Using silica fume-based activator in geopolymer binder production for improved sustainability

The effectiveness of using silica fume based sodium silicate in the production of metakaolin (MK) and ground granulated blast furnace slag (GGBS) based geopolymer was investigated. Un-densified silica fume was appropriately dissolved in a 10M sodium hydroxide solution to obtain an sodium silicate alternative (SSA) solution having the SiO2/Na2O molar ratio of 2. The SSA solution was used together with the 10M sodium hydroxide solution in 1:1 volume ratio as alkaline activator to elaborate MK and GGBS based geopolymer pastes with 1.5 and 0.7 alkaline activator/precursor material ratios respectively at ambient temperature of 23±2°C. Setting time, density, compressive strength, tensile strength, water absorption were the physical and mechanical tests performed on fresh and hardened pastes at 3, 7 or 28 days of curing. Powder of hardened paste at 28 days underwent thermogravimetric (TG/DTG) analysis at up to 1000°C. For comparison, similar geopolymer pastes were prepared with a standard commercial sodium silicate (SSC) having the same SiO2/Na2O molar ratio and submitted to similar tests. The silica fume based sodium silicate was found to be effective in geopolymer production as evidenced by the similar or slightly better performance in physical, mechanical and thermo-gravimetric properties when SSA was used compared to SSC. Un-densified silica fume could constitute suitable raw materials for sodium silicate production with additional sustainability potential in the reduction of embodied energy in the production process, since the by-product is not tumbled, heated or grinded and the dilution process with the sodium hydroxide solution happens at ambient temperature.