Influence of Drying on the Microstructure of Hardened Cement Paste: A Mercury Intrusion Porosimetry, Nitrogen Sorption and SAXS Study

Since the durability of concrete depends on the paste matrix, a sound knowledge of its pore structure is required. The most common methods to investigate the microstructure like mercury intrusion porosimetry (MIP) and gas sorption require the removal of free water in the pore space. However, the different drying methods used for this task either do not remove all free water or remove also chemically bound water and thus alter the microstructure. Small-angle X-ray scattering (SAXS) is one of the rare methods which allows to investigate the microstructure without complete water removal. Therefore, in order to compare the effect of different drying methods on the pore structure, hardened OPC paste samples (w/c = 0.5) were investigated with MIP, nitrogen sorption and for the first time with SAXS, after different drying was carried out. Since specific surfaces obtained for SAXS showed a similar trend as for MIP and nitrogen sorption, it can be concluded that SAXS can be utilized to predict the influence of drying on these methods as well. Solvent exchange appears to be a drying method that preserves the microstructure well, since for this the largest specific surface was obtained.