Experimental study on preparation of negative carbon filling material forwater protection mining by CO2 mineralization of coal⁃based solid waste

In the process of development and utilization of coal resources， large amounts of CO2， coal⁃fired fly ash， and other solid waste will be produced. In the “carbon peak， carbon neutral” prospect， if the coal⁃based solid waste backfill material can be mineralized using CO2 and filled the goaf to control the strata movement， it will enable to “turn waste into treasure”， thus promoting the development of green low⁃carbon coal industry. Aiming at the prob⁃ lem that it is difficult to meet the high temperature， high pressure and high calcium (“three high”) conditions required for CO2 mineralization at the mine filling site， a CO2 mineralized coal⁃based solid waste negative carbon fill⁃ ing material (NCFM) has been developed using conventional low⁃calcium fly ash (“three constant”) under ambi⁃ ent temperature and pressure conditions. In the process of the NCFM reaction， the raw materials first undergo a hydra⁃ tion reaction to generate the C—S—H gel， Ca(OH)2 and CaCO3， and then react with CO2 to generate the SiO2 sili⁃ ca gel. In this process， the basic activator accelerates the formation of C—S—H and silica gel， which makes the NCFM to have the characteristics of high early strength. As the NCFM further hydrates， the alkaline activator pro⁃ motes the pozzolanic reaction， leading to the formation of more hydration products， which enhance the long⁃term strength of the NCFM. Incorporating mechanical properties and microstructure of the NCFM via slump test， setting time test， uniaxial compressive strength ( UCS ) test， X⁃ray fluorescence ( XRF ) spectrum， X⁃ray diffraction ( XRD )， scanning electron microscope ( SEM )， X⁃ray spectrometer ( EDS ) analysis， and thermogravimetric analysis (TGA) test， the early and long⁃term strength development mechanism of the NCFM is elu⁃ cidated. The UCS of the NCFM samples with 80% fly ash content is the highest， reaching 2.70 MPa in 3 days and 5.12 MPa in 28 days. The maximum CO2 absorption of 1 g NCFM was 1.39 mg. The development of the NCFM nega⁃ tive carbon backfill material is feasible to utilize in the green mining， simultaneously promote the coal resource green mining in a collaborative way， such as the CO2 recycling and large⁃scale solid waste disposal， goaf reuse， and water protection. Thus， it enhances “carbon⁃coal substitution”， and speeds up the energy resources “double carbon” strategic target realization.