Development Of Lightweight Basalt Fiber Reinforced Strain-hardening Cementitious Composites And Its Thermal Insulation Properties Research

The commonly used spray applied fire-resistive materials for steel structure(SFRM)are brittle materials,which rapidly fail under extreme loads such as earthquake,impact and explosion.The weakened bonding strength between SFRM and steel matrix leads to the fireresistive materials debonding and spalling easily,which seriously hinders its practical application.High ductility fiber reinforced fire-resistive engineered cementitious composites(FR-ECC)have the advantages of non-flammability,no material base containing toxic gases generated by thermal decomposition,and good durability.It also has the advantages of high ductility and good crack resistance.In recent years,it has attracted extensive attention and research.However,due to the conventional FR-ECC mostly uses organic fiber,decomposition and gasification of organic fiber at high temperature will lead to ductility decline and cracking of fire-resistive materials,which affects the fire resistance of materials at high temperature and restricts the application of high ductility fiber reinforced engineered cementitious composites.Therefore,it is urgent to develop a kind of fire-resistive materials with high ductility,high fire resistance,and light heat insulation in order to improve the ductility and thermal conductivity of fire-resistive engineered cementitious composites at high temperature.In this paper,basalt fiber(BF)is used as cement-based toughening material to prepare a lightweight high ductility basalt fiber reinforced engineered cementitious composites(LWBF-ECC),which has strain-hardening characteristics and excellent heat insulation performance at high temperature.By combining macro test and micro analysis,the evolution mechanism of LWBF-ECC matrix structure change,mechanical properties and thermal insulation performance under normal temperature and elevated temperature environment was studied,and the high-temperature performance law and mechanism of LWBF-ECC were further discussed.The details are as follows:(1)In this study,calcium aluminate cement(CAC),ordinary portland cement(OPC)and slag calcium sulfoaluminate cement(S.SAC)were used as cementitious materials.The optimal water-cement ratio and light aggregate content corresponding to the three kinds of cement were determined through trial mixing,and different mixes were designed to prepare LWBF-ECC.Based on the optimized mixs,the basic mechanical properties of LWBF-ECC materials were tested at room temperature of 20°C for 28 days,including uniaxial tensile strength and ductility,uniaxial compression and flexural strength tests,and the basic mechanical properties of LWBF-ECC materials were tested at elevated temperatures(100°C,200°C,400°C,600°C and 800°C).(2)In order to measure the bonding performance of LWBF-ECC material to steel structure,the influence of cement type on coating bonding strength was studied by analyzing the interface transition zone,and the interface failure mechanism was analyzed.The bonding strength of LWBF-ECC and steel structure met the requirements of the specification and had good durability.(3)In order to study the influence of high temperature on the thermophysical properties of LWBF-ECC,the dry density and thermal conductivity of aluminate cement(CAC),ordinary portland cement(OPC)and slag sulfoaluminate cement(S.SAC)were tested at room temperature and elevated temperatures.The apparent characteristics of engineered cementitious composites under different temperatures were observed macroscopically,and the variation of dry density and thermal conductivity of LWBF-ECC with temperature elevated was studied.When the temperature was above 400°C,LWBF-ECC has better thermal insulation performance than SFRM and LW-ECC prepared by PVA fiber.The pores in the matrix are not obviously connected and have good crack distribution characteristics.(4)By means of TGA-DSC and CT microscopic experiment,the microstructure of LWBF-ECC after high temperature was studied,and the matrix structure changes of each component in the fire-resistive material under elevated temperatures were analyzed,including density change,hydration product change and pore distribution.The thermal conductivity of LWBF-ECC after elevated temperatures of different cement types were explained from the microscopic point of view.Compared with TG mass loss and density reduction,the large mass and strength loss of the specimen in the heating process mainly came from hydrate transformation and water of crystallization loss.At 400°C,small-size pores in cement-based materials increase and micro-cracks further expand.The distribution of pore size is consistent with the change of macroscopic thermal conductivity.In this study,engineered cementitious composites was used as fire-resistive matrix material and inorganic basalt fiber is used as toughening material to improve the ductility and thermal conductivity deterioration caused by melting decomposition of PVA fiber at high temperature.Compared with FR-ECC,LWBF-ECC still has strain hardening and multi-crack cracking characteristics at high temperature(the ultimate elongation is 0.78% and 0.32% at100°C and 200°C),and still has high residual compressive strength at 800°C.LWBF-ECC improves the heat insulation performance of FR-ECC at high temperature(thermal conductivity of 0.1151 W/m·K at 600°C,compared with 0.14 W/m· K for the FR-ECC prepared with PVA fiber and 0.2 W/m·K for the SFRM).The material meets the requirements of the specification for inorganic fire-resistive material and plays an active role in promoting the development of steel structure fire prevention and environment-friendly construction industry in China.