Optimization of heat cured fly ash/slag blend geopolymer mortars designed by “Combined Design” method: Part 1

In the current study, heat cured fly ash (FA)/ground granulated blast furnace slag (GGBFS) blend geopolymer mortars activated with sodium hydroxide (NaOH) were investigated. For this reason, 'Combined Design' method of Experimental Design has been introduced for the first time as an exceptional tool for the evaluation of the simultaneous effect of the parameters affecting different properties of the mortars. Empirical models were developed to predict the mortars flow workability, compressive strength and flexural strength as well as to find the optimum levels for the most relevant conditions. For this purpose, 40 design points with different FA/GGBFS ratios ranging from 0/450 g to 450/0g; water 180-225 g, NaOH 50-150 g and curing temperature 50-100 °C were generated. For each of the 40 design points, mortars flow workability, flexural and compressive strength were tested and investigated. In the end, Scanning Electron Microscopy (SEM) analysis of three optimal mortar samples, representative of 100% FA, ∼55/45 FA/GGBFS and 100% GGBFS was performed and analysed for the physical properties such as unit weight, water absorption and porosity. The generated models shows excellent correlation coefficients such as R2 = 0.95 for flow workability and R2 = 0.98 for compressive strength. The SEM analysis and physical test results comply with the model results, which show that “Combined Design” method is a very effective and timesaving method to reach very accurate results in the design of geopolymer mortars.