Synergistic effects of basalt powder and coal bottom ash on the strength development, durability, and microstructural stability of alkali-activated two-part geopolymer concrete

This research investigates the mechanical performance, durability characteristics, and microstructural features of basalt powder – coal bottom ash – based geopolymer mortar and concrete as sustainable alternatives to conventional cementitious systems. Geopolymer binders were developed using varying basalt powder – coal bottom ash proportions and liquid-to-binder ratios to identify an optimal composition. Mechanical properties, including compressive, split tensile, and flexural strengths, dynamic modulus, and Shore D hardness, were evaluated at curing ages up to 180 days. Durability performance was assessed through sorptivity, rapid chloride permeability, sulfate resistance, chloride resistance, and acid resistance tests. The results indicate that blended geopolymer systems significantly outperform reference concrete at all ages, with the optimal 50% basalt powder–50% coal bottom ash mix achieving the highest strength and stiffness due to efficient geopolymerisation, improved particle packing, and dense aluminosilicate gel formation. BCAC exhibited a 29% higher dynamic modulus and superior surface hardness compared to conventional concrete, confirming enhanced elastic efficiency and abrasion resistance. Durability results revealed reduced permeability and improved resistance to aggressive environments, including chloride, sulfate, and acidic exposures. SEM and EDX analyses confirmed the formation of a compact, well-polymerized aluminosilicate matrix with refined pore structure and reduced calcium-rich phases. The findings highlight that basalt powder – coal bottom ash two-part geopolymer concrete offers high mechanical performance, excellent durability, and long-term microstructural stability, supporting its potential for sustainable structural and infrastructure applications.