Properties of class F fly ash based geopolymer mortar produced with alkaline water

In this study class F fly ash based geopolymer mortar is produced with water containing an alkaline substance such as sodium bicarbonate (NaHCO3). Subsequently, compressive strength, density, absorption and porosity of the produced mortar mixture were tested after 28 days and compared with controlled geopolymer mortar produced with pure distilled water. A mixture of NaOH and Na2SiO3 solutions along with an elevated curing regime of 70 °C for 24 h was used to activate the geopolymerization process. The entire process involved in designing 21 mortar mixtures in 04 different batches by varying three processing parameters with different levels. The processing parameters and levels varied are as follows: Concentrations of sodium hydroxide solution expressed in molarity (10 M, 12 M and 14 M), the ratios of sodium silicate (Na2SiO3) to sodium hydroxide (NaOH) solution (1.0, 2.0 and 2.5) and varying concentrations of sodium bicarbonate (0.5 g/L, 1.0 g/L, 1.5 g/L, 2.0 g/L, 7.5 g/L and 15 g/L) spiked in distilled water. A ratio of 3.0 fine aggregate to fly ash was kept constant for all mortar mixtures. Results indicated that with 12 M NaOH solution, 1.5 proportion of Na2SiO3 to NaOH solution, 1.0 g/L concentration of sodium bicarbonate in distilled water, geopolymer mortar of 41 MPa compressive strength with low porosity and good quality of mixture along with a low rate of absorption could be produced. Up to a concentration of 1.5 g/L of sodium bicarbonate in water is beneficial and higher levels are detrimental towards the properties of geopolymer mortar.