Carbonated blast-furnace slag as supplementary cementitious material: Phase transition and effect on cement hydration

• The carbonation of blast furnace slag (BFS) decreased the pozzolanic reactivity. • Carbonated slag (CS)-blended pastes showed comparable 28-day compressive strength to BFS blends. • Carbonation of BFS improved the pore structure and the interfacial transition zone between CS and hydration products. • CS has the potential to improve the resistance of cementitious materials to external sulfate attack. Blast-furnace slag, which contains high levels of CaO (and MgO), holds potential as a feedstock for CO 2 capture and storage. This study investigates the phase transitions occurring during the wet carbonation of ground granulated blast-furnace slag (GGBFS), characterizes the physical and chemical properties of carbonated GGBFS (CS), evaluates its pozzolanic reactivity, and examines the microstructure and performance of cement pastes blended with GGBFS or CS. The findings reveal that aragonite and calcite, the primary carbonation products, precipitate on the surface of GGBFS, hindering its dissolution and thereby reducing both the pozzolanic reactivity and the early strength of cement pastes. Nevertheless, the 28-day strength of CS blended pastes is comparable to that of GGBFS blends, due to the improved pore structure and the interfacial transition zone between CS and hydration products, which is attributed to the stabilization of ettringite and the formation of monocarboaluminate and hemicarboaluminate phases induced by the calcium carbonate introduced through CS. This characteristic implies extra benefits of using CS against GGBFS apart from CO 2 sequestration, that is, the potential of CS to be used in projects requiring low early-age heat.