Synergistic modification of coal-based solid waste grouting material with SAP and nano-SiO2 for CO2 mineralization: Optimization, performance, and mechanisms

Coal-based solid waste mineralization materials generally face bottlenecks of low carbon sequestration efficiency and poor long-term stability. While superabsorbent polymer (SAP)-based internal curing has been proven effective in cement-based materials, its applicability in solid waste-based geopolymer systems remains controversial. In this work, a multi-component coal-based solid waste CO 2 mineralization grouting material was prepared using coal gangue, fly ash and desulfurization gypsum as raw materials, carbide slag and sodium silicate as activators, and SAP and nano-SiO 2 as functional modifiers. Formula optimization and comparative analysis under standard and mineralization curing were performed. Under standard curing, SAP locked up the early alkaline solution and retarded geopolymer skeleton formation, reducing the 28d compressive strength. Under mineralization curing, SAP improved the strength retention ratio and achieved a maximum carbon sequestration capacity of 115.1 g/kg. SEM-EDS results show that SAP suppresses Na⁺ migration and promotes preferential reaction of CO 2 with Ca 2+ to form calcite. TG-DTG and XRD results further confirm lower Ca(OH) 2 residue and higher calcite content, supporting the alkaline reserve slow-release function. BET and SEM results indicate that SAP retains pore water and promotes gel formation around released-water voids, as evidenced by the pore structure evolution in which in-situ-formed calcite fills mesopores while SAP-released voids and calcite growth stress reshape macropores, collectively demonstrating enhanced internal mass transfer and CO 2 mineralization within the matrix. These combined effects prevent premature geopolymerization termination and promote deep CO 2 mineralization, providing a theoretical and experimental basis for backfill-mineralization synergistic CO 2 sequestration in coal mine goafs using coal-based solid wastes.These combined effects prevent premature geopolymerization termination and promote deep CO 2 mineralization, providing a theoretical and experimental basis for backfill-mineralization synergistic CO 2 sequestration in coal mine goafs using coal-based solid wastes.