Spent Coffee Grounds–Fly Ash Geopolymer Used as an Embankment Structural Fill Material

Abstract

The drinking of coffee forms a deep-rooted pastime in many communities worldwide. However, the culture of coffee drinking generates vast quantities of organic waste that ends up in landfills. Current research trends are inclined towards recycling of waste materials into alternative construction materials, hence the need to research sustainable uses for spent coffee grounds. Coffee grounds (CG) are highly organic with a very high percentage of biodegradable material. The objective of this research was to study the strength development of CG when used as a geopolymer stabilized embankment structural fill material aiming for a better understanding of geopolymer stabilization of highly organic material. Fly ash (FA), being a silica and alumina rich material, was used as a precursor. A liquid alkaline activator, L, being a sodium hydroxide-sodium silicate solution was used for alkali activation of FA in the CG-FA geopolymer. Factors found to affect strength development of the CG-FA geopolymer were: (1) the ratio of sodium hydroxide and sodium silicate in the activator liquid; (2) the curing time; (3) the replacement ratio of FA in the CG; (4) the alkalinity of the activator liquid used; and (5) the curing temperature. Optimally, FA can constitute up to 30% of the CG-FA mix for efficient geopolymerization to occur. The concentration of sodium hydroxide can be increased up to 12 mol before the strength development-to-alkalinity ratio decreases. The highest strength was found to occur when the curing temperature was 50°C. By introducing 30% of FA into CG, an efficient geopolymer can be synthesized with a $\mathrm{L}/\mathrm{FA}$ ratio of 1.8 and a ${\mathrm{Na}}_2{\mathrm{SiO}}_3:\mathrm{NaOH}$ ratio of $50:50$, which provides the maximum 7-day strength. California Bearing Ratio tests conducted on the CG-FA geopolymer, confirm that it is possible to use geopolymer stabilized CG as an embankment structural fill or subgrade material. Geopolymerization products will further encapsulate and bind the organics in CG, eliminating their potential to biodegrade, which attest to the efficacy of geopolymers as an alternative stabilization agent for highly organic soils.