Preparation And Properties Of Porous Cement-based Fireproof Materials In Tunnels

Fireproof coating for tunnels have been widely used as a measure of fire protection in many aspects,but they have the disadvantages of low bond strength and high toxicity.In this paper,lightweight and non-toxic porous aluminate cement-based material was prepared with Na₂O₂,which replaces inorganic thick Fireproof coating or organic foaming material to achieve the purpose of reducing self-weight and improving fire resistance.The preparation parameters of porous aluminate cement-based material were determined according to the compressive strength,dry density,porosity and thermal conductivity of porous aluminate cement-based material,and XRD,SEM and image analysis were used to clarify the mechanism of fire-resistant,and the porous aluminate cement-based material was modified by fly ash and wollastonite to finally obtain porous aluminate cement-based material with excellent fire resistance,which laid a theoretical foundation for the application of porous aluminate cement-based material in tunnel fire prevention.The research results showed when the mass of Na₂O₂ occupies 3%of the aluminate cementand the water-cement ratio（m_w:m_C）is 0.53,the optimum porous aluminate cement-based material can be prepared.And its compressive strength is 0.68 MPa,the dry density is 445.96 kg·m^-3,the porosity is 78.17%,and the thermal conductivity is 0.1389W·（m·k）^-1.Based on this parameter,the fire endurance of the porous aluminate cement-based material before and after freeze-thaw cycle could meet the index requirements of the tunnel fireproof material in the standard GB28375-2012"Fireproof Coating for Concrete Structures".The average pore size of porous cement-based materials is 212 um,and the pore structure uniformly distributed in the samples.There are feldspars in the hydration products of porous aluminate cement-based material,which is beneficial to maintain a stable structure in a fire environment,reduce the number of cracks caused by high temperature,and increase the fire endurance of porous aluminate cement-based material.Fly ash was used to improve its mechanical properties and prepare porous FA-CA cement-based materials.When the amount of fly ash occupies 20%of the aluminate cement and the water-cement ratio is 0.69,the optimum porous FA-CA cement-based material can be prepared.After 15 freeze-thaw cycles,the strength loss is 1.5%lower than that of porous aluminate cement-based material.At the same time,its fire endurance still meets GB28375-2012"Fireproof coating for concrete structure".The sodium hydroxide in the sample is consumed and reacts with the Si-O-Si and Si-O-Al structures of fly ash to produce phases such as C-S-H gel,C-A-S-H gel and garnet,which promotes the development of strength of porous FA-CA cement-based materials.The pore structure distribution of porous FA-CA cement-based materials is obviously more uniform than that of porous aluminate cement-based material,and the average pore diameter can be reduced to 171μm.Wollastonite was used to improve its toughness and prepare porous FA-CA-S cement-based materials.When the amount of wollastonite occupies 20%of the aluminate cement,the porous FA-CA-S cement-based material has the maximum flexural strength.The fire resistance of porous FA-CA-S cement-based materials before and after freeze-thaw cycles still meets GB28375-2012"Fireproof coating for concrete structure".The addition of wollastonite did not change the reaction system of porous FA-CA cement-based materials.In addition,the fibrous wollastonites are interlaced to form a good network structure and evenly distributed in the sample,which can effectively improve the pore structure and enhances the fire resistance of porous FA-CA-S cement-based materials.