The performance of a fire resistant coating for tunnel passive fire protection under successive severe thermal loading is 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. Its performance was assessed by subjecting a concrete slab with a five cm thick K-geopolymer coating layer into successive RijksWaterStaat (RWS) fire incidents. During the first test, the maximum measured temperature in the K-geopolymer/concrete interface was 250 °C, which is 130 °C lower than the RWS test requirement, while, during the second fire test, the maximum temperature was almost 370 °C, which is still lower than the RWS requirement proving the effectiveness of the material as a thermal barrier. In addition, the material retained its structural integrity, during and after the two tests, without showing any mechanical or thermal damages.

In this paper, the development of fire-resistant geopolymers for the passive fire protection of buildings is investigated. The fire-resistant geopolymeric materials were based on the alkali activation of a metallurgical slag produced in primary copper industry with a highly alkaline solution of potassium hydroxide. In order to decrease the density of the developed geopolymers, their foaming or the addition of lightweight aggregates was also studied. The physical, mechanical and thermal properties of the developed materials were determined and their performance upon exposure to fire was tested according to the ISO-834 standard time–temperature curve, which is considered as appropriate for testing fire protection systems employed in buildings. During the whole test of the materials performance against fire, the temperature at the interface between concrete and geopolymer was recorded below 180 °C, as required by the ISO test followed, proving effectiveness for the developed materials to be used as a thermal barrier in buildings and constructions. After testing, the geopolymers kept their structural integrity, without presenting any significant macroscopic damage.