Development of Geopolymer-Based Weld Backing Using Recycled Industrial Waste for A36 Steel Welding Using GMAW Process

Geopolymer is an alkali-activated aluminosilicate material that combine ceramic-like thermal performance with low thermal conductivity, high-temperature resistance, while remaining castable and structurally adaptable. It is synthesized from calcined kaolin (metakaolin) and fly ash, which react with alkaline solutions to form strong covalent bonds through geopolymerization. This material offers a viable alternative to conventional refractories and imported ceramic products.In this study, a geopolymer material was developed for welding applications and utilized as a weld backing strip for gas metal arc welding (GMAW) of ASTM A36 carbon steel. Weld backing strips are essential for achieving full penetration and consistent root quality in large-scale steel fabrication, particularly in structural, shipbuilding, and heavy industrial construction. The geopolymer binder consisted of 35 wt% metakaolin, 15 wt% fly ash, and a 1:1 ratio of 10 M NaOH and sodium silicate solution. To enhance thermal resistance, river sand, fine glass powder, or recycled SAW flux was incorporated as an external solid phase. Geopolymer specimens were thermally cured and fired at 500 °C to eliminate moisture and organic. Moreover, it was heated to 900 °C to simulate welding heat exposure. Microstructural, mineralogical, and functional group transformations were evaluated using XRD, SEM, and FTIR, while mechanical strength, thermal conductivity, and density were also assessed. The results indicated that glass-enhanced geopolymer exhibited the lowest thermal conductivity (0.89 W/m·K) and highest compressive strength retention after firing, owing to its partial crystallinity and preserved amorphous phase. Flux-based composites showed extensive ceramic phase formation, while sand-based composites retained high thermal conductivity and suffered severe strength loss. Welding trials confirmed that geopolymer backings effectively supported root bead formation with no cracking.