Effects of Hydraulic Materials on the Performance Evolution of Carbonated High-Volume Magnesium Slag Mortars

Magnesium slag (MS) is a solid by-product during magnesium production using the Pidgeon process. Around 5–6 million tons of magnesium slag was produced in China in 2023, which accounted for 83% of the total disposal of magnesium slag worldwide. To explore the innovative and high-end application of MS in building materials, this study investigated the preparation of calcium carbonate cementitious composites produced by high-volume (80%) MS and 20% of traditional ordinary Portland cement (OPC), low-carbon cement–calcium sulfoaluminate cement (CSA), or green cement–alkali-activated materials after CO2 curing. The effects of OPC, CSA, and AAM on the performance evolution of MS blends before and after carbonation curing were analyzed. The results indicated that AAM contributed to a superior initial strength (7.38 MPa) of MS composites after standard curing compared to OPC (1.18 MPa) and CSA (2.72 MPa). However, the lack of large pores (around 1000 nm) in the AAM-MS binder caused the slowest CO2 penetration during the carbonation curing period compared to the OPC- and CSA-blended samples. Less than 3 days were required for the full carbonation of the CSA- and OPC-blended MS mortar, while 7 days were required for the AAM blends. After carbonation, the OPC-blended MS exhibited the highest strength performance of 51.58 MPa, while 21.38 MPa and 9.3 MPa were reached by the AAM- and CSA-blended MS mortars, respectively. OPC-blended MS composites exhibited the highest CO2 uptake of 13.82% compared to the CSA (10.85%) and AAM (9.41%) samples. The leaching of Hg was slightly higher than the limit (<50 µg/L) in all MS mortars, which should be noticed in practical application.