Multi-Objective Optimization of Alkali-Activated Concrete Using Fly Ash and GGBFS through Desirability Function Analysis

This study presents the development and multi-objective optimization of alkali-activated concrete (AAC) using fly ash (FA) and ground granulated blast furnace slag (GGBFS) under ambient curing conditions (30–35℃, 60–70% RH). Sodium hydroxide (NaOH) with varying molarities (8M, 12M, 16M) was used along with sodium silicate to activate the FA/GGBFS blends (100:0, 75:25, 50:50, and 25:75%). The mechanical properties, such as compressive strength, split tensile strength, and flexural strength, were evaluated after 28 d of curing. The results showed that increasing the NaOH molarity significantly enhanced the mechanical performance, with the 50% FA–50% GGBFS mix at 16M yielding the highest compressive strength (71.4 MPa), split tensile strength (3.84 MPa), and flexural strength (12.3 MPa). However, increased molarity reduces the workability owing to shorter setting times and higher viscosity. To optimize the performance holistically, Desirability Function Analysis (DFA) was used to simultaneously evaluate six parameters: compressive strength, split tensile strength, flexural strength, cylindrical compressive strength, cost, and CO₂ emissions. Cost analysis focused on activators, whereas CO₂ emissions were estimated using literature-based factors (1.5 kg CO₂/kg NaOH and 0.07 kg/CO₂ GGBFS). Equal weighting was applied to all the six variables. The highest composite desirability (0.893) was achieved by the 8M NaOH, 50% FA–50% GGBFS mix, which offered strong mechanical properties (58.3 MPa compressive strength), low cost (US $84.86/m³), and moderate CO₂ emissions (49.71 kg/m³), making it the most sustainable and structurally efficient blend. Meanwhile, the 12M–50% FA–GGBFS mix showed the highest mechanical strength and ranked second overall, balancing strength and sustainability, and was especially suitable for structural applications. This study demonstrates that AAC mixes can be tailored to meet both economic and environmental goals without compromising performance. It also confirms that ambient curing with balanced FA/GGBFS ratios enables practical AAC production for real-world applications.