Effect of the Na₂SiO₃/NaOH Ratio and NaOH Molarity on the Synthesis of Fly Ash-Based Geopolymer Mortar

Geopolymers are inorganic polymeric substances, which are synthesised from raw materials rich in silicates and aluminates. When these raw materials activated with alkaline solution, a complex aluminosilicate gel forms due to polymerization process. However, the proportion in which the alkaline solutions used largely controls the geopolymerisation process. Thus, it is essential to understand the influence of alkaline solutions on the behaviour of geopolymers. With this in mind, an attempt was made to study the effect of Na₂SiO₃ to NaOH ratio, NaOH molarity on synthesis of fly ash based geopolymer mortar. Compressive strength and morphological characteristics were established, and the influence of (a) Na₂SiO₃ to NaOH ratio (viz., 1, 1.5, 2, and 2.5), and (b) NaOH molarity (viz., 10M, 12M, 14M, 16M, and 18M) were investigated on these properties. Results show that samples at Na₂SiO₃ to NaOH ratio of 1.5 exhibit maximum compressive strength values. It has been found that the compressive strength increased with increase in NaOH molarity till 16M and thereafter, strength found decreased with further increase in NaOH molarity. Morphological studies using environmental scanning electron microscope (ESEM) on the samples cast with different Na₂SiO₃ to NaOH ratios reveal that hardened mortar samples contain unreacted fly ash particles and the amount of unreacted particles seen increased with increase in Na₂SiO₃ to NaOH ratio.