Experimental and Statistical Optimization of Low-carbon Binder Concrete under Varying Alkaline Activator Conditions

This study investigates the mechanical and microstructural performance of fly ash–GGBS-based geopolymer concrete activated using sodium carbonate and sodium silicate in varying ratios (1:1, 1.5:1, and 1:2), with a conventional sodium hydroxide–sodium silicate mix serving as the control. Workability was evaluated through slump tests, while compressive strength was measured at 7, 14, and 28 days. The control mix (NaOH:Na2SiO3 = 1:2.5) achieved a 28-day compressive strength of 48.6 MPa, while the carbonate-based mixes recorded strengths of 44.1 MPa (1:1), 36.3 MPa (1.5:1), and 41.2 MPa (1:2). SEM–EDS analyses confirmed that the 1:1 mix exhibited a dense matrix with a balanced distribution of Si, Al, Na, and Ca, indicative of the formation of both N–A–S–H and C–A–S–H gels. XRD patterns further corroborated the presence of these hydration products, with reduced unreacted phases in the optimized mix. Statistical evaluation using Response Surface Methodology (RSM) and ANOVA identified the alkaline activator ratio as a significant factor influencing compressive strength (p < 0.05), with the model achieving a high coefficient of determination (R2 = 0.985). The predicted and experimental strengths showed close agreement, validating the model's predictive capability. Results demonstrate that a 1:1 Na2CO3:Na2SiO3 ratio provides an optimal balance between mechanical performance and sustainability, making it a promising alternative for high-strength, eco-friendly geopolymer concrete applications.