Experimental assessment and optimization of mix parameters of fly ash-based lightweight geopolymer mortar with respect to shrinkage and strength

Abstract

The aim of the study is to determine the impact of various NaOH molarities, sodium silicate-to-sodium hydroxide ratios, binder amounts and the age on the compressive strength and free shrinkage of fly ash based lightweight geopolymer mortars. After experimental evaluation, the data were used to model and optimize mix proportions of lightweight geopolymer mortars through response surface method. NaOH molarities were determined as 6 M, 8 M, and 10 M. sodium silicate-to-sodium hydroxide ratios were chosen as 0.5, 1.5, and 2.5 and binder amounts were used as 600, 700, and 800 kg/m3. For the same variables, except the binder amount, the compressive strengths of geopolymer pastes were also determined. The usage of lightweight aggregate as a replacement of river sand (25%) affected the results due to pore structure and its strength. Thus, it was concluded that strength and shrinkage of the samples reduced in comparison to the geopolymer mortars only including sand. The increase of molarity improved compressive strength of pastes and mortars. However, higher sodium silicate-to-sodium hydroxide ratio resulted in lower strength values. Geopolymer mortars with higher binder amounts and NaOH molarities showed a lower drying shrinkage strains, while resulted in increase of the autogenous shrinkage. The effect of sodium silicate-to-sodium hydroxide ratios were also observed in both shrinkage types. Besides, the optimum variables based on maximum strength and minimum shrinkage strain values were reached in optimisation study conducted with response surface method (RSM). The optimum values for the parameters of NaOH molarity, binder amount and sodium silicate-to-sodium hydroxide ratio were specified as 9.892 M, 600 kg and 0.5, respectively. The corresponding desirability value for this optimisation was obtained as 0.833 within the acceptable range.