Innovative Recycling of Lime Slaker Grits from Paper-Pulp Industry Reused as Aggregate in Ambient Cured Biomass Fly Ash-Based Geopolymers for Sustainable Construction Material

Lime slaker grits and biomass fly ash are solid wastes produced by the Kraft paper-pulp industry that are commonly disposed of in landfill. However, recent studies and European regulations discourage such disposal practices. This work investigates an alternative and innovative way to recycle and reuse these wastes in the production of green geopolymeric mortars intended for application in the construction industry. Here, biomass fly ash was used as the main source of alumino-silicate in the binder precursor (70 wt.% substitution to metakaolin), and grits (ranging from 1–12.5 mm, as provided by the industry) were reused as aggregate. Aggregate granulometry was also completed by using a commercial natural siliceous sand (<1 mm). Mortars using sand only were prepared for comparative reasons. The implemented mix was designed to investigate the influence of the grits on the mortar’s properties such as its binder/aggregate ratio, workability, bulk density, water sorptivity, and compressive strength. At the same time, waste reuse was analysed in light of its limitations and potentialities. Moreover, in the pursuit of sustainability, the manufacturing process that was followed was highly cost-effective in ambient curing conditions (20 °C, 65% RH), which avoided the use of any external source of energy as commonly used in geopolymers processing. The achieved results proved that the combined use of these wastes, which to date has hardly been explored, along with ambient manufacturing conditions increases the material sustainability. The produced mortars are suitable for innovative applications in various fields, with a particular focus on construction and contribute to the circular economy.