Geopolymerization behavior of fine iron-rich fraction of brown fly ash

Iron rich brown fly ash has been segregated into (a) coarse, and (b) fine size fractions using air classification. The suitability of fine iron-rich fly ash (CFA) has been evaluated for the synthesis of geopolymer. The behavior of CFA based geopolymer in relation to reactivity, NaOH concentration and sodium silicate to sodium hydroxide (S/H) ratio has been studied. The concentrations of NaOH (6, 8, 10 and 12 M) and S/H ratios (0.33, 1.0 and 3.0) were the variable parameters. Calorimetric studies revealed that the total heat release increased with NaOH concentration, but decreased with the increase in S/H ratio. The geopolymer synthesis was done by thermal curing at 60 °C for 24 h. It was found that the compressive strength increased with an increase in the NaOH concentration. The increase in S/H ratio from 0.33 to 1.00 resulted in a significant increase in the compressive strength, but a further increase from 1.00 to 3.00 resulted in a slight decrease in compressive strength. High strength geopolymer mortar up to 70 MPa at 28 days was obtained, when the mixture was formulated with 12 M NaOH and S/H ratio of 1.00. From SEM-EDS studies, an increase in the degree of reaction and formation of geopolymer gel was observed with increase in NaOH concentration. Also it revealed that the geopolymer gel obtained was ferro-sialate based polymers with chemical formula close to [Ca, Na]-[-Fe-O-]x-[-Si-O-Al-O-]1−x-[-Si-O-]y, wherein “x” is less than or equal to 0.5, and “y” is a value between 0 and 25. Formation of the compact microstructure has resulted in improved strength properties of geopolymer.