Using in-situ observation to understand the leaching behavior of Portland cement and alkali-activated slag pastes

This paper presents the leaching behavior of hydration products and unhydrated particles in Portland cement (PC) and alkali-activated slag (AAS) systems with a new in-situ observation method. After leaching in NH4Cl solution, obvious decalcification can be found in unhydrated C3S and C2S in PC, while the unhydrated slag keeps stable in AAS. The Ca in the gel of AAS is more vulnerable to leaching than that in PC due to the lack of phases with buffering capability like portlandite. In addition, the Mg-Al layered double hydroxide, which mainly exists in the dark rim of AAS, is not stable in NH4Cl solution. The in-situ observation method proposed in this study provides a new methodology to investigate the leaching behavior of different phases directly and helps to understand the effect of leaching on cementitious materials from a new perspective, it can therefore be used to instruct the design of durable cementitious systems.