Effects of spent coffee grounds as partial soil replacement on the strength, stiffness, and microstructure of foamed...

Foamed lightweight soil (FLS) is a cement-based lightweight material valued for its low density and ease of placement. This study investigates the combined effects of water content and spent coffee grounds (SCG) as a partial soil replacement on the engineering properties of FLS. Mixes were prepared with water-cement (w/c) ratios of 0.6 and 0.8 and SCG replacement levels of 0 to 3% by equivalent volume of the dry soil fraction. Physical and mechanical properties were evaluated through density measurements, unconfined compressive strength (UCS) tests, stress-strain behavior, secant modulus (E₅₀), and microstructural observations using SEM-EDX. All mixes achieved fresh densities within the target range of 950-1050 kg/m³. Increasing the w/c ratio from 0.6 to 0.8 improved strength and stiffness, with 60-day UCS increasing from 355 kN/m² to 417 kN/m² and E₅₀ from approximately 9422 kN/m² to 10,036 kN/m². The inclusion of up to 3% SCG further enhanced UCS and stiffness within the investigated range. SEM-EDX observations indicated a hydrated cementitious matrix containing Ca-, Si-, O-, Al-, and S-rich regions. However, the identification of specific hydration products was treated as morphology and element-based inference because EDX alone cannot provide definitive phase confirmation. The mix with 0.8 w/c and 3% SCG showed the best performance among the tested mixtures, demonstrating the potential feasibility of SCG as a low-dosage waste-derived modifier for FLS.