Potassium based geopolymer for passive fire protection of concrete tunnels linings

[Display omitted] •A fire resistant potassium based geopolymer was developed.•FeNi slag doped with pure alumina was used as raw material.•The K-geopolymer possess adequate mechanical properties.•Subjected to thermal loading the K-geopolymer was resistant to the RWS curve.•The test proved that the K-geopolymer can put an effective heat flux barrier. The design of a fire resistant coating for tunnel passive fire protection and its performance under thermal loading are presented. The material falls under the class of potassium based geopolymers (K-geopolymer) and was prepared by mixing ferronickel (FeNi) slag, doped with pure alumina, with a highly alkaline potassium hydroxide aqueous phase. The physical, mechanical and thermal properties of the K-geopolymer were determined and compared to those of some commercially available fire resistant materials. Its behavior upon exposure to fire was assessed by subjecting a concrete slab, coated with a 5cm thick K-geopolymer layer, to thermal loading under the RijksWaterStaat (RWS) temperature–time curve, which is considered as the most severe prescribed tunnel fire scenario. During the test, the geopolymer/concrete interface temperature remained under 280°C, which is 100°C lower than the RWS test requirement, proving the effectiveness of the material as a thermal barrier. In addition, the K-geopolymer retained its structural integrity after the test, without any significant macroscopic damage.