Distribution of Water in Synthetic Calcium Silicate Hydrates - Langmuir

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Distribution of Water in Synthetic Calcium Silicate Hydrates C. Roosz,†,‡,§ S. Gaboreau,*,‡ S. Grangeon,‡ D. Prêt,† V. Montouillout,∥ N. Maubec,‡ S. Ory,∥ P. Blanc,‡ P. Vieillard,† and P. Henocq§ †

UMR CNRS 7285 IC2MP, Université de Poitiers, Equipe HydrASA, rue Albert Turpain, Bat B8, 86022 Poitiers, France Environment and Process Division, BRGM, 3, avenue Claude Guillemin, F-45060 Orléans Cedex 2, France § Andra, 1/7 rue Jean Monnet, Parc de la Croix Blanche, 92298 Châtenay-Malabry Cedex, France ∥ CNRS-CEMHTI UPR 3079, 1D Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France ‡

S Supporting Information *

ABSTRACT: Understanding calcium silicate hydrates (CSHs) is of paramount importance for understanding the behavior of cement materials because they control most of the properties of these man-made materials. The atomic scale water content and structure have a major influence on their properties, as is analogous with clay minerals, and we should assess these. Here, we used a multiple analytical approach to quantify water distribution in CSH samples and to determine the relative proportions of water sorbed on external and internal (interlayer) surfaces. Water vapor isotherms were used to explain the water distribution in the CSH microstructure. As with many layered compounds, CSHs have external and internal (interlayer) surfaces displaying multilayer adsorption of water molecules on external surfaces owing to the hydrophilic surfaces. Interlayer water was also quantified from water vapor isotherm, X-ray diffraction (XRD), and thermal gravimetric analyses (TGA) data, displaying nonreversible swelling/shrinkage behavior in response to drying/rewetting cycles. From this quantification and balance of water distribution, we were able to explain most of the widely dispersed data already published according to the various relative humidity (RH) conditions and measurement techniques. Stoichiometric formulas were proposed for the different CSH samples analyzed (0.6 < Ca/Si < 1.6), considering the interlayer water contribution.

1. INTRODUCTION Nanocrystalline calcium silicate hydrates (CSHs) are the main binding phase of cement-based materials. They have received much attention from the scientific community in the last decade because of the ubiquitous use of cements in numerous applications, such as, for instance, radioactive waste repositories and geothermal and gas storage. They are responsible for the mechanical performance of cement matrices, providing cohesion to the material and contributing to its physical and chemical properties. As all of these properties are influenced or controlled by the CSH structure, a sound understanding of CSH properties may allow us to predict and possibly adapt the properties of cement materials. CSH consists of sheets of calcium atoms in 7-fold coordination,1 covered by chains of Si atoms in tetrahedral coordination. The degree of polymerization of the Si chain increases as the Ca/Si ratio decreases.2 The crystallites have two or three layers stacked along c*3,4 and a layer-to-layer distance varying from 14 to 9 Å,5 possibly because of (i) interstratification of different layer spacing,6 (ii) variable Sibridging tetrahedra,7 or (iii) variable hydration (possibly related to the interlayer water content with respect to the drying process).4 The importance of quantifying the water content in the CSH structure is further exemplified with the recent work, which demonstrated that water confined within and between © 2016 American Chemical Society

CSH crystallites plays a crucial role in the cohesion of cement materials.8 The role of confined water was demonstrated through Grand Canonical Monte Carlo and molecular dynamics simulations, where changes in the amount, location, and physical state of water under extreme conditions influence the sustainability of the cementitious materials.9 Finally, the impact of confined water on shear strength has also been simulated by analyzing water dissociation with a molecular dynamics approach.10 Because CSHs are minute, the proportion of external to internal (interlayer) surfaces is non-negligible, and consequently, the proportion of water sorbed on external surfaces cannot be neglected. This led to the development of different nomenclatures that aimed to describe water sorbed on CSH surfaces. For example, Taylor11 distinguished “evaporable” and “nonevaporable” water, whereas Allen et al.4 further distinguished adsorbed and physically bound water and outlined the importance of quantifying the water content in CSH to determine the composition and the density of the solid particles. More recently, a water classification based on water sorption in cement paste divided water into four categories.12 Received: March 5, 2016 Revised: May 11, 2016 Published: June 9, 2016 6794

DOI: 10.1021/acs.langmuir.6b00878 Langmuir 2016, 32, 6794−6805

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with three pressure sensors (133 kPa, 1.33 kPa, and 13.3 Pa). With the long acquisition time (more than one week), the free space in the sample cell was continuously corrected by monitoring the pressure changes in a neighboring empty cell induced by the changing water level in the water bath. In the case of the water vapor isotherm, a long acquisition time (7 days per isotherm) was required because of the slow equilibrium kinetics. Consequently, only CSH 0.6, 1.2, and 1.6 were analyzed. Nitrogen gas adsorption−desorption cycles at 77 K were determined using a Micromeritics ASAP 2050 volumetric adsorption analyzer. Samples were initially outgassed at 40 and/or 100 °C for 24 h under a residual pressure of 1.6 × 10−6 Pa and later outgassed after the isotherm acquisition at 150 °C under vacuum to obtain the dry sample weight to normalize the isotherm. The two adsorbates N2 and water (H2O) were used to describe the texture of solids, following classical Brunauer−Emmett−Teller (BET) theory and a generalized t-plot method.23 In the case of N2 sorption at 77 K, a widely used thickness equation24 was applied to calculate the t curve. For water adsorption, isotherms were compared to t standard curves obtained from data on a number of nonporous or macroporous adsorbents25 with energetic BET C constants similar to that of our samples. The experimental test isotherms were then redrawn as a t curve, that is, a plot of the volume of gas adsorbed as a function of t, in others words, the standard multilayer thickness on the reference nonporous material at the corresponding P/P0, in the absence of capillary condensation. The t method allows for the determination of the micropore volume and the total surface area. Powder XRD. XRD analysis was performed on randomly oriented powders. The measurements were on oven-dried, freeze-dried, and fully hydrated samples at different RHs. A data set was also obtained for CSH 1.2 to better constrain hydration phenomena. CSH 1.2 was equilibrated for two weeks at different RHs of interest for understanding water isotherms, by following the adsorption− desorption path. The sample was initially dried at 40 °C under vacuum for 72 h, with a residual pressure of 10−4 Pa. The fully hydrated state was also acquired. The samples stored at different RHs were analyzed using airtight specimen holder rings with domelike X-ray transparent caps to preserve the RH from the environment over the time of analysis. The XRD patterns recorded on a Bruker D8 advance diffractometer using CuKα radiation (λ = 1.5418 Å) were measured in continuous scan mode over the 3.5−75° 2θ range. Data were averaged every 0.02° 2θ (equivalent counting time 10 s). Data were simulated using software based on the matrix formalism developed by Drits and Tchoubar,26 adapted to the study of lamellar phases suffering from various types of structural defects27 and previously applied to the study of magnesium silicate hydrates (MSHs).28 NMR. 1H NMR experiments were performed at 17.6 T on a Bruker Avance 750 spectrometer equipped with a very-high speed 1.3 mm MAS probe. The 1H MAS spectra were acquired at a spinning rate of 60 kHz using a rotor-synchronized Hahn-echo sequence (τ fixed to 8 rotor periods). This sequence removes almost all of the probe signals, without modifying the spectra. Scans (128) were accumulated with a recycling delay of 20 s, and the chemical shifts were referenced relative to tetramethylsilane (TMS) solution. All of the spectra were deconvoluted with individual Gaussian−Lorentzian peaks using the Dmfit program29 taking into account all of the information available in the literature dealing with cement phases.30 Because the NMR spectra are quantitative, integrating the different components leads to the relative amount of each proton species. The rotors were packed directly in a H2O-free glovebox to avoid contamination by ambient humidity. TGA. Thermal analysis was performed on a TGA−differential thermal analysis (DTA) instrument (Setaram SETSYS Evolution) using 200−300 mg of each sample. The freeze-dried samples were heated from room temperature to 1000 °C either with a heating rate of 10 °C/min or with plateaus of several hours at 150, 700, and 1000 °C. The derivative thermogravimetric (dTG) spectra were deconvoluted. The results of the dTG deconvolutions are proposed to help the

Finally, another complexity results from the fact that CSH occurs as a compacted material in which various pore sizes are observed and influence water behavior and, in turn, mechanical properties. For instance, changing the relative humidity (RH) leads to disjoining effects with low relative humidity, as a result of a change in the abundance of interlayer water.13 Some pioneering work already has discussed how drying affects water removal14−17 and induces CSH swelling and shrinkage in cement paste.18 Most of these old studies described how adsorbed and interlayer water evolved until the drying process and/or the vapor pressure conditions. To summarize, the water structure in CSH is not well understood and is still debated. Because the structural configuration and/or adsorption of water molecules at CSH surfaces are involved in the physical properties of hardened cement materials, we need to understand their quantitative distribution, how water content governs mechanical properties, how drying shrinkage19,20 occurs with volume changes and,17 the Young’s modulus,18,21 and how chemical composition affects thermodynamic data.22 This study aims to contribute to better characterization of the different types of water in CSH. We propose a combined approach to quantify the water content and structure of CSH as a function of its Ca/Si ratio. Thermal gravimetric analyses (TGA) were complemented by high-resolution nuclear magnetic resonance (NMR) spectroscopy, so we acquired high-resolution 1H fast magic-angle spinning (MAS) spectra. We combined gas adsorption (N2 and water) with X-ray diffraction (XRD) acquisitions under variable RH conditions to follow the drying and rewetting cycle.

2. MATERIALS AND METHODS 2.1. Synthesis of Samples. All of the CSH samples in this study were prepared by mixing calcium hydroxide (Ca(OH)2, Prolabo) heated at 1000 °C for 24 h and amorphous silica (SiO2,Aerosil 200, Degussa). We used ultrapure water (Milli-Q 18 MΩ), boiled and outgassed under a N2 flux prior to being added in the glovebox. CSH samples, having theoretical Ca/Si ratios ranging from 0.6 to 1.6, were synthesized using a precipitation method at 22 °C in a N2saturated glovebox. In what follows, these samples are labeled CSH X, where X stands for the target Ca/Si ratio. The homogenized reactants were mixed with distilled and CO2-free water at a water/solid ratio of 50. Each synthesis tube was shaken in tightly closed polyethylene (PE) vessels for one month at 22 °C. After centrifugation, the samples were filtered (0.22 μm Millipore Millex-VV, PVDF). The gels were stored under different drying conditions. Samples were either (i) freeze-dried at −70 °C or (ii) oven-dried to reach a “dried state” or (iii) pressed between paper filters, representing a “fully hydrated” state. The fully hydrated state was achieved by picking the centrifuged gel from the equilibrium solution (without any dehydration step) and by pressing the solid between two paper filters. The samples were then stored in closed containers in the glovebox under a N2 atmosphere. 2.2. Sample Characterization. Electron Probe Microanalyzer (EPMA). Quantitative electron probe microanalyzer analyses were performed with a CAMECA SX FIVE electron microprobe using a 15 kV acceleration voltage, 30 nA probe current, and 1−2 μm spot size. Kα Ca and Si radiations were analyzed simultaneously with thallium acid phtalate (TAP) and pentaerythritol (PET) analyzing crystals. The counting time was 40 s. Analyses were performed on compacted pellets. For each prepared sample, 100 analyses using a ZAF matrix correction algorithm were performed and averaged. These data allow calculating the atomic Ca/Si ratio of the CSH samples. All of the results are presented in Table S1. Nitrogen and Water Vapor Adsorption Volumetry. Complete water vapor adsorption−desorption isotherms at 25 °C were obtained using a BELL Belsorp-Max volumetric adsorption analyzer equipped 6795


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Table 1. Energetic Constant and Surface Area from BET Theory and the t-plot Method of the Analyzed CSH Samples Measured from Nitrogen and Water Vapor Isothermsa nitrogen adsorption volumetry t plot

BET

samples CSH CSH CSH CSH CSH

0.6 0.8 1.0 1.2 1.6

Ca/Si ratio

grain density (g/cm3)

C

Vm (cm3/g STP)

0.69 0.82 1.06 1.23 1.42

2.13 2.22 2.26 2.42 2.37

290 160 160 260 125

93 81 62 44 53

SBET (m2/g)

St total surface area (m2/g)

St without μpores (m2/g)

μpores surface area (m2/g)

volume μpores (cm3/g STP)

maximum volume adsorbed (cm3/g STP)

94 58 45 62 20

23 14 11 13 5

1451 1163 806 545 634

408 420 326 356 357 299 271 273 228 194 199 137 231 229 209 water vapor adsorption volumetry

t plot

BET C

a

samples

Ca/Si ratio

CSH 0.6 CSH 1.2 CSH 1.6

0.69 1.23 1.42

50 50 30

3

Vm (cm /g STP)

108 53 45

2

SBET (m /g)

total surface area (m2/g)

adsorption σ = 14.8 Å2

adsorption σ = 14.8 Å2

desorption σ = 14.8 Å2

431 214 180

400 210 177

558 364 308

Grain density was measured with helium pycnometry.

10 nm within the layer plane,3 the SBET evolution could be explained by considering an increase of the particle size from 4 (CSH 0.6) to 8 nm (CSH 1.2), meaning an increase of the number of layers coherently stacked along c. The BET C values do not have any dependence on the Ca/Si ratio; this indicates that the nature of the adsorbate/adsorbent interaction is the same for all samples, which is quite usual for N2 (Table 1). The total surface areas, the external surface areas without micropores, and the micropore surface areas have also been calculated from the t-plot curves. The change in the total surface area is in agreement with the BET values with slightly higher values. At relative pressure higher than the B point34 when reaching a monolayer, a slope decrease is observed, indicating the presence of micropores (t plot not shown). The volume of micropores decreases from CSH 0.6 to CSH 1.6 (Table 1). Water Vapor Volumetry. The adsorption−desorption curves are displayed in Figure S1. On the adsorption branch, the relative water vapor pressure range 0−0.75 corresponds to the adsorption of water molecules on the external surfaces of CSH. Over this range, two to three layers of H2O molecules (i.e., V/Vm) form on the external surfaces and eventually fill up the mesopores (Figure 1). On the desorption branch, the relative water vapor pressure range 0.95−0 mainly corresponds to water desorption from mesopores and external surfaces. Unlike the N2 isotherms, the desorption branch does not coincide and remains parallel to the adsorption branches, even at low P/P0. A remaining and constant amount of water does not desorb. It is trapped at high P/P0 and highly bound to the CSH, possibly through hydrogen bonding, as previously shown.35,36 This constant amount of remaining water trapped in/on CSH crystals at a high RH of around 0.95 is attributed to the reincorporation of interlayer water that has been released during the initial drying of samples at 40 or 150 °C under vacuum (RH almost equal to 0%). Such strong hysteresis for the interlayer swelling of CSH totally differs from the behavior observed for the swelling of clay, with only slight hysteresis/

reader identify the main components of outgassing over the range of temperature. Special attention was paid to the effect of drying on the water content. Accordingly, acquisitions were performed on freeze-dried and fully hydrated samples, equilibrated at different RHs.

3. RESULTS 3.1. EPMA Analysis. All EPMA analyses are reported in Table S1. About 100 analyses were performed on each CSH sample. Results are given in elemental weight % (Ati wt %) and mol % (Ati mol %) for Si and Ca with the associated stoichiometric oxygen. The weight % was first converted into mol % to calculate the atomic Ca/Si ratio. The values agreed with the modal fraction of CaO and SiO2 salts used for each CSH sample synthesis. 3.2. Gas Adsorption. The adsorption/desorption isotherms of the different CSHs are shown in Figure S1. According to the IUPAC classification, the isotherms are displaying type-II behavior and an H3 hysteresis loop.31 The reversible type-II isotherm is the normal form of the isotherm obtained with a macroporous adsorbent. The type-II isotherm represents unrestricted multilayer adsorption build-up and the occurrence of mesopores. The type H3 loop, which does not exhibit any limiting adsorption at high P/P0, is coherent with the aggregates of platelike particles, giving rise to slit-shaped pores, and proves that the samples are mesoporous.31,32 Isotherms gave linear BET plots from 0.03 to 0.3 P/P0 for all tested samples (data not shown). N2 Volumetry. The calculated BET specific surface areas (SBET) and associated monolayer capacity (Vm) decrease with increasing Ca/Si from 408 (Ca/Si = 0.6) to 194 m2/g (Ca/Si = 1.2). At higher ratios, CSH 1.6 displays a higher value (231 m2/ g) than CSH 1.2 (Table 1). This could be because of the precipitation of portlandite nanoparticles33 and detected by TGA (Figure 4A-3), during the drying step, which tends to increase the surface area of the CSH 1.6. According to a platelike morphology and an isotropic size of the coherent scattering domains, obtained from the XRD pattern fitting, of 6796


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“micropores” seen by N2 would be high energy sites. Next, we considered the t plots calculated from the standard isotherms proposed by Hagymassy et al.,25 and the number of layers (V/ Vm) was deduced from the interpolated statistical thickness of the film adsorbed on the surface. From the latter, the quantity adsorbed in the multilayer was recalculated (Figure 1) for each isotherm, based on the SBET obtained on the adsorption branch. The calculated number of layers (V/Vm) was then plotted onto the measured isotherm, giving rise to the multilayer adsorbed water from P/P0 ranging from 0 to 1 and the contribution of the capillary condensate water in mesopores upon the adsorption branch (Figure 1). Note that up to a P/P0 of 0.7 upon adsorption the adsorbed water in the multilayer corresponds to the total water content measured by volumetry. 3.3. Powder XRD. The XRD patterns are typical for CSH hydrates,3,6,37 exhibiting in few broad and asymmetric maxima that can be indexed as 00l reflections and hk bands. The main differences between the various CSH 1.2 XRD patterns acquired at various RHs are the position and intensity of the 7.2−9.3 and 30° 2θ maximum. The maximum at 7.2−9.3° 2θ corresponds to the (001) reflection using the structure model from Grangeon et al.,3 so how it varies in position depends on the layer-to-layer distance. When dried, this position is at 9.6 Å. No change in this position was observed when RH is increased up to 40%. At 98% RH, the (001) reflection is at 10.5 Å, lower than the value obtained in the fully hydrated state (12.2 Å). Upon desorption, no change was seen from 98 to 40% RH. This change displays nonreversibility in the hydration process of interlayers with strong hysteresis of interlayer swelling in agreement with the proposed interpretation of water isotherms. Partial swelling after drying was observed previously,17 suggesting that the hydrates change during drying. Although qualitative observation of the variation in the position of the (001) reflection may be used as a proxy for the variation in the layer-to-layer distance, it must be used with special care with nanocrystalline structures, as is the case here. Indeed, many parameters induce significant shifts in the position of the (001) reflection, even though the layer-tolayer distance remains constant, including variation in the mean number of layers stacked coherently (i.e., in the crystallite size along c*), (micro-) strains, or interstratification. In addition, in nanocrystalline structures, the position of the (001) reflection, in Angstroms, cannot be taken as the value of the layer-to-layer distance because of the contributions from many parameters, including the Lorentz factor. Consequently, the layer-to-layer distance must be determined using quantitative modeling of XRD patterns. We did this and determined layer-to-layer distances of 12.3 Å (instead of 12.2 Å) for water-saturated conditions and 9.5 Å (instead of 9.6 Å) for dried conditions (Figure 2), indicating a difference of 2.7 Å, by considering a coherent scattering domain of 10 nm within the layer plane. Although such results highlight the variation in the interlayer water content as a function of relative humidity, from XRD, we could not determine whether water was sorbed at (or removed from) the CSH external surface. 3.4. 1H NMR. 1H MAS NMR results are presented in Figure 3A,B. The spectra in Figure 3A were acquired on a sample having a Ca/Si ratio of 1.2 (CSH 1.2) that was first freeze-dried and then stored for one month at 95% RH. Several acquisitions of 42 min each were performed over one day. In this experiment, we derived benefit from the low airtightness of the MAS 2.5 mm rotors. During the rotation, the rotor is licked by several bars of dry air, and the sample temperature reaches 60−

Figure 1. Water vapor adsorption/desorption isotherms of CSH 0.6, 1.2, and 1.6. The water adsorbed in the multilayer (field with black points) was recalculated from the t plots (red line) calculated from the standard isotherms proposed by Hagymassy et al.25 and the number of layers (V/Vm) deduced from the interpolated statistical thickness of the film adsorbed on the surface. Capillary condensated water upon adsorption is represented by the striped domain. The red crosses represent the quantification of water measured from TGA data at 150 °C (normalized on a dried mass) upon direct dehydration from a fully hydrated sample.

shifts of water uptakes corresponding to crystal swelling/ shrinkage between adsorption/desorption branches. The calculated BET specific surface areas (SBET) for the adsorption branch decrease with increasing Ca/Si from 431 (CSH 0.6) down to 180 m2/g (CSH 1.6) (Table 1), in agreement with the N2 external surface area SBET and increasing particle size. These values are based on a classical adsorbate cross-sectional area (σ) for which a water area of 14.8 Å2 was used. The BET C energy constant calculated at the adsorption range from 50 to 30 means strong water affinity for the CSH external surfaces, in agreement with values used for highly bound water.35,36 An increase of the surface area measured on the desorption branch was detected and ranges from 558 to 308 m2/g for CSH 0.6 and 1.6. This could confirm the hydration of the interlayer space. The t plots have been built up by selecting the standard isotherm25 on the basis of the adsorbate/adsorbent interaction quantified by the BET C constant value (Table 1). Unlike the N2 t plots, no downward deviations were observed (data not shown). Consequently, micropores were not detected; the 6797


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ongoing dehydration process and thus for the presence of free water (easily evacuated under a gas flux), which could be attributed to capillary condensation (for P/P0 > 0.75 on isotherms) and adsorbed water on external surfaces. As the dehydration process progresses, we can distinguish a broad shoulder ranging up to 10 ppm, meaning the existence of at least two proton species. The broad shoulder is particularly clear in the spectrum of the freeze-dried sample, where the main signal appears as a Gausso−Lorentzian centered at 4.4 ppm overlooking a very broad signal. These two components could be attributed to the protons of the water molecule engaged in different interactions with the framework, mainly in multilayer plus a small amount of interlayer water regardless of the water distribution identified by water isotherms and XRD, the narrower signal corresponding to the most mobile molecule weakly coupled. For the sample dried at 100 °C under vacuum, this contribution has almost completely disappeared. The remaining signal at 4.4 ppm corresponds to a small amount of water, in strong interaction with the network. With such drying conditions, XRD results show that interlayers are collapsed but could bear some trapped water molecules. This remaining water could also be adsorbed on external surfaces, especially on strongly energetic adsorption sites located at the border of crystals.41 Additional contributions located between ∼2 and ∼0 ppm can be seen and will be specifically studied by analyzing the spectra of samples having different Ca/Si ratios. Figure 3B shows the 1H MAS NMR spectra of CSH 0.6, 1, 1.2, and 1.6 samples oven-dried at 100 °C under vacuum. The samples were packed in a H2O-free glovebox just before their analysis. All of the spectra were normalized to the mass of CSH dried at 100 °C under vacuum. Despite our experimental

Figure 2. (A) XRD patterns of CSH 1.2 in the fully hydrated state (state B) and under dried conditions (40 °C under vacuum, state A) at different characteristic RHs identified on isotherms by following the adsorption desorption path of water isotherms. (B,C) Real positions were calculated for the fully hydrated and dried states by modeling (red curve) the XRD patterns.

70 °C because of the frictional force. This treatment likely progressively dehydrates the sample. The spectra of a freezedried sample (which was not allowed to rehydrate) and of a sample that was oven-dried at 100 °C under vacuum were also acquired for comparison (Figure 3A). The main differences between the spectra are located on the broad signal centered at around 5 ppm, which could be attributed to protons from water molecules.38−40 During the course of the repeated acquisitions and thus presumable outgassing on the sample that was initially freeze-dried and then left for equilibration at 95% RH, the intensity of the signal located at 5 ppm decreased, whereas the Gausso−Lorentzian lineshape was preserved. This argues for an

Figure 3. (A) 1H MAS NMR spectra of (i) freeze-dried CSH 1.2 (black line), (ii) equilibrated at 95% RH, measured over a day (dotted lines), and (iii) oven-dried at 100 °C under vacuum (red line). (B) 1H MAS NMR spectra from CSH 0.6, 1.0, 1.2, and 1.6 dried at 100 °C under vacuum and the main chemical shifts are labeled H1−H5. The thumbnail represents the results of the fitting deconvolution (normalized on the dried mass of the sample). (C) Sketch of the proposed CSH structure model of the proton attribution according to the increasing Ca/Si ratio. 6798


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Langmuir conditions, a probe−rotor contribution was always present on the spectra. We identified it by acquiring the NMR spectrum of the empty rotor. Moreover, all spectra exhibit a broad contribution centered at around 4.4 ppm, which, as discussed above, can be assigned to water molecules in strong interaction with the network. The additional contributions, located at lower ppm values, were isolated using spectral deconvolution. To help the description, an example of these two contributions is given in Figure 3B, being understood that their intensity will differ according to the samples. We identified five narrow components (H1−H5) corresponding to hydroxyl groups in addition to the broad contribution due to the instrumentation setup (rotor). Note that the total intensity of the spectra decreases with increasing Ca/Si ratio because the sample density increases (Table 1, i.e., the number of moles of CSH analyzed decreases). How the integrated area of each component changes as a function of the sample Ca/Si ratio is shown in the thumbnail of Figure 3B. The H2 contribution generally decreases with the increase of the Ca/Si ratio, whereas H3 and H5 increase, and H1 and H4 remain about constant. Peak assignment is made difficult by the complex CSH structure and the multiplicity of possible proton configurations. Nevertheless, on the basis of the available literature,39,40,42 it is assumed that H5, H4, and H3 (δiso around 0.5, 0.8, and 1.4 ppm) can be assigned to H atoms connected to Ca−O, whereas H2 and H1 (δiso around 2.1 and 3.4 ppm) are probably related to the Si−OH. Finally, given that the degree of Si chain depolymerization increases with increasing Ca/Si ratio in the sample,43 H2 silanol is assumed to be attributable to the proton linked to the Si-bridging tetrahedra. This species is only present in CSH with a Ca/Si ratio lower than 1.2, whereas some bridging tetrahedra remain at higher ratios43 (see Figure S2). This change means that protons linked to Si-bridging tetrahedra (Si−O−−H+) are probably replaced by Ca2+ ( Si−O−−Ca2+) to fulfill the charge compensation requirements, explaining the increase of H5 species (assigned to CaOH) with increasing Ca/Si ratio. Finally, H3 is assumed to be a proton linked to a Ca layer to compensate for the progressive removal of Si-bridging tetrahedra (Figure 3), and all other components, whose abundance remains constant (H1 and H4) over the range of the Ca/Si ratio investigated here, could be assigned to protons bound to the edge of the particles (Figure 3). 3.5. Thermogravimetric Analysis. Figure 4 displays typical TG and dTG curves of freeze-dried CSH samples with Ca/Si ratios of 0.6, 1.2, and 1.6. Different weight losses can be identified from the dTG curves. Usually, the water outgassing over increasing temperature is linked to the energetics interactions of water with solid surfaces. In this way, the main weight loss, centered at 100 °C, is attributed to the volatilization of water weakly bound to the surface, whereas the shoulder at a lower temperature (50 °C) corresponds to more external sheets of water in the multilayer. The weight loss centered at around 200 °C with an onset at 150 °C is attributed to highly bound water. The loss ranging from 150 to 700 °C would correspond to dehydroxylation whose weight loss part should be because of Portlandite (CaOH2) at 450 °C.44 The weight loss at above 700 °C is typical of carbonates.45 Note that temperature boundaries are approximate and depend on the heating rate and on how the water interacts with solid surfaces. Data were also obtained on a freeze-dried CSH 1.2 equilibrated at different RHs for one month (Figure 4B). TG curves were acquired between 25 and 300 °C, in the range of water molecules outgassing. For RH < 80%, TG curves

Figure 4. (A) TG and dTG curves of freeze-dried CSH 0.6, 1.2, and 1.6 samples. Deconvolutions are given to illustrate the main outgassing range of temperature and the detection of accessory phases such as portlandite and calcium carbonates. (B) TG and dTG curves of freezedried CSH 1.2 equilibrated at different RHs are also illustrated over a temperature ranging from 25 to 300 °C. (C) Symbols are the water content measured at 150 °C upon desorption on fully hydrated CSH, equilibrated at 11 and 33% RH. The dotted and black lines represent the adsorbed water on external surfaces at 11 and 33% RH, upon the adsorption branch, obtained from water vapor isotherms.

displayed the same behavior as for the previous freeze-dried CSH samples (Figure 4A), with weight loss ranging from 20 to 25% wt. Over RH > 80%, weight loss at 150 °C reached 50% wt and the dTG curve shows a large domain of outgassing water with an onset at 50 °C, which overlaps that of the highly bound water. This feature is probably linked to a smearing effect on the release of such a large amount of capillary water at a high heating rate.46 The water content being highly dependent on the drying process and the RH conditions, a data set was also acquired on fully hydrated CSH samples equilibrated at 11 and 33% RH upon desorption. For these samples, no heating rate was used, only plateaus at temperatures corresponding to the steps previously highlighted (dehydration and dehydroxylation) from the previous acquisitions and XRD data were used. Samples were heated at 150, 700, and 1000 °C. For each temperature, the weight loss was measured until it did not change anymore with time. The weight loss at 150 °C, normalized to the dried mass of CSH, is reported for the samples equilibrated at 11 and 33% RH in Figure 4C. The weight loss is systematically higher for samples equilibrated at 33% and for samples with low Ca/Si (∼22% wt for Ca/Si = 0.6) compared to samples with a high Ca/Si ratio (∼17% wt for Ca/Si = 1.6). The difference in weight loss between 11 and 33% RH tends to decrease from low (∼7.7% wt for Ca/Si = 0.6) to high (5.6% wt for Ca/Si = 6799


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Table 2. Summary of the TG Results Measured for CSH 0.6, 0.8, 1.0, 1.2, and 1.6 Samples, Freeze-Dried and Pressed between Paper Filters (Fully Hydrated) Equilibrated at 11 and 33% RH freeze-dried 11% RH


0.6 0.8 1.0 1.2 1.6

fully hydrated 11% RH

33% RH

Ca/Si ratio

mass loss (150 °C) (g/100 g dried)




total mass loss (1000 °C) (g/100 g dried)

mass loss accessory phases (1000 °C) (g/100 g dried)

0.69 0.82 1.06 1.23 1.42

11.2 12.2 12.5 10.3 10.9

14.8 13.7 12.2 13.1 11.4

22.5 21.7 19.1 18 17

30.4 29.6 27.3 26.3 26.6

32 30.8 28.6 27.6 28.1

1.6 1.2 1.4 1.3 1.5

discussed how the position of the (001) reflection changes as a function of the Ca/Si ratio.5,7,15,17 In this study, sample preparation and RH conditions were shown to have a significant impact on the layer-to-layer distance. For example, this varies nonlinearly with RH from 9.5 to 12.3 Å in dry and fully hydrated states, respectively (Figure 5A), for CSH having a Ca/Si ratio higher than 1. Such a change has an impact on the water content measured by TGA, for which most of the published data were given after different vacuum drying processes (F drying, D drying, or P drying47), so underestimating part of the interlayer water, as shown with the TGA results (Table 2) in the case of freeze-dried samples. The impact of the drying process was here further assessed by studying the how the CSH 1.2 crystal structure changed upon drying at 100 °C under vacuum and upon freeze-drying, the so-called F drying,47 using 29Si solid-state NMR spectroscopy (Figure 5B). The spectra of CSH 1.2 display the typical shift position lines centered at −79 and −85 ppm corresponding, respectively, to Q1 and Q2 silicon environments. Interestingly, both lines are broader in the case of the dry sample, leading to a broader chemical shift distribution due to a broader silicon environment distribution. This broadening could then be attributed to structural strains, for example, sheet bending28 and/or collapsing affecting the density of the local order of Si atom-neighboring bonds,21 whereas the long-range order remains unaffected as the ratio of Q1/Q2 peak intensities was not modified (Figure 5B). This is supported by the XRD as decreasing basal distance was observed in both cases (Figure 2). The drying process also complicates the identification and quantification of hydroxyl groups, which we achieved using single pulse 1H MAS NMR, following previous studies which used this method to characterize the different hydrogen species in hydrates and cement materials.38,39,47 1H NMR is very sensitive but presents two main drawbacks: a narrow range of chemical shifts and a very strong dipolar homonuclear coupling. In the presence of water, the spectrum will be dominated by the H2O signal, and all of the hydroxyl species can supposedly interact with H2O molecules. The result is often broadening and a line shift, making analyses difficult. The use of very highfield spectrometer associated with very high-MAS speed (i.e., 60 kHz) has dramatically improved the spectral resolution, but only the complete removal of adsorbed water molecules (Figure 5C) allows us to discriminate the different O−H signals in the sample (Figure 3B). Another effect of removing interlayer water molecules upon drying is the interlayer space collapsing. This depends on the drying conditions (Figure 5A) and influences subsequent water adsorption capacities (Figure 5D). In particular, CSH samples

1.6) Ca/Si ratios. It is consistent with multilayer water whose adsorbed amount increases with the external surface area. Table 2 summarizes the results of the different data sets. The data obtained from the “fully hydrated samples”, both equilibrated at 11 and 33% RH, were added on the water adsorption isotherms (Figure 1). The data both at 11 and 33% RH, obtained upon desorption from fully hydrated samples, are above the desorption branch of the water adsorption isotherms. These data are used in the calculation of the interlayer water content hereafter.

4. DISCUSSION 4.1. Effect of Drying on the CSH Structure. This work emphasized the impact of the drying process on the structure of the CSH. Figure 5 summarizes the impacts of the drying processes on the water content and the associated structure modifications. TGA, XRD, NMR spectroscopy, and water adsorption were used to probe these changes. We examined crystallographic, chemical, and physical aspects. The layer-to-layer distance is a direct indicator of the impact of the drying process. Several authors have for a long time

Figure 5. (A) XRD patterns of the CSH 1.2 sample under different drying conditions (freeze-dried, dried at 40 °C under vacuum and 150 °C, and fully hydrated). (B) 29Si MAS NMR spectra of the synthetic CSH 1.2 sample freeze-dried and dried at 100 °C under vacuum. (C) 1 H MAS NMR spectra of the CSH 1.2 sample freeze-dried and dried at 100 °C under vacuum. (D) Water vapor isotherms of the CSH 1.2 sample dried at 40 and 100 °C under vacuum. 6800


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Figure 6. (A) Water vapor adsorption/desorption isotherm of CSH 1.2 with the associated distribution of water. The blue line is the calculated quantity of water adsorbed in the multilayer recalculated from the standard isotherms proposed by Hagymassy et al.25 upon adsorption. The red line represents the capillary and adsorbed water in the multilayer upon desorption, obtained by subtracting a constant remaining interlayer water from the desorption branch. (B) Curves represent the quantity of water adsorbed obtained by the subtracting the adsorption branch from the desorption one. (C) Schematic structure of CSH with a sketch proposed to explain the distribution of water in CSH according to the evolution of RH.

have a lower capacity to uptake water when they are first dried at 100 °C than when they are dried at 40 °C. This behavior is in line with previous studies,16,17,48 which showed that the dehydration/rewetting cycles are accompanied by a nonreversible change in the layer-to-layer distance. The volume of water present in the samples also depends on the RH conditions (Table 2), with the amount of outgassed water increasing with RH. This is due to the amount of water molecules adsorbed on the external surfaces of the CSH increasing with RH. Finally, from TGA data at 150 °C (Table 2) and water vapor isotherms data (Table 1), it can be calculated that 2−3% of interlayer water was removed from samples equilibrated at 11% RH as compared with those equilibrated at 30% RH, in agreement with discussion in earlier studies.14 This observation was recently confirmed49 based on 1 H NMR50 and molecular simulations.8 For instance, it has been demonstrated that for fully hydratedsamples equilibrated at a RH of 11% a part of the water was already removed from the interlayer space.8,14,49 The same behavior was observed for freeze-dried samples, where both the layer-to-layer distance (Figure 5A) and the water content measured by TGA (Table 2) indicated that part of the interlayer water was removed. 4.2. Water Distribution. Microstructural models have been proposed in the past to describe the CSH microstructure. Powers and Brownyard51 proposed that the CSH gels could be

composed of particles layered with evaporable and nonevaporable water. Feldman and Sereda52 deduced from the discrepancy in measured N2 and water specific surface areas that CSH can be described as layered compounds. In this model, interlayer water explains the volume contraction occurring upon water removal. The presence of interlayer water was then confirmed in C−(A)−S−H.11,15−17,53 More recently, it has been shown that the position of the (001) reflection in CSH XRD patterns shifts toward low d values with the increase of the Ca/Si ratio.6,54 Strong data scattering is seen in these studies, probably as a result, for a given Ca/Si ratio, of varied water content,5,21 considering the drying process. Note that the apparent d spacing cannot be straightforwardly related to a layer-to-layer distance as, for example, samples may have significantly different mean numbers of layers coherently stacked along c*.3,6 Like any layered compound, CSH displays hydrophilic external and internal (interlayer) surfaces55 because of their structural disorder and the substantial amount of nonbridging oxygen atoms that provide acceptor sites for hydrogen bonds. The surface of CSH is also covered by hydroxyl groups from silanols (SiOH) and CaOH (Figure 3), which are also able to establish hydrogen bonds.36,56 Consequently, water multilayers can form at CSH surfaces, as observed on the type-II water vapor isotherms (Figures 1 and 6A). Such behavior depends on 6801


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Table 3. Weight Distribution of Water in CSH 0.6, 0.8, 1.0, 1.2, and 1.6 Samples and Deduced Stoichiometric Formula of the Different CSHs Based on a Defective Tobermorite-Like Model


0.6 0.8 1.0 1.2 1.6

samples CSH CSH CSH CSH CSH a

0.6 0.8 1.0 1.2 1.6

Ca/Si ratio

single adsorbed water layer (g/g dried)

multilayer adsorbed water at 98% RH (g/g dried)

0.69 0.82 1.06 1.23 1.42

0.081 0.071a 0.051a 0.038 0.039

0.40 0.35a 0.26a 0.21 0.19

hydroxylic water (OH) (g/g dried) 0.079 0.079 0.082 0.083 0.096

Ca/Si ratio

Ca

Caouter

Si

O

0.69 0.82 1.06 1.23 1.42

0.69 0.72 0.83 0.92 0.96

0.00 0.10 0.23 0.31 0.46

1.00 1.00 1.00 1.00 1.00

2.53 2.58 2.84 3.01 2.97

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

OH 0.55 0.57 0.67 0.73 0.91

± ± ± ± ±

0.07 0.08 0.09 0.10 0.10

interlayer water (g/g dried) 0.105 0.117 0.112 0.122 0.119

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

H2Ointerlayer 0.69 0.83 0.89 1.06 1.12

± ± ± ± ±

0.07 0.08 0.09 0.10 0.10

Values are interpolated from the measured isotherms of CSH 0.6, 1.2, and 1.6.

close to 12.2 Å (Figure 2). This behavior could be related to the layered structure of the CSH with outgassing of part of the interlayer water and/or interstratification of hydrated layers. 4.3. Water Content. Even though water distribution has been discussed for a long time,14,51,52 the respective amounts of hydroxyls, interlayer, adsorbed, and capillary water are still debated. Richardson5 has recently proposed a compilation of data displaying the mole of water per Si (H2O/Si) against the Ca/Si ratio of CSH. This compilation was discussed by57 arguing with a reason that those data, although presented as H2O/Si, count for water molecules and hydroxyls. All of these data, presented in Richardson,5 were widely dispersed, even for the same Ca/Si ratio, and were discussed according to the drying process. Given our results, this varied dispersal could be explained by considering that the extracted data were either acquired from different CSH conditioning (fully hydrated, freeze-dried, and dried under vacuum or N2 flux) or from different techniques and/or different analytical parameters (1H NMR, TGA at 110 or 1000 °C, SAXS/SANS, and molecular simulation). Moreover, most of the time, the RH conditions that could impact the water content according to the adsorbed water on external surfaces were not specified. We propose water quantification here, including water molecules and hydroxyls. In particular, we distinguish between adsorbed and interlayer water, and we quantify the hydroxyls. Interlayer water was quantified according to XRD, water vapor isotherms, and TGA data from CSH samples pressed and equilibrated at 30% RH, avoiding removing a part of the interlayer water, upon desorption. The total water is expressed relative to the mass of water measured at 150 °C (Table 2) and is normalized per mass of dry CSH. The masses of adsorbed water on external surfaces at 30% RH measured with water vapor isotherms were previously deduced by the t-plot approach (Table 1). This water on external surfaces is subtracted from the total to deduce the interlayer contribution. The hydroxyl content was obtained in the same way by combining XRD, TGA, and 1H NMR data. The total weight loss between 150 and 700 °C was normalized according to the mass of dry CSH. This quantity of hydroxyls (Table 3) encompassed all hydroxyls that are detected in the TGA analysis, in the range of 150 °C, where collapsing of the interlayer space is observed (Figure 5), to 700 °C, where CSH

the specific surface area, which here ranges from 400 to 200 m2/g according to the Ca/Si ratio (Table 1). The amount of adsorbed water layers increases until condensation occurs in mesopores. The capillary condensation is clearly dependent on the pore size of the CSH gels as described in cement materials.12,49 According to the previously described models and the data obtained in the present study, a sketch is proposed in Figure 6 to explain the distribution of water in CSH. From the initial dried sample (step six in the Figure 6A,C), the range of relative water vapor pressure 0−0.75 (step two) corresponds to water vapor adsorption on external surfaces. The amount of adsorbed water depends on the specific surface area and thus the Ca/Si ratio of the CSH samples. The range of relative water vapor pressure 0.75−0.95 (step 3), corresponds to the adsorption of multilayer hydrates on the external surfaces with the mesopores filling up and the hydration of the interlayer space as probed by the XRD (Figure 2) upon adsorption. The capillary condensated water upon desorption and the water in the interlayer space were distinguished by subtracting the adsorption branch from the desorption (Figure 6B). The calculated curves display relatively linear evolution from P/P0 ranging from 0.1 to 0.3, assuming remaining interlayer water to be constant. This value of remaining interlayer water was subtracted from the desorption branch, displaying the water adsorbed on the external surfaces and the capillary contribution upon desorption (red curve in Figure 6A). The curves in Figure 6B also show that the hydration of the interlayer could take place at P/P0 close to 0.95 where for all curves the capillary condensated water starts decreasing. XRD data raises the possibility that the hydration of the interlayer space of initially dried CSH samples is partial, as previously demonstrated.15−17 However, this contribution is highly bound to CSH surfaces. On the desorption branch, the range of relative water vapor pressures 0.75−0.05 (step 4) corresponds to water desorption in the mesopores and on external surfaces, whereas the removal of water highly bound in the interlayer space was not probed (step 5). XRD results indicate that the hydration of the interlayer space is partial, with an apparent position of the (001) at 10.5 Å even at 95% RH in the rewetting cycle (Figure 2), whereas under fully hydratedconditions, the apparent position of the (001) is 6802


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and interparticle pore spaces, comparable to the distinction made for clay.61 Nevertheless, the distribution and quantification of water constituting the CSH microstructure are still debated. The application of a multiple complementary analytical approach in this study has allowed us to reconsider earlier models for the water distribution in CSH and to propose quantification. We have used high-resolution NMR to probe the hydroxyl component of the CSH structure. Silanol groups with protons linked to the bridging tetrahedron are identified at a Ca/Si ratio below 1.2. At a higher Ca/Si ratio, these groups are no longer present. The disappearance of the bridging tetrahedron is accompanied by an increase of H atoms connected to Ca−O with CaOH, compensating the charge deficit as outer sphere calcium located on external or internal surfaces. The structural disorder and the substantial number of nonbridging oxygen atoms make CSH-present hydrophilic surfaces sensitive to relative humidity. So as that for clay minerals, the texture of CSH is influenced by the drying method. The amount of probed water is found to depend on the initial drying conditions for which the quantity of water is different in the case of dehydration or rewetting cycles. The difference related to the hydration of the interlayer space is only partial in case of rewetting and seems to depend on energy barriers. We used water vapor isotherms and XRD and TGA measurements to quantify the water molecule distributions, that is, the respective amount of water adsorbed on external surfaces, interlayer space, and the hydroxyl groups. We were able to give the stoichiometric formula of the different CSHs synthesized in this study from this quantification, considering the hydration contributions. An accurate quantification of water distribution in CSH allows us to reconsider some mechanisms in cement materials, such as mechanical, transfer, and thermodynamic properties. However, some issues remain unresolved. Low pressure adsorption isotherms62 could be used to explain the micropores and or high energy sites seen by N2. Similarly, vibrational spectroscopy, such as infrared or Raman spectroscopy, under controlled water pressure and neutron diffraction could be used to distinguish hydration ranges, as already demonstrated for clay minerals.63

melts (as shown by XRD analysis, Figure S3). All of the adsorbed water layers are in turn obtained with the water vapor isotherms. The contributions of adsorbed water for the CSH 0.8 and 1.0 were deduced by interpolating the measured values (Table 3), with the assumption that their specific areas decrease linearly from 0.6 to 1.2, as shown by isotherms. Table 3 summarizes the weight distribution of water of CSH 0.6 to 1.6. All of the water contributions were plotted on a diagram displaying the H2O/Si according to the Ca/Si ratio adapted from Richardson’s study.5 Only data with available information on sample conditioning and analytical parameters were kept. Some data, recently published from molecular simulations,58 SAXS/SANS experiments,4 1H NMR, and TGA57 or crystallographic considerations,59 were added (Figure 7). According to this representation, the different contributions

Figure 7. Plots of H2O/Si against Ca/Si with the gray diamonds are adapted from ref 5. The blue circles are for interlayer water, the light blue ones are for interlayer water and a single adsorbed monolayer, and the purple ones are for the contribution of hydroxyls added to the previous contributions, calculated from this work.

of quantified water (hydroxyls and interlayer and adsorbed water molecules) are superimposed on these previously published data. From this, we explain all of the scattering data, for which the interlayer water explained the lower trend is in quite good agreement with crystallographic data, the SAXS/ SANS measurements and the molecular simulations. The upper part of the data is explained by considering the total water content, considering the hydroxyls and varying degrees of adsorbed water (Figure 7). From this quantification of water, we have proposed the stoichiometric formulas of the different CSHs (Table 3) based on a tobermorite structure59,60 and only considering the interlayer contribution. On the basis of these assumptions and the 1H NMR results, even though it was widely accepted that CSH corresponds to a defective tobermorite-like structure, outer sphere calcium (Caouter) was differentiated from the sheet of seven coordinated calcium polyhedra without asserting if they are located on external or internal surfaces.5,44 In the stoichiometric formulas, we display the total amount of hydroxyls and to consider the interlayer water content.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00878. Nitrogen and water vapor adsorption/desorption isotherms of CSH 0.6, 1.2, and 1.6 samples, XRD patterns of CSH 0.8, CSH schematic layer structure,29Si solidstate NMR spectra of CSH samples, and experimental data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is a part of a Ph.D. study initiated, followed, and granted by Andra during the GL-Thermochimie project in the

5. CONCLUSION It is now well accepted that CSHs are particles composed of layers. Accordingly, distinction can be made between interlayer 6803


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framework of the Andra/BRGM scientific partnership. Frederic Villieras and Manuel Pelletier are thanked for the fruitful discussion on gas adsorption. Financial support from the TGIRRMN-THC Fr3050 CNRS for conducting the research is gratefully acknowledged. S. Grangeon acknowledges funding from the French National Research agency (ANR - Grant ANR-14-CE01-0006; NACRE Project).

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Distribution of Water in Synthetic Calcium Silicate Hydrates - Langmuir

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