Optimization of formulation ratios of geopolymer mortar based on metakaolin and biomass fly ash

This study investigated the factors affecting the properties of geopolymer (GP) mortars through a two-stage experimental program created through central composite design of experiments. Stage I focused on the effects of SiO2/Al2O3, H2O/Na2O, and total water-to-total solids (W/S) ratios on mortars formulated using metakaolin (MK) as the only geopolymerization precursor, while Stage II evaluated the impact of introducing biomass fly ash (BFA) as a second precursor. Most importantly, the SiO2/Al2O3 ratio was defined based on raw materials' dissolution tests rather than on their starting chemical composition, setting this study apart from most published literature. The mortars' properties of interest, taken as response variables, were the flow test spreading diameter, and the efflorescence level and compressive strength after 7 days of curing under ambient conditions. The results of Stage I, regarding GP mortars based on MK only, show that SiO2/Al2O3, H2O/Na2O, and W/S all had a statistically significant effect on compressive strength. Regarding the efflorescence level, SiO2/Al2O3 had the most significant effect, H2O/Na2O showed a small contribution, and W/S effect was not statistically significant. In stage II, the incorporation of BFA led to (MK + BFA)-based mortars with lower spreading diameter and compressive strength, and higher efflorescence levels, with % BFA being the most important factor affecting these three response variables. Based on the developed models, appropriate combinations of factor values that simultaneously maximize the compressive strength while maintaining the spreading diameter and efflorescence levels within suitable limits were found: GP mortars based only on MK with SiO2/Al2O3 = 4.5, H2O/Na2O = 19.0, and W/S = 0.17 are expected to achieve 53 MPa of compressive strength at 7 days; (MK + BFA)-based GP mortars with % BFA = 39.0, H2O/Na2O = 15.5, SiO2/Al2O3 = 4.5 and W/S = 0.17 are expected to achieve 30 MPa. These results provide valuable insights into the factors affecting the properties of GP mortars and can be useful for optimizing formulations for different applications.