domain: phys spi phys.cond

text:
Ilya V Yakovlev

Stanislav S Yakushkin

Mariya A Kazakova

Sergey N Trukhan

Zoya N Volkova

Alexander P Gerashchenko

Arcady V Ishchenko Andrey S Andreev

Oleg N Martyanov

Olga B Lapina

Arcady V Ishchenko

Jean -Baptiste D'

Espinose De Lacaillerie

Superparamagnetic behaviour of metallic Co nanoparticles according to variable temperature magnetic resonance

Introduction

Cobalt nanoparticles and composite materials based on them are extensively studied and have already found applications in various scientific and industrial fields including catalysis 1-5 , electromagnetic absorptive materials 6,7 , energy storage materials 8-10 , and medicine 11,12 . The usage properties of an ensemble of Co nanoparticles depend on their size, morphology, crystalline and magnetic structures, properties that may actually all be connected to each other. They also depend on the particle size distribution of the ensemble and on interparticle interactions 13-15 . Thus, predicting the particles' morphology and crystalline structure in specific synthesis conditions is of crucial importance for their successful application.

The main characterization techniques that can be used for studying Co nanoparticles include X-Ray diffraction (XRD), transmission electron microscopy (TEM), scanning probe microscopies, magnetic resonance techniques (ferromagnetic resonance (FMR) 16,17 and 59 Co internal field nuclear magnetic resonance ( 59 Co IF NMR) 18-22 ). All these techniques have their advantages and disadvantages. The application of XRD is limited for very small or defective nanoparticles due to diffraction pattern broadening; local microscopic techniques characterize a very small fraction of the sample, which is not always representative of its entirety. FMR and 59 Co IF NMR techniques are particularly dependent on the particle size distributions. Small superparamagnetic particles give rise to a very sharp signal in FMR, sometimes making large ferromagnetic particles indistinguishable. On the contrary,

Co IF NMR provides information only about particles large enough to be ferromagnetic and mostly about the biggest ones due to the volumetric origin of the signal and higher enhancement factor.

Recently, Liu et al. 22 have proposed a technique of temperature differential nuclear ferromagnetic resonance (TDFNR) that allows to "cut out" a narrow part of the distribution and study it separately by subtracting 59 Co IF NMR spectra recorded at different temperatures. This technique is based on the thermally induced ferromagnetic to superparamagnetic transition. Above a given temperature, particles smaller than a certain critical size experience thermal "flips" of magnetization during the experimental time and become invisible by 59 Co IF NMR. However, to efficiently employ this technique for particles a few nanometers in size one needs to be able to achieve very low temperatures (liquid helium temperature), which may not be achievable with every experimental setup or too expensive due to the high price of liquid helium. Thus, we have turned our attention to another spectroscopic technique -ferromagnetic resonance (FMR). Please do not adjust margins

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Even though it cannot provide full information on the structure of the particles, its sensitivity can be three orders of magnitude higher, since in this case the electronic spin magnetization is probed directly.

In this work, we have attempted to simultaneously apply 59

Co IF NMR and FMR to investigate Co nanoparticles contained inside multi-walled carbon nanotubes (MWCNTs) and establish the application limits of both methods. In our previous works 23,24 , factors which affect the formation of Co nanoparticles in the MWCNTs structure such as CNT dimensions, impregnation solution concentration and postsynthetic acidic treatment, were discussed in detail. Herein, we could thus concentrate on Co/MWCNT samples containing metallic Co nanoparticles localized inside the MWCNT channels.

59

Co IF NMR experiments performed at room temperature and at 30 K, and FMR experiments performed at elevated temperatures up to 600 K, provided the data on the blocking temperatures from which the critical sizes for superparamagnetic behaviour of encapsulated particles could be estimated. These results were efficiently compared with the particle size distribution obtained using high-resolution TEM.

Experimental Co/MWCNT hybrid material synthesis

Multi-walled carbon nanotubes with specific structural properties and morphology were synthesized using a wellestablished procedure already described in detail by some of us elsewhere 25 . In brief, the catalytic chemical vapor deposition technique was employed for the growth of nanotubes via pyrolysis of ethylene on the surface of Fe-Co catalyst at 680 °С. The needed morphology of MWCNTs was obtained by using Al 2 O 3 -supported catalyst with 30 wt.% of the active component. The catalyst residues were removed by boiling the obtained MWCNTs in a solution of 15% hydrochloric acid for 4 hours with continuous stirring. After that, the MWCNT samples were washed with distilled water until the suspension reached a neutral pH value. The purity of MWCNTs obtained after the acid purification was 99.7% (as determined by X-ray fluorescence analysis). After that, the nanotubes were treated with concentrated nitric acid for 2 hours to create openings and defects on their surface in order to facilitate Co deposition.

According to high-resolution transmission electron microscopy (HRTEM) data (Fig. 1), the obtained nanotubes were built up of 5-7 carbon monolayers with the inner and outer diameters (ID and OD, respectively) of the tubes amounting to approximately 3.9 nm and 7.2 nm correspondingly. Using the method of acid-base titration described by Boehm 26 the content of COOH groups in oxidized MWCNT was determined and measured to be 2.7 units per nm 2 . The specific surface area of oxidized MWCNTs was 360 m 2 /g. Co nanoparticles were deposited inside the MWCNTs by incipient wetness impregnation with an aqueous solution of Co(NO 3 ) 2 ⋅6H 2 O (98%, Sigma-Aldrich) and subsequent reduction under H 2 flow. The procedure for preparing of Co/MWCNT samples was described in detail earlier 23,24 . The resulting sample contained 4.2 wt.% of Co (as determined by X-ray fluorescence analysis). For further studies by 59 Co IF NMR and FMR, the reduced samples were transferred to ampoules that were sealed without contact with air immediately after the reduction procedure.

Characterization techniques

BET surface area was measured by nitrogen adsorption at 77 K using an ASAP-2400 Micromeritics instrument. X-ray fluorescent analysis was performed on an ARL Perform'X (Thermo Scientific) equipped with a rhodium anode. Elemental content was estimated using a standardless UniQuant procedure 27 . High-resolution transmission electron micrographs (HRTEM) for statistical analysis were recorded using JEM-2010 microscope (Jeol, Japan) with acceleration voltage of 200 kV and spatial resolution of 0.194 nm. HRTEM data was analyzed using FIJI software package.

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Please do not adjust margins experiments were performed using a Bruker Avance III spectrometer and a home-built probe in the absence of external magnetic field. The experiments were conducted at 30 K and 300 K to differentiate the signal of the particles according to their blocking temperature 28

. A custom-built auto-tuning device was used to avoid the time-consuming procedures of manual tuning and matching. A -τ- echo sequence was used for broadband excitation with a  pulse length of 1 μs and a 6 μs interpulse delay. The echo signals were recorded varying the carrier frequency stepwise each 500 kHz to cover the entire spectral width. After performing Fourier transform, phase and baseline corrections, the real parts of obtained echo signals were integrated. The results of the integration were then assigned as the intensities of the signal at each frequency. The spectra were also recorded with several different power levels of RF irradiation according to the established procedure for obtaining quantitative 59 Co IF NMR spectra 29 . The variation of temperature did not cause any significant Q factor change due to the fact that the FWHM of any individual Fourier-transformed echo remained constant between 30 K and 300 K. Ferromagnetic resonance (FMR) spectra were recorded using an X-band Bruker EleXsys 500 spectrometer equipped with ER 4114HT high-temperature cavity with digital temperature control unit BVT3000. Continuous-wave mode was employed with 4 Oe modulation at frequency of 100 kHz. For the FMR experiments, the sample was sealed in a quartz ampoule with 2 mm inner diameter after the reduction. With this diameter, the Q factor decrease due to conductivity of the sample remains negligible. Heating of the sample also led to a Q factor decrease of less than 5%. Considering this, the precision of quantitative measurements was satisfactory for all measurements.

Results and Discussion

High-resolution transmission electron microscopy

Figure 2 shows a HRTEM micrograph recorded for the Co/MWCNT hybrid material. Remarkably, all of the Co nanoparticles in the sample were found inside the carbon nanotubes. This means that the acidic treatment of the nanotubes creates enough inner channel openings for efficient Co ion penetration. The fact that the Co nanoparticles are located inside the inner channels implies the presence of strong capillary forces during Co impregnation. Since the particle shapes in the sample are not spherical, we have analysed the micrographs using a bivariate particle size distribution. The resulting distribution of particles over their long axis (C) and their short axis (A) as well as the distribution of their aspect ratios (C/A) is given in Figure 3. According to these data, the median aspect ratio of Co nanoparticles was equal to ~1.6. Thus, the particles in the sample were slightly elongated, confirming previous qualitative observations [START_REF] Andreev | Espinose de Lacaillerie[END_REF] .

HRTEM data proved that the Co particles were not spherical, thus we propose using a non-spherical model for the correct determination of particle volumes. Indeed, because of the axial symmetry imposed by the confinement within the carbon nanotubes, they can be represented as prolate spheroids. Taking this into account results in a realistic particle volume distribution (Fig. 4), which is the physically relevant property when considering effects related to magnetic ordering of the material. Co nuclei located inside the domains and domain walls experience two different mechanisms of RF field enhancement leading to a drastic increase of the signal originating from and near domain walls 30,31 . When a Co particle is small enough that the energy barrier of domain wall creation becomes too high, the conservation of single magnetic domain becomes more This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins energetically favorable (a commonly reported value for the multi-/single-domain limit at RT is 70 nm for a spherical particle 32 ).

The resonance frequency of Co inside a single domain particle increases with respect to the one of Co in a multidomain particle, due to the presence of uncompensated demagnetization field, which is aligned parallel to the hyperfine field 33 . Finally, when the size of the particle is small enough that its magnetic anisotropy energy becomes comparable to the thermal energy, the characteristic time of random "flips" of the magnetization that occur due to thermal fluctuations may become smaller than the characteristic time of the experiment (usually tens of microseconds in NMR). That leads to a complete loss of NMR signal, since the spin quantum states at the start of the experiment and at its end are different (thus averaging the magnetization to zero during the time of experiment). Such state of particles is called superparamagnetic, and the critical volume V of the particle, above which its magnetization becomes "blocked" during the experimental observation time at a given temperature T B is described by the Arrhenius-type Equation (1) originally proposed by Néel 34 :

(1)

where k B is the Boltzmann constant, K is the anisotropy constant of the material (5*10 6 erg*cm -3 for hcp Co), τ m is the characteristic measurement time, and τ 0 is the characteristic "flipping attempt" time (usually considered to be between 10 reconstructing in principle a volume size distribution. This procedure is based on the assumption that Equation (1) derived for spherical particles constitutes a valid approximation when the particles aspect ratio is not too high (as opposed for example to particles in 24 , where their high aspect ratio provides for a considerable magnetic shape anisotropy).

The fact that 59 Co IF NMR depends on particle sizes can be illustrated by comparing the room temperature 59

Co IF NMR spectra of three samples with similar Co content but different particle sizes (Fig. S2). In the sample of the present study (ID ≈ 4 nm nm, OD ≈ 7 nm), the Co nanoparticles are located inside carbon nanotube channels and are thus limited in size. The nanotubes of higher OD (9 and 18 nm) contained the bigger Co nanoparticles located inside the inner channels (Id ≈ 4 and 7 nm, respectively) and on the outer surface. For the smallest OD = 7 nm, the majority of the Co particles were undetectable by 59 Co IF NMR as their state is superparamagnetic at RT and at the NMR timescale. The NMR intensity was thus considerably smaller than for the ones in MWCNT of bigger OD. Indeed, in the approximation of spherical particles in the absence of substantial magnetic shape anisotropy using a characteristic time for 59 Co IF NMR experiment of about 20 μs and τ 0 of 10 -10 s, we can estimate from Eq. 1 the room temperature superparamagnetic limit (critical diameter of a spherical particle corresponding to the blocking volume V) to be D crit = 5.8 nm. The 59 Co IF NMR spectrum taken at 30 K displays a much higher intensity and was shifted toward higher frequencies compared to the room temperature spectrum (Fig. 5). Bloch's law for spontaneous magnetization explains such increase of resonance frequency, while the intensity increased according to Boltzmann distribution. However, after taking both of these factors together with the change of T 2 relaxation time (for more information on T 2 measurement see Supplementary) into account for a quantitative comparison of signal intensities, the amplitude of the 30 K spectrum at 216.5 MHz (resonant frequency of single-domain fcc Co particles) remained about 40 times higher (Fig. 5,red). This can be explained by the additional superparamagnetic Co particles that have undergone blocking into ferromagnetic state with the decrease in temperature.

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Please do not adjust margins At 30 K, the estimate of the critical particle size given by Eq. 1 is 10 nm 3 . However, the actual critical volume may be somewhat bigger than estimated from Eq. ( 1) due to magnetic anisotropy associated with the slight non-sphericity of the particles and surface effects. To account for these morphological effects, Liu et al. 22 have used an empirical 2.5 multiplier on the diameter for the critical blocking size predictions in order to match them to TEM observations. However, in our work, the critical size limits seemed to coincide well with the results for particle size distribution obtained by TEM, i.e. the particles with volumes between 10 and 100 nm 3 (which should become "blocked" in 59

Co IF NMR experiment with decrease in temperature from 300 K to 30 K) correspond to the major part of cobalt volume determined from the HRTEM data. Therefore, we assumed that the increase of signal intensity did indeed arise from magnetic blocking of the majority of nanoparticles in the sample. Thus, for the considered aspect ratios (Fig. 3), the spherical assumption remained valid.

Apart from particle size considerations given by the analysis of spectra intensity, 59

Co IF NMR also provides the information on the magnetic and crystalline structures of the particles. In line with previously published assignments 21 , the spectrum recorded at 30 K exhibited a typical line shape, consisting of a Pseudo-Voigt profile at 216.5 MHz that corresponds to single-domain fcc-Co particles and a complex line shape corresponding to hcp-Co phase (219-225 MHz). The latter line could also correspond to stacking faults in fcc-Co packing since they are indistinguishable from hcp-Co. This together with the magnetocrystalline anisotropy makes the line corresponding to hcp-Co much wider compared to fcc-Co 36,37

. No well-defined resonances at 213 MHz corresponding to domain walls occurring in multi-domain Co particles were present in the spectrum, which is not surprising considering the particle size distribution. A decomposition of the spectrum into a Pseudo-Voigt line corresponding to fcc-Co and the remaining signal obtained by subtracting the fcc-Co line from the spectrum resulted in roughly 60% fcc-Co content. The spectrum recorded at RT displayed a much weaker intensity resulting in a less reliable decomposition (possible decompositions are shown in Fig. S3). Nonetheless, the fcc-Co intensity was visibly lower compared to the 30 K spectrum, which is consistent with the fact that hcp packing stability increases with the nanoparticle size increase In order to further investigate superparamagnetic particles in the sample, a series of ferromagnetic resonance (FMR) spectra at room and elevated temperatures were recorded (Fig. 6). At room temperature, there was a broad asymmetric signal in FMR spectrum, with two notable features. The absorption in the high-field region (2000-5000 Oe) can be attributed to the single-domain ferromagnetic nanoparticles interacting with the microwave radiation. The absorption by ferromagnetic particles in the low-field region (0-1000 Oe) is often considered non-resonant. Thus, the origin of that part of the signal is rather complex and difficult to simulate.

With the temperature increase, both the low-field and high-field signals changed. A narrow intense line appeared in the high-field region at 3200 Oe starting from 400 K. When the temperature rose, the high-field line narrowed, followed by a low-field signal intensity decrease. These effects resulted from the superparamagnetic behaviour of Co nanoparticles: with the temperature increase, more nanoparticles exhibited superparamagnetic behaviour resulting in the increase of the narrow component. Simultaneously, the signal in the low-field region decreased since temperature fluctuations unblocked the magnetic moments of nanoparticles.

To analyze the superparamagnetic transition of the Co nanoparticles with the temperature change, the high-field part of the signal was simulated (Fig. 7) as a superposition of two Lorentzian-shape absorption lines: a narrow one, corresponding to "unblocked" nanoparticles (the double

6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins integral is called I unblocked ), and a broad one corresponding to "blocked" nanoparticles (the double integral is called I blocked ).

The double integral of the narrow component is growing together with the temperature increase, supporting the idea of superparamagnetic nature of the signal (Fig. S4).

Critical size calculations

Based on the I unblocked /I blocked ratio from the experimental FMR spectra, it is possible to choose the cut-off volume (x) such that the ratio between the total volumes of particles with V < x and of particles with V > x is equal to I unblocked /I blocked = 0.05, the ratio observed in FMR at 600 K. The obtained value of x in this case is 17 nm 3 . Thus, the critical volume V of a Co nanoparticle for FMR spectroscopy at 600 K is 17 nm 3 . Since the FMR experiments were performed using X-band spectrometer, the characteristic measuring time τ m can be estimated as 10 -10 s. These experimental values provided an estimate of the thermal relaxation time τ 0 using the Neel equation (Eq. 1) as:

which is in reasonable agreement with the commonly reported values of 10 -9 to 10 -12 s 35 . Furthermore, this value of the critical volume of 17 nm 3 observed at 600 K (FMR, 600K) allowed us to predict using Néel equation ( 1) the value of the critical volume at 300 K (FMR, 300 K) as:

Then, following the same logic using (Eq. 1) and using the thermal relaxation time τ 0 obtained by FMR, we can estimate the critical volumes for 59 Co IF NMR spectroscopy at 300 K V(NMR, 300 K) and 30 K V(NMR, 30 K) as follows:

The obtained values of V crit for NMR spectroscopy differ from the predictions made earlier (10 nm 3 at 30 K and 100 nm 2 at 300 K) by a factor of about 2, but nonetheless, this range still covers the major part of the sample volume as determined from the cumulative volume graph (Fig. 8). According to these values, particles corresponding to 89% of total Co volume become thermally blocked in the 30 K -300 K temperature range, which is consistent with the significant intensity increase on the 59 Co IF NMR spectra. While for magnetic properties the physically relevant size is the particle volume, for most nanoparticle applications, for example supported heterogeneous catalysis, the size of interest is the linear size or the particle diameter. We thus also derived from the HRTEM data an effective spherical diameter distribution (Fig. 9) using:

Obviously, the particle volume varies as the cube of the effective diameter so that the later parameter is much less sensitive and the discrepancy of a factor 2 in the volumes translates in a difference of only 25% in the effective Please do not adjust margins Please do not adjust margins diameters. This value was significantly lower than the 250% factor used by Liu et al. on Fischer-Tropsch catalysts 22 . In our case, the combination of FMR and 59 Co IF NMR performed thus satisfactorily to characterize all Co particles from the linear size distribution given by HRTEM images analysis. This might be due to the better defined spheroidal geometry of the particles under consideration. Co IF NMR allows observation of particles solely in ferromagnetic state, and its operational range is limited to temperatures below room temperature due to inherently low sensitivity and long experimental time. This means that all spherical particles bigger than approximately 6-7 nm contribute to the 59 Co IF NMR spectrum taken at room temperature. The main advantage of this technique is its capacity not only to analyze particle sizes, but also to probe the crystalline and magnetic structure of the particles.

On the other hand, both ferromagnetic and superparamagnetic particles give rise to FMR signal making this technique less particle size selective. However, distinguishing the signals in FMR is not obvious. Moreover, an imprecision of thermal relaxation time τ 0 determination at different temperatures may cause the discrepancy between the predicted and experimentally determined values of blocking sizes for 59 Co IF NMR. Nevertheless, the sensitivity of FMR can be three orders higher compared to 59 Co IF NMR since it is the spin magnetization that is probed in this case. This facilitates doing FMR experiments at elevated temperatures, which extends the range of reachable critical blocking sizes to some extent. The probed blocking size range is indeed the main obstacle for these techniques due to a very weak dependency of the linear blocking size on the temperature (cubic root) and even weaker dependency on the measurement time (cubic root of logarithm).

Conclusions

Based on the image analysis by HRTEM, Co/MWCNT hybrid material possesses a relatively narrow size distribution of metallic Co nanoparticles with most of the particles being less than 6 nm in effective diameter. This was the result of particle confinement inside the channels of nanotubes that was observed on HRTEM micrographs. According to the difference between 59 Co IF NMR spectra recorded at 30 K and 300 K, fcc crystal structure is stabilized in Co nanoparticles smaller than 6 nm.

Using this sample, we have demonstrated that despite some experimental drawbacks, 59

Co IF NMR and FMR are two complementary techniques that in combination provide a very powerful and, most importantly, bulk-averaged method for structural characterization of a large range of Co particle volume distributions based on HRTEM data analysis. Due to its high sensitivity, FMR can be used for particle size distribution investigation even at elevated temperatures, which increases the number of particles in superparamagnetic state. At the same time,

Co IF NMR is a technique that provides unique information on the crystalline and magnetic structure of the particles. Moreover, the relatively long experimental time in 59 Co IF NMR allows putting Co nanoparticles into superparamagnetic state in a much wider size range compared to FMR. However, these techniques possess drawbacks that limit the accuracy of critical blocking size determination, namely, very low signal-to-noise ratio in 59 Co IF NMR and presence of non-resonant absorption signal in FMR spectra that leads to somewhat inaccurate decomposition of spectra. Furthermore, because the blocking temperature does not depend only on size but also on shape, caution must be used when discussing particle sizes using 59 Co IF NMR and FMR without using supplementary non-bulk techniques such as HRTEM.

Figure 1 .

 1 Figure 1. HRTEM micrographs of the oxidized carbon nanotubes (A-B) together with a schematic side view (C).

Figure 2 .

 2 Figure 2. HRTEM micrographs of the Co/MWCNT hybrid material taken at different magnifications. Co nanoparticles are visible as dark grey areas. 59 Co Internal Field Nuclear Magnetic Resonance ( 59 Co IF NMR)

Figure 3 .

 3 Figure 3. Left -distribution of particle long axes (C), middle -distribution of particle short axes (A), right -distribution of particle aspect ratios (C/A).

Figure 4 .

 4 Figure 4. Co nanoparticle volume distribution obtained from the experimental A and C axes distributions provided by HRTEM.

35 .

 35 The volume dependence of the blocking temperature expressed in Eq (1) is the basic principle underlying the determination of particle size distribution by 59 Co IF NMR. Any given temperature corresponds to a critical volume above which particles are ferromagnetic. 59 Co IF NMR signal intensity at a given temperature is thus proportional to the total volume of particles larger than the corresponding critical volume among other factors. Consequently, collecting the 59 Co IF NMR spectra at different temperatures allows

Figure 5 .

 5 Figure 5. 59 Co IF NMR spectra of Co/MWCNT material recorded at room temperature (black, intensity multiplied by a factor of 10 for clarity) and 30 K (red) corrected for the temperature dependence of resonance frequency and amplitude. Experimental points are depicted with circles; interpolated lines are drawn for clarity. The 30 K spectrum is shifted into the lower frequency by about 5 MHz to account for the resonance frequency shift caused by the increase of the spontaneous magnetization according to the Bloch's law. The spectral intensities were corrected to account for Curie's law and relaxation time difference.

Figure 6 .

 6 Figure 6. FMR spectra of Co/MWCNT hybrid material taken at various temperatures. The intensity of the resonant components of the signal grows due to gradual unblocking of ferromagnetic particles and due to narrowing of the signal caused by temperature increase.

38 .

 38

Figure 7 .

 7 Figure 7. Experimental FMR spectra (black) at several chosen temperatures along with the simulated lines (shown in color). Red line represents the unblocked superparamagnetic nanoparticles; green line represents the blocked ferromagnetic particles; blue line is the sum of two simulated lines.

Figure 8 .

 8 Figure 8. Depiction of particle blocking range for IF NMR between 300 K and 30 K. Blue bars represent the HRTEM volume distribution. Solid red line is the cumulative volume.

Figure 9 .

 9 Figure 9. Recalculated spherical model Co nanoparticle DEff size distribution obtained from the experimental A and C axes distributions provided by HRTEM. Dashed lines represent the critical particle sizes for FMR and 59 Co IF NMR calculated from the obtained critical volumes.

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This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3

Acknowledgements

The authors are grateful for the support of RFBR grant № 20-33-90184, and a budget project of Boreskov Institute of Catalysis. I.V.Y. acknowledged the French Embassy in Moscow for a PhD fellowship. Pascal Scholzen (ESPCI Paris) is thanked for rich discussions on 59 Co IF NMR.

Conflicts of interest

There are no conflicts to declare.


M Palacios

S Gismera

M M M Alonso

F Brumaud

Puertas

J B D'espinose De Lacaillerie

B Lothenbach

A Favier

C Brumaud

F Puertas

Early

Keywords:

published or not. The documents may come

Introduction

For a concrete formulation to be acceptable, it is not sufficient to meet the mechanical strength, volume stability or durability requirements laid down in national and international standards. It must also be delivered on site with sufficient time for mixing, transport, casting and consolidation. This means that it must exhibit suitable workability and rheology [START_REF] Tattersall | The Rheology of Fresh Concrete[END_REF][START_REF] Kovler | Properties of fresh and hardened concrete[END_REF].

Worldwide environmental concerns are driving the study and development of new construction materials with a lower carbon footprint. Alkali-activated cements and concretes are considered among such materials in which, compared to traditional cements and concretes, the portland clinker content of the binder is substantially lowered and replaced by industrial waste and by-products. Although many studies have been conducted on the mechanical strength and durability of alkali-activated binders [START_REF] Provis | Geopolymers and related alkali-activated materials[END_REF][START_REF] Shi | Recent progress in low-carbon binders[END_REF][START_REF] Palomo | A review on alkaline activation: new analytical perspectives[END_REF][START_REF] Puertas | Alkali-activated fly ash/ slag cements: Strength behaviour and hydration products[END_REF][START_REF] Robayo-Salazar | Alkali-activated binary concrete based on a natural pozzolan: physical, mechanical and microstructural characterization[END_REF], much fewer have explored their rheology [START_REF] Pacheco-Torgal | Handbook of Alkali-Activated Cements[END_REF]. One particularly intriguing phenomenon is the peculiar rheological behaviour of sodium silicate-alkali activated slag (SS-AAS) cementitious systems. Earlier studies showed that the workability and rheology of these materials are closely related to the alkalis concentration and silica modulus of the activating solution [START_REF] Puertas | Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution[END_REF]. A faster loss of fluidity occurs as the SiO 2 /Na 2 O and the percentage of Na 2 O increases. Furthermore, under a constant shear rate, a fast increase and consequent decrease of the shear stress was observed in SS-AAS pastes prepared with sodium silicate solutions containing 4% Na 2 O and silica modulus between 1 and 1.5, while only a fast increase of the shear stress occured for silica modulus above 1.5 [START_REF] Puertas | Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution[END_REF]. Palacios et al. [START_REF] Palacios | Effectiveness of Mixing Time on Hardened Properties of Waterglass-Activated Slag Pastes and Mortars[END_REF] showed that lengthening mixing times improved the workability of SS-AAS systems, while enhancing mechanical strength and reducing drying shrinkage (a significant problem in these alkaline systems).

The rapid loss of workability and the especially short setting times observed in SS-AAS pastes, mortars and concretes have often been attributed to primary C-(A)-S-H formation, which would be the product of the immediate chemical reaction between the Ca 2+ ions dissolving out of the slag into the highly alkaline medium and the silicate ions already present in the activator [START_REF] Huanhai | Kinetic study on hydration of alkali-activated slag[END_REF][START_REF] Palacios | Rheology and setting of alkali-activated slag pastes and mortars: effect of organic admixture[END_REF]. It has been hypothesized that the formation of this C-(A)-S-H would lead to the flocculation of the slag particles and to flow loss. However, empirical evidence of the existence of the primary C-(A)-S-H has proved elusive to date, and this issue constitutes the key aim of this study.

For this purpose, the evolution of the composition of the pore solution and solids during the early reactivity of SS-AAS pastes was monitored and correlated with the time evolution of the rheological properties of the pastes during the first 5 h of reaction. Furthermore, the impact of the mixing time on the rheological properties of SS-AAS pastes has been explored. The new scientific understanding provided by this study will enable us in the future to develop new strategies to extend the workability of SS-AAS cements.

Experimental

Materials

A granulated vitreous blast furnace slag supplied by Ecocem (France) was used in this study. Its chemical composition was determined by Xray fluorescence (XRF) using a Philips PW-1004 XRF spectrometer (see Table 1). Fig. 1 presents the particle size distribution of the slag dispersed in isopropanol measured by laser light scattering (Malvern MasterSizer S equipment) with a D v50 value of 10.42 μm.

A commercial sodium silicate solution (27.0 wt% SiO 2 , 8 wt% Na 2 O and 65 wt% H 2 O; supplied by Merck) and NaOH (98% purity from Panreac) were used for the preparation of the activator solution. Sodium silicate, NaOH and ultrapure water (ρ = 18.2 MΩ⋅cm, TOC around 1-2 ppb) were mixed in the appropriate amounts to obtain an alkaline sodium silicate (SS) solution with 4% Na 2 O by weight of slag, SiO 2 /Na 2 O = 1.5 and H 2 O/Na 2 O = 11.1. This solution composition was chosen because it has been reported in previous studies [START_REF] Puertas | Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution[END_REF][START_REF] Palacios | Rheology and setting of alkali-activated slag pastes and mortars: effect of organic admixture[END_REF] to result in pastes with poor rheological properties.

Sample preparation and test performed

SS-AAS pastes were prepared by mixing at room temperature 80 g of blast furnace slag with 44 g of the alkaline sodium silicate activator (liquid/solid ratio = 0.55) for 30 s at 200 rpm with a mechanical stirrer using an anchor geometry (6 cm of diameter) in a plastic container with an internal diameter of 9 cm. The pastes were afterwards mixed during 3 min at 700 rpm. The test carried out on these pastes are described below.

Rheological behaviour

Three different rheological tests were carried out on SS-AAS pastes:

▪ Measurements at constant shear rate: Pastes were placed in a Haake Rheowin Pro RV1 rotational viscometer fitted with a grooved Z38/S cylindrical rotor blade. The evolution of the apparent viscosity of the SS-AAS pastes was tested for 70 min at a constant shear rate of 100 s -1 at 25 • C ± 0.1 • C. ▪ Small amplitude oscillatory shear (SAOS) measurements were carried out using a Malvern Kinexus Lab+, a stress-controlled rheometer equipped with serrated parallel plates [START_REF] Coussot | Rheometry of Pastes, Suspensions, and Granular Materials: Applications in Industry and Environment[END_REF] at room temperature (23 ± 0.1 • C). The diameter of the plates was 25 mm and the gap between plates was 1 mm. After mixing, the paste was poured on the lower plate of the rheometer before the upper plate was set into position and covered to limit evaporation. The sample was sheared at 100 s -1 for 30 s and after a 10 s resting time, the measurement sequence started. The limits of the linear viscoelastic range (LVR), where the rheological properties of the materials are not strain-or stress-dependent, were first determined by identifying the critical value of strain γ c through a strain sweep test: a strain amplitude (from 0.0001% to 100%) at a constant frequency of 1 Hz was applied and the end of the linear elastic regime was defined as when the storage modulus G' falls to 90% of the plateau value [START_REF] Nachbaur | Dynamic mode rheology of cement and tricalcium silicate pastes from mixing to setting[END_REF]. The viscoelastic property G' (storage modulus) of the SS-AAS slag pastes was then investigated in a large time domain by applying for 70 min a constant shear strain (lower than γ c ) and a constant frequency of 1 Hz to the material, and by recording the subsequent stress. This test is used as an indication of the timedependent structural changes within the material. ▪ Effect of the mixing time on the rheology of SS-AAS pastes: pastes were prepared by mixing 500 g of slag with 275 g of the alkaline sodium silicate solution as described in Section 2.2. ##Pastes were initially mixed at 200 rpm during 30 s and afterwards mixed during 3 or 5 min at 700 rpm. The evolution of the shear stress over time of the pastes was measured using a VISKOMAT NT (Schleibinger) equipped with a vane geometry and a constant shear rate of 100 s -1 was applied during the measurement. The use of this equipment enabled to test bigger batches of paste and confirm the anomalous rheological behaviour of SS-AAS pastes previously measured by using the Haake Rheowin Pro RV1 rotational viscometer.

Pore solution composition

SS-AAS pore solutions were extracted at different times by using the steel die method and filtered with a nylon 0.45 μm membrane filter. An aliquot of the samples was immediately acidified with HNO real pH value by a factor of 0.3 to 0.5 pH units due to the alkali error caused by the presence of high Na-concentrations [START_REF] Traynor | Methodology for pH Measurement in High Alkali Cementitious Systems[END_REF].

Moreover, static NMR 27 Al measurements were carried out on a Bruker Avance III 500 spectrometer at 11.7 T. Spectra were taken in a commercial static broadband Bruker probe. All spectra were recorded following a one pulse excitation with a recycle delay of 1 s and 80 number of transients. The pulse length was determined to be 3.75 μs by taking the maximum of the nutation curve of aluminium in a 0.1 M Al (NO 3 ) 3 aqueous solution. This solution also served as the external standard for calibrating the signal intensity and the chemical shift scale. The paste was introduced into a 5 mm diameter PTFE NMR tube immediately after mixing. At regular time intervals, the signal was recorded as the reaction progressed. Since the sample was not spinning, due to the strong quadrupolar coupling of 27 Al (a spin 5/2) in the slag, only the dissolved aluminium contributed to the signal. Solid reaction products of weak quadrupolar coupling might also contribute to a certain extent but this was neglected. This methodology has already been successfully applied to metakaolin geopolymer pastes [START_REF] Favier | Espinose de Lacaillerie, A multinuclear static NMR study of geopolymerisation[END_REF]. The 27 Al resonance integrated intensity was calibrated with respect to an Al (NO 3 ) 3 aqueous solution of known concentration. As the liquid fraction of the sample is evolving over time as it is consumed during the hydration, the amount of Al in solution measured by NMR was not converted into an effective pore solution concentration.

Microstructure of pastes

Solvent replacement is the most widely applied method to stop the hydration of cementitious materials. However, organic solvents might interact with the silicates of the activator present in the pore solution of the pastes at early times; in SS-AAS pastes this was previously identified by Chen et al. [START_REF] Chen | Method to stop geopolymer reaction[END_REF]. In the present paper, a deeper study of the artefacts introduced by the organic solvents on SS-AAS microstructure has been done and an alternative method to stop of the reaction in fresh sodium silicate-activated materials has been applied.

The microstructure of quenched pastes was analysed at different reaction times over the first 5 h by the following techniques:

• Fourier-transform infrared spectroscopy (FTIR) spectra were obtained by using a Nicolet 6700 spectrometer (Thermo Scientific). KBr pellets were prepared by mixing 1 mg of sample and 200 mg of KBr.

Frequencies were scanned in the range of 4000-400 cm -1 , with a resolution of 4 cm -1 . • 29 Si and 27 Al magic angle solid nuclear magnetic resonance (MAS NMR) spectra were acquired on a BRUKER MSL 400 spectrometer. A 4-mm (outer diameter) ZrO 2 rotor was used at a spinning frequency of 10 kHz. The 29 Si and 27 Al spectra were obtained at resonance frequencies of 79.49 and 104.2 MHz, respectively. Chemical shift values were referenced to tetramethylsilane (TMS) and to a 1 M solution of AlCl 3 as external standards for 29 Si and 27 Al, respectively. In the 29 Si MAS NMR measurements the pulse length was 4.5 μs, the recycle delay 10 s and the number of scans 5600. For the 27 Al MAS NMR measurements the π/6 pulse length was 2 μs, the recycle delay 5 s and the number of scans 1200. • SS-AAS pastes were observed by scanning electron microscopy (SEM) with a Hitachi S-4800 in secondary electrons mode, attached to an Bruker XFlash 5030 EDX microanalysis unit. Prior to imaging, the specimens were carbon coated. • The BET surface area (SSA BET ) measurement was done using a BET multi-point nitrogen physisorption equipment (ASAP 2420). The samples were previously degassed in an external degassing station at 40 • C under N 2 flow for 16 h [START_REF] Mantellato | Reliable specific surface area measurements on anhydrous cements[END_REF]. • Thermogravimetric analysis was performed with a TGA-DSC-DTA Q600 from TA Instruments. Around 40 mg of samples was heated in an alumina crucible from 30 to 1100 • C with a heating rate of 10

• C/min under N 2 atmosphere (100 ml/min).

Thermodynamic modelling

Gibbs Energy Minimization software (GEMS-PSI) [START_REF] Kulik | GEM-Selektor geochemical modeling package: revised algorithm and GEMS3K numerical kernel for coupled simulation codes[END_REF] was used to determine the phase assemblage of the SS-AAS paste over the first 5 h of reaction. The GEMS-PSI [START_REF] Thoenen | Thermodynamic Database 12/07[END_REF] and the Cemdata18 [START_REF] Lothenbach | Cemdata18: a chemical thermodynamic database for hydrated Portland cements and alkali-activated materials[END_REF] thermodynamic databases were used to calculate the solid phases precipitated; the C-N-A-S-H phase was modelled using the so-called "CNASH" model [START_REF] Lothenbach | Cemdata18: a chemical thermodynamic database for hydrated Portland cements and alkali-activated materials[END_REF][START_REF] Myers | A thermodynamic model for C-(N-)A-S-H gel: CNASH_ss. Derivation and validation[END_REF], which considers the uptake of alkali and aluminium in C-S-H phases. The data were completed with data for Na-containing zeolites [START_REF] Ma | Synthesis, characterization, and thermodynamic study of selected Na-based zeolites[END_REF]. It should be noted that the general thermodynamic database used in [START_REF] Thoenen | Thermodynamic Database 12/07[END_REF] describes the aqueous silica complexes at high silica concentration only poorly, due to the uncertainty associated with the data for polynuclear silica species. The formation of quartz, pyrite, goethite, and hematite in the hydrated cements was suppressed in the calculations for kinetic reasons. The composition of hydrated slag was modelled based on slag composition as determined by XRF data (Table 1) and the nominal composition of the sodium silicate solution used.

The measured concentrations of the pore solution were used to calculate the saturation index (SI) of different solids to assess which solids could potentially precipitate. The SI with respect to a solid is given by log(IAP/K S0 ), where IAP is the ion activity product calculated from activities derived from the measured concentrations in the solution and the solubility product, K S0 , of the respective solid [START_REF] Thoenen | Thermodynamic Database 12/07[END_REF]. A positive saturation index implies oversaturation indicating that the solid could form, while a negative value indicates undersaturation, the respective solid cannot form or will dissolve. As the use of saturation indices can be misleading when comparing phases, which dissociate into a different number of ions, "effective" saturation indices were calculated by dividing the saturation indices by the number of ions participating in the reactions [START_REF] Lothenbach | Influence of limestone on the hydration of Portland cements[END_REF]. The SI values for hydroxy-sodalite (Na 8 Al 6- Si

6 O 24 (OH) 2 ⋅2H 2 O), Na-chabazite (Na 2 Al 2 Si 4 O 12 6H 2 O), natrolite (Na 2 Al 2 Si 3 O 10 2H 2 O), and faujasite × (Na 2 Al 2 Si 2.5 O 9 6.2H 2 O), hydro- talcite (Mg 4 Al 2 (OH) 14 ⋅3H 2 O
), CNASH (the SI were expressed with respect to CaO(SiO 2 ) 1.5 (Na 2 O) 0.3125 (H 2 O) 1.1875 , the end-member of the C-N-A-S-H solid solution with the highest oversaturation), M-S-H, and strätlingite (Ca 2 Al 2 SiO 2 (OH) 10 ⋅3H 2 O) were divided by 22, 8, 7, 6.5, 12, 4.75, 5.5 and 6 respectively to obtain effective SI. The activity coefficients of the aqueous species were calculated using the extended Debye-Hückel equation, which is applicable up to approx. 1 to 2 M ionic strength [START_REF] Merkel | Groundwater Geochemistry: A Practical Guide to Modeling of Natural and Contaminated Aquatic Systems[END_REF]:

log 10 γ i = -A γ z 2 i ̅̅ I √ 1 + ȧB γ ̅̅ I √ + b γ I + log 10 X jw X w
γ i represents the activity coefficient, z i the charge of the species i, A γ and B γ are temperature-and pressure-dependent coefficients, I is the effective molal ionic strength, X jw the molar quantity of water and X w the total molar amount of the aqueous phase. Using NaOH as background electrolyte, the common ion size parameter ȧ equals to 3.31 Å and b γ , a short-range interaction parameter, to 0.098 [START_REF] Helgeson | Theoretical prediction of the thermodynamic behavior of aqueous electrolytes by high pressures and temperatures; IV, calculation of activity coefficients, osmotic coefficients, and apparent molal and standard and relative partial molal properties to 600 degrees C and 5kb[END_REF].

Results

Rheological behaviour

Fig. 2 shows the evolution of the apparent viscosity of the SS-AAS pastes over the first 70 min of reaction. During the first 40 min, a steady decrease of the apparent viscosity was observed as the structure broke down with the applied shear. Afterwards, a sharp increase of the apparent viscosity occurred that reached a maximum at around 47 min, with a subsequent reduction of this rheological parameter. This rise of the viscosity occurred before the initial setting of the paste (occurring when a sufficient number of interparticle bridges are formed) at around 80 min (see Table S1 in supplementary material). However, it is worth highlighting that rheological and setting time measurements were carried out in different conditions, under a continuous shear in the former and in static conditions in the latter. A similar evolution of the viscosity has been measured by using a Viskomat NT rheometer equipped with a vane geometry (see Fig. S1), confirming that no artefact was involved in this anomalous rheological behaviour.

The sharp increase of the viscosity could be related to (a) an increase of the viscosity of the interstitial fluid [START_REF] Huanhai | Kinetic study on hydration of alkali-activated slag[END_REF]; or (b) the interparticle bridges formed by the early reaction products growing onto the slag particles. Additional rheological tests performed on quartz sand (with a similar particle size distribution to the slag) mixed with the sodium silicate solution (the same than the one used in the preparation of SS-AAS pastes) have shown a constant apparent viscosity of the paste over the test (see Fig. S2 in Supplementary material). This meant that the rise of the viscosity of SS-AAS pastes was related to the initial chemical reactivity of the slag in contact with the alkaline solution. In fact, the calorimetry curve shown in Fig. S3 (in Supplementary material) showed after 30 min a first peak (with a maximum at 1 h) thus demonstrating the initial reactivity of the slag. This initial peak has been widely explained in the literature to correspond to the initial dissolution of the slag to form a primary C-S-H [START_REF] Huanhai | Kinetic study on hydration of alkali-activated slag[END_REF], something that we will further explore in this paper.

To gain further insights into the reaction and structuration of the pastes in its fresh state, SAOS measurements were carried out [START_REF] Roussel | Recent advances on yield stress and elasticity of fresh cement-based materials[END_REF]. As shown in Fig. 3, where the storage modulus G' and the loss modulus G" are plotted as a function of the strain, just after mixing, the SS-AAS paste had a critical strain γ c of 0.01%. The critical strain establishes the limit of the linear viscoelastic regime (LVR) of the pastes: that is the strain at which the initial structure of the material has been sufficiently modified to produce the rupture of particle network of interactions [START_REF] Roussel | Steady state flow of cement suspensions: a micromechanical state of the art[END_REF]. The order of magnitude of the critical strain thus highlights the ability of the structural network to be deformed under stress and defines the nature of interactions. The small critical strain observed here, substantially lower than the one usually reported for Portland cement [START_REF] Roussel | The origins of thixotropy of fresh cement pastes[END_REF] or geopolymer systems [START_REF] Favier | Espinose de Lacaillerie, A multinuclear static NMR study of geopolymerisation[END_REF], is associated with short range links between the particles [START_REF] Roussel | Steady state flow of cement suspensions: a micromechanical state of the art[END_REF]. Fig. 4 shows the evolution of the storage modulus G' over the reaction time at a constant shear strain of 0.005% (lower than γ c ). This parameter did not significantly change during the first 17 min of reaction, afterwards a slight increase was observed and a sharp rise of G' took place after 40 min of reaction. This indicated a fast structuration of the system after this time, at a time which matched the increase of the viscosity shown in Fig. 2. The measurement was stopped after 70 min, at which time, the modulus reached the limit of the rheometer at 300 kPa.

Pore solution composition 3.2.1. Analysis of the pore solution over time

The evolution of the pH and composition of the pore solution of the SS-AAS paste over the first 5 h of reaction are shown in Table 2. The pH values remained almost constant over time, with a value between 13.1 and 13.3. An increase in the concentrations of Al, Ca and Mg was observed during the first 30 min because of the initial dissolution of the slag. Afterwards, a significant drop in the concentration of these elements, together with Na and Si, occurred. After 5 h of reaction the concentrations of Al, Ca and Mg were, respectively, 71%, 98% and 99% lower than those measured at 30 min. This was related to a change in the balance between the slag dissolution and the precipitation of the hydration products. It is worth highlighting, that the drop of these main elements in the pore solution occurred above 30 min, when the viscosity and the storage modulus increased (see Fig. 2 and Fig. 4).

In-situ monitoring of the aluminium species in the pore solution over time was performed by static liquid-state 27 Al NMR. This technique enables to detect primarily the aluminium in the pore solution while the signal coming from the solid phases is minimized. As shown in Fig. 5, at 15 min of reaction, almost no tetrahedral aluminium species were detected in the interstitial fluid. Afterwards, an abrupt raise of the amount of aluminium signal occurred reaching a maximum at around 37 min. From that moment on, the amount of aluminium in solutions steadily decreased until 90 min of reaction, after which it remained constant within experimental error (±3 μmol). The evolution of Al in solution measured by ICP-OES and NMR matched and a drop of Al concentration is detected at the very moment when the elastic modulus of the paste increased.

Effective saturation indexes over time in SS-AAS

The determination of saturation indices from the concentrations determined in the pore solutions by ICP-OES offers the possibility of assessing independently which solid phases could form or are expected to dissolve. The effective saturation indices are plotted in Fig. 6 The positive effective saturation indices suggest that strätlingite could precipitate during the first hours, when the Al concentrations are relatively high, but any strätlingite would be destabilised at later ages when the solutions become undersaturated (negative SI).

The calculated saturation indices generally increased with time up to 30 min, but showed a decrease after 60 min indicating the nucleation of solids within this timeframe, consistent with the observed changes in viscosity and storage modulus.

Arrest of the reaction of sodium silicate-activated slag pastes

To correlate the rheological properties of SS-AAS pastes with the formation of hydration products over time, it is essential to effectively arrest the reaction of the pastes to be analysed without introducing artefacts. This is a relevant issue at early stages when high concentration of silicates is found in solution that might interact with organic solvents.

Solubility of sodium silicate solution in organic solvents

The solubility of the sodium silicate solution in organic solvents, commonly used to stop cement hydration such as acetone, ethanol and isopropanol was initially investigated. Organic solventsodium silicate mixes with a weight ratio of 1:10 and 1:1 were prepared; the former shown in Fig. 7 while the latter in Fig. S4 in the supplementary material. After contact of both compounds, the solution became turbid and a geltype product formed at the bottom of the glass tubes. The amount of geltype product depended on the type and amount of organic solvent, being the highest for ethanol-containing mixes and increasing with the solvent content. This observation agreed with previous studies [START_REF] Jung | One-step synthesis of structurally controlled silicate particles from sodium silicates using a simple precipitation process[END_REF] that considered the hydrolysis and condensation reactions of sodium silicate in alcohol solvents such as ethanol.

Fig. 8 shows the FTIR spectrum of the sodium silicate solution and of the precipitate formed after contact with the organic solvents. The spectrum of the sodium silicate solution shows a main band at 1004 cm -1 assigned to Si -O asymmetric stretching vibration of [SiO(OH) 3 ] - The measured pH values in the presence of 1 to 3 M Na are expected to underestimate the real pH value by a factor of 0.3 to 0.5 pH units due to the alkali error [START_REF] Traynor | Methodology for pH Measurement in High Alkali Cementitious Systems[END_REF]. b Measurement error ± 10%. and [SiO 2 (OH) 2 ] 2-(Q 1 and Q 2 , respectively) species and two bands at 459 and 619 cm -1 associated to the deformation of Si-O-Si of various Q n species [START_REF] Osswald | FTIR spectroscopic study on liquid silica solutions and nanoscale particle size determination[END_REF]. The FTIR spectra of the precipitate showed a shift of the band associated to the asymmetric stretch of Si-O from 1004 to 1040-1080 cm -1 with respect to the initial sodium silicate solution that confirmed the higher polymerization degree of the precipitated product with respect the starting sodium silicate solution.

The possible formation of this condensation product on the slag surface might introduce several artefacts during the characterization of fresh SS-AAS pastes (see Figs. S5 and S6 in the supplementary material). Therefore, an alternative method, described in the section below, has been used to stop the reactivity of SS-AAS pastes.

Method to stop reaction of fresh SS-AAS pastes

SS-AAS pastes were prepared as described in Section 2.2 and after 12 min of reaction, three different methods to stop the reaction were applied:

Solvent replacement with acetone and ethanol.

The paste was initially treated during 45 s with acetone using a paste:solvent ratio of 3:10. Afterwards the solvent was vacuum filtered through a nylon filter with a size pore of 0.45 μm and the acetone treatment was repeated once. Finally, the solid was treated with 10 ml of ethanol during 15 s. After further filtration, the solid was vacuum filtered and dried in a desiccator under vacuum until constant weight.

3.3.2.2.

Freeze-drying. SS-AAS paste was directly submerged into liquid nitrogen and after 1 h introduced in a freeze dryer device (TELSTAR CRYODOS) that keeps the temperature at -25 • C and pressure at 17 mPa. The sample was removed after 24 h.

Water and isopropanol treatment. SS-AAS paste was firstly

mixed with ultrapure water, ρ = 18.2 MΩ⋅cm, TOC around 1-2 ppb, (1 g of paste and 20 g of ultrapure water). The solid was vacuum filtered, and the water treatment repeated. With this procedure, the non-reacted sodium silicate was removed and the reaction of the slag stopped. After vacuum filtration, the solid was washed with isopropanol (solid:isopropanol = 1:10), filtered and transferred to a desiccator under vacuum until constant weight. A similar procedure was previously proposed by Chen et al. [START_REF] Chen | Method to stop geopolymer reaction[END_REF]. We further discuss the benefits of using this alternative method in comparison with standard protocols used for Portland cement systems.

The obtained solids were analysed by SEM, TGA, FTIR and N 2 adsorption techniques. From the micrographs in Fig. 9a, the presence of a precipitate on the surface of the slag treated with acetone-ethanol can be observed. This may be attributed to the precipitation of the silicate species in alcohol and acetone [START_REF] Chen | Method to stop geopolymer reaction[END_REF] described in Section 3.3.1. Furthermore, a layer of an unknown product is detected on the slag surfaces when the freeze-drying method is applied (see Fig. 9b). The formation of this layer is not observed when slag is activated with NaOH and freeze dried as shown in Fig. S7 (supplementary material); which suggests that the sodium silicate solution was the main responsible for this artefact. With both treatments, the formation of products on the slag surface leads to a decrease of the SSA BET with respect to the starting slag. In particular, SSA BET decreased from the initial 1.27 m 2 /g to 0.7-0.5 m 2 / g (see Table 3). In addition, a weight loss of around 4% has been measured for the samples treated with acetone-ethanol and freezedrying; which is rather high weight loss for samples that have only reacted over 12 min with typically small amount of reaction products.

In contrast, no precipitate was identified in samples treated with water-isopropanol, leading to a similar SSA BET as observed for the initial slag, and to a small weight loss in the TGA analysis that fits with the early studied hydration time (12 min). Fig. 10 shows the FTIR spectra for the anhydrous slag and the pastes 20 min after exposure to different treatments. The very wide band observed in all cases between 730 cm -1 and 1200 cm -1 was associated with the ʋ 3 (Si-O) stretching vibrations in the SiO 4 tetrahedra [START_REF] Puertas | Alkali-activated fly ash/ slag cements: Strength behaviour and hydration products[END_REF]. The spectra exhibited a second wide band at 509 cm -1 , generated by the bending vibrations (ʋ 4 (O-Si-O)) [START_REF] Mozgawa | Spectroscopic studies of alkaline activated slag geopolymers[END_REF]. The asymmetric stretching vibration bands attributed to the Al -O bonds in the AlO 4 groups present in the slag appeared at 700 cm -1 to 690 cm -1 [START_REF] Socrates | Infrared and Raman Characteristics Group Frequencies[END_REF].

A comparison between the spectra for the anhydrous slag and the SS-AAS pastes after different treatments revealed the band at 730 cm -1 to 1200 cm -1 to be significantly wider in two types of samples, those treated with acetone and ethanol and those exposed to freeze-drying. That broader band would denote the presence of a lower order reaction product, such as a gel containing the organic solvents (see Fig. 8) or the layer of sodium silicate visible in in the freeze-dried samples (Fig. 9c). In contrast, the bands attributed to the Al-O bonds in the AlO 4 groups appeared to remain unaltered in the pastes, irrespective of the method used to stop the slag reaction.

The above results led to the conclusion that the initial washing of the SS-AAS pastes with water and subsequent treatment with isopropanol is the best method to arrest the reaction of SS-AAS pastes at early ages. Contrary to the organic solvent exchange and freeze-drying methods, no precipitates have been detected on the slag surface. Consequently, this alternative method was chosen to investigate the evolution of microstructure of early SS-AAS pastes in the next section.

Microstructural characterization of pastes over time

Table 4 shows the weight loss in the temperature range between 50 • C and 500 • C attributed to the bound water of the initial hydrates for the SS-AAS pastes over the first 5 h of hydration. The four samples showed a mass loss peak at around 650 • C attributed to the presence of CaCO 3 in the initial slag due to its weathering; the TGA curves are shown in Fig. S8 in the supplementary material.

At 12 min, the weight loss was very similar to the one of the starting slag, confirming the low advancement of the reaction at this age. In contrast, after 46 min and 5 h of reaction, a weight loss of around 2% and 4%, respectively, were measured that infers the formation of reaction products at the time when the storage modulus and viscosity of SS-AAS pastes dramatically rises.

Fig. 11 shows the FTIR spectra (in the 1700-400 cm -1 region) of the starting slag and the SS-AAS pastes up to 5 h of reaction. The spectra for the 9 min and 20 min pastes were very similar to the spectrum for the anhydrous slag, further confirming the low degree of reaction at such short activation times. After 46 min and 60 min, both the ʋ 3 (Si -O) (at around 968 cm -1 ) and the ʋ 4 (O-Si-O) (at around 500 cm -1 ) bands widened, denoting lower short-range structural order in the pastes studied. Five hours into the reaction, the ʋ 3 (Si-O) band at 980 cm -1 was narrower and shifted to higher wavenumbers than in the original slag, indicating the formation of more highly polymerized reaction products.

The 29 Si MAS NMR spectra for the anhydrous slag and its hydrated pastes are reproduced in Fig. 12, whilst the spectrum for the sodium silicate solution used to activate the slag is shown in Fig. S9. The decomposition results are given in Table 5. The anhydrous slag was the primary source of the signal at -75.5 ppm in all the samples. In addition, a broad shoulder was present after 46 min. It was centered around -85.5 ppm extending between about -80 ppm and -105 ppm. This chemical shift range corresponds to the resonances of the Q n (mAl) (n = 1,2,3,4 and m = 0, 1, 2 3) units associated with all the silica containing moieties outside the slag (in slag, mainly Q 0 environments are present [START_REF] Puertas | A model for the C-A-S-H gel formed in alkali-activated slag cements[END_REF]), namely the solution silicate oligomers (at around -92 ppm) and the alumino-silicate gel, at around -89 ppm (Q 3 (1Al)) and -99.5 ppm (Q 4 (4Al)) [START_REF] Favier | Espinose de Lacaillerie, A multinuclear static NMR study of geopolymerisation[END_REF]. The signals associated with C-A-S-H would also overlap with this region, so we cannot totally discard the presence of a small amount of C-A-S-H at this reaction time. After 5 h, the intensity of the signal at -85.5 ppm increased. Consequently, the evolution of the 29 Si MAS NMR spectra were associated with alumino-silicate gel [START_REF] Favier | Espinose de Lacaillerie, A multinuclear static NMR study of geopolymerisation[END_REF] and C-A-S-H product, which would increase with reaction time.

The 5 h 27 Al MAS NMR spectra obtained for the anhydrous slag and the SS-AAS pastes are reproduced in Fig. 13. The main peak narrowed as reaction proceeded. In glass, the breadth of the resonance of quadrupolar nuclei can be directly related to structural disorder via the Czjzek model [START_REF] J.-B. D'espinose De Lacaillerie | MAS NMR spectra of quadrupolar nuclei in disordered solids: the Czjzek model[END_REF]. Consequently, the observed narrowing could be due either to the passage of aluminium from a vitreous environment to a more organized C-A-S-H one, or to the preferential dissolution of the more disordered fractions of the slag. Actually, both phenomena are likely to occur simultaneously.

The microstructural surface of the slag particles was characterised by scanning electron microscopy (SEM) before and during the reaction with the sodium silicate solution (see Fig. 14). The SEM micrographs showed that the slag particles roughened as the reaction proceeded. After 46 min a very compact granular structure could be distinguished across the entire surface of the slag particles. This would confirm the formation of hydrates on the slag surface from around 40 min of slag reaction.

Thermodynamic modelling

Thermodynamic modelling was also used to calculate the kind and amount of hydrates that can be expected to form upon slag reaction as summarised in Fig. 15. The main hydrates predicted are C-N-A-S-H containing Na and Al, and a zeolitic precursor (here represented by hydroxy-sodalite), in agreement with different experimental and modelling results [START_REF] Haha | Influence of slag chemistry on the hydration of alkali-activated blast-furnace slagpart I: effect of MgO[END_REF]. In addition, a minor amount of M-S-H, brucite and FeS (troilite) are predicted; the formation of a hydrotalcite like phase is predicted at above 20% slag reaction. The formation of C-A-S-H and hydrotalcite is observed commonly in alkali activated slags [START_REF] Ben Haha | Influence of activator type on hydration kinetics, hydrate assemblage and microstructural development of alkali activated blast-furnace slags[END_REF], as well as the precipitation of ill-defined N-A-S-H instead of a crystalline zeolite [START_REF] Provis | Geopolymers and related alkali-activated materials[END_REF]. The formation of ill-defined and rather disordered tetrahedral-coordinated aluminosilicates is more likely to occur on shorter timeframes instead of crystalline zeolites as their formation is kinetically limited [START_REF] Lothenbach | Zeolite formation in the presence of cement hydrates and albite, physics and chemistry of the earth[END_REF]. Thus, at early times, where we have a low degree of slag reaction of up to 20% the formation of ill ordered N-A-S-H gel and C-N-A-S-H, in agreement with the NMR results, can be expected to dominate the hydrates formed and thus the rheological properties.

Discussion

Several studies [START_REF] Puertas | Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution[END_REF][START_REF] Puertas | Alkali-activated slag concrete: fresh and hardened behaviour[END_REF][START_REF] Kashani | The interrelationship between surface chemistry and rheology in alkali activated slag paste[END_REF] have addressed the poor rheological properties of SS-AAS cementitious systems, that worsen with the increase of siliceous modulus and alkali concentration [START_REF] Puertas | Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution[END_REF]. The formation of an early C-(A)-S-H has been pointed as the main responsible of the fast loss of fluidity of SS-AAS cements, however, no experimental evidences for that have been shown up to date. In this study, we correlate the rheology of SS-AAS systems with the evolution of the pore solution composition and hydration products formed over time. Furthermore, the impact of the mixing time on the rheological properties of these alkaline cements was investigated.

In a first step, an alternative protocol to stop the early reaction of SS-AAS had to be developed, as the sodium silicate in the pore solution precipitates in the presence of organic solvents normally used to stop the reactivity of Portland cements (see Figs. 7 and8). Two consecutives washing of the SS-AAS pastes with ultrapure water (in a solid:water ratio 1:20) and a subsequent treatment with isopropanol and vacuum drying have been proved to be an optimum way to stop the early reactivity of SS-AAS without introducing artefacts. This method is mainly suitable for fresh samples (with high content of silicates in the pore solution). Otherwise, organic solvent exchange can be used in SS-AAS systems with longer reactions times and low silicate concentration in solution.

Correlation between the rheological properties and the evolution of the pore solution composition and reaction products

Small amplitude oscillatory shear measurements have enabled to monitor the structuration of SS-AAS pastes with a solution containing SiO 2 /Na 2 O = 1.5 and 4% Na 2 O by weight of slag. Over the first 30-40 min of reaction, an increase of aluminate species, Ca and Mg, due to slag dissolution, was measured by 27 Al NMR and ICP-OES (Fig. 5 and Table 2). At this stage, FTIR and NMR analysis confirmed the absence of hydration products, explaining an almost constant value of the measured storage modulus (see Fig. 4). However, after 30-40 min of reaction, a drastic drop of the concentrations of Al, Si, Ca and Mg in the pore solution occurred, indicating a supersaturation of the solution and the precipitation of reaction products that led to a sharp increase of the storage modulus of SS-AAS. By 29 Si MAS NMR (Table 5), the proportion of silicon of unreacted slag is estimated to be only about 79%. Since the initial amount of silicon in slag in the reaction mixture was 86% (the rest coming from the activator), that means that the advancement of hydration of the silicate part of the slag at 45 min was 8%. This number, obtained by 29 Si NMR, does not consider aluminates but can be taken as rough estimate of the advancement of slag reaction. This amount of hydration products was small but enough to modify the structure of the paste.

The combination of FTIR and NMR measurements together with thermodynamic modelling has enabled us to identify the nature of these Fig. 12. 29 Si MAS NMR spectra of the starting slag and SS-AAS pastes over 5 h of reaction. Subtracting the contribution of the slag from the spectra reveals resonances in the range of -85 ppm corresponding to alumino-silicate gels and C-A-S-H; their intensity increased with time.

Table 5

Analysis of the 29 Si MAS NMR spectra of the starting slag and SS-AAS pastes over 5 h of reaction. early hydration products. After 40 min of reaction, FTIR (Fig. 11) and 29 Si MAS NMR (Fig. 12) data confirmed the formation of a reaction product highly polymerized, in particular, a broad signal centred at around -85 ppm in the 29 Si MAS NMR could indicate the formation of an aluminosilicate gel and C-A-S-H. This agrees with the thermodynamic modelling results that predicts, at the early stages of the slag reaction, the co-existence of an ill-defined N-A-S-H and C-N-A-S-H. Both, the illdefined N-A-S-H and C-N-A-S-H formed from the reaction of the Na + and silicate species originally in the solution with the Ca 2+ ions and aluminate species dissolved from the slag. The removal of Mg and silicate ions can be also linked to the precipitation of M-S-H as predicted by thermodynamic calculations; however, this might be present in very low amounts, and it was not detected by FTIR or NMR. At 5 h of reaction, the amount of hydration products increased as shown in the FTIR spectra (Fig. 11) and 29 Si MAS NMR (Fig. 12). In particular, the FTIR spectra showed a narrowing of the ʋ 3 (Si -O) band at 980 cm -1 with the increase of the hydration time that indicates the formation of a more organized C-N-A-S-H, as also observed by 27 Al MAS NMR. While in Portland cement systems the nucleation of C-S-H in the contact areas between particles leads to an increase in the shear modulus at very low degrees of hydration [START_REF] Roussel | The origins of thixotropy of fresh cement pastes[END_REF], in SS-AAS systems, the formation of an ill-defined N-A-S-H and C-A-S-H of a few nanometers is the main responsible of the rapid flow loss. The formation of an aluminosilicate gel has been also concluded to be the responsible for the increase of the elastic modulus in sodium silicate-activated metakaolin systems [START_REF] Favier | Espinose de Lacaillerie, A multinuclear static NMR study of geopolymerisation[END_REF].

In contrast, in NaOH-AAS, no silicate species are initially in solution and the concentration of silicate and aluminium species as well as Ca ions slowly increases as slag dissolves. Thermodynamic calculations (shown in Fig. 16) predicted that no N-A-S-H is expected to form in such NaOH-AAS systems while experimental data indicated that the precipitation of C-A-S-H can take several hours [START_REF] Gong | In situ quasi-elastic neutron scattering study on the water dynamics and reaction mechanisms in alkali-activated slags[END_REF], which together would explain why the NaOH-AAS systems remain fluid for longer time and have a longer setting times than SS-AAS systems.

The next question to address is, why does the viscosity decrease when a constant shear is applied to SS-AAS cements as observed in Fig. 2? According to the results, the slag starts to dissolve from the first moment of contact with the alkaline solution, releasing Ca, Al, Si and Mg into the pore solution. Once supersaturation is reached, a N-A-S-H precursor gel and C-(N)-A-S-H are formed leading to the flocculation of the particles. However, the attractive forces are relatively small, probably because of the low amount of hydrates formed, and a shear is able to break down the particles interaction. A sketch of the proposed mechanism is shown in Fig. 17. The use of sodium silicate solutions with a higher SiO 2 /Na 2 O and/or higher percentage of Na 2 O would lead to the faster and higher amount of the reaction products, inducing higher attraction forces between particles that cannot overcome even by applying a constant shear as previously shown by Puertas et al. [START_REF] Puertas | Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution[END_REF].

Impact of the mixing time on the rheological properties of SS-AAS pastes

An increase of the mixing time has been previously concluded to delay the setting times of SS-AAS [START_REF] Palacios | Effectiveness of Mixing Time on Hardened Properties of Waterglass-Activated Slag Pastes and Mortars[END_REF]. For this reason, we have studied the effect of increasing the mixing time from 3.5 min to 5.5 min on the evolution of the shear stress of SS-AAS pastes. As shown in Fig. 18, an extension of 2 min of the mixing time delays around 30 min the time of appearance of the sharp increase of the shear stress (and consequently apparent viscosity). Again, the structure breaks down afterwards under shearing. The area under both curves is relatively similar independently of the applied mixing time (734 and 864 Pa . s, for pastes mixed during 3.5 and 5.5 min, respectively). However, a lower intensity of the peak is observed for pastes mixed for longer time. This might involve that the number of flocs formed over time is the same but they are formed at lower speed when pastes are mixed for a longer time.

The extension of the fluidity of SS-AAS systems is essential to easily place them in practice. Current superplasticizers have been proved to lose their dispersing properties when sodium silicate is used as activating solution in alkali-activated slag systems. Their lack of solubility in these alkaline solutions and the highly competitive adsorption between the superplasticizers and the silicates of the activating solution [START_REF] Marchon | Molecular design of comb-shaped polycarboxylate dispersants for environmentally friendly concrete[END_REF] are the main responsible of their ineffectiveness. As shown in this study, a longer mixing leads to an extension of the period where the system remains fluid, however, extra mixing is not enough and an alternative strategy must be found. From the results obtained in this study, we now understand that it is essential to identify effective retarding admixtures that delay the slag reactivity and the early formation of the N-A-S-H precursor gel and C-N-A-S-H. The addition of maleic acid [START_REF] Brough | Sodium silicate-based alkaliactivated slag mortars: Part II. The retarding effect of additions of sodium chloride or malic acid[END_REF] or nano-ZnO [START_REF] Garg | Mechanism of zinc oxide retardation in alkali-activated materials: an in situ X-ray pair distribution function investigation[END_REF] has been probed in the literature to delay the reaction of sodium silicate-activated slag cements. While the former does not affect to the nature of the hydration products, the latter slows down the reactivity of the slag by forming calcium zincate at the early stage of the hydration and depleting the calcium in solution to form C-N-A-S-H. We need now to further expand these studies and understand the impact of effective retarders on the fluidity of SS-AAS cements.

Conclusions

In this paper, a combination of experimental characterization techniques, to study the pore solution composition and microstructure of SS-AAS pastes, and thermodynamic calculations have been applied to understand their anomalous rheological properties. The initial precipitation of an ill-defined N-A-S-H (precursor of N-A-S-H gel) and C-N-A-S-H cause the fast increase of the storage modulus, and loss of fluidity, after 40 min of reaction. Both reaction products are formed from the reaction of the Na + ions and silicate ions in the initial activating solution with the Ca 2+ and aluminate species dissolved from the slag.

If a shear is applied, the viscosity of SS-AAS pastes dramatically decreases after reaching a maximum value. This allows to infer that the attractive forces between the particles with the early hydrates are low, probably due to the small amount of reaction products formed at the early stages, and deflocculation takes place.

An increase of the mixing time delays the time at which the dramatic increase of the shear stress occurs and the flocculation rate of SS-AAS pastes.

An alternative method has been used to arrest the reaction of sodium silicate-activated slag (SS-AAS) pastes that involves an initial treatment of the SS-AAS pastes with water to remove the sodium silicate in solution and decrease the pH, and a subsequent treatment with isopropanol. This has been proved to avoid artefacts for the microstructural analysis of the SS-AAS pastes.

Fig. 1 .

 1 Fig. 1. Particle size distribution of the blast furnace slag.

Fig. 2 .

 2 Fig. 2. Evolution of apparent viscosity over time in SS-AAS pastes. There is a transient increase of viscosity around 40 min. This was indicative of the temporary formation of a structure that eventually broke down under shear.

Fig. 3 .

 3 Fig. 3. Strain sweep of SS-AAS slag paste for the determination of the critical strain. Measurements were carried out immediately after the mixing stage. Log scales in both axis have been used.

Fig. 4 .

 4 Fig. 4. Evolution of the storage modulus over time for the SS-AAS paste prepared using a sodium silicate solution with a SiO 2 /Na 2 O = 1.5 and 4%wt Na 2 O by weight of slag.

  . The positive SI values show that the solutions were at all-time oversaturated with respect to M-S-H, C-N-A-S-H and several Na-containing zeolites: hydroxy-sodalite (Na 8 Al 6 Si 6 O 24 (OH) 2 ⋅2H 2 O), natrolite (Na 2 Al 2 Si 3 O 10 2H 2 O), Na-chabazite (Na 2 Al 2 Si 4 O 12 6H 2 O) and faujasite X (Na 2 Al 2- Si 2.5 O 9 6.2H 2 O), indicating that these solids could potentially form. The solutions were also oversaturated with respect to brucite, hydrotalcite and siliceous hydrogarnet (Ca 3 Fe 2 (SiO 4 ) 0.84 (OH) 8.64 ); data not shown.

Fig. 5 .

 5 Fig. 5. Evolution of Al in the pore solution of SS-AAS pastes determined by ICP-OES (in mmol/l) and 27 Al MAS NMR (in μmol). ICP and NMR data cannot be directly compared since one measures a concentration and the other a quantity but the trends clearly matched.

Fig. 6 .

 6 Fig. 6. Calculated effective saturation indices as a function of hydration time. A positive saturation index indicates oversaturation, a negative value undersaturation.

Fig. 7 .

 7 Fig. 7. Mixes of organic solvent:sodium silicate solution in a ratio of 1:10. The formation of a white gel-type product is observed. Reference tube contains exclusively the alkaline sodium silicate solution (SS).

Fig. 8 .

 8 Fig. 8. FTIR spectra of the alkaline sodium silicate solution (SS) before and after treatment with ethanol, acetone and isopropanol in a ratio of organic solvent:SS of 1:1.

Fig. 9 .

 9 Fig. 9. (a) Starting blast furnace slag and slag particles after treatment of the SS-AAS pastes with: (b)Acetone-Ethanol; (c) Freeze-drying and (d) Water-isopropanol. SS-AAS pastes were tested after 12 min of reaction.

Fig. 10 .

 10 Fig. 10. FTIR spectra of the SS-AAS pastes treated with: (a) Acetone-Ethanol; (b) Freeze-drying and (c) Water-isopropanol. SS-AAS pastes were tested after 20 min of reaction.

Fig. 11 .

 11 Fig. 11. FTIR spectra of the SS-AAS over time.

Fig. 14 .

 14 Fig. 14. SEM image of the starting slag and SS-AAS over 5 h of reaction; (a, b) starting slag; (c, d) at 12 min; (e, f) at 46 min and (g, h) at 5 h. After 46 min, the formation of reaction products is observed on the slag surface.

Fig. 15 .

 15 Fig. 15. Modelled volume of hydrates formed in the sodium silicate containing solution as a function of the degree slag reaction. The formation of a poorly ordered N-A-S-H precursor gel instead of the calculated OH-sodalite is expected.

Fig. 16 .

 16 Fig. 16. Modelled volume of hydrates formed in the NaOH-AAS pastes (with 4%wt Na 2 O by weight of slag) as a function of the degree slag reaction.

Table 1

 1 3 2% wt. to prevent the precipitation of solids and analysed by Inductively Coupled Plasma -Optical Emission Spectrometry ICP-OES (VARIAN 725 ICP-OES). Pore solutions were diluted with HNO 3 2% wt. in a proportion of 1:50 to measure Al, Ca, Fe, Mg and K; and 1:2000 to measure Si and Na concentrations.The pH of the pore solution was measured on the remaining solution at 25 • C ± 0.1 • C with a Crison pH meter (pH Basic 20). The electrode was calibrated with standard buffers (pH 4, 7 and 10). The measured pH values in the presence of 1 to 3 M Na are expected to underestimate the Chemical composition (wt%) of the blast furnace slag.

		CaO	SiO 2	Al 2 O 3	MgO	Fe 2 O 3	MnO	SO 3	Na 2 O	K 2 O	TiO 2	P 2 O 5	LoI	IR
	wt%	38.5	38.7	11.2	8.6	1.1	0.3	1.0	0.6	0.4	0.4	0.1	-1.0	0.0015

LoI: Loss on ignition measured at 1000 • C according to EN 196-2:2014. The negative value is due to the oxidation of sulphides to sulphates; IR: insoluble residue according to UNE 80230:2010.

Table 2

 2 Measured composition of pore solutions from SS-AAS over the first 5 h of reaction.

	Reaction time (minutes)	pH a	Al b	Ca b	Mg b	Fe b	Na b	Si b
			(mmol/L)	(mmol/L)	(mmol/L)	(mmol/L)	(mmol/L)	(mmol/L)
	15	13.1	17.20	47.77	16.37	0.46	2779	2173
	30	13.2	27.75	69.61	23.00	0.49	2198	2187
	60	13.2	22.97	24.85	9.16	0.11	1673	1567
	90	13.2	18.25	6.58	3.01	0.01	1567	1356
	120	13.3	16.34	3.17	2.44	0.01	1919	1076
	180	13.3	12.74	3.83	1.13	0.03	1499	911
	240	13.3	9.32	1.17	0.38	0.01	1169	563
	300	13.3	7.90	0.70	0.21	0.00	1544	512
	a

Table 3 SSA

 3

	Method to arrest the	Acetone-Ethanol	Freeze	Water-
	reaction		drying	isopropanol
	SSA BET (m 2 /g)	0.73	0.57	1.52
	Weight loss (%) determined	3.8	4.3	0.3
	by TGA

BET of SS-AAS pastes after 12 min of reaction using different methods to stop their reaction. The starting slag has a SSA BET of 1.27 m 2 /g and a weight loss of 0.2%.

Table 4

 4 Weight loss (%) of the starting slag and SS-AAS pastes at different reaction times measured by TGA.

	Range of temperature ( • C)	Weight loss (%)
		Starting slag	SS-AAS pastes
			12 min	46 min	5 h
	25-500	0.25	0.34	1.96	3.82
	500-1000	0.50	0.38	0.58	0.57

  Fig.13.27 Al MAS NMR of the SS-AAS pastes at different reaction times. The resonances narrowed with time indicating the formation of products with higher ordering and aluminium site symmetry than the original slag.

								Starting slag
								46 min
								90 min
								5 h
						100	80	60	ppm	40	20	0
		Starting	46	90	5 h	Assignment
		slag	min	min
	δ (ppm)	-75.5	-75.5	-75.5	-75.5	Q 0
	Width	13	13	13	13
	(ppm)
	Integral	100	79	77	66
	(%)
	δ (ppm)		-85.5	-85.5	-85.5	Unresolved hydrates and
						silicate oligomers
	Width		24	24	24
	Integral		21	23	34
	(%)

Acknowledgement

The authors thank Prof. Habert (ETHZ) for constructive discussion. Dr. Francesco Caruso (SIK-ISEA) is thanked for helping to develop a protocol for the ICP-OES measurements. Dr. M. Palacios acknowledges

CRedit authorship contribution statement

Marta Palacios: Conceptualization, Methodology, Writing-original draft, Supervision, Funding adquisition; Sara Gismera: Investigation, Methodology; M.M. Alonso: Methodology, Supervision, Revision of the manuscript; Jean-Baptiste d'Espinose de Lacaillerie; Methodology, Writing-original draft, Barbara Lothenbach: Methodology, Writingoriginal draft; Aurelie Favier: Investigation, Revision of the manuscript; Coralie Brumaud: Investigation, Revision of the manuscript, Francisca Puertas: Conceptualization; Supervision; Writing-original draft, Project administration; Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.cemconres.2020.106302.