Influence of Metakaolin Replacement on the Early Compressive Strength of Heat-Cured Fly Ash–Based Geopolymer Mortar

Geopolymer materials have gained significant attention as sustainable alternatives to traditional concrete, as they reduce carbon dioxide emissions associated with the cement production industry. This environmentally friendly material consists of industry waste materials that have been chemically activated by an activation solution. This study attempts to evaluate the effects of replacing metakaolin (MK) in fly ash (FA)-based geopolymer mortars with sand at 10%, 20%, and 30% with an emphasis on early strength within the first few hours. Na2SiO3 (2:1) and 12 M NaOH were combined to create an alkaline activator. After being formed into 50 x 50 x 50 mm cubes, the samples were first heat-cured at 90°C for initial durations. At two, five, and twenty-four hours, the bulk density and compressive strength were measured. The results showed that density gradually decreased over time, and that replacing some of the sand with MK significantly increased both early and later compressive strengths while initially reducing density. The increased active aluminosilicate content and quicker N-A-S-H gel formation are responsible for this improvement. MK20 showed the best workability, microcompactness, and reactivity. On the other hand, because of its higher viscosity and potential for microporosity, MK30 was marginally weaker than MK20 but showed greater strength in comparison to the reference. The results support the use of medium replacement proportions of MK in applications that require both structural stability and quick strength growth