Acid Resistance of Fly ash based Geopolymer mortars

Geopolymers are a new promising binder manufactured by activation of a solid aluminosilicate source material with a highly alkaline activating solution and aided by heat. Fly ash, considered to be a waste material is rich in silica and alumina and hence can be used as a source material for manufacture of Geopolymers. These binders have been reported to achieve high early strength and better durability as compared to Ordinary Portland cement based counterparts. An experimental study was conducted to assess the acid resistance of fly ash based Geopolymer mortar specimens having percentage Na2O ranging from 5% to 8% of fly ash. The program consisted immersion of specimens in solutions of 10% Sulfuric acid and 10% Nitric acid up to a period of 24 weeks and evaluation of its resistance in terms of surface corrosion, residual alkalinity, changes in weight and compressive strength at regular intervals. Geopolymer mortar samples did not show any noticeable change in colour and remained structurally intact though the exposed surface turned slightly softer. Through Optical microscope, corroded surface could be seen which increased with duration of exposure. Samples almost lost its alkalinity after exposure in the acid solution within 12 weeks and showed very low weight loss in the range from 0.81% to 1.64% in Sulfuric acid and from 0.21% to 1.42% in Nitric acid. Compressive strength loss at the end of test varied from 44% to 71% and 40% to 70% in Sulfuric acid and Nitric acid respectively. Results obtained in the present study indicate that Geopolymers are highly resistant to both Sulfuric and Nitric acid.