Effects of mineralogical changes in BOFS during carbonation on pH and Ca and Si leaching

Abstract

Basic oxygen furnace slag (BOFS) contains different alkaline Ca-bearing minerals that lead to high CO2 sequestration and high pH. The high alkalinity of BOFS is an environmental issue. In this study, we investigated the relationship between accelerated carbonation, pH, and leaching by analyzing changes in mineral composition during carbonation process. Results indicate that carbonation of Ca-bearing minerals in BOFS was mainly from, from highest to lowest, Ca(OH)2, β-C2S and C4AF. Ca(OH)2 was most likely to be carbonated because of its high solubility. Carbonation greatly reduced the alkalinity of BOFS, from an initial pH of 12.73 (non-carbonated BOFS) to a final pH of 9.36. When the pH value was <10, carbonation was hindered by lack of CO32−. The increase in the degree of carbonation greatly reduced Ca leaching but significantly increased Si leaching. The pH value and Ca leaching from non-carbonated BOFS were determined by Ca(OH)2. The pH value and Ca leaching from carbonated BOFS tended to equilibrate with a saturated CaCO3 solution, and Si leaching was determined by amorphous SiO2 generated during carbonation.