Very high strength (120MPa) class F fly ash geopolymer mortar activated at different NaOH amount, heat curing temperature and heat curing duration

•Influence of activator concentration on geopolymer mortar compressive and tensile strength was investigated.•Influence of heat curing temperature and duration on geopolymer mortar compressive and tensile strength were investigated.•Very high compressive strength (120MPa) and tensile strength (15MPa) geopolymer were obtained.•For a constant curing temperature, optimal activator content was existed for maximal strength.•For a constant activator content, optimal temperature was existed for maximal strength. In this laboratory work, high compressive and flexural tensile strength of alkali activated fly ash geopolymer mortars were presented. Class F fly ash was used throughout the study. NaOH was used as alkali medium that provides high pH value. Also, the factors influencing the compressive and flexural tensile strength were investigated. A total of 216 fly ash geopolymer mortar samples were prepared. Heat curing temperature, heat curing duration and alkali (Na) concentration were chosen as the influencing parameters of strengths. Mortar mixture parameters were 3 and 1/3 for sand–binder ratio, and water–binder ratio, respectively. Na concentrations of the mortar mixtures were changed from 4% to 20% with 2% increment step. Heat curing temperatures were changed from 45 to 115°C with 10°C increment step. Heat curing durations were chosen as 24, 48 and 72h. For each combination of influencing parameter, three prismatic specimens with 40mm×40mm×160mm dimensions were prepared using a three-cell mortar cast. After heat curing period in a laboratory oven, the samples were left to cool down to room temperature, then compressive and flexural strengths were measured as described in its respective standard. Very high compressive and flexural tensile strength obtained, which were as high as 120 and 15MPa respectively.