Influence of Water Activity on Belite (ß-C₂S) Hydration

The hydration of the two most reactive phases of ordinary Portland cement (OPC), tricalcium silicate (C3S), and tricalcium aluminate (C3A) is successfully halted when the activity of water (αH) falls below critical thresholds of 0.70 and 0.45, respectively. It has been established that the reduction in relative humidity (RH) and αH suppresses the hydration of all anhydrous phases in OPC, including less explored phases like dicalcium silicate, that is, belite (β-C2S). However, the degree of suppression, that is, the critical threshold, for β-C2S, standalone has yet to be established. This study utilizes isothermal microcalorimetry and X-ray diffraction techniques to elucidate the influence of αH on the hydration of β-C2S suspensions via incremental replacements of water with isopropanol (IPA). Experimentally, this study shows that with increasing IPA replacements, hydration is increasingly suppressed until eventually brought to a halt at a critical threshold of approximately 27.7% IPA on a weight basis (wt.%IPA). From thermodynamic estimations, the exact critical αH threshold and solubility product constant of β-C2S (KC2S) are established as 0.913 and 10-12.68, respectively. This study enables enhanced understanding of β-C2S reactivity and provides thermodynamic parameters during the hydration of β-C2S-containing cementitious systems such as OPC-based and calcium aluminate-based systems