Dissolution and geopolymerization of synthetic fly ash glass

Two fly ash glasses were synthesized and dissolved in KOH at a water/solid ratio of <= 0.3. We measured reaction kinetics, gel formation, and compressive strength. Mathematical modeling revealed that two consecutive solid-state diffusion processes controlled the reaction progress. A fast diffusion process determined the rate at which gel formed on the glass surface. Formation of gel in the interstices between glass particles slowed the reaction progress. The mixtures set and hardened. Compressive strength increased. A slower diffusion process took over as soon as most of the interstices were filled with gel. This diffusion process controlled the growth of compressive strength in the longer term. Compressive strength and its growth rate depended on Si/Al in the glass and on macro-porosity of the geopolymer, much less so on the mass of gel. The higher Si/Al supported the formation of a more compact gel, leading to higher and more rapidly growing compressive strength than at the lower Si/Al.