Optimizing carbonation efficiency and mechanical properties of low-calcium cementitious materials with MgSO4 addition

This study investigates the effects of magnesium sulfate (MgSO₄) addition on the carbonation efficiency and mechanical properties of low-calcium CO₂-sequestering cementitious material (LCC), which is prepared by calcining a mixture of 78.2% limestone and 21.8% sandstone at 1275 °C for 2 h. LCC samples were prepared with varying concentrations of MgSO₄ solution (0, 0.5, 1, 2, and 3 mol/L) and subjected to CO₂ curing for 24 h. The carbonation behavior, compressive strength, and microstructural characteristics were examined using XRD, TGA, FT-IR, SEM, and LF NMR techniques in combination. Results demonstrate that the addition of MgSO₄ significantly influences the carbonation process and mechanical performance of LCC. Optimum performance was achieved after subjecting the paste prepared with LCC at a concentration of 0.5 mol/L MgSO₄ to a carbonation period lasting 24 h. This resulted in a notable increase in compressive strength by approximately 28% (145 MPa) compared to control samples along with an observed enhancement in CO₂ uptake by around 4%. Microstructural analysis reveals that the inclusion of MgSO₄ promoted the formation of more stable carbonate phases such as Mg-calcite and vaterite while also enhancing silica gel polymerization within the matrix structure of LCC materials. Additionally, it was found excessive concentrations (>1 mol/L) of MgSO₄ led to decreased carbonation efficiency and reduced strength due to gypsum formation as well as limited pore water availability. This study provides valuable insights into optimizing the carbonation process of LCC materials while demonstrating the potential efficacy of MgSO₄ as an effective additive for enhancing the performance of low-carbon CO₂-sequestering cementitious materials.