Effective recycled binder produced upon thermal activation to enhance mechanical properties of fly ash – based geopolymer mortars

Growing interest in sustainable construction has driven the efforts to recycle concrete and hardened cementitious materials, including geopolymer binders. However, re - use of geopolymer binders is limited by their low reactivity and weak performance results. In this study, thermal activation of recycled binders derived from hardened fly ash – based geopolymer pastes and concrete, was employed to enhance binder reactivity for re - use. Raw recycled binders were calcined at 250 – 550 °C and used to prepare geopolymer mortars. Various mechanical properties were measured. At the optimal calcination temperature found to be 400 °C, compressive strength remarkably increased by 91% and 102% for the paste - derived and concrete - derived recycled binders, respectively. Calcination of the recycled binders also led to reduced pore volume of the resulting mortars, although drying shrinkage increased reaching 0.201%. Evidently, thermal treatment effectively enhances performance of recycled geopolymer binders for viability toward sustainable production. • Raw recycled binders were inadequate due to weak reactivity • Compressive strength increased by up to about 90% owing to binder calcination • Binder calcination showed pessimum effect on strength, with 400 °C being optimal • Drying shrinkage significantly increased due to binder calcination