Corrosion durability performance of green patch repair mortar made by mixture of fly ash and bagasse ash based geopolymer

This study evaluates the corrosion durability performance of sustainable patch repair mortars using a geopolymer binder derived from fly ash (FA) and bagasse ash (BA). Aimed at reducing environmental impact and supporting the circular economy, the research investigates how varying FA-BA precursor ratios affect the corrosion resistance of reinforced concrete repair materials. Mortar cubes (15×15×15 cm) were prepared with two embedded steel bars at 3 cm and 5 cm cover depths. Six binder combinations were tested: 100% FA; 90% FA + 10% BA; 80% FA + 20% BA; 70% FA + 30% BA; 60% FA + 40% BA; and 50% FA + 50% BA. Each was evaluated in triplicate under three environmental exposure conditions: dry, wet, and cyclic dry–wet. Corrosion activity was monitored over three months using half-cell potential measurements in accordance with ASTM C876. The results show that the 80% FA + 20% BA mixture provided the best corrosion resistance, indicated by the highest and most stable potential values across all exposures. This enhancement is attributed to the amorphous silica content in BA, which promotes the formation of a dense and chemically stable N-A- S-H (sodium aluminosilicate hydrate) gel matrix. The resulting microstructure likely improves impermeability and protects embedded steel by strengthening the passive layer. These findings demonstrate the effectiveness of FA-BA-based geopolymers as durable, low-carbon patch repair materials. The work supports the United Nations Sustainable Development Goals (SDGs), particularly Goals 9 (Industry), 11 (Sustainable Cities), 12 (Responsible Production), and 13 (Climate Action).