Mycelium biocomposites from spent coffee grounds, rice husk, and recycled paper for temporary eco-road guideposts: Microstructure-property relationships, fire resistance, and outdoor durability

• MBCs from fibrous RP and particulate SCG/RH were developed and characterized • MBC/RP achieved highest strength (1.67 MPa) and V-0 fire resistance • Fibrous RP enhances ductility (0.58 mm/mm); SCG fillers increase stiffness (2.41 MPa) • X-ray µCT and SEM images reveal substrate-morphology links to porosity and durability • MBC/RP suited for dry outdoor use; 1:4 scale guidepost prototype was fabricated Mycelium biocomposites (MBCs) offer a sustainable, zero-waste alternative for non-structural construction materials. This study investigated the influence of substrate morphology on MBC properties by using two novel waste streams: fibrous recycled paper (RP) alone and a particulate co-substrate of high-content spent coffee grounds (SCG) mixed with rice husks (RH), mainly for non-structural construction components (road guidepost). Three compositions of MBCs were fabricated: (MBC/RP, MBC/SCG50-RH50, MBC/SCG80-RH20). Compression test, water absorption, and fire resistance performance were characterized alongside microstructural analyses via SEM and X-ray μCT. Experimental results disclosed that substrate morphology critically governs MBC performance. MBC/RP achieved the highest compressive strength (1.67 MPa) at high strain 0.58 mm/mm and an excellent V-0 fire rating due to dense mycelial entanglement with fibrous substrate and protective char layer formation. Conversely, MBC/SCG-RH groups exhibited lower strength (0.25–0.46 MPa) and fire resistance. Nevertheless, MBC/SCG80-RH20 achieved the highest stiffness (2.41 MPa) and exhibited brittle behavior, linked to SCG-RH particle interlocking that created a closed-pore structure (58.27–61.61% porosity) and significantly lower water uptake (130%) than open-pored MBC/RP (272% water uptake and 52.87% porosity). Accordingly, MBC/SCG-RH groups are better suited for biodegradable packaging while MBC/RP was the only candidate satisfying non-structural construction materials specifications. Despite susceptible to high moisture, MBC/RP maintained structural integrity in dry environments, demonstrating a functional lifespan exceeding three months. The practical feasibility was validated by successfully fabricating an initial 1:4 scale MBC/RP road guidepost prototype. These findings confirm the potential of tailoring waste resources to meet mechanical, fire performance, and degradability for non-load-bearing outdoor applications.