The role of Na2O dosage in iron rich fly ash geopolymer mortar

It has been reported that the use of Class F fly ash with high iron content can reduce compressive strength when used to produce geopolymer concrete. This limitation has constrained the utilization of iron-rich fly ash in the production of geopolymeric materials. To provide increased clarity, this study investigates the compressive strength and microstructural development of iron-rich fly ash geopolymer mortar employing a range of sodium hydroxide/sodium silicate activator ratios. The study assesses Na2O dosage of 7.5, 10 and 15% with an alkali modulus (AM) ranging from 1.00 to 1.875 and identifies optimum performance using Analysis of Variance and Signal to Noise techniques. A Na2O dosage of 10% and AM 1.375 gave the optimum strength at 7, 28 and 90-days. This gave a compressive strength of 44.9 MPa (28-days) and 53.5 MPa (90-days). For the optimum mix design, reduction in strength was evident with high iron incorporation into the geopolymer matrix. A higher Na2O dosage resulted in increased dissolution and a decrease in average pore size, but an increase in total porosity. Moreover, a higher concentration of Na2O also correlated with increased carbonate products, in the form of carbonation and efflorescence. In addition, specimens synthesized with a higher concentration of Na2O dosage indicated a potential for high thermal durability.