Effect of Material Composition and Alkali Activator Modulus on Mechanical Properties and Reaction Process of Coal-Based Solid Waste Geopolymer

Abstract

This study investigates the effects of coal-based solid waste/ground granulated blast furnace slag mass ratios (90/10,70/30,50/50) and water glass modulus (1.0, 1.2, 1.5) on geopolymer fluidity, setting time, and compressive strength. Fly ash (FA), coal gangue (CGP; 20% of coal-derived solid waste), and finely ground granulated blast furnace slag (GGBS) particles are used as composite precursors. The reaction mechanism was elucidated through XRD, TGA, and SEM-EDS analyses. Slag content is the main influencing factor, as increasing its proportion from 10% to 50% enhances the 28-day compressive strength of the geopolymer by 250%. Increased slag promotes the formation of additional C-A-S-H gel and results in a denser matrix. In contrast, excessive amounts of coal gangue can lead to an increase in porosity due to its high crystallinity and low reactivity, which inhibits improvements in mechanical properties. The optimal formulation (coal-based solid waste/ground granulated blast furnace slag 50/50; activator modulus 1.5) achieves a 28-day compressive strength of 32.6 MPa. This study provides theoretical and technical support for the rational utilization of coal-based solid waste and the development of cementitious materials from industrial solid waste.