Effect of slag, silica fume, and metakaolin on properties and performance of alkali-activated fly ash cured at ambient temperature

Alkali-activated fly ash (AAFA) is usually cured at elevated temperature, which limits its wide field applications. However, several studies have shown that properties of AAFA cured at ambient temperature can be significantly improved by incorporating different cementitious additives. This research investigated the impact of substituting a small portion of fly ash with different potential cementitious additives such as slag, metakaolin, and silica fume on various properties of AAFA mixtures at ambient curing conditions. Toward that end, several experimental observations including workability, geopolymerization heat released, compressive strength, Young's modulus, electrical resistivity, chloride permeability, and pore structure distribution were made. The effects of alkaline solution concentration combined with the different chemical compositions on mechanical properties and permeability characteristics were also investigated. An ordinary Portland cement was also included in the testing program as a comparative reference. Test results showed that the AAFA mixtures exhibited lower heat as compared to ordinary Portland cement mixture, and the results of heat evolution were in line with early-age strength development. The inclusion of metakaolin resulted in a significant improvement in the early strength and stiffness. The inclusion of slag led to the improvement of the mechanical and permeability properties of AAFA cured at ambient temperature, and the effect of sodium silicate (SS) to sodium hydroxide (SH) solution ratio was found to be more profound when the slag was used as an additive. Incorporating silica fume in the AAFA mixtures decreased the mechanical properties and increased the volumetric percentage of large capillary pores.