# Impact of alkalis and shrinkage-reducing admixtures on hydration and pore structure of hardened cement pastes

Hafsa Rahoui, Ippei Maruyama, Matthieu Vandamme, Jean-Michel Pereira, Martin Mosquet

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## To cite this version:

Hafsa Rahoui, Ippei Maruyama, Matthieu Vandamme, Jean-Michel Pereira, Martin Mosquet. Impact of alkalis and shrinkage-reducing admixtures on hydration and pore structure of hardened cement pastes. Cement and Concrete Research, 2024, 184, pp.107620. 10.1016/j.cemconres.2024.107620. hal-04662681

HAL Id: hal-04662681

https://hal.science/hal-04662681v1

Submitted on 26 Jul 2024

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# Impact of alkalis and shrinkage-reducing admixtures on hydration and pore structure of hardened cement pastes

Hafsa Rahoui<sup>a</sup>, Ippei Maruyama<sup>b</sup>, Matthieu Vandamme<sup>c</sup>, Jean-Michel Pereira<sup>c</sup>, Martin Mosquet<sup>a</sup>

## Abstract

The hydration and pore structure of hardened cement pastes containing various amounts of alkali and hexylene glycol -a shrinkage-reducing admixture (SRA)- are studied. Until three months, SRA retards cement paste hydration regardless of alkali content; after seven days at most, this retardation diminishes with time. Alkalis increase the hydration degree at early ages for all pastes. The pore structure coarsens with the SRA: both the specific surface area and the volume of pores with a 5 nm entry size decrease in the presence of the SRA. The magnitude by which the SRA impacts the pore structure reduces with alkali.

In alkali media, the C-S-H gel uptakes alkalis and aluminum; modifying the C-S-H structure, the gel pore volume, and the interlayer space. The SRA depletes the alkalis from the solution and may increase the C-S-H alkali uptake, which could lead to changes in the gel pore volume and specific surface area.

**Keywords:** Alkalis, shrinkage-reducing admixture, pore structure, hydration

2010 MSC: 00-01,99-00

## 1. Introduction

Throughout its service life, concrete is subject to several types of shrinkage, including drying shrinkage. Drying shrinkage, which is mainly driven by the hardened cement paste [1,2], can be detrimental to the performance of structures [3–5]. Also, microcracking induced by drying shrinkage in restrained conditions [6,7] (by the presence of aggregate or reinforcement) impacts the durability of cement-based materials [8].

Drying shrinkage of hardened cement pastes (HCP) and alkali-activated materials depends on multiple factors, among which is alkali content. For cement-based materials [9–13], a higher alkali content is usually associated with a higher shrinkage and a higher risk of cracking. Alkali-activated materials are prone to higher

<footnote><sup>a</sup> Holcim Innovation Center - Saint-Quentin Fallavier, 38291, France</footnote>

<footnote><sup>b</sup> Graduate School of Engineering, The University of Tokyo, Tokyo, Japan</footnote>

<footnote><sup>c</sup> Navier, Ecole des Ponts, Univ Gustave Eiffel, CNRS, Marne-la-Vallée, France</footnote>
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autogenous shrinkage and drying shrinkage [14,15] than ordinary Portland cement systems.

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Controlling the drying shrinkage and the associated risk of cracking is key for the durability of cement-based materials and novel materials. Shrinkage-reducing admixtures (SRAs) are among the shrinkage mitigation strategies studied for cement-based materials and prove beneficial in reducing both shrinkage and cracking [16–18]. Ran et al. [19] evaluated four different types of SRAs (diethylene glycol, dipropylene glycol, diethylene glycol-monobutyl ether, and diethylene glycol-tert-butyl ether) at 2% dosage. These SRAs reduced the drying shrinkage by up to 44.4%, except for diethylene glycol, which did not show any reduction in shrinkage. Zhang et al. [20] evaluated the ability of an SRA (based on a hydrocarbon compound and a glycol ether) to reduce the drying shrinkage of mortars with ordinary Portland cement, moderate-heat Portland cement, and low-heat Portland cement. The performance of SRA depends on both the cement used and the dosage of SRA. The ability of the SRA to reduce the drying shrinkage is correlated to the impact of SRA on the pore structure. Reduction in total drying shrinkage of HCP is observed in the presence of SRAs. Such reduction varies from 30% to 50% [16–18,21,22] depending on the mix design parameters (e.g., water-to-cement ratio (w/c), type of cement, SRAs types, and SRAs dosage).

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How the performance of SRAs may depend on the cement parameters is not explored sufficiently, even though the performance of these admixtures in alkali media may be compromised. For instance, adding up to 1% of an SRA (namely hexylene glycol) to alkali-activated slag systems cured for 28 days did not significantly reduce their total drying shrinkage [23]. In contrast, adding 1% (per mass of slag) of an SRA based on polypropylene glycol to another alkali-activated slag system cured for 48 hours reduced its total drying shrinkage by roughly 50% [14]: the authors argue that the fact that the SRA reduces the drying shrinkage of samples cured for short periods of time is because the SRA retards hydration [23,24].

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An analysis of the pore structure is required to understand the potential impact of alkalis on an SRA's ability to reduce shrinkage. Shrinkage of cement-based materials depends on the developed pore structure, namely the distribution of pores and specific surface area [25–28]. Hansen [25] identified two key mechanisms for drying shrinkage: capillary stresses and the Gibbs Bangham adsorption effect; the pore structure and the surface area impact the capillary stresses and the adsorption stresses, respectively. Juenger and Jennings [26] found that irreversible drying correlates to surface areas and pore volumes. Jennings et al. [27] found that at relative humidity higher than 25%, the capillary pores control the shrinkage response, while at relative humidity <25%, interlayer spaces are key to explaining the shrinkage of cement-based materials. Maruyama et al. [28] found that both reversible and irreversible drying shrinkage can be predicted based on a linear relationship to the water vapor surface area.

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Both alkalis and SRA impact the cement hydration, the pore structure of hardened cement-based materials, and the shrinkage (i.e., both autogenous shrinkage and drying shrinkage). Alkalis increase the hydration rate at early ages and may lead to retardation of hydration in the long term [29,30]. The acceleration of hydration at early ages is linked to an acceleration of dissolution and precipitation of phases in the presence of alkali [31]. Alkalis influence the pore structure of cement-based materials

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and the morphology of hydrates. The volume of small pores (i.e., with a radius smaller than 4 nm) and the specific surface area decrease in the presence of alkalis [29,32]. However, an increase in total pore volume is reported in the case of a material with w/c equal to 0.35 [33]. The capillary pores do not percolate anymore in the presence of alkalis in materials at low w/c ratios [34]. Finally, the morphology of the formed C-S-H gel is impacted by the presence of alkalis: C-S-H is reported to be coarser and present a divergent needles morphology [29,35]. These observed trends can depend on the type of alkali salt used and its dosage.

Regarding the impact of alkalis on hydration and pore structure, we identify the following knowledge gap: although the effect of alkalis on hydration and pore structure is well investigated, the link between these parameters and shrinkage is not clearly established. In addition, the influence of alkalis on both hydration and pore structure may depend on the type of alkali (potassium, sodium) [33]. The total alkali content (including soluble alkali) of regular clinkers, expressed in equivalent Na₂O, usually ranges from 0.2 to 1.5% [36]. However, the ranges of alkali content investigated in the scientific literature are often higher than the ranges of alkali content occurring ordinarily in cement [12,37].

SRAs often retard hydration [15,32–34]. Eberhardt [38] observed that such retardation of the degree of hydration is more prominent for the silicate phases and SRA adsorbs on the portlandite, inducing both retardation of hydration and changes in the morphology of the formed portlandite. Rajabipour et al. [41] explained such retardation effect by the interaction of the SRA with the pore solution composition. Furthermore, SRAs impact the pore size distribution of cement-based materials as well as their specific surface area [16,42,38,18]. However, Ai and Young [43] report little to no impact of an SRA on microstructure. Zhang et al. [20] observed a slight increase in total pore volume for low-heat Portland cement with SRA, while a decrease in the total pore volume was observed for ordinary Portland cement. Ling et al. [44] observed a coarsening of pore structure with the addition of SRA for both Portland cement paste and fly ash-based geopolymer. The absence of a consensus on the impact of SRAs on microstructure may be linked to several causes: the materials used (e.g., various types of cement or SRA), the curing conditions and age of samples as well as the preparation of the samples prior to pore structure characterization. Finally, the morphology of portlandite is modified in the presence of an SRA, where the size of portlandite crystals is smaller with SRA than without SRA [45].

For what concerns the impact of alkalis on an SRA's performance to reduce shrinkage, we identify the following knowledge gap: although the ability of an SRA to reduce shrinkage has been extensively investigated [18,38,43,46,47], how the performance of SRAs depends on the cement type, particularly their alkali content, is not established. Further, the interaction of SRA and alkali in pore solution and subsequent implications on hydration and pore structure have not been sufficiently explored. The influence of SRAs, alkalis, and the combination of SRAs and alkalis on the microstructure of hardened cement paste is critical to understanding the implications of each of these parameters on drying shrinkage and the durability of cement-based materials.

Hence, in this study, we investigate how an SRA (i.e., hexylene glycol) and alkalis, taken separately and combined, impact the pore structure of hardened cement pastes by means of gas adsorption measurement and mercury intrusion porosimetry. The
hydration of cement pastes is also analyzed using X-ray diffraction and thermogravimetric analysis. Target alkali content ranges from 0.2% to 1.2% of sodium equivalent per mass of unhydrated cement to span a variety of alkali contents encountered in the field. Investigation of the pore structure of hardened cement pastes at various ages provides insight into the positioning of SRA molecules in the pore structure of the cement paste of various alkali contents.

## 2. Materials and methods

We consider cement pastes made with cement with three different alkali contents (i.e., low, medium, and high alkali content). Various amounts of SRA are added to the cement pastes (i.e., 0%, 4%, and 8% of the mass of cement). We characterize the hydration and pore structure of the nine types of samples at various ages ranging from 7 days to 1 year.

### 2.1 Materials

We use an Ordinary Portland Cement (OPC) from Le Teil, France. Table 1 provides its chemical composition. Its total alkali content expressed in equivalent Na₂O is 0.23%, and the soluble alkali content in equivalent Na₂O is 0.16%. We call it low alkali cement.

Some cement is enriched in alkali with potassium sulfate K₂SO₄ to obtain a medium alkali cement and a high alkali cement. Potassium sulfate is added to the mixing water to reach the desired soluble alkali content. The medium alkali cement is obtained by adding 1.4% of K₂SO₄ in clinker mass to the OPC cement. The resulting alkali content is 0.73% of equivalent Na₂O. The high alkali cement is obtained by adding 2.8% of K₂SO₄ in clinker mass to the OPC cement. The resulting alkali content is 1.23% of the equivalent Na₂O.

<table><caption>**Table 1 Chemical composition of Ordinary Portland Cement clinker used in this study, in mass fraction, obtained with X-ray Fluorescence (XRF) and X-ray Diffraction (XRD)**</caption><thead><tr><th colspan="2">Composition from XRF</th><th colspan="2">Composition from XRD</th></tr><tr><th>Component</th><th>Mass fraction, %</th><th>Phase</th><th>Mass fraction, %</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>22.5</td><td>Alite C<sub>3</sub>S</td><td>69.4</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>2.9</td><td>Belite C<sub>2</sub>S</td><td>17.6</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>2.25</td><td>Ferrite C<sub>4</sub>AF</td><td>7.1</td></tr><tr><td>CaO</td><td>66.97</td><td>Aluminate C<sub>3</sub>A</td><td>1.1</td></tr><tr><td>MgO</td><td>0.85</td><td>Calcite C<sub>2</sub>C</td><td>1.9</td></tr><tr><td>K<sub>2</sub>O</td><td>0.17</td><td>Gypsum C<sub>3</sub>S<sub>2</sub>H<sub>2</sub></td><td>0.5</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.12</td><td>Anhydrite C<sub>2</sub>S</td><td>1.2</td></tr><tr><td>SO<sub>3</sub></td><td>2.22</td><td>Hemihydrate C<sub>2</sub>S<sub>3</sub>H<sub>0.5</sub></td><td>0.4</td></tr></tbody></table>
The shrinkage-reducing admixture (noted 'SRA') used in this study is hexylene glycol, also called 2-methyl-2,4-pentanediol, sourced from Sigma Aldrich. Three amounts of SRA are added to the material: 0%, 4%, and 8% of the mass of cement. Cement pastes are prepared following the procedure described in [45,48,49] with a solution-to-cement ratio of 0.55, where solution refers to the mixture of water and SRA. The cement pastes are mixed every 30 minutes until reaching a creamy consistency. The total mixing time depends on the mix, ranging between 6 hours to 12 hours. The creaminess of the consistency is evaluated by the eye. This protocol is used to minimize segregation and bleeding of cement pastes and ensure homogenous cement pastes, given the high water-to-cement ratios used (0.47 to 0.55).

We note PP-L, PP-M, and PP-H, the OPC samples with low alkali content, medium alkali content, and high alkali content, respectively, to which no SRA is added. We note SR4-L, SR4-M, and SR4-H, the samples with 4% SRA added and with low alkali content, medium alkali content, and high alkali content, respectively. We note SR8-L, SR8-M, and SR8-H, the samples with 8% SRA added and with low alkali content, medium alkali content, and high alkali content, respectively. Table 2 provides the composition of all samples normalized to the content of unhydrated cement by mass.

<table><caption>Table 2 Composition of the studied nine types of cement pastes, expressed per gram of unhydrated cement</caption><thead><tr><td>Sample</td><td>Water (g/g)</td><td>SRA (g/g)</td><td>K<sub>2</sub>SO<sub>4</sub> (g/g)</td></tr></thead><tbody><tr><td>PP-L</td><td>0.55</td><td>-</td><td>-</td></tr><tr><td>SR4-L</td><td>0.51</td><td>0.04</td><td>-</td></tr><tr><td>SR8-L</td><td>0.47</td><td>0.08</td><td>-</td></tr><tr><td>PP-M</td><td>0.55</td><td>-</td><td>1.4</td></tr><tr><td>SR4-M</td><td>0.51</td><td>0.04</td><td>1.4</td></tr><tr><td>SR8-M</td><td>0.47</td><td>0.08</td><td>1.4</td></tr><tr><td>PP-H</td><td>0.55</td><td>-</td><td>2.8</td></tr><tr><td>SR4-H</td><td>0.51</td><td>0.04</td><td>2.8</td></tr><tr><td>SR8-H</td><td>0.47</td><td>0.08</td><td>2.8</td></tr></tbody></table>

Samples are cured in sealed aluminum bags and kept at 20 °C until characterization. We characterize samples cured for 7, 14, 28 days, 3 months, 6 months, and 1 year.

## 2.2 Hydration characterization methods

The samples are dried to stop the hydration from progressing to measure the hydration degree of cement pastes at the desired age (i.e., 7, 14, 28 days, 3 months, 6 months, and 1 year) and quantify the formed hydrates. Stopping hydration is required to prevent hydration from progressing during the characterization. The drying method
to stop hydration efficiently used here was freeze drying [50]. This method consists of reducing the temperature of the cement paste by liquid nitrogen quenching, then decreasing the pressure by placing the sample under a vacuum, and finally gradually increasing the temperature back to the ambient temperature while keeping the sample under a vacuum. This process allows the sublimation of the formed ice. The vacuum pump used during the process could apply a pressure theoretically as low as 6.7×10⁻⁴ mbar. All samples were freeze-dried except the 6-month-old samples. These samples were not dried before XRD and thermo-gravimetric analysis (TGA) measurements to enable the identification of the impact of the SRA on the formed hydrates independently of the pre-drying impact [51]. Also, at this age, the evolution of the hydration degree during the characterization is likely negligible.

After hydration stopping, the samples were crushed and sieved below 100 µm for the TGA measurements and below 63 µm for the XRD measurements. The powder was then tested directly by XRD and TGA to quantify the formed hydrates and assess the hydration degree of the samples.

We used the X-ray diffractometer X'Pert Pro X'Celerator detector. The 2θ ranges scanned from 5° to 65° with a scan speed of 0.016°/min. We used an internal reference to quantify the identified phases: powder cement paste was mixed with a known quantity of corundum, which serves as a reference to quantify the crystalline phases. Thermogravimetric analysis was conducted on the crushed cement pastes. The measurements were conducted under a nitrogen environment with a flow of 20 mL/min and a heating rate of 10 K/min, using an open vessel. The temperature ranged from 20 °C to 1000 °C.

XRD measurements and TGA measurements are combined to assess the hydration degree α, following [52]:

$$ \alpha = 1 - \frac{W(t)/(1 - W_w)}{W_0} $$

where W(t) is the mass fraction of anhydrous phases (i.e., C₃S, C₂S, C₃A, and C₄AF) of the hydrated cement paste at the given time t, W₀ is the mass fraction of anhydrous phases of the clinker before hydration (i.e., W₀ ~ 1, as other minor phases are present in the unhydrated cement, see Table 1), and Ww stands for the mass fraction of non-evaporable water in the hydrated cement paste. W(t) and W₀ are measured by XRD. Ww is measured by TGA and corresponds to the mass loss between 20 °C and 550 °C. In the case of hardened cement paste that is not predried prior to XRD and TGA, Ww includes the mass fraction of evaporable water. Note that in the case of cement pastes containing SRA, SRA decomposes at temperatures of 200 °C, and Ww includes the decomposition of SRA during the TGA test.

## 2.3 Pore structure characterization methods

We characterized the pore structure of the various materials at 3 months of age with nitrogen sorption, water sorption, and mercury intrusion porosimetry. Those various techniques explore different fractions of the HCP pore structure. According to Muller [53], mercury intrusion porosimetry (MIP) explores the capillary pores and a fraction of the gel pores. According to a recent work by Kurihara and Maruyama [54],
the nitrogen adsorption method investigates the gel pores, while water sorption
investigates most of the total area of C-S-H, except the spaces between stacked layers.

For pore structure evaluation, cement pastes were dried before measurement. This pre-drying may affect the obtained results. Zhang and Scherer [55] reviewed and conducted a comparative study of various drying techniques (i.e., freeze drying, solvent exchange using various solvents) and found that solvent exchange using isopropanol preserved the pore structure best; this method was previously recommended by several authors [51–54]. Moreover, Muller [53] investigated vacuum drying, oven drying at 50 °C and 110 °C, freeze-drying, and solvent exchange. He also confirmed that solvent exchange using isopropanol preserved the pore structure best, comparing measured pore volume to capillary pore volume obtained by ¹H-NMR relaxometry measurements.

For solvent exchange, we used the protocol defined in [53]. Samples were sawed into small pieces of 2 mm thickness. They were then immersed in isopropanol for one week. The isopropanol was renewed after one day and after three days. After isopropanol immersion, the samples were placed in a desiccator for 1 week and subjected to a vacuum at room temperature. The theoretical pressure of the vacuum pump was 6.7×10⁻⁴ mbar.

For nitrogen sorption measurements, the samples were coarsely crushed into pieces of size below 1.1 mm. An amount of 100 mg was then vacuumed for 24 hours. Nitrogen sorption measurements were conducted with a Belsorp Mini 2 provided by Microtrac-bel, and the relative pressure $p/p_0$ ranged from 0.01 to 0.99.

For mercury intrusion porosimetry, the samples were coarsely crushed into pieces of size below 1.1 mm. The sample, of about 1 g, was left under vacuum for 24 hours. The device used for MIP measurements was of type Autopore IV provided by Micromeritics. The maximum reached pressure was 414 MPa, which corresponds to a pore entry diameter of 3 nm. The assumed contact angle for mercury is taken at 130°. Values of 130° to 140° are frequently used for the contact angle for MIP measurements [50].

For water sorption measurements, the sample was ground manually with a mortar and pestle and then sieved. The fraction of the sample with a size larger than 25 µm and smaller than 75 µm was used for the measurements. An amount of ~25 mg was dried under vacuum at room temperature for 45 min prior to measurement. Water sorption measurements were conducted with a VSTAR vapor sorption analyzer (Quantachrome Instruments) at a temperature of 20 °C. At least two measurements per sample were conducted.

As already mentioned, the methods used in this work to characterize the cement paste pore structure require pre-drying. These pre-drying methods (i.e., solvent exchange and vacuuming at room temperature) impact the pore structure. However, in a relative manner, assuming that the impact introduced by the pre-drying on cement pastes is similar from one cement paste to another, we consider that the trends observed on the pore structure remain valid.
## 3. Results

### 3.1 Impact of SRA and alkali on hydration degree of cement

The hydration degree of the 9 types of cement paste at ages of about 3 months (i.e., 91 days precisely) and 6 months (i.e., 180 days precisely) is measured. The hydration degree of cement pastes in terms of SRA content for the various alkali contents is shown in Figure 1. This hydration degree decreases with the addition of the SRA. This decrease is observed at any given alkali content and can reach 15% when we compare 3-month-old low-alkali-content cement pastes containing 8% SRA with low-alkali-content plain pastes of the same age.

<figure><img src="image_3.png" /><figcaption>**Figure 1** Hydration degree of plain pastes and pastes containing SRA at various alkali contents at 3 and 6 months.</figcaption></figure>

Since a decrease in the hydration degree is observed at 6 months for cement pastes containing the SRA, we investigate the hydration degree versus the curing time from 7 days to 1 year. Figure 2 shows the hydration degree of plain pastes and pastes containing 8% SRA at low alkali, medium alkali, and high alkali content. At 1 year, the hydration degree of cement pastes with 8% SRA is slightly lower than the hydration degree of plain pastes. The hydration of the cement pastes in the presence of the SRA is significantly retarded at early ages; however, hydration is never stopped, as it keeps on increasing over time.
<figure><img src="image_4.png" /><figcaption>Figure 2 Hydration degree of pastes with various alkali contents. Light-shaded symbols are for plain pastes, and dark-shaded symbols are for pastes with 8% of SRA.</figcaption></figure>

At early ages (i.e., up to 14 days), increasing alkali content leads to an increase in the hydration degree. For pastes containing SRA, the increase in the hydration degree linked to an increase in alkali content persists even at later ages. The impact of the alkali on the hydration degree is observed when the hydration degree of the plain pastes is lower than 0.8. The impact of the alkali on the hydration of plain pastes supports the findings of Jawed and Skalny [29], who reported the acceleration of hydration in the presence of alkalis for up to 7 days. Juenger and Jennings [32] also reported an increase in hydration of cement paste with high content of alkali (where they modified the alkali content by adding NaOH to the mix water) at an early age (i.e., less than 1 day); however, the same authors reported that at later ages hydration is retarded in the presence of alkalis. More recent work [60,61] also reports a decrease in the hydration degree at later ages.

Although the impact of SRAs on the hydration of cement pastes was reported by several authors [18,35–37], few investigated the causes of this impact on hydration. Among the potential causes for the retardation of hydration by SRAs are the potential adsorption of SRAs on anhydrous phases or hydrates and the impact of SRAs on the pore solution composition. We should recall that the addition of SRAs to mixing water was done by replacing the equivalent mass of water with the SRA; the actual water-to-cement ratio is, therefore, lower for pastes containing the SRA than for plain paste: this ratio decreases from 0.55 for the plain pastes to 0.47 for the pastes containing 8% of SRA. The water-to-cement ratio of the samples in the present study remained higher than 0.44, which is sufficient to warrant complete hydration [62]. In
section 4.2, we further discuss the influence of the water-to-cement ratio and SRA on
hydration.

## 3.2 MIP pore size distribution of cement pastes

MIP measurements (when dealing with intrusion of mercury) enable the
assessment of a pore entry size instead of a pore size [63]; this phenomenon is also
referred to as the ink-bottle effect. In fact, when intruding mercury into a constricted
pore, the pressure required to intrude the pore is the pressure needed to intrude the
entry of the pore. The corresponding pore volume, in this case, is the pore volume of
the large pore, and the corresponding diameter is that of the small entry since intruded
mercury will access both the small entry and the large pore. The obtained pore size
distribution is expressed in terms of pore entry size; the volume of intruded pores is
given in terms of ml per ml of dry sample.

Results of pore size distribution obtained by MIP measurements on plain pastes
with various contents of alkali are shown in Figure 3.

<figure><img src="image_5.png" /><figcaption>Figure 3 Pore size distribution and cumulative porosity obtained with MIP for plain pastes with various alkali content.</figcaption></figure>
Results show that plain pastes present a bimodal distribution at pore entry diameters of ~15 nm and ~5 nm, except for low-alkali-content plain paste, for which a third family is identified at pores of entry size of 300 nm. The volume of the pores of size ~15 nm increases with increasing alkali content, while the volume of the pores of size ~5 nm is larger at the lowest alkali content than at the other alkali contents. The family of pores of pore entry size of 300 nm is only present for low alkali content cement paste, which can be linked to the percolation of the pore network in the case of low alkali content. Bentz [34] found that high alkali content leads to a depercolated capillary pore network, whereas at low alkali, capillary pores are percolated.

The pore size distribution of pastes containing the SRA at a given alkali content is shown in Figure 4 and Figure 5 for low and high alkali contents, respectively. Plain paste at low alkali content presents a family of pores at 300 nm, which is not observed for other pastes; increasing the SRA content (see Figure 4) or the alkali content (Figure 3) reduced the volume of these large capillary pores. At low alkali content, for pastes containing the SRA, the volume of pores of entry diameter ~5 nm decreases with an increasing SRA content, while the volume of pores of size ~15 nm increases. The cumulative porosity for pastes containing 4% and 8% SRA is higher than the cumulative porosity of plain paste. Both pastes at 4% and 8% of SRA show a total porosity of 32%, whereas the porosity of plain paste is 28%. Hence, the presence of the SRA coarsens the pore structure. These effects are further discussed in section 4.3.

<figure><img src="image_6.png" /><figcaption>Figure 4 Pore size distribution and cumulative porosity obtained by MIP on cement pastes at low alkali content at 3 months age.</figcaption></figure>

The pore size distribution of pastes with SRA at high alkali content, shown in Figure 5, shows that increasing the SRA content does not impact the volume of pores with an entry of ~15 nm, whereas the volume of pores with an entry of ~5 nm is slightly reduced in the presence of the SRA. The coarsening of the pore structure of cement pastes containing SRA is more notable at low alkali content than at high alkali content.
<figure><img src="image_7.png" /></figure>

**Figure 5** Pore size distribution and cumulative porosity obtained by MIP on cement pastes at high alkali content.

### 3.3 Influence of SRA, alkali, and combination of alkali and SRA on the specific surface area of hardened cement pastes

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The specific surface area (SSA) of cement pastes evaluated by nitrogen adsorption analyzed by the Brunauer-Emmett-Teller (BET) method [64] is plotted in terms of the SRA content in Figure 6 and given in Table 4. For the three contents of alkali, the SSA decreases with increasing content of SRA. The decrease of the specific surface area is significant in case of low alkali content and can reach up to 60% in the case of samples containing 8% of SRA. Moreover, increasing the alkali content also leads to a decrease in the specific surface area. This decrease is more important for the plain pastes than for the pastes containing SRA.

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<figure><img src="image_8.png" /><figcaption>Figure 6 Specific surface area of cement pastes at 3 months age, evaluated with BET method applied to nitrogen sorption isotherms.</figcaption></figure>

The BET constant $C_{BET}$ is related to the affinity of the gas to the solid. It should be noted that the samples were pre-dried by vacuuming at room temperature, without any solvent exchange prior to the measurement. For the plain pastes, this BET constant ranges from 71 to 83 and decreases significantly in the presence of the SRA (see Table 3). In fact, for samples with an SRA content of 4%, the constant $C_{BET}$ reaches a value of ~ 30; for samples with an SRA content of 8%, the constant $C_{BET}$ reaches a value of ~ 22. This impact of the SRA on the BET constant $C_{BET}$ indicates that, for cement pastes that contain the SRA, when the samples are pre-dried by vacuuming at room temperature, some SRA may remain adsorbed on the solid surfaces. This potential adsorption of SRA on the pore walls does not lead to a reduction in the total porosity. In fact, MIP results on cement paste prepared by solvent exchange show that the presence of SRA increases the total porosity of the cement pastes.
<table><caption>Table 3 BET constant of cement pastes obtained from nitrogen sorption isotherms</caption><thead><tr><th>Sample</th><th>BET constant<br>C<sub>BET</sub> (-)</th></tr></thead><tbody><tr><td>PP-L</td><td>70</td></tr><tr><td>SR4-L</td><td>30</td></tr><tr><td>SR8-L</td><td>22</td></tr><tr><td>PP-M</td><td>83</td></tr><tr><td>SR4-M</td><td>32</td></tr><tr><td>SR8-M</td><td>23</td></tr><tr><td>PP-H</td><td>82</td></tr><tr><td>SR4-H</td><td>30</td></tr><tr><td>SR8-H</td><td>22</td></tr></tbody></table>

The specific surface area is also assessed using the BET model applied to water sorption isotherms. The results, displayed in Figure 7 and given in Table 5, confirm that the SSA decreases significantly when the SRA is added to the cement pastes. The impact of the alkali content on the SSA, evaluated by water adsorption, of plain pastes is minor: the SSA obtained from the water sorption isotherms is not correlated with alkali content, in contrast to the SSA obtained from the nitrogen sorption isotherms, which decreases with an increasing alkali content.

The SSA obtained from the water sorption isotherms (see Figure 7) is higher than the one obtained from the nitrogen sorption isotherms (see Figure 6) by a factor of 2 to 3. This observation is well known, not only for cement pastes [50] but also for other porous materials. The fact that the measured SSA depends on the adsorbate can be linked to several causes [50]. Water molecules (whose size is around 3 Å), which are smaller than nitrogen molecules (whose size is around 3.5 Å), may access pores that are inaccessible to nitrogen. Another explanation proposed is that nitrogen adsorption measurements would require longer equilibration times than water adsorption measurements [65,66] because of the temperature at which the measurements are conducted (i.e., at -195.7 °C for nitrogen and at 20 °C for water vapor). Also, the dipolar nature of water makes it strongly attracted to the ionic surfaces of the hydrates, while nitrogen is less attracted to these surfaces because of its quadrupolar nature. Feldman and Sereda [67] indicated that measuring the specific surface area with nitrogen allows measurements of a "true" surface area since water can enter the C-S-H interlayer while nitrogen cannot. However, Odler [66] found that the specific surface area measured by water adsorption is closer to the total surface area of cement-based materials than the one measured by nitrogen adsorption. Recently, Kurihara and Maruyama [54] showed that nitrogen sorption could assess most gel pores, while water sorption could assess most of the total surface area of C-S-H, except the surface between C-S-H stacked layers.
<figure><img src="image_9.png" /><figcaption>**Figure 7** Specific surface area of cement pastes at 3 months age, evaluated from BET analysis of water sorption isotherms.</figcaption></figure>

The decrease of the SSA with an increasing alkali content, evaluated from nitrogen sorption isotherms, is consistent with observations by other authors [29,32]. This decrease can be linked to the impact of alkalis on the microstructure of hardened cement paste and potential changes in the morphology of formed C-S-H gel. Such modifications will be detailed in the light of the literature in the discussion section.

The SSA obtained from water sorption is almost independent of the alkali content. Considering that this SSA is representative of the surface of the whole C-S-H gel without the spaces between stacked layers, this finding is consistent with the fact that, at a given SRA content, the hydration degree of mature pastes is almost independent of the alkali content (see Figure 2).

Independent of the alkali content, the addition of the SRA to the pastes leads to a decrease of the SSA evaluated from nitrogen adsorption (see Figure 6) or from water adsorption (see Figure 7). For this latter, adding 4% or 8% of SRA to the paste makes little difference.

# 4. Discussion

## 4.1 *Impact of alkalis on pore structure*

Alkalis are reported to impact the capillary pores and gel pores. In fact, Sant et al. [33], who used a cement containing 1.8% K₂SO₄ (0.6% Na₂O eq) and enriched with
either sodium or potassium hydroxide 0.5 M, reported an increase at 28 days of the volume of capillary pores upon addition of alkalis. They also reported that the most representative sizes of pore entries are not impacted by alkalis. This observation is consistent with the results presented here (see Figure 3). However, Mota Gassó et al. [60] reported an impact of alkalis on the pore size distribution obtained by MIP, namely that a higher porosity and smaller characteristic size of pore entry are observed when alkalis are added to the paste.

These changes in the capillary pores and gel pores are potentially attributed to the impact of the alkalis on the C-S-H morphology. In fact, the microstructure of the C-S-H gel formed is influenced by both alkali content and alkali type. When the content of potassium hydroxide is high, a coarse C-S-H structure is observed [29] with divergent needles instead of foil-like C-S-H.

The uptake of ions by C-S-H in alkali media can lead to changes in the structure of C-S-H. Studies on synthetic C-S-H showed that the alkalis uptake increases at low Ca/Si ratios [68]; at high Ca/Si ratios (>1.2), the uptake of alkalis is suppressed. The uptake of alkalis may also lead to a replacement of divalent calcium ions by monovalent alkalis, which decreases the silica chain length, may increase the interlayer spacing [35,68], and reduces the gel pores volume [60]. However, it should be noted that the C-S-H stoichiometry is unchanged by the addition of alkalis [31].

Other studies on synthetic C-S-H with a Ca/Si ratio of 1.0 showed an increase in aluminum uptake with increasing concentration of aluminum [69]. The presence of alkali favors this incorporation of aluminum, which is found in bridging positions. In the presence of alkalis, partial replacement of calcium ions by alkalis leads to an increase of the interlayer and a shorter silica chain. Later, L'Hôpital et al. [70] showed an increase of the basal spacing by 2Å when aluminum was incorporated in the synthesized C-A-S-H gel at Ca/Si ratios ranging from 0.2 to 1.6. An increase of basal spacing by 2Å, intrinsic to the presence of aluminum in C-S-H gel, is also reported in [71]. Studies on synthesized C-A-S-H gels in the presence of sodium hydroxide also showed an increase in the basal spacing with aluminum incorporation [72]. The promotion of aluminum uptake by alkalis is explained by the substitution of Si⁴⁺ for Al³⁺ in C-S-H gel, while alkalis balance the charge by adsorption/binding [73]. However, it should be noted that the uptake of alkalis is found to be lower in pastes with high Ca/Si ratios than in pastes with low Ca/Si ratios [74].

Hence, increasing the alkali content favors the uptake of alkalis and aluminum, leading to changes in the C-S-H interlayer and gel space. The increase of the interlayer spacing and the decrease of the gel pore volume in the presence of alkali could explain the decrease of SSA obtained from the nitrogen sorption isotherms with increasing alkali content (see Figure 6), as well as the reduced gel pore volume observed in MIP measurements (see Figure 3). The SSA obtained from the water sorption isotherms (see Figure 7) shows less dependence on the alkali content. This little dependence may be related to the fact that water vapor molecules can access the C-S-H interlayer space and gel pores and to the fact that the hydration degrees of the plain pastes at the given ages of testing did not depend significantly on the alkali content.
500

## 4.2 Impact of SRA on hydration

To investigate the impact of water-to-cement ratio on the kinetics of hydration at long-term ages, i.e., beyond 1 month, the degree of hydration obtained from this work and literature [75–80] is plotted in terms of w/c ratio in Figure 8, Figure 9 and Figure 10 for ages of 28 days, 3 months and 1 year, respectively.

505

<figure><img src="image_10.png" /><figcaption>Figure 8 Degree of hydration of cement pastes at the age of 28 days from this work and literature data [75–79].</figcaption></figure>

510

<figure><img src="image_11.png" /><figcaption>Figure 9 Degree of hydration of cement pastes at the age of 91 days from this work and literature data [75,76].</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 10 - Degree of hydration of cement pastes at the age of 365 days from this work and literature data [75,79,80].**</figcaption></figure>

In Figure 8, the magnitude by which the degree of hydration of samples at 28 days age decreases with w/c ratio is lower compared to the decrease of degrees of hydration for cement pastes containing SRA at the various alkali content, and most specifically for low alkali content. At 3 months of age, Figure 9 shows that the degree of hydration decreases with the w/c ratios; such decrease is lower for cement pastes from literature (without SRA) compared to the reduction observed for cement pastes with SRA at the various alkali content. Results of Wong and Buenfeld [76] show a slight increase in the degree of hydration with w/c.

Results for samples at 1 year age, shown in Figure 10, indicate that the hydration degree increases with increasing w/c for plain pastes and pastes containing SRA by a similar magnitude.

The retardation of hydration of cement pastes by SRA pastes reduces with time. Comparing our data to the literature, in addition to the effect of the w/c ratio, SRA may contribute to the retardation of hydration at 28 days and 3 months. At the age of 1 year, the role of SRA in reducing the degree of hydration cannot be isolated from the effects of reducing w/c in light of current data and literature findings. The experimental measurement of the degree of hydration, which was not performed in this study, is important for clearly distinguishing the role of SRA in hydration retardation independently from the impact of the water-to-cement (w/c) ratio.

The decrease of degree of hydration in late ages is linked to the lack of space [81]: for low w/c, pores filled by water are depleted, for high w/c ratios, the water filled pores are refined to the point that hydrates cannot precipitate in this confined space. In this work, the pore space of pastes containing SRA is not more refined than the pores space of plain pastes (based on MIP measurements).

It should be noted that the presence of SRA in the solution and the potential increase of SRA concentration due to water consumption by hydration reduces the water activity. Such impact of SRA on the water activity may also lead to a retardation of the hydration.

Eberhardt [38] performed experiments with several SRAs, among which a commercial SRA composed mainly of DPTB dipropylene tert-butyl ether. The author used leaching experiments to show that SRAs do adsorb on cement hydrates, mostly
on portlandite. This adsorption explains the retardation of hydrates of silicate phases in his study. However, Ai and Young [43], who used a commercial SRA based on DPTB, found no evidence of the adsorption of SRAs on cement hydrates or anhydrous phases.

Rajabipour et al. [41] studied the impact of a commercial SRA, consisting mainly of poly-oxyalkylene alkyl ether, on the pore solution of cement paste. Their results showed that the presence of the SRA in the extracted pore solution of cement pastes lowered the amount of dissolved alkalis in the pore solution, which may explain the decrease in the hydration rate of cement paste containing SRAs, as alkalis act as accelerators of hydration at early ages.

The portlandite and ettringite amount are larger in the presence of SRA than in its absence (cf. supplementary data). This was reported by other workers as well [82]. The influence of SRA on the pore solution composition (depleted alkalis and sulfates) may explain this effect. In addition, the morphology of both portlandite and ettringite can be modified by SRA: a finer ettringite [83] and more dispersed smaller crystals of portlandite [45] are reported for cement pastes containing SRA.

In our study, the hydration of cement pastes containing an SRA is only retarded and does progress through time; the impact of the SRA on hydration through adsorption on anhydrous phases or on formed hydrates may not be sufficient to explain the retardation of hydration if we assume that adsorption occurs irreversibly: irreversible adsorption of SRA on cement would prevent (and not retard) hydration of some cement. Furthermore, the impact of the SRA on the composition of the pore solution was studied in our previous work, and the SRA was found to be leachable [48]. Also, the alkalis are depleted from the pore solution in the presence of the SRA. The impact of the SRA on hydration may be linked to the impact of the SRA on the pore solution composition rather than the adsorption on unhydrated cement or on the formed hydrates.

## 4.3 Impact of SRA on pore structure

The effect of SRAs on the pore structure of cement pastes may be linked to three features: 1) the retardation of hydration, 2) steric effects (i.e., the presence of the SRA at pore entries (which would render the pore inaccessible), or at the surface of the cement hydrates), and 3) the impact on pore solution composition and hence potential changes to the C-S-H gel.

The decrease of SSA observed for cement pastes containing SRA (see Figure 6 and Figure 7) may be partly due to the lower hydration degree of these pastes and partly due to the intrinsic presence of SRA in cement pastes. In this study, SRA retarded the hydration (see Figure 1 and Figure 2), as many others have already observed [18,35–38].

Figure 11 displays the SSA obtained from the nitrogen adsorption for the 9 types of 3-month-old cement pastes versus the measured hydration degree. We note a linear relationship between SSA and hydration degree, regardless of alkali content. This
linear relation indicates a correlation between hydration degree and SSA obtained from nitrogen measurements.

<figure><img src="image_13.png" /><figcaption>**Figure 11** Specific surface area of 3-month-old cement pastes and of a 9-month-old cement paste cured at a relative humidity of 95% vs. measured hydration degree. Light shade, medium shade, and darker shade correspond to plain pastes, pastes with 4% of SRA, and pastes with 8% of SRA, respectively.</figcaption></figure>

To investigate the origins of the observed decrease of SSA with SRA content independently from the observed decrease of hydration degree, we analyze the hydration degree of plain cement paste and of a cement paste with 8% of SRA cured at 95% relative humidity for more than 9 months. Under curing at a relative humidity of 95%, hydration of cement pastes progresses [84], and the hydration degrees of the low-alkali pastes with and without SRA are almost equal: 0.94 for the low-alkali paste without SRA and 0.92 for the low-alkali paste with 8% of SRA. However, the SSA of the sample with 8% of SRA (evaluated by nitrogen adsorption) remains lower than that of the plain paste: in the case of the plain paste, the SSA reaches 111 m²/g, whereas, for the paste with 8% of SRA, the SSA reaches 69 m²/g. Furthermore, the hydration degrees of the 3-month-old plain paste and of the 9-month-old paste with SRA cured at 95% relative humidity are comparable. However, in spite of this similar hydration degree, the SSA of the sample with the SRA remains lower than that of the plain paste: the SSAs measured from the water sorption are 193 m²/g and 166 m²/g for the 3-month-old plain paste and the 9-month-old paste containing 8% of SRA and cured at 95% relative humidity, respectively. The coarsening of pore structure is, therefore, not solely linked to the hydration retardation, but is intrinsic to the presence of the SRA, in the sense that, at a given hydration degree, the presence of the SRA decreases the SSA of the paste.

The impact of the presence of the SRA on the developed pore structure, as evaluated by means of water sorption and nitrogen sorption, may indicate the positioning of the SRA molecules at the level of cement hydrates. As observed on water sorption isotherms (see Figure 7) and nitrogen sorption isotherms (see Figure 6), a significant impact of the SRA on the pore structure of the cement paste is noted.
Note that solvent exchange was only used prior to MIP measurements, not prior to water sorption and nitrogen sorption measurements. For those sorption experiments, samples were prepared by vacuuming at room temperature without any prior solvent exchange.

In what follows, a primary approach is made to assess the positioning of SRA at the level of the cement hydrates. The SRA may be located in the large pores of cement paste where bulk water exists and it may also be adsorbed on solid surfaces. The schematic in Figure 12 is a visual proposal for the potential positioning of SRA in the C-S-H microstructure.

<figure><img src="image_14.png" /><figcaption>**Figure 12** Schematic representation for C-S-H microstructure in low-alkali paste (left figure) and in paste with SRA or of high alkali content (right), adapted from [60]. Red squares stand for interlayer water, and green triangles for gel water. The SRA's potential positioning is indicated by gray star markers.</figcaption></figure>

The BET constant $C_{BET}$ of the samples, evaluated from nitrogen sorption isotherms, decreases for cement pastes that contain the SRA (see Table 3). This decrease in the BET constant indicates that the SRA may be adsorbed on the surfaces of the hydrates during the drying process. Such a hypothesis is consistent with the idea that the SRA hinders adsorption sites (such that surfaces seen by nitrogen molecules, although existing, are lower than in plain pastes) or makes some pore volume inaccessible to nitrogen/water molecules. Water sorption isotherms on a model material, namely zeolite, showed that the SRA reduces not only the surface area but also the pore volume [48,85]. Such observation is a strong indicator that the SRA may also be placed at the entry of some pores, making them inaccessible.

Based on a rudimentary approach, the estimated size of the used molecule in our study is ~ 0.6 nm. The volumetric mass density of liquid SRA is $ρ_{SRA} = 0.92$ g/cm³, assuming that the molecule can occupy a volume of a cube of characteristic size $l$, where $M_{SRA}$ is the molar mass of SRA and $N_A$ is the Avogadro number. This value is consistent with data on polyethylene glycol (PEG) in aqueous solution [86].
The water-sorption-based SSAs of cement pastes containing SRA exhibit a decrease of SSA for pastes containing SRA compared to plain pastes and a plateau between 4% and 8% of added SRA (see Figure 7), in contrast to the N₂-sorption-based SSAs, which exhibit no plateau. This plateau of the SSA measured by water sorption for pastes containing SRA may indicate that in addition to the inaccessible surfaces of C-S-H to water vapor [54], 1) SRA leads to an increase of the inaccessible surfaces or 2) the surfaces of C-S-H are intrinsically reduced in the presence of SRA. The first hypothesis aligns with the SRA's ability to render some adsorption sites or pores spaces inaccessible to water vapor molecules. For the second hypothesis, the current data set does not allow us to confirm the ability of the SRA to modify the surfaces of the cement hydrates.

Finally, by investigating the composition of the pore solution extracted from hardened cement pastes containing the SRA, we observed in a previous study that the SRA leads to a depletion of alkalis from the solution [48]: this observation is consistent with results obtained by Rajabipour et al. [41]. Therefore, the SRA may favor alkali uptake by C-S-H, leading to an effect similar to that of alkali, as detailed in section 4.1: the SRA may increase the alkali uptake by C-S-H, leading to changes in the C-S-H morphology with a potential increase of the interlayer spacing and a decrease of the gel pore volumes. Such effects of alkali uptake are more significant in pastes with low Ca/Si ratios. Furthermore, the mass balance of cement paste composition before and after hydration based on XRD and TGA measurements allowed estimation of the Ca/Si ratio of cement pastes, suggesting that the Ca/Si ratio decreases in the presence of SRA from ~1.7 to ~1.5 [87], regardless of alkali content. The SRA may modify the C-S-H structure, with a potential increase of interlayer space, a decrease of gel pores volume, and an occupation of adsorption sites.

# 5. Conclusions

In this study, the effect of an SRA (i.e., hexylene glycol) and alkalis on the hydration and pore structure of hardened cement pastes is characterized by various methods: XRD, TGA, MIP, nitrogen sorption isotherms, and water sorption isotherms.

Increasing the alkali content increases the hydration degree of cement pastes at early ages (i.e., up to 14 days). At later ages, the impact of alkali on the hydration degree is not significant, which is consistent with the findings of the literature. The alkalis are also found to impact the pore structure: the volume of pores with an entry diameter of about 15 nm increases slightly with alkali content, while the N₂-sorption-based SSA decreases.

The SRA leads to a retardation of hydration for any alkali content. This retardation is persistent until three months. The difference in the hydration degree between plain pastes and pastes containing SRA decreases with time, as shown by the hydration degree measured after 7 days.

The SRA impacts the pore structure of cement pastes regardless of alkali content. In the presence of the SRA, the MIP results show that the characteristic pore entry
size of 15 nm shifts towards larger sizes. Also, the volume of pores with a characteristic entry size of 5 nm decreases with an increasing SRA content. Likewise, the specific surface area obtained from nitrogen sorption measurement decreases with an increasing SRA content.

The specific surface area obtained from water sorption isotherms reaches a plateau when displayed as a function of the amount of SRA added, regardless of alkali content. Gel pores and capillary pores of pastes at low alkali content are more impacted by the presence of the SRA than pastes at medium or high alkali content.

The influence of SRA and alkali on the microstructure of cement pastes is linked to the impact of these parameters on the hydration of cement pastes and on the formed C-S-H gel structure. The increase in alkali content and SRA may lead to an increase in the interlayer space and a decrease in the gel porosity.

The pore structure of hardened cement paste depends on the SRA, alkali, and the combination of SRA and alkali. The magnitude by which the SRA impacts the pore structure depends on the alkali content, and it suggests that the SRA's ability to reduce shrinkage at a given relative humidity could also depend on the alkali content.

Understanding the impact of SRAs and alkalis on the pore structure of hardened cement pastes could provide a better understanding of the impact of those physical parameters on drying shrinkage and on the performance of SRAs in alkali media.

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Appendix:

<table><caption>Table 4 Tabulated values of SSA and BET constant $C_{BET}$ of samples evaluated by nitrogen sorption isotherm</caption><thead><tr><th rowspan="2">Samples</th><th rowspan="2">Curing conditions</th><th colspan="2">Nitrogen as adsorbate</th></tr><tr><th>SSA (m²/g)</th><th>BET constant<br>$C_{BET}$</th></tr></thead><tbody><tr><td>PP-L</td><td rowspan="10">Sealed for 3 months</td><td>103</td><td>70</td></tr><tr><td>SR4-L</td><td>71</td><td>30</td></tr><tr><td>SR8-L</td><td>43</td><td>22</td></tr><tr><td>PP-M</td><td>74</td><td>83</td></tr><tr><td>SR4-M</td><td>60</td><td>32</td></tr><tr><td>SR8-M</td><td>39</td><td>23</td></tr><tr><td>PP-H</td><td>62</td><td>82</td></tr><tr><td>SR4-H</td><td>54</td><td>30</td></tr><tr><td>SR8-H</td><td>27</td><td>22</td></tr><tr><td>PP-L-95</td><td rowspan="2">At 95% RH for more than<br>9 months</td><td>111</td><td>82</td></tr><tr><td>SR8L-95</td><td>69</td><td>39</td></tr></tbody></table>

1000

<table><caption>Table 5 Tabulated values of SSA and BET constant $C_{BET}$ of samples evaluated by water sorption isotherm</caption><thead><tr><th rowspan="2">Samples</th><th rowspan="2">Curing conditions</th><th colspan="2">Water vapor as adsorbate</th></tr><tr><th>SSA (m²/g)</th><th>BET constant<br>$C_{BET}$</th></tr></thead><tbody><tr><td>PP-L</td><td rowspan="10">Sealed for 3 months</td><td>193</td><td>21</td></tr><tr><td>SR4-L</td><td>145</td><td>31</td></tr><tr><td>SR8-L</td><td>145</td><td>36</td></tr><tr><td>PP-M</td><td>181</td><td>23</td></tr><tr><td>SR4-M</td><td>156</td><td>29</td></tr><tr><td>SR8-M</td><td>141</td><td>30</td></tr><tr><td>PP-H</td><td>206</td><td>24</td></tr><tr><td>SR4-H</td><td>149</td><td>30</td></tr><tr><td>SR8-H</td><td>137</td><td>28</td></tr><tr><td>PP-L-95</td><td rowspan="2">At 95% RH for more than<br>9 months</td><td>192</td><td>22</td></tr><tr><td>SR8L-95</td><td>166</td><td>30</td></tr></tbody></table>