Strength and hardening characteristics of activated fly ash mortars

Utilisation of fly ash in raw materials would contribute to the elimination of an environmental problem and to the development of new high-performance materials. It is necessary to study the binders obtained by chemical activation of pozzolanic materials that could act as a substitute for existing cement. Fly ash contains a glass phase. Because it is produced under high temperatures, it is a chemically stable material. Fly ash consists mostly of SiO₂ and Al₂O₃, and it assumes an oxide form on the inside of the fly ash. Because this reaction does not occur by itself, it is necessary to supply it with additional OH⁻ from alkali activators. The current paper summarises the strength development of fly ash mortars under varying chemical activators, curing temperatures and curing ages. Also, reaction products of the alkali-activated fly ash were observed, using scanning electron microscopy and X-ray diffraction analysis. The main reaction product in mortar, activated with NaOH and water glass was zeolite of the type Na₆(AlO₂)₆(SiO₂)₁₀12H₂O. Feldspar [(Na, Ca)(Al, Si)₂Si₂O₈] and hydrated calcium silicate (such as C–S–H or C–A–H gel) were also produced. Compressive strengths of 52·2 MPa and 57·3 MPa were observed at 3 days and 28 days, respectively, under high-temperature (90°C) curing.