Using fly ash to partially substitute metakaolin in geopolymer synthesis

Metakaolin-based geopolymers containing between 0 and 40% fly ash were prepared, to study the influence of fly ash substitution on the reaction process and products. At constant liquid/solid ratio, the partial replacement of metakaolin by fly ash reduces the rate of reaction, but the reaction continues for an extended duration and so is still able to achieve a relatively high extent of reaction and provide a comparable 28-day compressive strength when the replacement is less than 30%. The replacement of 10% of the metakaolin by fly ash gives an increased reaction extent (as measured by isothermal conduction calorimetry), and also a higher 28-day compressive strength, by 15%. The porosity of the geopolymer increases with the level of fly ash replacement, and pore refinement and ongoing reaction of fly ash particles during the first month of curing are also apparent.