Water Sorption Isotherms and Thermodynamic Characteristics of Hardened Cement Paste and Mortar

The moisture sorption characteristics of two cement-based materials: hardened cement paste and mortar were investigated at 30, 50 and for water activity ranging from 0.05 to 0.95. The sorption isotherms were typical type II sigmoidal curves according to BET classification. In both adsorption and desorption, an increase in temperature resulted in lower equilibrium moisture contents at corresponding values of water activity. The sorption isotherms exhibited hysteresis during a period water activity from 0.05 to 0.95. Some models were applied for analysing the experimental data. Nonlinear regression analysis was used for the determination of the parameters in the equations. Estimated parameters and fitting ability for sorption models were evaluated. Sorption isotherm data were used to determine the thermodynamic functions such as isosteric heat of sorption, sorption entropy, spreading pressure, net integral enthalpy and entropy. The Clausius-Clapeyron equation was used to evaluate the isosteric heats of sorption.