Microstructural changes in blast-furnace cements with different replacement ratios under repeated drying and wetting cycles

Abstract

Temperature and humidity changes affect the durability of buildings. Experiments can be conducted to simulate the changes in the temperature and humidity conditions in real environments through repeated dry and wet cycles, revealing the microstructural changes in hardened cement. Calcium-silicate-hydrate (C–S–H) is a hydration product with the greatest influence on the physical properties of concrete, resulting in changes in the pore structure of hardened ordinary cement with repeated dry and wet cycles. However, blended cements with blast furnace slag produce less microstructural changes under dry and wet cycles than ordinary cement. In this study, blast-furnace cements with different replacement ratios were employed to investigate the causes of fine pore structure changes in blast-furnace cements under repeated dry and wet cycles. The hydration products were quantified using solid-state nuclear magnetic resonance (NMR) spectroscopy, and the pore size distribution was confirmed by mercury injection porosimetry. The ²⁷Al MAS NMR results showed an increase in the Al/Si content after repeated dry and wet cycles, and from the Al content of Al[Ⅳ], it is inferred that AlO₄ bonds to C–S–H and C–A–S–H is formed, which maintains the width of the C–S–H interlayer and reduces pore structure changes in the size range of 30–2000 nm than ordinary cement. In the future, blast furnace cement with different substitution rates may exhibit a better practical freezing resistance owing to reduction of microstructural changes with repeated drying and wetting of the cement.