Metakaolin-blended cement paste：Exploring early-age shrinkage and quantitative characterization

This study examined the effects of metakaolin (MK) on early-age tensile properties, shrinkage, and water evaporation of cement paste, and introduced a new parameter to characterize early-age total shrinkage. The aim is to assess the risk of early shrinkage cracking in MK-blended cement pastes starting from the initial setting under inadequate early curing conditions. Thermogravimetric analysis, isothermal calorimetry, and mercury intrusion porosimetry were used to investigate the effects of MK on cement paste hydration and pore structure. Results indicate that MK improves early-age tensile properties, with 1-day tensile strength increasing by 1.5%–11.7% and tensile strain capacity by 3.4%–10.7%. However, it also causes a substantial rise in autogenous and total shrinkage, with total shrinkage increasing by 26.3%–147.9% at 1 day. Since the tensile strain capacity increase is much smaller than the shrinkage increase, MK may heighten the risk of early-age shrinkage cracking. MK addition also increased water evaporation, and higher dosages led to greater moisture loss. Meanwhile, as its pozzolanic reaction progressed, calcium hydroxide and chemical bound water (CBW) decreased with increasing MK content. MK significantly affected the pore size distribution (PSD), helping to explain its influence on water evaporation and mechanical strength. For the first time, a novel parameter r t , combining CBW, water evaporation, and PSD, is proposed to elucidate the internal mechanism by which MK influences early-age total shrinkage. This parameter tracks changes in capillary pore size due to evaporation and hydration, effectively revealing the trend of early total shrinkage variation in blended cement pastes.