Elucidating hardening mechanism of carbonatable binders prepared with steel slag based on distribution of microhardness

Abstract

To ascertain the hardening process of carbonatable binders prepared with steel slag, this study examined the distribution of carbonation products on the surface of steel slag particle and the microhardness distribution of both the particles and the compact. The experimental results indicated that carbonation initiates in the active area rich in calcium silicate phase, surrounded by the RO phase (a solid solution formed from FeO, MgO, MnO, and other divalent metal oxides). In this region, calcium carbonate gradually grows, condenses, and spreads outward, eventually covering the surface of steel slag particle. Therefore, the microhardness of the carbonated steel slag particle shows a significant statistical distribution, corresponding to the calcium silicate phase, RO phase, and carbonation products, respectively. After carbonation, the carbonation matrix of compact forms progressively, including the voids between particles (microhardness below 100 HV 0.05), the carbonated area (microhardness between 100 and 200 HV 0.05), the transition zone (microhardness between 200 and 350 HV 0.05), and the uncarbonated area (microhardness above 350 HV 0.05). Moreover, through the statistical analysis of microhardness distribution of various region, the gradient change of carbonation reaction degree of compact is accurately obtained, which demonstrates the hardening process from the outside inward of carbonatable binders compact. This research sheds new light on the elucidation of hardening process of carbonatable binders.