Mineralogical and microstructural response of hydrated cement blends to leaching

Recent advances in concrete technology have enabled the manufacturing of hydrated cements blended with high levels of supplementary cementitious materials (SCMs). These composites can exhibit mechanical and physical properties similar to ordinary Portland-based cements; yet their equivalent performance in “corrosive” environments has to be proven. In this paper, we describe mineralogical, microstructural and geochemical alteration patterns of hydrated cement pastes, despite adequate curing, containing 10 wt-% up to 70 wt-% replacement of Portland cement by SCMs, due to combined leaching and carbonation attack for 182 days. Such knowledge is highly relevant for assessing degradation features of steel-reinforced concrete in tunnels. The dissolution of portlandite, katoite and tobermorite as well as recrystallization of C-S-H caused the development of a leached layer around the specimen‘s surface. Calcite, vaterite and hydrotalcite precipitated within the altered zone, but no passivation effect due to clogging of pore space by these deposits was observed. The thickness of the altered layer, the amounts of portlandite dissolved and CaCO₃ phases neo-formed, the decrease in the Ca/Si molar ratio of C-S-H and the increase in total porosity were highest in pure cement paste. All hydrated cements blended with different types and levels of SCMs (including metakaolin, silica fume, limestone, granulated slag, and their combinations) have behaved better than the pure cement paste, which demonstrates the equivalent performance of these blended mixes in weakly aggressive environments.