Experimental study on calcium carbide residue as a combined activator for coal gangue geopolymer and feasibility for soil stabilization

• CCR could be used as a potential replacement of NaOH to activate coal gangue geopolymer. • Optimal CCR content and NaOH:Na 2 SiO 3 mass ratio = 20% and 50:50. • The compressive strength depended on the types and amounts of reaction products and pore structure. • CG-CCR geopolymer is feasible to enhance the strength of soft soil. • The exponential correlation between strength of stabilized soil and CG-CCR geopolymer content is proposed. This study investigates the use of calcium carbide residue (CCR) as a potential alternative to NaOH for activating coal gangue (CG) geopolymers, both of industrial waste. A series of experiments were carried out to evaluate the effects of CCR content, NaOH:Na 2 SiO 3 mass ratios, liquid to solid ratio (L/(CG + CCR)), and curing time on the strength development of CG-CCR geopolymers. Moreover, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP), and field emission scanning electron microscopy (FESEM) were used to analyze the reaction products and microstructural characteristics of CG-CCR geopolymer. Accordingly, the CG-CCR geopolymers were applied to stabilize soft soil. Experimental results indicate that CCR could significantly enhance the compressive strength of CG-CCR geopolymer. A CCR content of 20% correlates to the greatest increase in 3-d compressive strength (327.6%) compared with pure CG geopolymer (CCR = 0%). The optimal NaOH:Na 2 SiO 3 mass ratio is 50:50, while the compressive strength increases with decreasing L/(CG + CCR) and increasing curing time. Based on the analysis of microstructure (XRD, FTIR, TGA, MIP, and FESEM), it was inferred that CCR could promote the polymerization reaction and increase the reaction products (primarily N-A-S-H and C-A-S-H gels) of CG-CCR geopolymers, which are responsible for the strength development. Additionally, CG-CCR geopolymer has a positive impact on the stress–strain relationship and strength of stabilized soil. This study demonstrates that CCR can be used as a partial replacement for NaOH in generating CG geopolymer, which can provide an effective path for disposing of CG and CCR as well as lead to more practical applications for geopolymer in soil stabilization.