Effects of thermal conditioning and alkali molars on the properties changes of mesoporous fly ash geopolymers

The physicochemical changes in fly ash geopolymers represent a complex reaction that continues to spark extensive debate. This study focuses on the mechanical and durability properties of these materials, evaluating their potential applications across various industries and environments. Specifically, it investigates the evolution and structural changes of a geopolymer blend synthesized from Class F fly ash (FA), with an emphasis on the coupled effects of NaOH molarity, curing temperature, and preservation conditions in both alkaline and non-alkaline solutions. Experimental results reveal that NaOH concentration and curing temperature significantly influence the hydration reaction, initial strength, and pore structure development of FA geopolymers. For instance, the Si/Al ratio ranged from 1.97 to 2.68, confirming the mesoporous aluminosilicate nature of the composition. Curing in an alkaline NaOH solution facilitated secondary reactions of FA particles, thereby preventing drying, shrinkage and cracking. Conversely, immersion in non-alkaline distilled water, lacking dissolved alkali cations, did not produce a robust gel network through hydrolysis. The leaching experiments and pore distribution analyses identified the timing of secondary precipitation, highlighting the direct impact of NaOH molarity on the mesoporous structure. The kinetics and hardening of the gel products with compressive strengths ranging from 8.9–77.8 MPa were influenced by OH⁻ concentration (8 and 10 mol/L), curing temperature (60°C), immersion duration (0–48 h), and solution alkalinity within specific limits. These parameters promoted accelerated setting times and self-repair of cracks within the FA geopolymer matrix. The findings demonstrate the potential for efficient recycling of FA in the production of sustainable building materials, supporting their application in diverse environmental conditions.