Predicting sorption isotherms from thermodynamic calculations

Accurate sorption/desorption isotherms for cementitious materials are important in predicting drying shrinkage, moisture transport, ionic transport, freezable water content, and the service life of concrete. This paper develops a framework for constructing water sorption isotherms for hydrated cementitious pastes from the outputs of thermodynamic modeling and a pore partitioning model (PPM). Thermodynamic modeling helps quantify the solid phases and pore space in the hydrated matrix. The PPM provides the volume of evaporable water in crystalline hydrates, the total volume of gel water, the volume of capillary water, and volume of pores due to chemical shrinkage. The sorption isotherm is constructed from information on the evaporable water present in individual phases at each RH, water adsorbed on C-S-H, water in pores with kelvin radius of 2–5 nm, capillary water, and water in pores due to chemical shrinkage and air voids. The Brunauer-Skalny-Bodor (BSB) model is used to calculate the water adsorbed on the C-S-H. This model predicts the sorption isotherms from the literature to within an error of 2–19 %. The areas for future work and the challenges in predicting the desorption isotherms are discussed.