Effect of elevated temperature on strength and phase stability of geopolymers from fly ash, bagasse ash, and calcium carbide residue

• Developed geopolymers from FA, BA, and CR as green cement alternatives. • Optimal mix (0.67FA–0.16BA–0.16CR) achieved 28.24 MPa and thermal stability. • Higher strength correlated with lower water absorption and porosity. • XRD and TGA showed phase transitions and decomposition at 600 °C. • SEM/EDS confirmed microstructure and elemental homogeneity post-heating. This study evaluates the thermal resistance of alkali-activated geopolymers synthesized from industrial by-products—fly ash (FA), bagasse ash (BA), and calcium carbide residue (CR). The materials were activated using 5 M NaOH and sodium silicate, then subjected to elevated temperatures of 100 °C, 200 °C, 400 °C, and 600 °C for 60 min. Compressive strength, water absorption, and thermogravimetric weight loss were assessed to determine thermal performance. The 0.67FA–0.16BA–0.16CR mix achieved the highest strength of 28.24 MPa at ambient conditions and retained better stability after thermal exposure. At 600 °C, strength deterioration was observed across all mixes, linked to increased porosity and decomposition of gel structures. XRD and TGA analyses confirmed phase transformations and CO₂ release between 150 °C and 600 °C. The results highlight the critical role of precursor composition in enhancing the mechanical and thermal performance of sustainable geopolymer materials.