Influence of low vacuum condition on mechanical performance and microstructure of hardened cement paste at early age

To explore the influence of low vacuum on the mechanical performance and microstructure of cement-based materials at early age, the mechanical properties, phase composition and multi-scale pore structure of 7d-aged hardened cement pastes exposed to low vacuum and normal air environment were studied, and the relation between mechanical performance and microstructure was analyzed. The results show that low vacuum can significantly retard the strength growth of paste at early age, and lead to the decline of strength with exposure time increasing. Moreover, the mechanical performance of cement pastes in low vacuum condition was generally weaker than that under normal air environment. Furthermore, the number of gel pores decreased, while the most probable aperture and total porosity increased after low vacuum exposure, and more micron-scale macropores and millimeter-scale defects appeared. The multi-scale variation of pore structure is the dominating reason for the delayed development and degradation of mechanical performance under low vacuum environment. In addition, the phase composition of cement pastes did not change during low vacuum exposure, and the strength of pastes was highly correlated with the total amount of some key hydration products (e.g. C-S-H gel, ettringite, calcite et al.).