EFFECTS OF COMPOSITION OF FLY ASH-BASED ALKALI-ACTIVATED MATERIALS ON COMPRESSIVE STRENGTH: A REVIEW
Qingbo Tian
Abstract
A fly ash-based alkali-activated material (FAAM) is an important member of the alkali-activated geopolymeric family. The compressive strength is one of most important mechanical properties for FAAMs in its use as a construction material. The effects of the components of the precursors from fly ash, an alkali-activator, or additives on the compressive strengths of FAAMs have been reviewed. The SiO₂/Al₂O₃, Al₂O₃/Na₂O, and water/solid (W/S) ratios are crucial in developing the compressive strengths of FAAMs. The strength map, as a function of the change in the SiO₂/Al₂O₃ and SiO₂/Na₂O ratios has been established. There is a critical value of the SiO₂/Al₂O₃ ratio at about 4.20 to 4.30, in which the increase or decrease of the compressive strengths at a constant SiO₂/Al₂O₃ ratio with various Al₂O₃/Na2O ratios is found. The active CaO resource, originating from a ground blast-furnace slag (GBFS), ordinary Portland cement (OPC), or chemical agents, such as Ca(OH)₂ or CaO, is of benefit to the improvement in the strength of FAAMs: however, to avoid the dominant formation of C-A-S-H, N-C-A-S-H, and C-S-H gels instead of N-A-S-H and A-S-H gels, an optimal addition of 7.5% to 10% OPC and a 15% to 20% GBFS replacement is recommended by considering the setting time, workability, and strength development. The effects of Fe₂O₃ in the fly ash and the silica on the compressive strengths of the FAAMs are also generalised.