Optimizing Mechanical and Thermal Properties of Slag-Based Geopolymer Fiber Boards via Fiber Pretreatment and Reinforcement Type

This study aims to optimize the physical, mechanical, and thermal properties of 100% Ground Granulated Blast Furnace Slag (GGBFS) based geopolymer wood-composite panels. Pine fibers were utilized as the primary reinforcement matrix, while glass and hemp fibers were introduced as secondary reinforcements at varying proportions (3%, 6%, 9% by weight). The research investigated the effects of fiber pretreatments (hot water vs. 1% NaOH) and reinforcement hybridization. Results indicate that GGBFS successfully geopolymerized, forming a hybrid N-A-S-H and C-A-S-H gel network. Quantitative analysis revealed that 9% glass fiber reinforcement yielded the highest mechanical performance, achieving a Modulus of Rupture (MOR) of 10.05 N/mm2 and Internal Bond (IB) strength of 1.32 N/mm2, alongside superior water resistance (1.0% Thickness Swelling). Conversely, while hemp fiber inclusion reduced mechanical strength (MOR: 5.77 N/mm2 at 9%), it significantly enhanced thermal insulation, reducing thermal conductivity to 0.10 W/m·K. It was observed that aggressive NaOH pretreatment caused alkali-induced degradation of pine fibers, negatively impacting the composite’s integrity compared to hot water treatment. This study demonstrates the feasibility of tailoring 100% slag-based geopolymer composites for either structural (glass-reinforced) or insulating (hemp-reinforced) applications using industrial by-products.