Humidity‐dependent dimensional change and microstructure of hardened cement paste incorporating low‐kaolinite calcined clay

Abstract

Calcined clay‐blended cementitious systems are likely to exhibit greater shrinkage than Portland cement systems, yet few studies have quantitatively evaluated this behaviour under equilibrium humidity conditions. This study investigates hardened cement paste with 30 wt.% replacement by Thai low‐kaolinite calcined clay (approximately 25% kaolinite) using water vapor sorption isotherms (WVSI), dimensional change measurements under controlled relative humidity, and SEM‐EDS. WVSI results showed that the calcined clay system had greater water vapor sorption and pore volume than the Portland cement system, indicating increased gel porosity. The dimensional change isotherms revealed larger shrinkage across all humidity levels for the calcined clay system, primarily due to greater moisture loss. The shrinkage per unit mass loss crossed between the two systems at RH 60–80%, corresponding to Kelvin diameters of 4–10 nm, consistent with differences in pore size distribution. SEM‐EDS confirmed enhanced Al incorporation into C‐A‐S‐H and progressive reaction rim formation around calcined clay particles.