Effects of seawater, limestone powder and calcined clay on hydration kinetics and shrinkage behavior of ultra‐high‐performance engineered cementitious composites

Abstract To elucidate the mechanisms governing volumetric stability and optimization of ultra‐high‐performance engineered cementitious composites, this study investigated the coupled effects of seawater and limestone calcined clay cement (LC 3 ) on hydration kinetics, pore structure, and shrinkage behavior. Internal temperature and relative humidity monitoring, shrinkage measurements, isothermal calorimetry, thermogravimetric analysis (TGA), and mercury intrusion porosimetry (MIP) were conducted to characterize hydration and microstructural evolution. Hydration kinetics were analyzed using the Knudsen and Krstulović‐Dabić three‐stage framework, while shrinkage was decomposed into autogenous and drying components using a Kohlrausch‐Williams‐Watts (KWW) formulation and a tanh‐based drying model. The results show that seawater accelerates early hydration, shifts the peak degree of conversion to a lower level, and advances the induction‐to‐diffusion transition, leading to increased autogenous and drying shrinkage. In contrast, metakaolin promotes the formation of C‐(A)‐S‐H through pozzolanic reactions, while limestone powder provides filler and carboaluminate effects; their combined incorporation refines and homogenizes the pore network, significantly mitigating shrinkage. The kinetic fingerprint—characterized by n , , and the induction‐diffusion transition position—is related to shrinkage behavior, indicating that LC 3 prolongs the induction stage and weakens diffusion‐controlled hydration. MIP and TGA results further demonstrate that higher chemically bound water contents and lower portlandite contents are associated with refined pores and reduced connectivity, increasing the drying time scale. Overall, the LC 3 blend offers an effective low‐carbon strategy for designing UHPECC with reduced shrinkage.