Application of water vapor sorption measurements for porosity characterization of hardened cement pastes

Water vapor sorption can be used to study important properties of porous materials including specific surface area and pore size distribution (PSD). However, the data analysis is somewhat inconsistent in literature. In this work, the important factors influencing the analyzed results using sorption data were reviewed. Water vapor sorption measurements were then applied to two hardened cement pastes and one model porous material MCM-41. The specific surface area was calculated based on different equations accounting for multilayer adsorption and the PSD was analyzed from both the absorption and the desorption isotherms for comparison. The calculated specific surface area was quite dependent on which equation is considered for multilayer adsorption. For the studied hardened cement pastes, three characteristic peaks were found in the calculated PSD curves from the desorption isotherms with corresponding radii of 1.4, 1.8 and 3.0 nm while the peak at 1.4 nm was missing in the PSD curves calculated from the absorption isotherm. The network theory, suggesting desorption is controlled by the pore entry sizes while absorption is controlled by the interior pore sizes, can be used to explain some of the results.