Hydration and microstructure evolution of cement paste in low vacuum environments

Low vacuum is one of the most challenging service environments for cementitious materials, which would affect their hydration and microstructure, and thereby impair strength and durability. In this study, the hydration phases and microstructural evolution of pastes based on Portland cement (PC), sulphoaluminate cement (SAC), and tricalcium silicate (C3S, the clinker of PC) were investigated under low vacuum conditions. The results show that rapid mass loss could be observed at the initial stage of low vacuum treatment, which was mainly attributed to the dissipation of free water. After that, the mass loss rate was greatly slowed down and maintained at a relatively low level, which was primarily due to the loss of chemical bound water in hydration products. As a result, C3S samples, with the slowest early hydration procedure, exhibited the highest mass loss, which was followed by PC and SAC samples. The dissipation effect by low vacuum treatment resulted in the inhabitation of cement hydration, and correspondingly the contents of hydration products remained almost constant, or even decreased especially for ettringite (AFt). Moreover, the examination of the hydration product morphology demonstrates that under low vacuum conditions, the needle-like morphology of the hydration product C-S-H transferred gradually from divergence to convergence or underwent collapse, while the AFt crystals experienced degradation and then transformed into an amorphous state. This consequently led to the sample after low vacuum treatment exhibiting increased total volume of pores less than 100 nm and elevated percentage of pores with sizes from 50 nm to 100 nm. Overall, this research holds great scientific significance in guiding the engineering application of concrete in low vacuum environments.