Recycled glass (RG) is a demolition waste, rich in silica and with a high shear strength and has been used as a replacement material for sand in various construction applications. Spent coffee grounds (CG) is a waste material derived from brewing coffee and has been recently studied as a recycled construction material, due to its physical resemblance to sandy soils. Geopolymerization, is a green process which produces cementitious compounds using aluminosilicate-rich materials and alkaline liquids. In this research, a new geopolymer construction material was produced using RG as a supplementary filler material to stabilize CG. Fly ash (FA) and slag were used as the precursors to induce geopolymerization in this new RG + CG construction product. To maximize the potential strength of the geopolymer, fine RG was added into geopolymer mixes in proportions of 25%, 50%, and 75% to observe the effects on the final RG + CG product strength. The mixes were compressed into cylindrical specimens, cured at room temperature (i.e., 21 °C) and 50 °C for 7 and 28 days, and tested for their unconfined compressive strength (UCS) to observe the effect of the various RG replacement ratio on the strength of CG geopolymers. Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) were used to further explain the role of RG in influencing strength development. Higher RG contents were found to lead to higher UCS values. A lower liquid-to-precursor (L/P) ratio was required to achieve a saturation point in strength development. RG was found to provide mechanical strength and supplementary chemical bonding strengths by dissolving and contributing Si+ ions to form geopolymeric substances.
