Research On Thermal Properties And Thermal Fatigue Of Cement-based Materials

The climate in Qinghai Tibet Plateau and Xinjiang is changeable,the temperature difference is large and the humidity is low.The surface cracking of concrete buildings is common.Concrete bridges and piers in this area have been subjected to temperature cycling more than in other parts of China,and the thermal fatigue caused by temperature cycling promotes the cracking of concrete surface.The cracking of this kind of concrete structure is mainly related to the uncoordinated deformation and temperature gradient field of aggregate and cement stone when the temperature changes.The temperature deformation and internal temperature gradient field of concrete are determined by its thermal expansion coefficient and thermal conductivity.However,at present,the theoretical and model research on the thermal conductivity and thermal expansion coefficient of cement-based materials is not deep enough,and the thermal properties of some phases in cement paste are still unknown,metakaolin,limestone powder and glass powder are widely used in cement-based materials,but their effects on the thermal conductivity and thermal expansion coefficient of cement-based materials are less studied.At the same time,there are few reports on the influence of aggregates on the thermal fatigue of cement-based materials.These problems can affect the development and application of concrete in Qinghai Tibet Plateau and Northwest China.The thermal properties of clinker phase and hydration product phase in cement paste are calculated by molecular dynamics method.On this basis,the thermal conductivity and thermal expansion coefficient of cement paste are calculated by multiphase composite model.Comparing the test results,it is found that the self consistent model can accurately predict the thermal conductivity of cement paste,and the relative error between the prediction results of the model and the test values is between 0.2%～4.7%;Turner model is suitable for the prediction of thermal expansion coefficient of cement paste,and the relative error between the prediction results and the test results is less than 6%.Based on the study of the thermal conductivity and thermal expansion coefficient of pure cement paste,the effects of metakaolin,glass powder and limestone powder on the thermal conductivity and thermal expansion coefficient of cement paste are studied.The mechanism of three mineral admixtures in cement paste is studied by means of thermal analysis,mercury injection and scanning electron microscope.The results show that compared with pure cement paste,the thermal conductivity of cement paste with 25% metakaolin increases by 54.6% and the coefficient of thermal expansion decreases by 25.2%,indicating that the adding metakaolin is beneficial to alleviate the difference between the coefficient of thermal expansion of cement paste and aggregate in concrete.On the other hand,compared with pure cement paste,the thermal conductivity of cement paste with 25% glass powder and limestone powder decreases by 24.1% and 36.3% respectively,and the thermal expansion coefficient decreases by 31.6% and 36.3% respectively,which is mainly related to the increase of porosity and the decrease of calcium hydroxide content in cement paste.Based on the research results of thermal properties of cement paste and aggregate,the effects of aggregate type,dosage and particle size on thermal conductivity and thermal expansion coefficient of mortar and concrete are explored.Considering the interface transition zone,the calculation method of thermal conductivity and thermal expansion coefficient of mortar and concrete is proposed.The results show that the order of thermal conductivity of aggregate and cement stone is quartzite > granite >limestone > basalt > cement paste,which makes the thermal conductivity of mortar and concrete increase with the increase of aggregate content.The order of thermal expansion coefficient of aggregate and cement paste is: cement paste > quartzite >granite > basalt > limestone,which makes the thermal expansion coefficient of mortar and concrete decrease with the increase of aggregate content,and the difference between the thermal expansion coefficient of aggregate and cement stone matrix in quartzite concrete is small,while the difference between the thermal expansion coefficient of aggregate and cement stone in limestone concrete is large.When the type and content of aggregate are the same,the thermal expansion coefficient of mortar and concrete basically remains unchanged with the decrease of aggregate particle size,while the thermal conductivity of mortar and concrete decreases with the decrease of aggregate particle size,which is mainly related to the increase of interfacial transition zone content and interfacial thermal resistance effect in concrete.Based on the study of thermal expansion coefficient of cement paste,aggregate and concrete,the thermal fatigue damage law of concrete with different aggregates is studied.The degree of thermal fatigue damage of concrete is characterized by compressive strength,ultrasonic velocity and water absorption.The results show that with the increase of thermal fatigue times,the compressive strength of concrete decreases,the ultrasonic speed decreases,the water absorption increases,and the microcrack width in the surface and interface transition zone increases.The width of microcrack in the interface transition zone of concrete is about 5 times that of its surface microcrack,indicating that the thermal fatigue damage of concrete mainly occurs in the interface transition zone.When the number of fatigue cycles is the same,the damage degree of limestone concrete is the most serious,that of basalt concrete is the second,and that of quartzite and granite concrete is the least.This shows that the closer the thermal expansion coefficient of aggregate and cement paste is,the smaller the thermal fatigue damage degree of concrete is.Finally,it is proposed that the selection of appropriate aggregate rock type,particle size and dosage can effectively improve the macro performance of concrete after thermal fatigue cycle.