Utilisation of low-reactivity fly ash for fabricating geopolymer materials

Low-reactivity fly ash (LRFA), obtained from combustion in a low-temperature furnace, is a hazardous waste with low pozzolanic activity that is unsuitable for engineering applications. This study investigated the effects of mixing procedure (sequence mixing method (SMM) and conventional mixing method (CMM)) on the properties of a geopolymer made from LRFA blended with calcined kaolinite clay as a precursor. A mixture of sodium hydroxide and sodium metasilicate was used as the alkaline solution. It was found that the SMM was required to obtain LRFA geopolymers with sufficient strength for engineering applications. Using the CMM, the geopolymer had high porosity and low strength because gas bubbles were generated in the reaction between the alkaline solution and impurities in the LRFA during the hardening process. By contrast, in the SMM, the generated gas bubbles were released before the geopolymer hardened. As a result, the porosity was reduced and the strength was thus significantly increased. The 7-day strength of the geopolymer fabricated using the SMM was higher than the minimum requirement for Portland cement. In addition, the SMM reduced the setting time of the LRFA geopolymer, which is advantageous for rapid repair applications.