The Use of Bio-Carbonated Reactive Magnesia Cement-Solidified Construction and Demolition Waste for Water-Rich Goaf Filling

The disposal of construction and demolition waste (CDW) and the rehabilitation of water-rich goafs present two pressing challenges in sustainable mining and geotechnical engineering. This study introduces a bio-carbonated reactive magnesia cement (RMC) technology to solidify CDW for goaf backfilling applications. A comprehensive experimental program was conducted to evaluate the mechanical properties, water stability, and durability of bio-carbonated RMC-solidified CDW under simulated water-rich goaf environments. The unconfined compressive strength (UCS) of the 16% RMC specimens reached 3.12 MPa after 28 days, which was approximately 21% higher than that of the 12% Portland cement (OPC) control. Under dynamic water erosion conditions of 1.0 m/s for 72 h, the 16% RMC specimens showed a mass loss rate of 2.68% and a strength retention rate of 84.2%, both of which were superior to those of the 12% OPC control. The water–land strength ratio of the 16% RMC specimens reached 0.90 after 28 days of immersion, and the strength retention rate remained at 73.1% after 12 wet–dry cycles. Compared to the OPC control, these two indicators respectively represent a precise increase of 25% and approximately 60%. Microstructural analysis revealed that hydrated magnesium carbonates (HMCs), including nesquehonite and hydromagnesite, formed a dense spatial network that binds CDW particles and blocks pore channels. Additionally, a random forest model quantified the relative importance of RMC content, curing age, and carbonation degree on UCS, confirming that RMC dosage and the extent of carbonation are the dominant controlling factors. The optimal RMC content was determined to be 16%, balancing performance and cost. This study demonstrates that bio-carbonated RMC-solidified CDW is a technically viable and low-carbon backfill material with substantial CO2 sequestration potential for water-rich goafs.