Development of carbon-focused green low-carbon materials: Strength, microstructural evolution mechanisms, and...

Utilizing solid waste to prepare backfill materials for synergistic CO₂ mineralization and sequestration is a key emission reduction pathway for mines to achieve the 'dual-carbon' goals. Revealing the influence mechanism of CO₂ mineralization on the micro-to-macro multiscale evolution of backfill bodies is crucial. This study proposes a method for preparing a magnesium-coal-based solid waste carbon sequestration backfill material (MF) by mixing magnesium slag(MS), fly ash(FA), and coal gangue(CG), followed by carbonation curing. Through various experimental methods such as uniaxial compressive strength (UCS), fourier transform infrared (FT-IR), Thermogravimetry (TG), scanning electron microscopy (SEM), and acoustic emission (AE), the effects of different curing ages, magnesium slag content, and fly ash content on the strength, microstructure, deformation characteristics, failure mode, and acoustic emission characteristics of MF under carbonation curing conditions were investigated, and their carbon fixation capabilities were quantified. The results show that carbonation curing can significantly enhance the compressive strength of MF. The maximum strength after 28 days reaches 10.6 MPa, which is approximately 12 times that of uncarbonated MF, and the strength increases particularly rapidly in the early stage. After carbonation curing, a large number of carbonation products such as calcite (CaCO₃) and amorphous silica gel (SiO₂·nH₂O) are generated in MF, which act as mechanical locks to produce pore filling and can significantly improve the compactness of microstructure. This is the main reason for the increase in MF strength, and it also reveals the evolution law of MF strength from the perspective of microstructure. Appropriate increase in the content of magnesium slag can promote the carbonation reaction, but excessive fly ash will inhibit the deepening of the carbonation reaction due to blocking the diffusion channels of CO₂. After carbonation treatment, MF exhibits a predominant failure mode of tensile failure under uniaxial compression. Different MFs have distinct acoustic emission characteristics, which are all corresponding to their intensity and the stage of their microstructure evolution. The TG results indicate that the maximum carbon sequestration rate of MF is 16.34%, corresponding to a CO₂ sequestration capacity of 163.4 g/kg. This study demonstrates that carbonation treatment not only enhances the strength of MF and densifies its microstructure, but also has the function of mineralizing and sequestering CO₂. This research provides a sustainable new path for the development of high-strength, low-cost and low-carbon MF.