A comprehensive mechanical and physico-chemical characterization of fly ash-based geopolymers

• Optimizing fly ash-based geopolymer concrete for enhanced strength and durability. • In 28 days, geopolymers match Portland cement in compressive strength. • After 1 year, geopolymer concrete's compressive strength exceeds Portland cement's. • Optimization shows geopolymer concrete compressive strength can reach 64 MPa. • DoE shows the alkaline solution-to-fly ash ratio as the most influential factor. This work focuses on developing a predictive optimization method for geopolymer concrete, addressing both mechanical strength and water absorption. Despite numerous formulations proposed in the literature, no systematic method has been established to evaluate these properties simultaneously. This research addresses this gap by employing a Design of Experiments approach to systematically explore the effects of key variables such as NaOH molar concentration, sodium silicate-to-sodium hydroxide ratio, and alkaline solution-to-fly ash ratio. After 28 days, geopolymer concrete exhibits competitive compressive strength (geopolymer concrete: 25 MPa, reference Ordinary Portland concrete: 27 MPa), and after 365 days, its compressive strength surpasses that of traditional Ordinary Portland concrete (geopolymer concrete: 56 MPa, reference Ordinary Portland concrete: 27 MPa). Moreover, through Response Surface Methodology, an optimization model indicates that geopolymer concrete compressive strength can reach up to 64 MPa, with a strong influence from the alkaline solution-to-fly ash ratio. Additionally, the materials were characterized in terms of crystalline phases, surface chemistry, thermal stability, and surface area to gain a deeper understanding of the behaviour of these materials.