Response surface methodology for multi-objective optimization of fly ash-GGBS based geopolymer mortar

Neat fly ash based geopolymer require high temperature curing to obtain satisfactory properties. In order to improve its workability and mechanical properties under room temperature curing, taking ground granulated blast furnace slag (GGBS) substitution, alkaline solution to binder (S/B) ratio, and Na2SiO3 to NaOH solution ratio (SS/SH) as independent variables and compressive strength, flexural strength, and setting time as response target values, the response surface method (RSM) is used to design the test and establish the regression model. The multi-objective optimization of the comprehensive properties of geopolymer mortar was realized, followed by the verified experiments of the proposed optimized mix. To investigate the microstructure and composition of the hydration products, the optimally proportioned and neat fly ash based specimens were tested by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that the quadratic polynomial model was fitted with high accuracy and the GGBS substitution has a significant effect on the setting time and strength of the geopolymer. The compressive and flexural strengths were 80.46 MPa and 11.98 MPa when the slag was mixed at 75%, S/B = 0.45, and SS/SH = 1.5, which was 9% and 14.8% higher than the strength of neat fly ash based geopolymer mortar maintained at high temperature curing. SEM results show that the geopolymer added with GGBS performs a denser microstructure, which confirms the effectiveness of RSM in determining the optimal mix parameters of geopolymer.