Sustainable CO2-fixing hydraulic lime from high-magnesium limestone: mechanisms of strength enhancement, pore modification, and improved carbonation efficiency

To address environmental challenges posed by greenhouse gas emissions and the depletion of non-renewable high-grade limestone, this study investigates the use of high-magnesium limestone (HMLS) with varying MgO contents for producing CO₂-fixing hydraulic lime (CHL). The CHL consists of 0 %–15 % over-burned MgO (OM), 30 %–35 % Ca(OH)₂, and 55 %–65 % β-C₂S. The mechanical properties, phase composition, pore structure, and micromorphology of CHL were evaluated before and after accelerated carbonation (10 vol% CO₂, simulating industrial flue gas). Results demonstrate that CHL containing 5 %–15 % OM exhibited coarsened pores and higher fractal dimensions compared to OM-free CHL, facilitating CO₂ diffusion. This led to a 19.45 % increase in carbonation degree after 7 days and a 47.51 % improvement in compressive strength after 14 days. Additionally, OM altered CaCO₃ crystallization from cubic to rod-like, enhancing pore densification and strength. Accelerated carbonation consumed OM, partially converting it to magnesium carbonate and Mg-calcite. QXRD analysis revealed that 5 %–6.7 % of the OM in carbonated CHL had reacted. Autoclave expansion tests (AET) showed no volume deformation or cracking in hardened CHL, with stable compressive strength for samples containing ≤10 % OM. This confirms the feasibility of utilizing HMLS with ≤26 % dolomite content. The study highlights OM's critical role in improving carbonation efficiency and microstructure, providing a sustainable pathway for HMLS's utilization and high-performance CO₂-fixing binders.