Optimizing the compressive strength of low-grade hydrous clay and copper slag-based sustainable ternary blended geopolymer mortar

Ternary blended geopolymers represent a new class of sustainable binders with improved properties when compared to mono and binary blended geopolymers under ambient conditions. The study focuses on low grade industrial waste materials; locally sourced hydrous clay (HC) waste and copper slag (CS), to create novel Ternary Blended Geopolymer Mortars (TBGM). The addition of CS had a positive effect on strength development. The incorporation of 40 % CS as fine aggregate replacement, improved the proportion of HC in precursors from 10 % to 30 %. The design parameters considered were sodium silicate to sodium hydroxide solution ratio (SS/SH) at (2,2.5,3), the alkaline solution (AL) to binder (B) material ratio (AL/B) at (0.35,0.4,0.45), and the molarity of the sodium hydroxide solution (M) at (8,10,12) respectively. Box Behnken Design (BBD) with response surface methodology (RSM) was used to reduce the number of trial experiments to 15 from a total of 27 and, to statistically optimize and evaluate the interactions of SS/ SH, AL/B, and M on the performance of TBGM. The compressive strength of 72 N/mm2 at the SS/ SH, M and AL/B as 2.5, 12 and 0.4, respectively, was in good agreement with the experimental result of 78 N/mm2 at similar proportions. From the diagnostics and response plots of RSM, it can be inferred that the molarity of NaOH and AL/B and its combined interaction had more critical influence on the compressive strength of TBGM compared to SS to SH ratio. The microstructural and durability investigation further validated the optimization studies, as the optimum mortar specimen exhibited a denser microstructure with reduced shrinkage and sorptivity values.