Setting Time, Strength, and Bond of High-Calcium Fly Ash Geopolymer Concrete

In this paper, setting time, strength and bond of high-calcium fly ash geopolymer concrete were investigated. The high-calcium fly ash was from Mae Moh power plant in northern Thailand. Both sodium silicate solution and sodium hydroxide solution were used as alkali activators in every mix. Sodium hydroxide solution with 10 M, 15 M, and 20 M concentrations, sodium silicate to sodium hydroxide ratios of 1.0 and 2.0, alkaline liquid to fly ash ratio of 0.5 and two curing regimes viz., heat curing at 60±2°C for 24 h and room temperature curing at 23±2°C were used. The results indicated that fresh geopolymer concrete had short setting time of 28–58 min due to the presence of high calcium content of fly ash. In general, strengths and modulus of elasticity increased with the increase in NaOH concentration. For compressive strength, the optimum Na2O content was around 12% of fly ash. The high-strength geopolymer concrete with 28-day compressive strength of 54.4 MPa was obtained for mix with 15M NaOH. The modulii of elasticity of geopolymer concrete were related to the compressive strengths and comparable to those of portland cement concrete. The tensile splitting strength and bond strength were also related to the compressive strength and the values were higher than those of portland cement concrete. In particular, the bond strengths were significantly higher than those given by the current design code.