Sustainable fly ash geopolymer coatings: substrate-dependent interfacial mechanisms and functional performance

Abstract Coating extends the service life of infrastructure, reducing maintenance needs and supporting sustainability goals. Fly ash geopolymer coatings are promising alternatives to organic and cementitious systems due to their low-carbon, VOC-free composition and chemical compatibility with cementitious substrates. This study investigates the collective impact of mix chemistry, application method and substrate type governing the performance of geopolymer coatings. Coatings were applied by spraying and brushing at varying mix proportions onto steel, mortar, and plywood substrates. Results show that the S/L ratio controls microstructural integrity: low ratios promote shrinkage cracks, while an S/L ratio of 2.0 produces denser matrices and improved pencil hardness (HB). The substrate type emerged as the dominant factor in determining adhesion and durability. Mortar exhibited the strongest interfacial bonding, confirmed by cohesive failure (100%), localized interfacial elemental enrichment (Si, Al, Fe) near the substrate surface, and reduced wear (up to 55%). In contrast, steel and plywood showed higher adhesive failures, leading to increased wear. The application method (spray vs. brush) primarily affected coating uniformity but did not substantially alter performance trends. These mechanistic insights demonstrate that geopolymer coatings are inherently more compatible with cementitious substrates, where interfacial chemistry and surface roughness facilitate durable bonding. Although absolute mechanical performance remains modest compared to polymeric coatings, the valorisation of fly ash and low-VOC application establishes geopolymer coatings as sustainable surface protection for cement-based infrastructure.