Impact of calcium aluminate cement and calcium sulfate on gel C-A-S-H formation during the Portland cement hydration

2.1. Materials

The chemical compositions of the commercial CEM I 52.5 R PC and Fondu CAC, both supplied by Ciment Molins Industrial S.L., are presented in Table 1. The calcium sulfate used was a commercial hemihydrate provided by Algiss.

2.2. Composed studied

A binary cement was prepared with a PC/CAC ratio of 85/15, based on previous studies (2828. Torrens-Martín D, Fernández-Carrasco L. 2013. Effect of sulfate content on cement mixtures. Constr. Build. Mater. 48:144-150. https://doi.org/10.1016/j.conbuildmat.2013.05.106

29. Torrens-Martín D, Fernández-Carrasco L, Blanco-Varela MT. 2013. Conduction calorimetric studies of ternary binders based on Portland cement, calcium aluminate cement and calcium sulphate. J. Therm. Anal. Calorim. 114:799-807. https://doi.org/10.1007/s10973-013-3003-9–3030. Torrens-Martín D, Winnefeld F, Fernández-Carrasco L. 2021. Thermodynamic model for ternary OPC/CAC/Calcium Sulfate binders. Constr. Build. Mater. 302:124120. https://doi.org/10.1016/j.conbuildmat.2021.124120). This blend was evaluated without further additions, as well as with the incorporation of 3 wt% and 5 wt% of calcium sulfate. Two curing conditions were applied: curing in air at 20 °C and 95 % relative humidity, and curing under distilled water at 20 °C, both maintained until the corresponding testing age. A water-to-binder ratio of 0.4, as recommended by the Spanish and European standard UNE-EN 14647 for CAC-based cements, was used for both binary and ternary systems.

Hydration was halted by immersing the crushed paste in acetone, followed by filtration and rinsing with ethanol (3333. Zhang J, Scherer GW. 2011. Comparison of methods for arresting hydration of cement. Cem. Concr. Res. 41(10):1024-1036. https://doi.org/10.1016/j.cemconres.2011.06.003). The samples were then dried for 24 hours in a desiccator over silica gel. PC, CAC, and calcium sulfate were characterized by both X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) to determine their mineralogical and chemical compositions, respectively. The corresponding results are presented in Figure 1.

The majority minerals in PC were silicate phases alite and belite, aluminate C₃A and ferritic phases, cement plant-added calcium sulfate in the form of hemihydrate. The most prominent phase in the CAC was CA, followed by C₂AS and C₄AF and C₃FT as a minority phase; i.e., the standard composition for CAC (3434. Guirado F, Galí S. 2006. Quantitative Rietveld analysis of CAC clinker phases using synchrotron radiation. Cem. Concr. Res. 36(11):2021-2032. https://doi.org/10.1016/j.cemconres.2006.05.019). The diffractogram for calcium sulfate exhibited primarily hemihydrate, although traces of a calcium sulfate dihydrate were also observed.

In the infrared spectrum of anhydrous PC, the most prominent features arise from the vibrational modes of silicate and aluminate phases. The main band typically appears between 930 and 950 cm^–1 and corresponds to the asymmetric stretching vibrations of Si–O bonds in anhydrous silicate phases. In the spectrum of anhydrous CAC, the most intense band is observed between 850 and 880 cm^–1, attributed to the asymmetric stretching vibrations of Al–O bonds in monocalcium aluminate. Additional bands are detected in the 500–600 cm^–1 region, corresponding to Al–O bending modes. A shoulder near 950–970 cm^–1 may also appear, associated with Si–O stretching in gehlenite.

The infrared spectrum of calcium sulfate exhibits a strong band centered between 1110 and 1140 cm^–1, corresponding to the asymmetric stretching vibration of the sulfate group. A second, medium-intensity band appears around 600–670 cm^–1, attributed to SO₄^2– bending modes. Furthermore, two bands around 3400 cm^–1 and a single band at 1640 cm^–1, associated with O–H stretching and H–O–H bending vibrations, respectively, confirm that the calcium sulfate is in the hemihydrate form.

2.3. Methods

XRD analysis was performed using a PANalytical X’Pert PRO MPD DY 3197 diffractometer equipped with a secondary graphite monochromator (CuKα₁₂, flat sample). The instrument operated at 40 kV and 50 mA, with a step size of 0.02° and a counting time of 1 s per step, over a 2θ range of 5° to 60°. Semi-quantitative phase identification was carried out based on the intensity of the diffraction peaks.

Fourier-transform infrared spectroscopy (FTIR) was carried out using a Bomem ABB FTLA spectrometer equipped with a DTGS detector. Transmission spectra were recorded in the mid-infrared region (4000–350 cm^–1) with a spectral resolution of 4 cm^–1. For each sample, 32 scans were collected. Pellets were prepared by thoroughly mixing 1 mg of finely ground sample with 300 mg of potassium bromide. This analysis was used to identify the main functional groups and vibrational bands associated with both crystalline and amorphous components present in the raw materials.

Nuclear Magnetic Resonance (NMR) experiments were performed on a Bruker Avance 400 NMR spectrometer. The ²⁹Si spectra were acquired at a magnetic field strength of 9.4 T using an oscillating field of 4.5 kHz, with 8.8 μs pulses and a relaxation delay of 20 s.

To analyze the spectral complexity, a deconvolution procedure was applied to resolve overlapping bands and identify the main structural contributions. This mathematical treatment fits the composite absorption envelope using bell-shaped curves of predefined profiles (Gaussian, Lorentzian, logarithmic, or mixed). Initial parameters, such as the number of peaks, their positions, full width at half maximum (FWHM), relative intensities, and profile type, were defined based on the selected fitting function. These parameters were estimated through a preliminary analysis involving the calculation of the second derivative of the spectrum, followed by deconvolution according to the method proposed by Griffiths and Pariente (3535. Griffiths PR, Pariente GL. 1986. Introduction to spectral deconvolution. Anal. Chem. 5(8):209-215. https://doi.org/10.1016/0165-9936(86)80015-2). The second derivative allows the identification of inflection points and precise peak positions, which are subsequently refined through the deconvolution process. This approach enables the separation of overlapping vibrational signals, improving the interpretation of the structural evolution of the hydration gel.

3.1. C-S-H development in PC

To establish a comparison baseline among the different systems, PC paste was analyzed by FTIR at 6 hours, 7 days, 12 and 32 months. Figure 2 displays the infrared spectra obtained under both curing conditions. The general spectral shape is similar in both environments, with the exception of a sharp band at 873 cm^–1 observed in the air-cured samples, attributed to the bending vibration mode ν₄ of carbonate groups (CO₃^2–) (3636. Fernández-Carrasco L, Torrens-Martín D, Morales LM, Martínez-Ramírez S. 2012. Infrared spectroscopy in the analysis of building and construction materials. InTech. 357-372. https://doi.org/10.5772/36186). At 6 hours, the main band is centered at 930 cm^–1, corresponding primarily to the stretching vibrations of anhydrous silicate phases (3636. Fernández-Carrasco L, Torrens-Martín D, Morales LM, Martínez-Ramírez S. 2012. Infrared spectroscopy in the analysis of building and construction materials. InTech. 357-372. https://doi.org/10.5772/36186). As hydration progresses, this band diminishes in intensity. By 12 months, the main band shifts to 974 cm^–1, a position commonly associated with polymerized Q² units in the C–S–H gel (3737. Yu P, Kirkpatrick RJ, Poe B, McMillan PF, Cong X. 1999. Structure of calcium silicate hydrate (C-S-H): Near-, Mid-, and Far-infrared spectroscopy. J. Am. Chem. Soc. 82(3):742-748. https://doi.org/10.1111/j.1151-2916.1999.tb01826.x). At 32 months, a shoulder appears at 908 cm^–1, while the Q² band shifts to slightly lower wavenumbers. The 908 cm^–1 shoulder is typically assigned to Si–O stretching vibrations linked to the progressive polymerization of the C–S–H structure (3737. Yu P, Kirkpatrick RJ, Poe B, McMillan PF, Cong X. 1999. Structure of calcium silicate hydrate (C-S-H): Near-, Mid-, and Far-infrared spectroscopy. J. Am. Chem. Soc. 82(3):742-748. https://doi.org/10.1111/j.1151-2916.1999.tb01826.x

38. Bonaccorsi E, Merlino S, Taylor HFW. 2004. The crystal structure of jennite, Ca₉Si₆O₁₈(OH)₆·8H₂O. Cem. Concr. Res. 34(9):1481-1488. https://doi.org/10.1016/j.cemconres.2003.12.033

39. Kirkpatrick RJ, Yarger JL, McMillan PF, Ping Y, Cong X. 1997. Raman spectroscopy of CSH, tobermorite, and jennite. Adv. Cem. Based Mater. 5(3-4):93-99. https://doi.org/10.1016/S1065-7355(97)00001-1

40. Masse S, Zanni H, Lecourtier J, Roussel JC, Rivereau A. 1993. ²⁹Si solid state NMR study of tricalcium silicate and cement hydration at high temperature. Cem. Concr. Res. 23(5):1169-1177. https://doi.org/10.1016/0008-8846(93)90177-B

41. García-Lodeiro I, Fernández-Jiménez A, Sobrados I, Sanz J, Palomo A. 2012. C–S–H Gels: Interpretation of ²⁹Si MAS-NMR Spectra. J. Am. Chem. Soc. 95(4):1440-1446. https://doi.org/10.1111/j.1551-2916.2012.05091.x–4242. Sáez del Bosque IF, Martínez-Ramírez S, Blanco-Varela MT. (2015). Calorimetric study of the early stages of the nanosilica - tricalcium silicate hydration. Effect of temperature. Mater. Construcc. 65(320):e070. https://doi.org/10.3989/mc.2015.06814 ). The shoulders are more clearly defined in the samples cured under water, suggesting that polymerization progresses more rapidly in this environment. Additionally, Q¹ units centered at 820 cm^–1 become detectable after 7 days, with higher intensity in the water-cured samples. A weak signal above 1070 cm^–1, characteristic of Si–O stretching in highly condensed Q³ silicates (3939. Kirkpatrick RJ, Yarger JL, McMillan PF, Ping Y, Cong X. 1997. Raman spectroscopy of CSH, tobermorite, and jennite. Adv. Cem. Based Mater. 5(3-4):93-99. https://doi.org/10.1016/S1065-7355(97)00001-1), was ruled out, as previous ²⁹Si MAS NMR studies confirmed that C–S–H gels polymerize linearly and consist predominantly of Q¹ and Q² units (4040. Masse S, Zanni H, Lecourtier J, Roussel JC, Rivereau A. 1993. ²⁹Si solid state NMR study of tricalcium silicate and cement hydration at high temperature. Cem. Concr. Res. 23(5):1169-1177. https://doi.org/10.1016/0008-8846(93)90177-B–4141. García-Lodeiro I, Fernández-Jiménez A, Sobrados I, Sanz J, Palomo A. 2012. C–S–H Gels: Interpretation of ²⁹Si MAS-NMR Spectra. J. Am. Chem. Soc. 95(4):1440-1446. https://doi.org/10.1111/j.1551-2916.2012.05091.x).

Figure 2 also includes a deconvolution analysis of the FTIR spectra from samples aged 32 months, aimed at evaluating the long-term evolution of the C–S–H gel in the PC system. The assignment of the deconvoluted bands was based on both spectral interpretation and literature references. The second derivative of the spectrum was used as a diagnostic tool to identify inflection points, providing evidence for the presence of distinct vibrational components. Based on this approach, three main groups of bands were distinguished.

The first group, located around 1030 cm^–1, is attributed to Q²(1Al) units, indicating the partial substitution of Si by Al within the silicate chains (1111. Barzgar S, Tarik M, Ludwig C, Lothenbach B. 2021. The effect of equilibration time on Al uptake in CSH. Cem. Concr. Res. 144:106438.https://doi.org/10.1016/j.cemconres.2021.106438, 4343. Kapeluszna E, Kotwica Ł, Różycka A, Gołek Ł. 2017. Incorporation of Al in CASH gels with various Ca/Si and Al/Si ratio: Microstructural and structural characteristics with DTA/TG, XRD, FTIR and TEM analysis. Constr. Build. Mater. 155:643-653. https://doi.org/10.1016/j.conbuildmat.2017.08.091–4444. Adamczyk A, Długoń E. 2012. The FTIR studies of gels and thin films of Al₂O₃–TiO₂ and Al₂O₃–TiO₂–SiO₂ systems. Spectrochim. Acta A Mol. Biomol. Spectrosc. 89:11-17. https://doi.org/10.1016/j.saa.2011.12.018). The second group, centered near 970 cm^–1, corresponds to Q² units, characteristic of the silicate network typically found in C–S–H gels (3737. Yu P, Kirkpatrick RJ, Poe B, McMillan PF, Cong X. 1999. Structure of calcium silicate hydrate (C-S-H): Near-, Mid-, and Far-infrared spectroscopy. J. Am. Chem. Soc. 82(3):742-748. https://doi.org/10.1111/j.1151-2916.1999.tb01826.x). Finally, a third group appears at approximately 908 cm^–1, associated with Si–O stretching vibrations and generally considered indicative of the degree of polymerization of the gel structure (3737. Yu P, Kirkpatrick RJ, Poe B, McMillan PF, Cong X. 1999. Structure of calcium silicate hydrate (C-S-H): Near-, Mid-, and Far-infrared spectroscopy. J. Am. Chem. Soc. 82(3):742-748. https://doi.org/10.1111/j.1151-2916.1999.tb01826.x

38. Bonaccorsi E, Merlino S, Taylor HFW. 2004. The crystal structure of jennite, Ca₉Si₆O₁₈(OH)₆·8H₂O. Cem. Concr. Res. 34(9):1481-1488. https://doi.org/10.1016/j.cemconres.2003.12.033

39. Kirkpatrick RJ, Yarger JL, McMillan PF, Ping Y, Cong X. 1997. Raman spectroscopy of CSH, tobermorite, and jennite. Adv. Cem. Based Mater. 5(3-4):93-99. https://doi.org/10.1016/S1065-7355(97)00001-1

40. Masse S, Zanni H, Lecourtier J, Roussel JC, Rivereau A. 1993. ²⁹Si solid state NMR study of tricalcium silicate and cement hydration at high temperature. Cem. Concr. Res. 23(5):1169-1177. https://doi.org/10.1016/0008-8846(93)90177-B

41. García-Lodeiro I, Fernández-Jiménez A, Sobrados I, Sanz J, Palomo A. 2012. C–S–H Gels: Interpretation of ²⁹Si MAS-NMR Spectra. J. Am. Chem. Soc. 95(4):1440-1446. https://doi.org/10.1111/j.1551-2916.2012.05091.x

–4242. Sáez del Bosque IF, Martínez-Ramírez S, Blanco-Varela MT. (2015). Calorimetric study of the early stages of the nanosilica - tricalcium silicate hydration. Effect of temperature. Mater. Construcc. 65(320):e070. https://doi.org/10.3989/mc.2015.06814 ).

The deconvoluted bands reveal a typical C–S–H gel structure resulting from PC hydration. The dominant contribution corresponds to Q² units, which account for nearly 90% of the total band area. In contrast, the presence of Q²(1Al) units is limited, representing approximately 10%, with slightly higher proportions observed in the samples cured under water. This behavior is attributed to the partial dissolution of aluminates, primarily present as C₃A, and their limited incorporation into the C–S–H structure (3939. Kirkpatrick RJ, Yarger JL, McMillan PF, Ping Y, Cong X. 1997. Raman spectroscopy of CSH, tobermorite, and jennite. Adv. Cem. Based Mater. 5(3-4):93-99. https://doi.org/10.1016/S1065-7355(97)00001-1).

3.2. Incorporation of Al in C-S-H formation

To evaluate the extent of aluminum incorporation into the C–S–H gel, a binder composed of 85/15 PC/CAC was analyzed by FTIR at 6 hours, 7 days, 12 months, and 32 months under two curing conditions (Figure 3). As observed in PC, the spectra exhibited similar overall profiles in both environments, except for a sharp band at 873 cm^–1 in the air-cured samples, attributed to the ν₄ bending vibration of CO₃^2– groups (3636. Fernández-Carrasco L, Torrens-Martín D, Morales LM, Martínez-Ramírez S. 2012. Infrared spectroscopy in the analysis of building and construction materials. InTech. 357-372. https://doi.org/10.5772/36186). At 6 hours, the signal at 930 cm^–1, assigned to ν₃(Si–O) stretching in anhydrous silicates, is more intense than in the PC samples. This increase in the anhydrous silicate band is attributed to the delayed hydration of silicates in the presence of aluminates, as reported in previous studies (2727. Torrens-Martí D. 2013. Mezclas ternarias de cemento Portland, cemento de aluminato de calcio y sulfato cálcico: mecanismos de expansión. [PhD thesis]. Universitat Politècnica de Catalunya. Barcelona. https://doi.org/10.5821/dissertation-2117-95267, 2929. Torrens-Martín D, Fernández-Carrasco L, Blanco-Varela MT. 2013. Conduction calorimetric studies of ternary binders based on Portland cement, calcium aluminate cement and calcium sulphate. J. Therm. Anal. Calorim. 114:799-807. https://doi.org/10.1007/s10973-013-3003-9, 4545. Gu P, Beaudoin JJ. 1997. A conduction calorimetric study of early hydration of ordinary Portland cement/high alumina cement pastes. J. Mater. Sci. 32:3875-3881. https://doi.org/10.1023/A:1018600412638). This interpretation is further supported by the presence of a weak shoulder at 970 cm^–1, corresponding to Q² units associated with early-stage gel formation. In the 85/15 samples, this Q² shoulder is barely detectable at 6 hours, whereas in the PC system, a more pronounced band is observed due to the more advanced state of hydration.

As hydration progressed, the band attributed to Q² units gradually shifted toward 977 cm^–1, adopting a sharper profile than that observed in PC. A sharper band typically reflects fewer overlapping contributions (4646. Nakamoto K. 2008. Infrared and Raman Spectra of Inorganic and Coordination Compounds: Theory and applications in inorganic chemistry. Wiley. New York.), suggesting the formation of a more homogeneous gel. The shoulder at 908 cm^–1, assigned to Si–O stretching vibrations, becomes visible at 12 months, earlier than in PC, indicating a more advanced degree of polymerization in the PC/CAC system. At longer curing times, a shoulder appears at 1030 cm^–1, which is characteristic of aluminosilicate structures. This band is attributed to the asymmetric and symmetric stretching vibrations of Si–O–Si and Si–O–Al bonds in (SiO₄)^4- and (AlO₄)^5- (1111. Barzgar S, Tarik M, Ludwig C, Lothenbach B. 2021. The effect of equilibration time on Al uptake in CSH. Cem. Concr. Res. 144:106438.https://doi.org/10.1016/j.cemconres.2021.106438, 4343. Kapeluszna E, Kotwica Ł, Różycka A, Gołek Ł. 2017. Incorporation of Al in CASH gels with various Ca/Si and Al/Si ratio: Microstructural and structural characteristics with DTA/TG, XRD, FTIR and TEM analysis. Constr. Build. Mater. 155:643-653. https://doi.org/10.1016/j.conbuildmat.2017.08.091–4444. Adamczyk A, Długoń E. 2012. The FTIR studies of gels and thin films of Al₂O₃–TiO₂ and Al₂O₃–TiO₂–SiO₂ systems. Spectrochim. Acta A Mol. Biomol. Spectrosc. 89:11-17. https://doi.org/10.1016/j.saa.2011.12.018), and it is detected earlier in water-cured samples. These observations confirm the formation of a C–A–S–H-type gel in PC/CAC binders. During hydration, aluminum from CAC is incorporated into the gel network, forming Q²(1Al) units, in which Al occupies a bridging position between one SiO₄ and one AlO₄ tetrahedron. Further confirmation of C–A–S–H formation is provided by the evolution of the Q¹ band, which shifts toward 810 cm^–1. In the PC/CAC spectra, the intensity of Q¹ units is lower than in PC, reflecting the development of longer silicate chains. Andersen et al. (66. Andersen MD, Jakobsen HJ, Skibsted J. 2003. Incorporation of aluminum in the calcium silicate hydrate (C− S− H) of hydrated Portland cements: A high-field ²⁷Al and ²⁹Si MAS NMR investigation. Inorg. Chem. 42(7):2280-2287. https://doi.org/10.1021/ic020607b, 4747. Andersen MD, Jakobsen HJ, Skibsted J. 2004. Characterization of white Portland cement hydration and the CSH structure in the presence of sodium aluminate by ²⁷Al and ²⁹Si MAS NMR spectroscopy. Cem. Concr. Res. 34(5):857-868. https://doi.org/10.1016/j.cemconres.2003.10.009) reported that Al for Si substitution in the gel promotes chain elongation. Therefore, the presence of CAC in PC hydration leads to Al incorporation, resulting in a more polymerized C–A–S–H gel structure with longer silicate chains.

The deconvolution analysis presented in Figure 3 focuses on three groups of bands: Group 1 includes signals attributed to Q²(1Al) units; Group 2 comprises the bands corresponding to Q² units; and Group 3 refers to a smaller band around 908 cm^–1, associated with Si–O stretching vibrations related to C–S–H polymerization. In both curing environments, a significant proportion of Group 1 is observed, indicating substantial incorporation of aluminum into the gel, consistent with the formation of C–A–S–H during hydration. This incorporation is more pronounced in the samples cured under water, which also exhibit a higher intensity for Group 3. These results suggest a more advanced degree of silicate chain polymerization under water-curing conditions. The results obtained for the PC system exhibit a significantly broader Q² band and a lower contribution of Q²(1Al) units compared to the PC/CAC system. This suggests a lower degree of polymerization and minimal aluminum incorporation into the C-S-H gel in the absence of CAC. These findings further confirm the role of CAC in promoting the formation of a more homogeneous and aluminum-rich C-A-S-H structure.

3.3. Effect of calcium sulfate in C-A-S-H formation

After confirming the formation of C–A–S–H gel in the PC/CAC binders, the influence of calcium sulfate on this process was evaluated. For this purpose, calcium sulfate was incorporated at 3 wt% and 5 wt% into the 85/15 PC/CAC blend. Figure 4 shows the FTIR spectra obtained at various curing ages under both environments. The initial effect of calcium sulfate addition is evident as early as 6 hours, where the signals associated with Q² units are more intense compared to the PC/CAC system. By 7 days, the main band also becomes broader, suggesting an acceleration in silicate hydration relative to the binary system. This observation is consistent with previous studies on PC, which report that calcium sulfate enhances the degree of silicate reaction by modifying the hydration kinetics (2727. Torrens-Martí D. 2013. Mezclas ternarias de cemento Portland, cemento de aluminato de calcio y sulfato cálcico: mecanismos de expansión. [PhD thesis]. Universitat Politècnica de Catalunya. Barcelona. https://doi.org/10.5821/dissertation-2117-95267, 2929. Torrens-Martín D, Fernández-Carrasco L, Blanco-Varela MT. 2013. Conduction calorimetric studies of ternary binders based on Portland cement, calcium aluminate cement and calcium sulphate. J. Therm. Anal. Calorim. 114:799-807. https://doi.org/10.1007/s10973-013-3003-9-3030. Torrens-Martín D, Winnefeld F, Fernández-Carrasco L. 2021. Thermodynamic model for ternary OPC/CAC/Calcium Sulfate binders. Constr. Build. Mater. 302:124120. https://doi.org/10.1016/j.conbuildmat.2021.124120, 4848. Zunino F, Scrivener K. 2022. The influence of sulfate addition on hydration kinetics and CSH morphology of C₃S and C₃S/C₃A systems. Cem. Concr. Res. 160:106930. https://doi.org/10.1016/j.cemconres.2022.106930

49. Quennoz A, Scrivener K. 2013. Interactions between alite and C3A-gypsum hydrations in model cements. Cem. Concr. Res. 44:46-54. https://doi.org/10.1016/j.cemconres.2012.10.018

50. Bergold ST, Goetz-Neunhoeffer F, Neubauer J. 2017. Interaction of silicate and aluminate reaction in a synthetic cement system: Implications for the process of alite hydration. Cem. Concr. Res. 93:32-44. https://doi.org/10.1016/j.cemconres.2016.12.006–5151. Yang SY, Yan Y, Lothenbach B, Skibsted J. 2021. Incorporation of sodium and aluminum in cementitious calcium-alumino-silicate-hydrate C-(A)-SH phases studied by ²³Na, ²⁷Al, and ²⁹Si MAS NMR spectroscopy. J. Phys. Chem. C. 125(51):27975-27995. https://doi.org/10.1021/acs.jpcc.1c08419). Moreover, prior research has demonstrated that the incorporation of calcium sulfate into PC/CAC systems promotes silicate hydration in a manner similar to that observed in plain PC (2727. Torrens-Martí D. 2013. Mezclas ternarias de cemento Portland, cemento de aluminato de calcio y sulfato cálcico: mecanismos de expansión. [PhD thesis]. Universitat Politècnica de Catalunya. Barcelona. https://doi.org/10.5821/dissertation-2117-95267

28. Torrens-Martín D, Fernández-Carrasco L. 2013. Effect of sulfate content on cement mixtures. Constr. Build. Mater. 48:144-150. https://doi.org/10.1016/j.conbuildmat.2013.05.106

29. Torrens-Martín D, Fernández-Carrasco L, Blanco-Varela MT. 2013. Conduction calorimetric studies of ternary binders based on Portland cement, calcium aluminate cement and calcium sulphate. J. Therm. Anal. Calorim. 114:799-807. https://doi.org/10.1007/s10973-013-3003-9–3030. Torrens-Martín D, Winnefeld F, Fernández-Carrasco L. 2021. Thermodynamic model for ternary OPC/CAC/Calcium Sulfate binders. Constr. Build. Mater. 302:124120. https://doi.org/10.1016/j.conbuildmat.2021.124120), a trend that aligns with the results obtained in this study.

In samples subjected to prolonged hydration, the main band attributed to Q² units appears at 970 cm^–1. Although this position is similar to that observed in PC, the band exhibits a sharper profile, suggesting fewer overlapping contributions and, consequently, a more homogeneous gel. The Q¹ band is detected at similar ages to those in the PC/CAC system, indicating a comparable progression of polymerization.A shoulder at 1030 cm^–1, indicative of aluminosilicate structures, is already present at early hydration stages, confirming the formation of a C–A–S–H gel. The signal corresponding to Q¹ units at 810 cm^–1 is weak and comparable in intensity to that observed in the PC/CAC system, where the formation of longer silicate chains had previously been identified.

To assess the structure of the C–A–S–H gel formed in the ternary system, a deconvolution analysis of the main FTIR band, located between 1090 and 890 cm^–1, was performed on the samples aged 32 months (Figure 4). As in the previous case, the band was deconvoluted into the same three groups: Group 1 (Q²(1Al)), Group 2 (Q²), and Group 3 (908 cm^–1, associated with Si–O stretching vibrations related to the degree of polymerization). The presence of calcium sulfate prevents the delay in silicate hydration observed in the binary system, thereby promoting the formation of a greater quantity of Q² units in the resulting C–A–S–H gel. This effect is reflected in the higher contribution of Group 2, centered at 970 cm^–1, which exhibits a larger area than Group 1. Nonetheless, aluminum incorporation remains significant, with Group 1, corresponding to Q²(1Al) units, representing approximately 30% of the total band area across all samples analyzed. Compared to the PC/CAC system, the relative contribution of Group 1 is lower, due to the enhanced silicate hydration promoted by calcium sulfate, which favors the formation of Q² units. The band distribution was similar in both curing environments. Group 3, associated with the degree of polymerization, showed comparable intensity across all samples, indicating a polymerization process analogous to that observed in air-cured PC/CAC systems previously studied.

To investigate the evolution of Qⁿ units during gel development, ²⁹Si NMR spectra were recorded for air-cured samples at 7 and 28 days. These time points were selected to capture the early and mid-term stages of C–A–S–H gel formation, during which the most significant changes in silicate polymerization and aluminum incorporation typically occur. Previous studies have shown that the structural parameters of the gel tend to stabilize beyond 28 days, making this time frame representative for assessing trends relevant to mechanical performance and long-term durability (66. Andersen MD, Jakobsen HJ, Skibsted J. 2003. Incorporation of aluminum in the calcium silicate hydrate (C− S− H) of hydrated Portland cements: A high-field ²⁷Al and ²⁹Si MAS NMR investigation. Inorg. Chem. 42(7):2280-2287. https://doi.org/10.1021/ic020607b, 1616. L’Hôpital E, Lothenbach B, Le Saout G, Kulik D, Scrivener K. 2015. Incorporation of aluminium in calcium-silicate-hydrates. Cem. Concr. Res. 75:91–103. https://doi.org/10.1016/j.cemconres.2015.04.007, 5252. Torres-Carrasco M, Palomo JG, Puertas F. 2014. Sodium silicate solutions from dissolution of glasswastes. Statistical analysis. Mater. Construcc. 64(314):e014. https://doi.org/10.3989/mc.2014.05213 ).

The study focused on the chemical shift range of –75 to –90 ppm, which corresponds to the region where Qⁿ unit signals appear as a result of C–A–S–H gel polymerization. The spectra presented in Figure 5 are characterized by two main peaks, attributed to the overlapping resonances of distinct Si sites within the gel. The deconvolution procedure was based on a simplified version of the defect tobermorite model (1616. L’Hôpital E, Lothenbach B, Le Saout G, Kulik D, Scrivener K. 2015. Incorporation of aluminium in calcium-silicate-hydrates. Cem. Concr. Res. 75:91–103. https://doi.org/10.1016/j.cemconres.2015.04.007, 2020. Kunhi A, Moutzouri P, Berruyer P, Walder BJ, Siramanont J, Siramanont J, Harris M, Negroni M, Galmarini SC, Parker SC, Scrivener K, Emsley L, Bowen P. 2020. The atomic-level structure of cementitious calcium aluminate silicate hydrate. J. Am. Chem. Soc. 142(25):11060–11071. https://pubs.acs.org/doi/10.1021/jacs.0c02988, 5353. Neto FM, Snellings R, Skibsted J. 2024. Aqueous carbonation of aged blended Portland cement pastes: Impact of the Al/Si ratio on the structure of the alumina-silica gel. Cem. Concr. Res.177:107428. https://doi.org/10.1016/j.cemconres.2024.107428-5454. Richardson IG, Brough AR, Groves GW, Dobson CM. 1994. The characterization of hardened alkali-activated blast-furnace slag pastes and the nature of the calcium silicate hydrate (CSH) phase. Cem. Concr. Res. 24(5):813-829. https://doi.org/10.1016/0008-8846(94)90002-7), in which three distinct sites were considered: Q¹, Q²(1Al), and Q² (this position included ${\text{Q}}_{\text{b}}^2$ and ${\text{Q}}_{\text{p}}^2$ units). These environments correspond to resonances centered approximately at –79.4, –81.4, and –84 ppm, respectively. Table 2 summarizes the deconvolution results obtained for the selected samples. The variation in peak intensities reflects differences in the degree of chain polymerization (Equation [1]) and in the Al(IV)/Si atomic ratio (Equation [2]) (5353. Neto FM, Snellings R, Skibsted J. 2024. Aqueous carbonation of aged blended Portland cement pastes: Impact of the Al/Si ratio on the structure of the alumina-silica gel. Cem. Concr. Res.177:107428. https://doi.org/10.1016/j.cemconres.2024.107428

54. Richardson IG, Brough AR, Groves GW, Dobson CM. 1994. The characterization of hardened alkali-activated blast-furnace slag pastes and the nature of the calcium silicate hydrate (CSH) phase. Cem. Concr. Res. 24(5):813-829. https://doi.org/10.1016/0008-8846(94)90002-7

55. Richardson IG. 2014. Model structures for C-(A)-S-H(I). Acta Cryst. B. 70(6):903-923. https://doi.org/10.1107/S2052520614021982–5656. Goñi S, Guerrero A, Puertas F, Hernández MS, Palacios M, Dolado JS, Zhu W, Howind T. 2011. Textural and mechanical characterization of C-S-H gels from hydration of synthetic T1-C₃S, β-C₂S and their blends. Mater. Construcc. 61(302):169–183. https://doi.org/10.3989/mc.2011.00511).

At early hydration stages, Q¹ units dominate the ²⁹Si NMR spectra, representing more than 50% of the total signal in all samples, except for the 5% calcium sulfate blend at 28 days, where their intensity decreases markedly. This reduction may indicate a loss of Ca from the C–A–S–H structure due to the high incorporation of Al, which also corresponds with an increase in silicate chain length. This is consistent with the known inverse relationship between mean chain length and the Ca/Si ratio in C–A–S–H phases (5555. Richardson IG. 2014. Model structures for C-(A)-S-H(I). Acta Cryst. B. 70(6):903-923. https://doi.org/10.1107/S2052520614021982, 5757. Skibsted J, Hjorth J, Jakobsen HJ. 1990. Correlation between ²⁹Si NMR chemical shifts and mean Si O bond lengths for calcium silicates. Chem. Phys. Lett. 172(3-4):279-283. https://doi.org/10.1016/0009-2614(90)85403-Y). The structural change is further confirmed by the high Al(IV)/Si ratio obtained for this sample. The intensity of Q² units is higher at 7 days than at 28 days, suggesting a significant early contribution to gel polymerization. This behavior can be attributed to the elimination of silicate hydration inhibition caused by the presence of calcium sulfate, which enhances the kinetics of early gel formation. The development of a C–A–S–H-type gel is also evidenced by the evolution of Q²(1Al) units. After seven days, these units account for less than 20% of the total signal, but their intensity increases substantially at 28 days. This finding suggests that aluminum is progressively incorporated into the gel by occupying tetrahedral bridging positions within the silicate chains. Moreover, this incorporation is more pronounced in samples with higher calcium sulfate content, leading to longer silicate chains and increased Al(IV)/Si ratios.

The outcomes of this investigation carry important implications for the design of blended cementitious materials, particularly in terms of durability, mechanical performance, and sustainability. Incorporating CAC into blended systems enhances the formation of a highly polymerized and homogeneous C–A–S–H gel, with longer silicate chains and elevated Al(IV)/Si ratios, especially when calcium sulfate is added to moderate hydration kinetics. Such a nanostructure is expected to reduce porosity and permeability and boost resistance to chemical degradation (e.g., chloride immobilization and sulfate or acid attack), consistent with findings that hydrated CAC improves chloride binding capacity in Portland cement pastes (5858. Lü Z, Tan H, Liu X, Chen P, Wang Y, Liang W. 2023. Effect of hydrated calcium aluminate cement on the chloride immobilization of Portland cement paste. J. Wuhan Univ. Technol. Mater. Sci. 38(6):1360-1371. https://doi.org/10.1007/s11595-023-2830-1). Studies of fibre-reinforced CAC–slag mortars also demonstrate remarkable durability against sulfuric acid exposure, reflecting the acid-resistant gel phases that form in these systems (5959. Fan W, Zhuge Y, Ma X, Chow CW, Gorjian N, Oh JA, Duan W. 2020. Durability of fibre-reinforced calcium aluminate cement (CAC)–ground granulated blast furnace slag (GGBFS) blended mortar after sulfuric acid attack. Materials, 13(17):3822. https://doi.org/10.3390/ma13173822). Moreover, CAC exhibits rapid early-age strength development, often reaching high strength within 24 h, which supports its use in time-sensitive construction or rapid repair applications (6060. Pöllmann H. 2012. Calcium aluminate cements–raw materials, differences, hydration and properties. Rev. Mineral. Geochem. 74(1):1-82. https://doi.org/10.2138/rmg.2012.74.1). The synergistic role of calcium sulfate in blended formulations not only accelerates hydration but enables fine-tuning of ettringite and AFm phase formation, balancing early strength gain with long-term stability and avoiding delayed ettringite formation at optimized doses (6161. Neto JDSA, Angeles G, Kirchheim AP. 2021. Effects of sulfates on the hydration of Portland cement–A review. Constr. Build. Mater. 279:122428. https://doi.org/10.1016/j.conbuildmat.2021.122428). These results contribute to the broader effort of designing alternative cementitious systems with improved performance and reduced environmental impact. Similar to recent studies on alkali-activated materials incorporating recycled aggregates (6262. Alonso MM, Rodríguez A, Puertas F. 2018. Viability of the use of construction and demolition waste aggregates in alkali-activated mortars. Mater. Construcc. 68(331):e164. https://doi.org/10.3989/mc.2018.07417), the development of C-A-S-H-rich binders through hybrid formulations offers a promising pathway toward more sustainable construction materials.