Comparative Study on the Enhancing Effects of CaSO4 and CaCl2 Supplementation on Red Mud Carbonation

[Geoengineering News]

‎https://www.frontiersin.org/journals/climate/articles/10.3389/fclim.2026.1732612/abstract

Authors: Yu Cheng, Shiying Yan, Lu Jin, Xu Wang, Ming Jia, Ping An, Binghui Sun

28 January 2026

Abstract 

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 (CaSO4) and calcium chloride (CaCl2) 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 CaSO4 and CaCl2 decreased to 8.6 and 8.0, respectively, with CaCO3 characteristic peak intensity increasing compared to pure RM; (3) For solid RM, the two calcium sources lowered the minimum pH to 8.8 (CaSO4) and 8.4 (CaCl2), 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 CaCO3 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.

Source: Frontiers