Intumescent alkali silicate and geopolymer coatings against hydrocarbon fires

This work focuses on the investigation of the fire protection of steel using inorganic intumescent alkali silicate and geopolymer (alkali aluminosilicate) coatings at temperatures relevant for hydrocarbon fires, as described in the UL1709 standard. Pure alkali silicate coatings based on Na, K, or a mixture of these with Li exhibited high initial expansion followed by melting. In comparison, Li-silicate coatings expanded less but demonstrated significantly higher thermal stability. Increasing the SiO₂/Na₂O molar ratio prolonged the fire protection time, explained by the lower melt formation proposed by global equilibrium calculations. The presence of melting in the high expanding alkali silicate systems limits their use in hydrocarbon fire conditions. In comparison to pure alkali silicates, geopolymer coatings with kaolin, metakaolin, and fly ash and additional CaCO₃ displayed a higher thermal stability confirmed by global equilibrium calculations. The kaolin-based coating provided the best fire protection with a critical time of 37.6 min, explained by its high expansion compared to metakaolin and fly ash-based coatings. Examining the influence of CaCO₃ and kaolin content suggests that an optimum exists for the kaolin coatings in terms of expansion, fire protection, and thermal stability. The best performing kaolin coating (37.6 min) had a lower fire protection compared to a state-of-the-art commercial organic hydrocarbon coating (44.2 min), caused by their differences in internal structure. The commercial coating expanded to a more compact microporous solid, while the kaolin coating qualitatively displayed a higher proportion of macropores. This in turn suggests that future work must be performed to further improve the internal structure of the kaolin-based coatings to ensure good fire protection.