Effects of drying and wetting on the microstructure of ordinary portland cement and blast furnace slag cement during drying-wetting cycles

The pore structure of hardened ordinary cement becomes coarser due to microstructural changes brought about by drying-wetting cycles. However, the production of blast furnace cement, which is a mixed cement, involves the production of calcium silicate aluminate hydrate (C-A-S-H); therefore, the microstructural changes are different from those of ordinary cement, and the degree of pore coarsening is small. Studies have also been conducted to examine the changes in the microstructure of ordinary and blast furnace cements by subjecting them to intermittent drying. However, no studies have been conducted on the influence of drying and wetting processes on the microstructure during drying-wetting cycles. In this study, microstructural changes in ordinary and blast furnace cements during high-temperature drying-wetting cycles were investigated. Solid-state NMR was used to analyze changes in the C-S-H structure. Additionally, pore structure and water content changes in the hardened material were examined using MIP and 1 H NMR. The results showed that for drying only, ordinary cement and blast furnace cement showed little change in C-S-H structure and pore structure from the first drying. Meanwhile, during the drying-wetting cycles, the microstructure converged to that between the initial dry and wet states. In this period, the C-S-H structure changed intermittently due to drying-induced polymerization and progressive moisture supply-induced hydration; however, the blast furnace cement reduced pore structural changes by producing more C-A-S-H. This study is expected to contribute to basic research on hardened cement in the future. • In the case of drying, the pore structure does not change after initial drying. • The pore structure exhibits convergent behavior upon drying and wetting. • Blast-furnace cement is easily recovered micropore by Al binding due to wetting. • In dry and wet, polymerization and hydration proceed as the cycle proceeds.