| The fire will cause serious damage to the building,leading to the deterioration of the structure and the reduction of bearing capacity.Cement-based materials is one of the most widely used building materials.The research on the influence of high temperature on the properties of cement-based materials is an important problem concerned by the academic and engineering fields.In this study,the cement mortar is used as research object,combined with traditional test methods and modern testing techniques,the evolution of microstructure of cement mortar,water transport behavior and mechanical properties were studied at different temperatures(105°C,200°C,400°C,600°C,800 °C).The main research contents and results are as follows:(1)The surface morphology of cement mortar after different temperature effects was observed by digital microscope.It was found that obvious cracks in the interface between paste and particle when the heating temperature exceeded 200?.The crack opening increased with the rise of the temperature.The microstructure of cement mortar subjected to different temperatures was tested and analyzed by mercury injection porosimetry.The microstructure of cement mortar was tested and analyzed by means of mercury porosimetry.The characterization parameters such as porosity,total pore surface area and pore size distribution curve were measured.The results show that the microstructure of cement mortar is obviously deteriorated by high temperature,the porosity,average and median pore diameter show an obvious growth trend with the rise of temperature.According to the classification method of pore diameter,the proportion of non-harmful pores,less harmful pores,harmful pores and more harmful pores in cement mortar after the action of different temperatures was statistically calculated.(2)The microstructure evolution of cement mortar in saturated state was monitored by using advanced nuclear magnetic resonance technology and T2 spectra of saturated samples were obtained.The study found that the peak values of T2 of high-temperature damaged cement mortar all tended to move to the left with the increase of saturation time,indicating that the internal pore diameter of cement mortar decreased gradually with the increase of saturation time.This is due to cement mortar high temperature products and water contact secondary hydration reaction.And through theoretical model pore diameter,the transverse relaxed time can be converted to the transformation of T2 spectrum compared with mercury injection data.It was found that the peaks of the two showed good consistency in the small pore range,a combination of these can well reflect the cement mortar under the effect of high temperature and high temperature and moisture function after injury in the process of its change rule of microstructure.(3)The capillary water absorption process of cement mortar after high temperature was characterized by weighing method.The results show that the temperature is more higher,the greater the water absorption per unit area of the mortar,and the initial capillary water absorption coefficient increases with the increase of the heating temperature.Based on the nuclear magnetic resonance test results of high-temperature damaged mortar during one-dimensional water absorption,a quantitative relationship between initial capillary water absorption coefficient and microstructure parameters of high-temperature damaged cement-based materials was proposed.(4)The effects of different temperatures on the compressive strength,flexural strength and splitting tensile strength of cement mortar were studied.The changes of sound velocity and dynamic elastic modulus in mortar under different temperature were studied by ultrasonic testing.The results show that the compressive strength,flexural strength,splitting tensile strength and dynamic elastic modulus of cement mortar change obviously with the rise of temperature.The empirical relationship between mechanical properties and operating temperature of high temperature damaged cement mortar was established by linear regression equation. |
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