Re – activation of recycled fly ash – based geopolymer binder to enhance mechanical properties

This research delved into employment of thermal activation as a technique of enhancing the reactivity of recycled binders produced from parent hardened geopolymer systems. The production process involved crushing hardened fly ash – based geopolymer pastes and concretes, followed by milling the resulting powder to produce non – calcined or raw recycled binders, that were in turn exposed to various high temperatures of 250° to 550 °C, then used to prepare geopolymer paste mixtures at activator to binder ratio of 0.5. The alkali activator used was a combined solution mixture of sodium silicate and 12 M sodium hydroxide. The recycled binders produced were characterised using particle size /shape analysis, Brunauer – Emmett – Teller surface area measurements and thermogravimetry, prior to microanalyses. The synthesized paste mixtures were evaluated to determine their setting times and compressive strength values, followed by analytical studies done using X – Ray Diffraction, Fourier – Transform Infrared Spectroscopy and Scanning Electron Microscopy. Results showed that setting times of the raw or calcined recycled binder – based geopolymer mixtures were a drastic 70% to 90% shorter than those of the control fly ash – based paste. It was found that calcination substantially enhanced geopolymeric reactivity of recycled binders, increasing the strengths of their mixtures by 36% to 54%. Interestingly, mixtures that were synthesized using the paste – derived recycled binder calcined at the optimal 400 °C temperature, exhibited strength development that was similar or comparable to that of the control fly ash – based geopolymer paste. • Calcination temperature of 400 °C was found to be optimal • Setting times of recycled binder-based geopolymer mixtures were 70-90% shorter • Mixtures made with calcined recycled binders gave strengths 36-54% higher • Raw recycled binders have weak reactivities and low strengths • Calcination enhances strength due to water loss and secondary geopolymerisation