Mix design and mechanical performance of geopolymeric binders and mortars using biomass fly ash and alkaline effluent from paper-pulp industry

This work investigates the use of biomass fly ash (BFA) and an alkaline effluent (AEF), both generated from the Kraft pulp industry, in the preparation of geopolymeric binders and mortars for construction applications. BFA replaced the metakaolin (MK) while the AEF substituted the distilled water used to dissolve NaOH pellets. The mix design aims to maximize the amount of both the wastes and to optimize the materials properties, such as workability and mechanical performance. At the same time, also the environmental impact decreases enhancing the materials’ sustainability and facilitating the circular economy. For the previously optimized BFA/MK ratio (70/30 wt.%) several NaOH/Na2SiO3 ratios and water to AEF substitutions were tested. The best performance was achieved for the alkaline activator having one part of NaOH and three parts of Na2SiO3. According to the main results, the biologic AEF can totally replace the distilled water in the NaOH preparation. Thus will increase the sustainability of the novel materials. Subsequently, the optimized binder was used – with and without AEF - to manufacture mortars with various binder/aggregate ratios. It is concluded that the formulations with a binder/aggregate mass proportion of 1:3 showed the best mechanical performance (compressive strength surpassing 20 MPa, class M20) and the replacement of distilled water with AEF did not affect the specimens final properties, but enhance the materials sustainability. Furthermore, in the pursuit of sustainability, manufacture and curing were conducted at ambient conditions (20 °C, 65% RH), avoiding any external source of energy, involving simple, reproducible, and low-cost processes.