evaporative surface by combining multiple images acquired in constant 347 conditions (back-scattered mode) at 150x magnification. 348 Samples contaminated according to procedure P1, and previously mapped with μ-XRF, were scratched 350 and drilled to assess the position of the salt crystallisation front. Non-contaminated Migné and Maastricht 351 samples were also used as a reference. 352 Scratching was performed using a WOMBAT Scratch Tool supplied by Epslog Engineering. The samples 353

	338	The final polishing was carried out with
	339	water-free diamond polishing suspensions (9, 3, and 1 µm). Polished surfaces were coated with a 15 nm
	340	carbon layer, then observed with an SEM FEI QUANTA 200 3D, at high vacuum, and 20 kV voltage.
	341	Additional samples from Maastricht limestone contaminated with Na2SO4 according to procedure P1-1S

(belonging to the same set analysed with IC and HMC) were studied with a Zeiss Gemini 300 SEM FE 342 equipped with an Oxford X-Max detector at high vacuum and 15 kV voltage. Polished cross-sections were 343 prepared by embedding the stone fragments in bi-component epoxy resin (EpoFix, Struer), grinding them

  1.1 wt.%. The 2 nd 395 rewetting step (P1-2W) induced more damage (ca. 2 wt.% of mass loss) and slightly more salt 396 efflorescence. In contrast with Migne, damage occurred in Maastricht samples contaminated with Na2SO4 397 with procedure P2. 398

	Procedure	Migné -Na2SO4	Maastricht -Na2SO4
		Photo
	P1-1S
		Damage/alteration	Scaling with subflorescence	Efflorescence, granular disintegration,
				and scaling with subflorescence
		Salt efflorescence	Powdery	Powdery
	Procedure Damage severity	Migné -NaCl ++	Maastricht -NaCl ++
		Damage extension	++	++
	P1-1S	Photo
		Photo
	P1-1W
		Damage/alteration	Efflorescence	Efflorescence
		Salt efflorescence	Powdery	(micro) cauliflower
	P1-1W	Damage severity Damage extension Damage/alteration Salt efflorescence	No damage -Bursting with subflorescence Crust	No damage Granular disintegration with -subflorescence Crust
		Damage severity Photo Damage extension	++++ ++++	++ ++++
		Damage/alteration Photo	Efflorescence	Efflorescence
	P2	Salt efflorescence Damage severity	Powdery (surface) and hollow crust No damage	Powdery No damage
		Damage extension	-	-
	P2-2W
		Damage/alteration Photo Salt efflorescence	Efflorescence Cauliflower	Contour scaling with subflorescence Powdery
		Damage severity	+	++
		Damage extension	+	++
		Damage/alteration	Efflorescence	Efflorescence
		Salt efflorescence	Powdery	Powdery
		Damage severity	No damage	No damage
		Damage extension	-	-

Figure 4: Summary of the macroscopic visual observations on the specimens contaminated with NaCl. 399 Scores of severity of damage: low (+) to high (++++). Scores of extension of damage: small (+) to large 400 (++++) surface area. 401

Table 2 :

 2 Period (in days  3) for evaporation of 80% of the water introduced during the contamination. 419

	Procedure	Migné		Maastricht
		NaCl	Na2SO4	NaCl	Na2SO4
	P1 -1S	37	24	10	10
	P1 -1W  2W	21	-	8	8
	P2	40	30	12	12
	420
	421

Table 4 :

 4 The same salt content (1 wt.% of the dry weight of the specimens) 599was used in all procedures to facilitate comparison. The accumulation procedures were tested in two 600 stones with very similar composition but different porous structure to enable an easier and reliable 601 comparison of the results (e.g., exclude differences in decay which could come from other damage 602 processes, such as chemical reactions, hygric dilation, etc.). Besides, both stones have a unimodal pore 603 size distribution, which simplifies further the comparison. Introducing a realistic salt content into the 604 material and ensuring that the salt accumulates in a thin layer of the material close to the evaporative 605 surface will lead to high salt pore filling. Hence, the damage propagation phase (entailing fast repeated 606 wet-dry cycles) will trigger damage that reproduces onsite salt crystallisation mechanisms. The 607 applicability of the most effective procedure (P1-1S) has been validated by D'Altri et al. [12], who 608 reproduced the protocol by a multiphase numerical model. The numerical model confirmed the results 609 obtained in the two limestones tested and indicated that the procedure is also valid for other types of 610 porous building materials. 611 Advantages and disadvantages of procedures P1 and P2 for accumulating salt in Migné and 612 Maastricht stones. 613

		Procedure	Advantages	Disadvantages
		P1-1S	-Accumulation of both salts in a thin	-Can cause slight damage with Na2SO4
			layer of material (ca. 1 mm)
			-Low amount of salt efflorescence
			-Technically easy to perform in a
			reasonable time
		P1-1W & 2W	-Rewetting once can promote further	-Can cause medium to high damage with
			accumulation of NaCl in the outer layer	Na2SO4
			of Migné	-Can cause redistribution of salt within the
				specimen thickness
				-Increases the amount of salt efflorescence
				-It is more time-consuming than P1-1S
		P2	-Higher salt enrichment in a thin layer of	-Can cause medium damage with Na2SO4
			material (ca. 1 mm) with Na2SO4	-Causes a higher amount of salt
				efflorescence than P1-1S
				-Is more time consuming than P1-1S
	614
	615	Pore clogging and the change of the surface properties due to efflorescence can affect the Stage II drying
	616	rate. The different transport properties between Migné and Maastricht, and the different salt solution

Table 5 :

 5 Advantages and disadvantages of each method of analysis used for assessing the salt distribution 639 in Migné and Maastricht stones. 640

	Methods	Advantages	Disadvantages
	IC	-Very high precision of ion quantity	-Expensive
	HMC	-Technically simple	-Requires a climatic chamber
		-Enables the analysis of a large number	-Time-consuming
		of samples at the same time	-May require further analysis to confirm the
		-Low cost	results
	µ-XRF maps	-High precision of salt distribution in the	-Expensive and time-consuming sample
		entire sample section	preparation
	SEM	-Very high precision of salt distribution	-Representativeness of sample (only small
		-Enables observing the morphology of	samples can be analysed)
		the salt crystals and the region in which	-Risk of disturbance of sample condition (salt
		salt crystallizes preferentially	distribution) during preparation
	DRMS* & ST	-Good precision on homogeneous	-DRMS requires performing several
		samples	measurements for obtaining reliable results
		-Technically simple and fast	-Precision may be affected by the
		-Micro-destructive	inhomogeneity of the material and the
			presence of hard minerals or vugs
	UPV*	-Non-destructive	-Requires a suitable geometry of transducers
			(pointed shape)
			-Requires a large number of specimens for
			obtaining reliable results
			-Accuracy may depend on stone
			characteristic and salts

* Can be performed in situ

In general, a combination of screening measurements, carried out by simple and cheap techniques, such 686 as HMC, DRMS, UPV, followed by further analysis on a selection of most relevant samples (chosen based 687 on the results from the screening) with more sophisticated techniques, such as IC, SEM, and µ-XRF seems 688 to be the most suitable and sustainable choice. 689

The future work of the TC ASC-271 will focus on the propagation phase.