Mechanical properties, shrinkage, and heat evolution of alkali activated fly ash concrete

Use of high calcium fly ash as a binder to replace the ordinary Portland cement possibly cause a false set for alkali-activated concrete. Therefore, the appropriate mixed proportion of alkali-activated concrete must be considered for further applications. This research investigated the mechanical properties, shrinkage, and heat evolution of concrete using high-calcium fly ash as a binder with two alkali activators: sodium silicate (Na2SiO3) and sodium hydroxide (NaOH). Ratios of sodium silicate to sodium hydroxide (Na2SiO3/NaOH) of 0.3, 0.4, and 0.5 by weight, and NaOH concentrations of 2, 4, 6, and 8 Molar were used. The ratio of liquid to binder was 0.7, and the fly ash content was 400 kg/m3. The results indicated that increasing the Na2SiO3/NaOH ratio and NaOH concentration improved the compressive strength of the concrete. The maximum value for the compressive strength, 27.5 MPa, was observed for a Na2SiO3/NaOH ratio of 0.5 and NaOH concentrations of 2 Molar at 90 days. The modulus of elasticity of alkali-activated fly ash concrete was found to be similar to that of conventional concrete. Additionally, the shrinkage of concrete tended to decrease with increasing NaOH concentration, and increasing the Na2SiO3/NaOH ratio led to increased concrete shrinkage. Finally, the heat evolution of alkali-activated fly ash concrete was found to be two times lower than that of concrete made from Ordinary Portland cement at the same compressive strength. The increase of NaOH concentration and Na2SiO3/NaOH ratio resulted in higher temperature rise of alkali-activated fly ash concrete.