Comparative study on the enhancing effects of CaSO4 and CaCl2 supplementation on red mud carbonation

Red mud (RM), a strongly alkaline solid waste generated during alumina production, can undergo carbonation with CO₂ for mineral sequestration. To investigate the promoting effect of external calcium sources on RM carbonation, desulfurization gypsum (CaSO 4 ) and calcium chloride (CaCl 2 ) were selected as supplements, and RM samples with/without these calcium sources were prepared. Experiments were conducted under various CO₂ concentrations (100, 15, 1%) and atmospheric conditions for both RM suspensions and solid-state RM (simulating open-air piles). The results showed that: (1) With increasing CO₂ concentration, the time for RM suspensions to reach pH equilibrium shortened (30 min for 100% CO₂ vs. 15 h for 1%), and the equilibrium pH decreased (to 6.8 for 100% CO₂ vs. 8.3 for 1%); (2) Under atmospheric conditions, the pH of RM suspensions supplemented with CaSO 4 and CaCl 2 decreased to 8.6 and 8.0, respectively, with CaCO 3 characteristic peak intensity increasing compared to pure RM; (3) For solid RM, the two calcium sources lowered the minimum pH to 8.8 (CaSO 4 ) and 8.4 (CaCl 2 ), ultimately stabilizing around 9.0, whereas pure RM remained at 10.1. The CO₂ sequestration capacities reached 45.3 g/kg and 47.2 g/kg, respectively, while forming a porous CaCO 3 coating on the RM particles. The calcium sources significantly enhanced the stability and durability of the carbonation reaction, providing a scientific basis for long-term CO₂ sequestration.