Water content and distribution in C-(A)-S-H as a function of Ca/Si ratio and relative humidity

Zhanar Zhakiyeva, Alexander E. S. Van Driessche, Saule Mergenbayeva, Nuray Abdullayeva, Aliya Abzal, Valérie Magnin, Gabriel Julio Cuello, Chang-Seon Shon, Alejandro Fernández-Martı́nez

2026

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DOI 10.26434/chemrxiv.15004685/v1
Data confidence 85 high
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Abstract

Calcium-(aluminum)-silicate-hydrate (C-(A)-S-H) phases are the primary binding components in cementitious materials, and their structure and water content depend on both composition and environmental conditions. This study examines the combined influence of Ca/Si ratio and relative humidity on the water content and structural evolution of synthetic C-(A)-S-H. First, the suitable curing and drying conditions of C-(A)-S-H were established. X-ray diffraction indicated that structural stabilization is achieved after extended curing, while low-temperature vacuum drying preserves the layered structure. These conditions were subsequently applied to all synthesized samples. XRD results show a systematic decrease in basal spacing with increasing Ca/Si ratio, accompanied by a significant reduction in specific surface area derived from water sorption measurements. Thermogravimetric and adsorption analyses reveal that the H₂O/Si ratio increases with both Ca/Si ratio and relative humidity, with capillary condensation becoming dominant at very high humidity levels. Overall, these results improve our understanding of how composition and humidity govern water retention and structural features in C-(A)-S-H, providing insights relevant for the design of more durable cementitious materials. Calcium-(aluminum)-silicate-hydrate (C-(A)-S-H) phases are the primary binding components in cementitious materials, and their structure and water content depend on both composition and environmental conditions. This study examines the combined influence of Ca/Si ratio and relative humidity on the water content and structural evolution of synthetic C-(A)-S-H. First, the suitable curing and drying conditions of C-(A)-S-H were established. X-ray diffraction indicated that structural stabilization is achieved after extended curing, while low-temperature vacuum drying preserves the layered structure. These conditions were subsequently applied to all synthesized samples. XRD results show a systematic decrease in basal spacing with increasing Ca/Si ratio, accompanied by a significant reduction in specific surface area derived from water sorption measurements. Thermogravimetric and adsorption analyses reveal that the H₂O/Si ratio increases with both Ca/Si ratio and relative humidity, with capillary condensation becoming dominant at very high humidity levels. Overall, these results improve our understanding of how composition and humidity govern water retention and structural features in C-(A)-S-H, providing insights relevant for the design of more durable cementitious materials.