Development and performance of ambient cured geopolymer concrete with low alkali activation for sustainable construction

This study aims to develop and assess the performance of an ambient-cured geopolymer concrete formulated using a low-alkali activation strategy to enhance sustainability and practical applicability in construction. The binder system consists of a 40:60 blend of GGBS:Fly ash, activated through a hybrid Na₂CO₃-Na₂SiO₃ solution operating under moderated alkalinity relative to highly caustic NaOH-based activators. An experimental program was conducted to assess the influence of activator molarity and carbonate-to-silicate ratio on the fresh, mechanical, durability and microstructural properties of the geopolymer concrete. The optimised mix achieved a compressive strength of 48.6 MPa at 28 days, a split tensile strength of 4.3 MPa, a flexural strength of 6.9 MPa and a static modulus of elasticity of 31.5 GPa, indicating its suitability for structural applications. Durability properties were evaluated by low water absorption (~3.2%), reduced sorptivity (0.70 × 10⁻⁴ mm/√s), improved abrasion resistance and 94% residual compressive strength retention after sulfuric acid exposure, indicating the formation of a dense and chemically stable matrix. Microstructural investigations like SEM, EDS, XRD and FTIR indicated the development of a compact hybrid C–A–S–H/ N–A–S–H gel network with improved silicate connection and reduced effective porosity. RSM was used to support the mix optimisation and demonstrated a strong correlation with mechanical and durability properties (R² > 0.98). Overall, these results demonstrate that ambient-cured low-alkali geopolymer concrete offers reliable performance and practical advantages for sustainable construction. • Ambient-cured geopolymer concrete developed using low-alkali Na₂CO₃–Na₂SiO₃ activation • Structural-grade strength and durability achieved without heat curing or NaOH • Activator chemistry (pH, EC, TDS, alkalinity) linked directly to concrete performance • Hybrid C–A–S–H/N–A–S–H gel formation validated through multi-scale characterisation • RSM-based optimisation provides practical guidance for sustainable building materials