Stiffness and deformation properties of spent coffee grounds based geopolymers

Spent Coffee Grounds (CG) are the granular organic waste material from the food and beverage industry resulting from the brewing of coffee, and are often disposed to landfills. Fly Ash (FA) is the primary by-product of coal-combustion power-plants used for energy generation. Slag (S), on the other hand, is an industrial waste generated from iron-ore refineries. In this research, geopolymers comprised of various mix designs of CG, FA, and S were developed via alkaline activation to create a green construction material out of these traditional waste materials. The intended use for this recycled construction material was as a subgrade fill, which if successfully implemented would considerably reduce the need of disposing such waste materials into landfills. In this article, key design parameters including Unconfined Compressive Strength (UCS), resilient moduli (M R ) and permanent deformation characteristics of CG-geopolymer remoulded cylindrical specimens are evaluated using simulated long-term Repeated Load Triaxial (RLT) tests. Regression parameters used in two M R prediction models, namely the Bulk Stress Model and 2-Parameter Model, were determined for the CG geopolymers. The developed 3-Parameter Model was found to be valid for predicting the M R of CG geopolymers. When cured at 21 °C, the M R for CG + FA geopolymers is generally lower than that of CG + S geopolymer specimens. Curing at 50 °C reduces this difference of M R values between FA and S geopolymers. At 50 °C curing, the M R of some mixes, notably 70CG:30FA + 90Na 2 SiO 3 :10NaOH, 70CG:30 S + 90Na 2 SiO 3 :10NaOH, and 70CG:30 S + 50Na 2 SiO 3 :50NaOH decreases after 28 days of curing while gaining increased UCS. This indicates that these geopolymers can gain improved ductility while developing high strengths under optimum curing conditions. This article evaluates key design parameters including the M R for CG geopolymer specimens, which enables coffee grounds to be used as green recycled construction materials in roads.