Water Confined in Cement Pastes as a Probe of Cement Microstructure Evolution

The properties of the water confined in a hydrating white cement paste have been investigated using low-temperature differential scanning calorimetry (LT-DSC) and low-temperature near infrared spectroscopy (LT-NIR). LT-DSC thermograms show, upon cooling, several exothermic peaks in the temperature range -10 to -42 degrees C, whose position and area depend on the hydration process, as a consequence of the cement microstructure evolution. The peaks have been interpreted in terms of Jennings' Colloidal Model-II for the hydrated calcium silicate (C-S-H) microstructure. Thermograms from samples aged up to two months from the preparation show an exothermic peak at -42 degrees C, typical of water confined in small gel pores (SGP). The LT-NIR results show that, at the beginning of the hydration process, water crystallizes as hexagonal ice and becomes amorphous as the setting process evolves. Both calorimetric and spectroscopic findings indicate that the water confined into the SGP porosity of the C-S-H phase (with dimension 1-3 nm) has properties very similar to those previously described for the interfacial water in zeolites, Vycor, and proteins. In particular, this confined water experiences a liquid-liquid crossover at -42 degrees C, passing from a high-density to a low-density liquid (HDL-LDL crossover).