Use of coal-fired slag in filling bodies with early strength for mining applications

The global output of coal-fired slag (CS) has far exceeded 300 million tons, and the annual output in China is 4.5 billion tons, which seriously affects the ecological security. Additionally, with the continuous increases in mine production capacities, the filling cost continues to rise, which seriously affects the normal production levels of the mines. To reduce the filling cost and make full use of the CS, this study was designed to realize diversified application of coal-fired slag (CS) as a backfill with early strength. First, physical and chemical analyses were conducted for coal-fired slag powder (CSP), fly ash (FA), flue gas desulfurization gypsum (FGDG) and mine waste rock (MWR). Based on the alkali excitation theory, mixture design, response surface theory and early strength testing of the backfill, the optimal ratio for the solid waste cementitious material and the backfill with early strength was determined. The primary and interaction effects of the backfill were analysed. Finally, the hydration mechanism for the CS-based cementing materials was revealed by monitoring the resistivity and water content and with scanning election microscopy (SEM), X-ray diffraction (XRD) and thermogravimetry/differential scanning calorimetry (TG-DSC) analyses of the hydration products. The results showed that (1) the total amount of SiO2 and Al2O3 in the CS was 61.08%, and the content of CaO was 17.3%, which indicated potential gelling activity. (2) The backfill with early strength was prepared by mixing CSP, FGDG and FA at 70%, 3% and 27%, with MWR and CS exhibiting a specific mass fraction (81%), binder-sand ratio (4:6) and CS-MWR ratio (5:5). The 3 d and 7 d uniaxial compressive strengths of the backfill were 1.7 MPa and 3 MPa. (3) The CS-MWR ratio of the backfill and the interaction between the CS-MWR ratio and binder-sand ratio significantly influenced on the early uniaxial compressive strength. (4) The early hydration process for the CS-based cementitious materials included the dissolution period, the induced formation and induction period and the condensation and hardening period. At 3 d, the hydration products were mainly generated by the CSP (the total amount of hydration products produced was 7.01%), while at 4 d-7 d, the FA was involved (the total amount of hydration products produced was 7.37%. The results of this research provide theoretical guidance for using CS to develop low-cost backfills with early strengths for application in mines.