Response surface methodology to model the effects of key synthesis parameter interactions on mechanical and microstructural properties of natural pozzolan-based geopolymers

This paper contributes to the experimental study of the mechanical behavior and microstructural properties of geopolymer (GP) binders based on natural Algerian Pozzolan. Twenty-seven geopolymer mixtures were produced to model the effect of four synthesis parameters, namely SiO2/Al2O3 molar ratio (Si/Al), NaOH concentration (NaOH), Na2SiO3/NaOH mass ratio (NS/NH), and curing temperature (T). The impact of their interactions on the compressive strength at 28 days (CS28) of synthesized geopolymers was analyzed using response surface methodology (RSM) and Box-Behnken design. This study was completed by microstructural characterization of selected samples using FTIR, XRD, and SEM-EDS analysis. A statistical analysis of the model shows a good correlation with experimental results presented by the coefficient of determination (R2=0.95). An optimum CS28 of 46.84 MPa was obtained with Si/Al, [NaOH], NS/NH, and T equal to 5.2, 13 mol, 2 and 80°C, respectively. The results show that among the possible interactions, there are four dependent ones, namely Si/Al× (NaOH), Si/Al×NS/NH, Si/Al×T, and (NaOH)×NS/NH, while the other two are independent. The microstructure study of the selected samples reveals significant changes characterized by structural reorganization and the formation of substantial quantities of the NASH gel binding phase in the optimum mixture, confirming the measured strength values.