Feasibility of Producing Class F Fly Ash Geopolymer Mortar with Alkaline Water Containing Sodium Carbonate (Na₂CO₃)

Abstract

This study investigated the feasibility of substituting demineralized water with alkaline water containing sodium carbonate (Na₂CO₃) with varying concentrations to prepare alkali reagent (sodium hydroxide solution), which was in turn used for synthesizing class F fly ash based geopolymer mortar. Geopolymer mortar cube specimens were cast using demineralized water, and demineralized water spiked with various concentrations of Na₂CO₃ as reference and test specimens, respectively. The concentration levels of Na₂CO₃ in engineered water considered were 0.5, 1.0, 1.5, 2.0, 7, 9, 11, 13, and 15 g/L. The concentration of the alkaline solution and the proportion of Na₂SiO₃ (water glass solution) to NaOH (sodium hydroxide solution) were fixed constant at 12 M and 1.5. The curing regime of 70 °C for 24 h without any rest period was considered to activate the geopolymerization process for test and reference specimens. The evaluation of compressive strength development after 28 days was interpreted based on the guidelines mentioned by Burea of Indian Standards. Subsequently, a comparison of the morphology and elemental composition of reference and test mortar mixtures was performed by scanning electron microscopic images and energy dispersive x-ray analysis. Results indicated that good strength geopolymer mortar could be developed by replacing demineralized water with alkaline water containing Na₂CO₃ of 1.0 g/L concentration along with 12 Molarity NaOH solution and 1.5 proportion of alkaline activator solution. The deviations in the compressive strength of the test geopolymer mortar mixture were due to a change in the microstructure and elemental composition which had arrived due to the presence of sodium carbonate in water.