Heat-resistant geopolymer derived from metakaolin–fly ash blend with bagasse ash addition: an innovative curved backing for steel pipe welding

Ceramic backing is used to support the weld groove and ensure welding quality. However, commercial ceramic backings are expensive and not reusable, limiting their applications and highlighting the need for improvement. This research presents the development of a heat-resistant geopolymer as an innovative curved backing for steel pipe welding. Geopolymers produced from metakaolin-fly ash blends improved their heat resistance by the addition of bagasse ash (BA) at contents of 0, 5, and 10 wt%. The results revealed that BA-added specimens showed lower thermal conductivity due to high porosity (22.76 ± 0.93%), larger pore size (5.63–10.12 nm), and the formation of nepheline phases with an increasing Si/Al ratio. The amount of BA was optimized to 10% to obtain a heat-resistant geopolymer with 29.85 MPa of compressive strength and 0.31 W/m·K of thermal conductivity, providing a good balance between strength and insulation. For the welding experiment, the 10% BA geopolymer was fabricated into a curved strip consistently with the weld-joint profile and successfully used as backing for gas metal arc welding (GMAW) of carbon steel pipe, exhibiting excellent thermal resistance and complete joint fusion. Welded joints showed high joint efficiency (89.28%) and tensile strength (555.63 MPa) was greater than the minimum specified tensile strength of the base metal according to ASME section IX. The hardness profiles of the weld and heat affected zone exhibited evenly distributed with hardness values in the range of 164.3–193.0 HV. These results indicate that BA-added geopolymer is a cost-effective and environmentally friendly alternative material for used as an innovative curved backing for pipe welding application.