Investigation of coal mine overburden as a sustainable fine aggregate in alkali-activated concrete

The environmental impacts associated with conventional concrete production and the increasing scarcity of natural resources have highlighted the need for alternative materials for sustainable concrete production. This study investigates the feasibility of using coal mine overburden (CMO) as a complete replacement for natural fine aggregate in one-part alkali-activated concrete(AAC). Fly ash (FA) and ground granulated blast furnace slag (GGBS) were used as aluminosilicate precursors, while sodium metasilicate (SMS) was employed as a powdered alkali activator. The effects of FA/GGBS ratio, activator dosage, and CMO replacement levels of 0%, 25%, 50%, 75%, and 100% were investigated in terms of mechanical, durability, dimensional, and microstructural performance. The selected mixture containing 100% CMO achieved a 28-day compressive strength of 48.99 MPa, together with a splitting tensile strength of 3.90 MPa, flexural strength of 5.60 MPa, bond strength of 14.74 MPa, and modulus of elasticity of 27.90 GPa. The measured drying shrinkage at 28 days was 0.037%, while the chloride and sulphate contents were 345.89 mg/L and 90.43 mg/L, respectively. SEM–EDS analysis revealed a relatively compact reaction matrix and elemental composition consistent with the formation of calcium- and sodium-aluminosilicate binding products. The embodied-energy assessment indicated a reduction of 26.96%, from 2434.8 to 1778.3 MJ/m³, while the estimated material cost decreased by 11.13%, from ₹5800.2 to ₹5154.5 per m³, relative to the reference concrete within the defined assessment framework. Overall, the results demonstrate the technical feasibility of incorporating 100% CMO as fine aggregate in the investigated one-part AAC system, while providing potential embodied-energy and material-cost benefits under the stated assumptions.