Fire Resistant K-Based Metakaolin Geopolymer for Passive Fire Protection of Concrete Tunnel Linings

A fire resistant metakaolin geopolymer is developed and its performance under thermal loading is examined in this paper. The geopolymer was prepared by mixing metakaolin, doped with solid SiO₂, with a highly alkaline potassium hydroxide aqueous phase in order to create a paste that was subsequently cured at 70℃ for a certain period of time. The addition of solid SiO₂ was important in order the geopolymer developed to be fire resistant for temperatures up to 1350℃. The mechanical, physical and thermal properties of the metakaolin geopolymer were measured and compared with available commercial fire resistant materials. The behavior of the metakaolin geopolymer upon exposure on fire was studied following the EFNARC guidelines for testing of passive fire protection for concrete tunnels linings. The geopolymer was subjected to the most severe fire scenario, the Rijks Water Staat (RWS) temperature-time curve. During the test, the temperature in the geopolymer/concrete interface remained under 220℃, which is 160℃ lower than the RWS test requirement, proving the ability of material to work successfully as an efficient thermal barrier. After the test the surface of the geopolymer had exfoliated and cracked, without losing its structural integrity. Thus, the concrete slab protected by the geopolymer did not appear any form of spalling or degradation of its compressive strength.