Performance of geopolymer mortar incorporating spent coffee grounds as a recycled building material: An experimental and predictive analysis

Spent coffee grounds ash (K) is investigated as a sustainable partial sand replacement (0 %, 5 %, 10 %, 15 %) in geopolymer mortars based on granulated blast furnace slag. This study assesses workability, mechanical performance, high-temperature resistance, and microstructure through experimental testing and statistical analysis. Key findings indicate that K content significantly influences workability, strength, and physical properties. Replacing sand with K reduced workability by up to 21 % due to its finer particle size and porosity. The K5 series, containing 5 % K, exhibited the highest compressive strength improvement, with gains of 8.9 % and 16.3 % at 7 and 28 days, respectively, compared to the control. However, higher K contents (10 %, 15 %) negatively impacted mechanical performance. At elevated temperatures, significant mass loss occurred up to 400 °C, after which it stabilized at 600 °C while maintaining structural integrity. Porosity and water absorption increased with K content, except in K5. Microstructural analysis revealed that the K5 series formed a dense, crack-minimal matrix, whereas K10 and K15 exhibited more microcracking and porosity. Statistical models confirmed that temperature had the greatest influence on compressive and flexural strengths, whereas K content significantly affected mass loss.