Optimising Compressive Strength: Investigating the Ideal Conditions for Sodium Silicate-Activated Fly Ash-GGBS-Based Binder

Sustainable binding materials, such as fly ash (FA) and ground granulated blast-furnace slag (GGBS), are gaining popularity due to their engineering properties to be used as a substitute for cement with low CO2 emission. Our previous study found that a mixture of FA-GGBS can be activated without sodium hydroxide (NaOH) in the alkaline solution and cured at ambient temperature without compromising compressive strength. The objective of the current study was to optimise the composition and curing conditions for sodium silicate-activated FA-GGBS mixture. The aim was to achieve mortar or concrete with higher compressive strength without using hazardous NaOH for alkaline activation. The investigation revealed that maintaining the sodium silicate content at 50% of the binder weight and the water/binder ratio at 0.3 resulted in excellent performance of the FA-GGBS-based concrete activated without NaOH. Additionally, adding 2% micro-silica improved the workability and early compressive strength of the alkali-activated binder mix. Trials with self-compacted concrete demonstrated minimum variability in compressive strength and density. The minimum compressive strength was 7% higher when cured at ambient temperature than the traditional curing method of immersing in water for 28 days and testing under saturated surface dry conditions. This is the first report of a systematic optimisation study to produce mortar or concrete products with better performance by activating an alkali-activated binder mix without highly corrosive sodium hydroxide and external heat sources, making it ideal for field applications.