Facile Preparation of Vaterite Dominated Incineration Fly Ash (IFA) by Slurry Carbonation and Its Performance As Supplementary Cementitious Material

Municipal solid waste incineration fly ash (IFA) can be treated by slurry carbonation and adopted as a supplementary cementitious material (SCM) to reduce the carbon emission of the cement industry. However, effects of control parameters of carbonation on the mineralogy and morphology of the treated IFA require further investigation. In this work, the control parameters were sequentially optimized, aiming at producing more chemically active vaterite during carbonation. The vaterite dominated IFA was subsequently used as an SCM and its performance was studied and compared with limestone (pure calcite). Results indicated that precipitation of vaterite was favored under lower temperature and higher water-ash ratio conditions. CO2 flow rate has relatively smaller impact on the mineralogy of the treated IFA, but a higher rate led to agglomeration of vaterite particles. Under the optimized condition, vaterite dominated IFA containing 60% vaterite and 10% calcite was obtained. For the blended cement, its early hydration was suppressed by calcium sulfite (CaSO3) in the IFA, but the additional sulfate (CaSO4) accelerated the ettringite formation. The vaterite calcium carbonate in the treated IFA stabilized ettringite and reacted with aluminate phases to form hemicarbonate, refining the pore structure. However, hemicarbonate formation was limited by availability of aluminum that was consumed to form ettringite. Compared to limestone, the vaterite dominated IFA behaved better performance as an SCM, and the Porland cement can be substituted by 20% without compromising the 3-day and 28-day compressive strength. Toxicity leaching tests indicated only Pb exceeded regulatory limits in raw IFA, while Pb leaching in mortar samples was greatly declined below 1% of the limit.