Introduction

C-S-H (Calcium-Silicate-Hydrate) is the predominant phase in hydrated Portland cement, constituting roughly 75% of its composition (Maddalena et al., 2019), varying in cement containing SCMs (Lothenbach; Scrivener; Hooton, 2011). It significantly influences the material’s strength, porosity, and mass transport properties. Despite decades of study, C-S-H remains not completely understood, especially its transformations during exposure to humidity and carbonation (John; Matschei; Stephan, 2018). The structure of C-S-H is frequently described as a defective tobermorite, which has a Ca/Si=0.83, meanwhile, in cement, the Ca/Si varies from 0.7 to 2.1 (Lothenbach; Nonat, 2015; Tajuelo Rodriguez et al., 2017).

The Ca/Si ratio controls C-S-H crystal chemistry, pH (10 to 12.4), basal space, crystallite size, mean chain length, bound and interlayer water, and is responsible, in part, for the cohesion of hydrated cement pastes (Pellenq; Van Damme, 2004). However, changes in humidity and/or temperature can modify the layer-to-layer distance, and the chemical and mechanical behavior of the system (Roosz et al., 2016), which will cause swelling or shrinkage (Gajewicz et al., 2016; Gajewicz-Jaromin et al., 2019; Holthausen; Raupach, 2018; Scrivener; Juilland; Monteiro, 2015).

Carbonation in hydrated cement occurs when carbon dioxide (CO₂) reacts with alkalis, mainly calcium from the calcium-rich phases, in the presence of water, changing the mineral assemblage and the microstructure. While the existing literature commonly identifies portlandite as the primary reactant with CO₂ (Phung et al., 2015), points out that C-S-H may carbonate even faster than portlandite, but its kinetics and effect on chemistry and microstructure of various C-S-H composition is not clear. CO₂ is in the atmosphere, and it is diluted in the mixing and pore water, a concentration that normally depends on the atmospheric pressure of the gas, which varies locally. CO₂ in mixing water allows carbonation without interference with diffusion mechanisms been a predominant dissolution precipitation process (Zajac et al., 2023).

The main aim of this paper is to explore the influence of Ca/Si ratio of synthetic C-S-H in its carbonation by in-situ XRD, in-situ FTIR, SEM and TGA measurements.

Methodology

Two C-S-H samples with Ca/Si ratios target of 0.83 and 1.5 were synthesized via mechanochemical process utilizing a Fritsch Pulverisette 6 classic line planetary ball mill. The reagents employed were pure calcium hydroxide (Ca(OH)₂) and amorphous silica commercially available from Sigma Aldrich. Calcium oxide (CaO) was obtained by calcining Ca(OH)₂ at 900 °C for 4 hours. The reagents were mixed with Milli-Q ultrapure water in a water-to-solid ratio of 4. This mixture was processed in a tungsten jar, undergoing grinding for a total duration of 48 hours, with each cycle consisting of 55 minutes of grinding and 10 minutes of rest, all conducted at 100 rpm. The resulting samples were subsequently dried in a vacuum oven at 40 °C. The XRD of the synthetic 0.83 and 1.5 C-S-H is presented in Figure 1.

Figure 1
XRD patterns from 0.83 and 1.5 C-S-H – the black line is the synthetic C-S-H and the gray line is the C-S-H with Kapton film

To perform in-situ XRD carbonation, dried C-S-H samples were mixed with pure water (2g/g) or pure water that had been pre-bubbled with CO₂ (resulting in a pH of around 5). After 2 minutes the paste was transferred into a custom-made sample holder produced by a 3D printer and immediately covered with a Kapton polymeric film (Figure 2). The in-situ XRD measurements were conducted using an Empyrean Panalytical diffractometer, operating at 40 kV and 40 mA with CuKα radiation (λ = 0.154 nm) at 25 °C, scanning in the range of 2θ 3°-45°, 0.04 rad soller slit, divergence slit fixed at 0.125, 100s per angle step, angle step of 0.02º, detector PIXcel 3D. During the scans, the samples were spun at 16 rpm around the vertical goniometer axis to enhance particle statistics and minimize preferred orientation. A 10-minute scan was recorded every 20 minutes for a total duration of 20 hours, starting immediately after the mixing of dry C-S-H with CO₂-bubbled water.

Figure 2
C-S-H pastes placed in the sample holder with Kapton film

Furthermore, ATR-FTIR analysis was performed using a Shimadzu IRTracer-100 model in the range of 600-4000 cm^-1. In-situ ATR-FTIR was performed following the same mixture procedure of in-situ XRD, and the paste was transferred to a sample-holder for liquid samples to avoid evaporation. The in-situ analysis was collected over 7 hours. After the in-situ analysis was completed, the samples were once again dried in a vacuum oven at 40 °C.

Thermal Gravimetric Analysis (TGA) was made from 25 to 1000 °C at a constant heat rate of 10 °C/min in an inert N₂ atmosphere. The test was conducted immediately after drying samples to avoid any carbonation during storage.

The mass loss from C-S-H dehydration was calculated between 100 °C and 400 °C. Since C-S-H lose water in a wide range of temperature and the water content depends on the drying process, literature does not have consensus, varying from 50 °C-600 °C (Scrivener; Snellings; Lothenbach, 2016), 105 °C-818 °C (Li et al., 2020), 25 °C-200 °C (Maddalena et al., 2019), 100 °C-350 °C (Jin et al., 2022). Equation 1 was used to determine the stoichiometric formula of the synthesized C-S-H. By TGA results, in 0.83C-S-H the mass loss was 18% and 1.5C-S-H was 20%. Thus, the stochiometric formulas of the synthesized C-S-H are CSH_1.3 (or 0.83CaO.SiO₂.1.3H₂O) and C_1.5SH₂ (or 1.5CaO.SiO₂.2H₂O).

Where:

W₁ is the mass loss from C-S-H, 18.01, 56.08 and 60.08 are the molar mass of a Ca/Si =1; and

“X” is the molecules of H₂O in the C-S-H structure.

Results and discussion

Figure 3a and b show the XRD measurements of 0.83 and 1.5 C-S-H samples over time. The peaks of 0.83 Ca/Si ratio sample, at approximately 6.3° (corresponding to 14Å), 14.3°, 29.2°, and 32.0° were observed, which can be attributed to the C-S-H phases. The ~29° peak can also be attributed to calcite. However, no decomposition peaks associated with CaCO₃ polymorphs or portlandite were detected in the thermal gravimetric analysis (TGA) results presented in Figure 4c, indicating the high purity of the C-S-H in this sample.

Figure 3
(a) In-situ XRD patterns of Ca/Si: 0.83 from 10 to 1080 minutes with pure water; and (b) In-situ XRD patterns of Ca/Si: 1.5 from 10 to 1080 minutes with pure water

Figure 4
(a) In-situ XRD patterns from 10 to 1080 minutes for sample Ca/Si=0.83; (b) zoom in the 3° to 10° and 28° to 34° range with baseline correction from 10 minutes and 20h carbonation (the background was subtracted for normalization); and (c) TGA and DTG results of 0.83 C-S-H measured on the dry state

Conversely, in the Ca/Si 1.5 sample, peaks corresponding to portlandite at 18° and 34° were observed in addition to the C-S-H peaks, as corroborated by the TGA results shown in Figure 5c.

Figure 5
(a) In-situ XRD patterns from 10 to 1080 minutes experiment (sample Ca/Si=1.5) in contact with water + CO₂; (b) zoom in the 27° to 35° range with baseline correction from 10 minutes and 20h carbonation (the background was subtracted for normalization); and (c) TGA and DTG results of 1.5 C-S-H measured on the dry state

The XRD pattern of C-S-H in pure water did not exhibit significant changes, aside from variations in peak intensities, possibly due to preferential orientation effects within the C-S-H structure.

Figure 4 presents the XRD measurements of 0.83 C-S-H with pure water + CO₂. Only minor changes were observed during the measurement compared with Figure 3a, primarily marked by an increase in C-S-H and calcite/C-S-H peak at 29°. It’s noteworthy that the basal space ratio remained unaltered during the test. The sharpening of the 6.3° peak over time is attributed to water incorporation and/or preferred orientation of the C-S-H, as variations in relative humidity are known to influence the position and intensity of 001 reflections (Gaboreau et al., 2020). Figure 4c presents the TGA and DTG results of the 0.83 C-S-H. It is worth mentioning that even considering the low availability of calcium in 0.83 Ca/Si ratio, there is an increase in calcite mass after carbonation. The wollastonite (related to the decomposition of C-S-H) peak remained constant. Furthermore, the mass loss prior to carbonation was much higher than after carbonation and the stoichiometric mass loss due to carbonation, indicating that carbonation makes it difficult for water to leave the sample, which can be seen with a shift in the C-S-H mass loss curve in the begin of the graph. The same tendency is not observed in 1.5 C-S-H.

Figure 5 illustrates the XRD pattern of C-S-H 1.5 over-time, which is remarkable different than the one with C/S 0.83. The 2θ of around 6.3° (14Å), 14.1°, 28.01° (anorthic afwillite crystalline-C-S-H) 29.1° and 31.4° corresponding to C-S-H remained constant in 1.5 synthesized C-S-H with water + CO₂. A vaterite peak presented in 32.7° and calcite in 31.4° indicate a previous carbonation also detected by the TGA measurements. Just 4 hours after mixing with water saturated with CO₂, the peaks around 6.3° (C-S-H) and 14.1°(C-S-H) began to diminish, which can be caused due to carbonation and progressive amorphization of the C-S-H structure (Qomi et al., 2021; Liang, 2022). Importantly, the crystalline peaks of afwillite and portlandite remained stable throughout the 20-hour carbonation process. These results agreed with the TGA measurements in Figure 5c, as afwillite mass loss does not change, and portlandite loses only 0.9% of the mass. However, peaks corresponding to calcite and vaterite (as shown in Figure 5b) became more pronounced and well-defined after 20 hours of carbonation, suggesting carbonation of the C-S-H. Given the potential overlap between the 29° peaks of C-S-H and calcite, the growth of CaCO₃ polymorphs was confirmed through FTIR spectra and TGA measurements. Since TGA shows a growth of 4.3% of calcite, and only 0.9% of portlandite was consumed, this implies that the CO₂ has reacted with the calcium from the C-S-H. This is certainly facilitated by the higher specific surface area of C-S-H compared to portlandite (Regnault; Lagneau; Schneider, 2009). Also, there is more Ca²⁺ available in the structure and in the interlayer for high-calcium C-S-H.

Figure 6 illustrates the FTIR spectra of 0.83 and 1.5 Ca/Si ratio C-S-H samples during the initial 7 hours of exposure to either pure water or water saturated with CO₂.

Figure 6
In-situ ATR-FTIR spectra 7h long measurements cover a range from 600 to 1800 cm^-1, with a zoomed-in section between 3500-3800 cm^-1 – these spectra correspond to: (a) Ca/Si 0.83 mixed with pure water; (b) Ca/Si 0.83 mixed with water saturated with CO₂; (c) Ca/Si 1.5 mixed with pure water; (d) Ca/Si 1.5 mixed with water saturated with CO₂ – to enhance clarity, the spectra of pure water and water saturated with CO₂ were subtracted from the spectra obtained over time

In the case of the Ca/Si 0.83 samples (Figure 6a and 6b), a sharp peak at 968 cm^-1 corresponding to Si-O stretching vibrations of Q² is observed. Additionally, a broad peak at 667 cm^-1 is noted, associated with Si-O-Si tetrahedral vibrations. Interestingly, there were no discernible differences in the 0.83 Ca/Si C-S-H spectra when exposed to CO₂ in water, likely due to limited availability of Ca ions for participation in the reaction.

Conversely, significant differences are observed in the spectra of Ca/Si 1.5 C-S-H. The sharp peak related to Si-O vibrations of Q² is shifted to 945 cm^-1. This shift in the main peak position between 0.83 and 1.5 Ca/Si ratios is attributed to the presence of dimeric silicates (945 cm^-1), more common in high Ca/Si ratios, and oligomeric silicates (968 cm^-1), primarily found in low-calcium C-S-H (John; Matschei; Stephan, 2018). A peak at 810 cm^-1 assigned to Si-O-Si symmetric stretching vibrations of Q¹ is also present in Ca/Si 1.5 samples, as expected (Richardson, 2008; Zarzuela et al., 2020). At 3641 cm^-1, the appearance of CH species in the solution is observed, which begins to appear after the first hour in contact with water due to calcium dissolution. However, this behavior does not repeat in the 1.5 Ca/Si C-S-H exposed to water saturated with CO₂, likely because the calcium ions are being converted to CaCO₃ through the carbonation process.

Upon mixing water saturated with CO₂ with 1.5 Ca/Si C-S-H, peaks associated with carbonates appear at 879 cm^-1, 1045 cm^-1, and 1087 cm^-1, possibly from vaterite (Chakrabarty; Mahapatra, 1999; Possenti et al., 2021).

There is a noticeable increase in the 945 cm^-1 peak, indicating an increase in Q² vibrations (see Figure 7). This increase in Q² vibrations is observed during the initial stages of carbonation, when the C-S-H is getting more polymerized, as reported by (Chen; Thomas; Jennings, 2006; Li et al., 2020; Liang, 2022; Liu et al., 2022). There is also an increase in non-carbonated C-S-H samples followed by a decrease after 4h, probably due to water incorporation and/or evaporation.

Figure 7
Transmittance over time at 968 and 945 cm^-1 peaks for 0.83 and 1.5 C-S-H, respectively

Figure 8 presents the infrared spectra of the two analyzed C-S-H before and after carbonation in dried state. Before carbonation, characteristic peaks at 968 cm^-1 (in 0.83 C-S-H) and the 945cm^-1 (in 1.5 C-S-H) corresponding to Si-O stretching vibrations of Q² was found.

Figure 8
ATR-FTIR spectra of dried synthetic C-S-H (0.83 and 1.5) at 10 min an 20h of exposure to water saturated with CO₂

In the 0.83 C-S-H phase, a broad peak in the 1500-1400 cm^-1 ranges and a sharp peak at 875 cm^-1 (emerging after 20 hours of carbonation) were indicative of CaCO₃ polymorphs (also seen in TGA results). Following carbonation, a visible peak appeared at 1100 cm^-1, related to silica gel formation, consistent C-S-H decomposition by carbonation (Saeki et al., 2024). Additionally, a peak at 663 cm^-1 was observed, associated with Si-O-Si vibrations.

In the case of 1.5 C-S-H, the primary Q² peak initially appeared at 945 cm^-1 and subsequently shifted to 962 cm^-1 after carbonation, suggesting a decalcification of C-S-H, as also noted by (Zajac et al., 2020). Q₁ bands at 1007 cm^-1 and 794-805 cm^-1, unique to 1.5 C-S-H due to its higher calcium content, are also clearly visible. Following carbonation, a broad band in the 1600-1350 cm^-1 range, attributed to CaCO₃ polymorphs, became evident. Since no aragonite peaks were observed in XRD measurements, this band probably is from calcite and vaterite. The sharpened peak at 873 cm^-1 became more pronounced after carbonation and was associated with calcite and vaterite. In both 0.83 and 1.5 C-S-H the main peak related to Q² decreased its intensity after carbonation due to C-S-H decomposition.

Figure 9 presents the SEM images from the samples. The two types of C-S-H exhibit granular structures with a homogeneous distribution. The morphology of these samples is undefined. The synthesis of C-S-H resulted in particles mostly bellow 5 µm in size. In C-S-H 1.50, crystals of portlandite in the form of pseudo-hexagonal prismatic (Galan et al., 2015) are observed.

Figure 9
SEM images from 0.83 and 1.5 C-S-H before and after carbonation

Upon carbonation, changes in the atomic structure of C-S-H are evident, as observed in the diffractograms. Specifically, for C-S-H 1.50, possible vaterite/calcite crystals are observed in cube shape (Figure 9g) and flower-like shapes (Figure 9h) (Oral; Ercan, 2018), a consequence of the higher availability of calcium in the C-S-H 1.5 structure.

Conclusions

In this work, the time-dependent effect of mixing synthetic C-S-H with C/S of 0.83 and 1.5 with water or water saturated with CO₂ was measured by in-situ X-ray diffraction, ATR infrared spectroscopy and thermogravimetric analysis. Based on the results the following conclusions can be made:

1. CO₂ saturated mixed with water cause changes in C-S-H with Ca/Si ratios of 0.83 and 1.50 microstructure over-time. The effect depends on the calcium availability in the C-S-H system, which is governed by Ca/Si ratio;

2. despite portlandite availability in 1.5 C-S-H the CO₂ reaction reduced the content of C-S-H is more rapidly than Portlandite, probably because there is more Ca²⁺ available in the structure and in the interlayer;

3. carbonation acidifies 1.5 C-S-H changing its 945cm^-1 band to 962cm^-1, similar to 0.83C-S-H main band.;

4. the methods used in this work proved to be effective for monitoring the development of carbonation and wetting in C-S-H during the first 20h; and

5. these findings contribute to our understanding of the carbonation and wetting behavior of C-S-H and offer insights into its reactivity, crystal formation, and structural changes, which are critical aspects in the context of cementitious materials and their interactions with CO₂.

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