Evolution of multi-scale pore structure of concrete during steam-curing process

Steam-curing at elevated temperature involves a complex hygrothermal coupling action, which greatly affects the hydration process, microstructure buildup and thus the mechanical and long-term properties of cement-based materials. This paper investigated the multi-scale pore structure characteristics of concrete, paste and paste-aggregate interfacial zone during steam-curing process by a combined method of microscopic image analysis, scanning electron microscopy and mercury intrusion porosimetry. The evolution model of pore structure of steam-cured concrete during steam-curing stage and corresponding mechanism are also discussed. The results show that the multi-scale pore structure evolution of concrete is remarkably influenced by the steam curing process, which can be divided into three stages. The initial free water content is one of the critical factors affecting the pore structure evolution of steam-cured concrete, especially in the interfacial transition zone. The morphology of hydrated cement pastes in interfacial transition zone is looser than that of matrix. The expansion pressure of vapor in the heating period of steam-curing process when water interior expands more severely in a higher initial free water content hinders the precipitation and diffusion of hydrates, bringing a relative weaker interfacial transition zone for steam-ctu-ed concrete. A hydration degree-dependent and water to cement ratio related theoretical model is established to characterize the pore structure evolution during steam-curing process, which is proved to be well consistent with the experimentally measured results.