Sustainable geopolymer composites based on blast furnace slag reinforced with coffee silverskin fibers: Structural, thermal and mechanical evaluation

This study reports the development of sustainable slag-based geopolymer composites as potential substitutes for Portland cement materials. The main environmental benefit arises from the reduction of CO 2 emissions achieved by using blast furnace slag (BFS) as the primary precursor. Geopolymer composites were prepared with a solid-to-liquid (S/L) ratio of 1.6, activated with sodium metasilicate (Na 2 SiO 3 ), and reinforced with 2, 3, and 5 wt.% coffee silverskin fibers, a lignocellulosic by-product from the coffee industry, in two conditions: untreated (SGC n ) and NaOH-treated (SGC NaOH ). The chemical, thermal, mechanical, and microstructural properties were investigated by X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA/DTG), Shore D hardness, compressive strength testing, and field-emission scanning electron microscopy coupled with energy-dispersive spectroscopy (FEG-SEM/EDS). Composites reinforced with NaOH-treated silverskin exhibited improved mechanical performance, with compressive strength increases of up to approximately 14% compared with the neat geopolymer, as well as enhanced thermal stability (T max ≈ 722 °C). Microstructural analysis revealed improved fiber–matrix interaction and a more compact geopolymer matrix in the treated systems. FTIR and XRF results confirmed the formation of a typical aluminosilicate geopolymer network and effective incorporation of the lignocellulosic fibers within the inorganic matrix. Overall, the results demonstrate that coffee silverskin can be effectively valorized as a reinforcing phase in low-cost, low-carbon, and mechanically robust BFS-based geopolymer composites, highlighting their potential for sustainable construction applications. • Sustainable blast furnace slag (BFS)-based geopolymer composites reinforced with coffee silverskin fibers were successfully developed. • NaOH-treated silverskin fibers improved fiber–matrix interaction and increased compressive strength by up to ∼14%. • Fiber loading and alkaline treatment significantly influenced thermal stability, with treated systems showing T max values up to ∼722 °C. • FEG-SEM/EDS revealed improved interfacial bonding and more compact geopolymer matrices after NaOH fiber treatment. • The results demonstrate the feasibility of valorizing coffee silverskin as a low-cost, low-carbon reinforcement for sustainable construction materials.