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

The hydration of the two most reactive phases of ordinary Portland cement (OPC), tricalcium silicate (C ₃ S), and tricalcium aluminate (C ₃ A) is successfully halted when the activity of water (α _H ) falls below critical thresholds of 0.70 and 0.45, respectively. It has been established that the reduction in relative humidity (RH) and α _H suppresses the hydration of all anhydrous phases in OPC, including less explored phases like dicalcium silicate, that is, belite (β-C ₂ S). 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 α _H on the hydration of β-C ₂ S 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 α _H threshold and solubility product constant of β-C ₂ S (K _{C 2 S} ) are established as 0.913 and 10 ^-12.68 , respectively. This study enables enhanced understanding of β-C ₂ S reactivity and provides thermodynamic parameters during the hydration of β-C ₂ S-containing cementitious systems such as OPC-based and calcium aluminate-based systems.