Intermittent curing of fly ash geopolymer mortar

• Intermittent curing at 70 °C applied on 4 steps 6 h each followed by 18 h at room temperature.
• Intermittent curing improved the UCS at the end of each curing step.
• The effect of resting period and added water content on UCS was investigated.
• Increasing the AL-to-FA content is sensitive to the Na2O-to-SiO2 mole ratio.
• Very low H2O-to-Na2O and high Na2O-to-SiO2 mole ratios have inverse effect on UCS.

The research work focuses on the production of type F fly ash based geopolymer using intermittent curing. Two different types of soluble sodium silicate and Na(OH) solution with three different mole ratios were used with a fixed ratio. Two different fly ash-to-alkaline liquid activator ratios were used with and without additional water content. Two different resting periods were checked prior to starting the curing regime. The curing temperature was set at 70 °C applied intermittently on 4 steps for 6 h each per day followed by 18 h rest at ambient temperature. Twenty-one different geopolymer mixtures were cast using a mixture of fly ash and natural sand at a fixed ratio. The gain of compressive strength was checked at age 24, 48, 72, and 96 h and 7 days. Intermittent curing proved to increase the compressive strength of all geopolymer mortar at the end of each curing step. Thirteen geopolymer mixtures exceeded the Egyptian Code of Practice limit set for the 7-day compressive strength at 27 MPa. The UCS is directly proportional to the increase of the specific gravity of the soluble sodium silicate used, the age, the Na(OH) solution mole concentration, the alkaline liquid activator-to-fly ash ratio, the resting period and the Na2O-to-SiO2 mole ratio. Yet, it is inversely proportional to additional water content, H2O-to-Na2O mole ratio and the water-to-geopolymer solids ratio.

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