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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 used as the primary reinforcement, while glass and hemp fibers were added as secondary reinforcements. The research investigated the effects of fiber pretreatments (hot water and 1% sodium hydroxide (NaOH) solution and varying proportions (3%, 6%, 9% by weight) of these secondary reinforcements on the slag-based geopolymer matrix. Properties such as density, water absorption (WA), thickness swelling (TS), thermal conductivity (λ), modulus of rupture (MOR), modulus of elasticity (MOE), and internal bond (IB) strength were determined. Furthermore, microstructural and chemical interactions were examined through Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM) analyses. Results indicate that GGBFS successfully geopolymerized upon alkali activation, exhibiting strong mechanical properties due to the formation of N-A-S-H and C-A-S-H gels. While hot water pretreatment provided a slight improvement in the fiber-matrix interface, NaOH pretreatment led to alkali-induced degradation, causing significant reductions in performance. Glass fiber reinforcement generally resulted in higher density and systematically increased MOR, MOE, and IB values, significantly enhancing the mechanical performance and water resistance of the composites. In contrast, hemp fiber reinforcement typically yielded lower density values and reduced mechanical properties due to fiber agglomeration and high hydrophilicity. However, HF-containing groups, particularly with 9% addition, showed a notable reduction in thermal conductivity (down to 0.10 W/m·K), suggesting potential for lightweight insulation applications. FTIR, XRD, and SEM analyses corroborated the effects of fiber-matrix interactions and microstructural defects on the observed macro-properties. This study provides valuable insights into utilizing industrial and agricultural wastes in geopolymer composites, supporting sustainable and carbon-neutral construction goals.