Design and Properties of Coal Gangue-Based Geopolymer Mortar

The feasibility of preparing coal gangue-based geopolymer mortar (CGBGM) with composite-activated coal gangue was experimentally explored in this paper. The effects of water-to-binder ratio (W/B), alkali content (A), and slag content (S) on the fluidity and performances of the CGBGM were experimentally investigated. The ANOVA method was employed to evaluate the significance level of influenced factors. Moreover, the microstructure, element compositions, and qualitative microanalysis of the CGBGM at various curing ages were verified by ESEM-EDS. Test results denoted that the mechanical properties of the CGBGM are superior to that of ordinary Portland cement mortar (OPCM). The flexural and compressive strength of the CGBGM rapidly increases before 7 d and then tends to slow afterward. According to the impact degree on the flexural and compressive strength of the CGBGM, the S is ranked first, followed by the A, and finally the W/B. The flexural and compressive strength of specimens with 40% slag increased by 45.97% and 90.75%, respectively, compared to the control group. However, the A and W/B have little effect on flexural strength. In addition, the hydration productions filled in the crevice and healed the entrapped microcracks in the hardened paste with the increase in curing ages, forming a dense microstructure. The Ca/Si decreased from 0.5 at 3 d to 0.06 at 28 d, and the Ca/Al decreased from 0.25 at 3 d to 0.05 at 28 d. Finally, the difference in drying shrinkage behavior between the CGBGM and OPCM was systematically analyzed.