Role of calcium compounds in BOFS on phase evolution and microstructural changes upon high-gravity carbonation

The high-gravity (higee) carbonation process has emerged as a promising method for enhancing the carbonation efficiency of basic oxygen furnace slag (BOFS). While this process has proven effective in carbonating BOFS, the underlying mechanisms are not yet fully understood. To investigate this, we selected two types of BOFS: calcium hydroxide-rich (CH-rich) and calcium silicate-rich (CS-rich) BOFS, to examine their differences in carbonation degree, phase evolution, and microstructural changes during higee carbonation. Our experimental findings reveal that higee carbonation significantly reduces the carbonation time for BOFS by 97.5 %. Notably, the CH-rich BOFS achieves a carbonation degree of 20 %, which can be attributed to ex-situ carbonation mechanisms that expose unreacted core phases, allowing for prolonged carbonation. This process enables CH-rich BOFS to reach near-complete decalcification within just 10 min. In contrast, the CS-rich BOFS exhibits less than half the carbonation degree compared to its CH-rich counterpart. This limitation arises from the rapid formation of tightly packed calcite and decalcified C-S-H on the particle surface, which restricts further carbonation via in-situ mechanisms. To address this challenge, we propose a short-term pre-wet carbonation process for CS-rich BOFS prior to higee carbonation. This integrated approach successfully increases the carbonation limit of CS-rich BOFS to 13-16 % by reducing the initial carbonation rate. Overall, higee carbonation reveals calcium compound-dependent mechanisms in BOFS, offering a roadmap for scalable CO₂ sequestration and sustainable slag valorization in the cement industry.