The risk of explosion associated with high-strength cement concrete under elevated temperature conditions underscores the urgent need for safer and more resilient alternatives in construction applications. This study focuses on developing environmentally sustainable, explosion-resistant concrete options. A high-strength alkali-activated mortar (HSAAM) using granulated blast furnace slag (GBFS) was produced, and subsequently, a lightweight, high-strength alkali-activated concrete (LWHSAAC) was developed by replacing 50 % of the sand volume in HSAAM with expanded polystyrene (EPS). The mixtures were prepared with consistent ratios of sodium silicate (Na₂SiO₃), sodium hydroxide (NaOH), and water for the alkali activation solution. Samples were subjected to three distinct temperatures—200 °C, 400 °C, and 600 °C—at a controlled heating rate of 10 °C/min for durations of 10, 20, and 30 min. The study examined the effects of two cooling regimes (furnace and water) on performance metrics, including residual compressive strength, splitting tensile strength, impact resistance, mass loss, thermal insulation characteristics, visual appearance, and microstructural integrity. Key findings include a 3.47 %–18.89 % increase in compressive strength at 200 °C for S0 and a 6.58 %–14.47 % increase for S50, while reductions in compressive strength at 600 °C reached up to 82.57 % for S50.