The effect of curing on the properties of metakaolin and fly ash-based geopolymer paste

Fly ash was used as the raw material in the preparation of geopolymer paste and metakaolin was used as a substitution material in different ratios. Metakaolin material was made of kaolin clay calcined at a temperature of 1000 °C and at a final temperature for 1 h in a laboratory-type electric arc furnace. The fly ash in the prepared mixture was substituted by metakaolin ranging from 10% to 40%. In the mixtures, NaOH was used as the reactive together with a 3-module sodium silicate solution to solve silica and alumina in the fly ash and metakaolin particles. A 9 M NaOH solution was used in the preparation of the geopolymer pastes and the liquid/solid ratio used in the mixtures was 0.36 ml/gr. The pastes were cast into a 25 by 50-mm cylinder mold. This process was performed under a 100 bar pressure using a hydraulic hand press. Afterwards, the samples were placed in a laboratory-type oven for thermal curing. The samples were subjected to curing at 60 °C and 80 °C (two different curing temperatures) for 2 h, 4 h and 24 h (three different curing times). Following curing, the samples were maintained under laboratory conditions at room temperature until the day of the physical and mechanical tests. 7-day geopolymer paste samples were used for the tests. As a result, the ideal curing temperature and curing time were decided to be 60 °C and 2 h for the production of the geopolymer paste. It was also determined that the compressive strength values of the samples subjected to curing reached up to 25.1 MPa and that a 40% metakaolin substitution provided a better geopolymerization and significantly improved compressive strength values.