Compressive strength of ash-based geopolymers at early ages designed by Taguchi method

In the present work, compressive strength of ash-based geopolymers has been designed by Taguchi method at 2 and 7 days of water curing. Three factors including oven curing temperature (at 3 levels of 25, 70, and 90 °C), oven curing time (at 3 levels of 2, 4, and 8 h) and sodium hydroxide (NaOH) concentration (at 3 levels of 5, 8, and 12 M) were considered. By utilizing L9 Taguchi array, 9 series of experiments were conducted on the prepared specimens. The aluminosilicate source was a mixture of fly ash and rice husk ash while the alkali activating was done by a mixture of NaOH and sodium silicate solution. The obtained results were evaluated by analysis of variance (ANOVA) method to determine the optimum level of each factor. In all produced specimens, the optimum level of oven curing temperature was always 90 °C to achieve the highest compressive strength. Furthermore, the optimum strength was obtained by applying light and middle concentration of NaOH in approximately all specimens. Finally, the oven curing time was not an important factor to determine the compressive strength. To validate the accuracy of the optimum conditions suggested by ANOVA, compressive specimens were made and tested in accordance to the optimum conditions for each of 2 and 7 days water curing regimes. The compressive strength acquired from this situation was higher than those of proposed in initial 9 series of experiments for each of 2 and 7 days water curing regimes.