Study On Mechanical And Thermal Comprehensive Properties Of Low Heat Cement Based Cementitious Material System

Low heat cement and ordinary cement mixed with mineral admixture are two common cementing materials for mass concrete engineering.Low heat cement has low hydration heat and low early strength,and its hydration behavior is different from that of ordinary cement.Its mechanical and thermal properties are more contradictory.At present,there are few reports on the process hydration of low heat cement and optimization research on the mechanical and thermal properties of its cementitious material system.In this paper,based on the law of mechanical strength,hydration heat and resistivity measured by experiments,the early hydration behavior of low heat cement and ordinary cement cementitious system is discussed,the influence of mineral admixture on the mechanical and thermal properties of low heat cement cementitious system is systematically studied,and the calculation model of compressive strength and hydration heat is established.At last,the comprehensive mechanical and thermal performance of low heat cement cementitious system is optimized from two angles of qualitative analysis and quantitative calculation.Specifically as follows:(1)Development law of mechanical and thermal properties of low heat cement and ordinary cement cementitious material under the same hydration heat in 3d and 7d are compared,and the difference of hydration behavior is analyzed by resistivity and hydration heat methods.Research results show that:Under the same 3d and 7d hydration heat conditions,the peak value of hydration temperature and the final hydration heat of the low heat cement are lower than that of ordinary cement cementitious material,and its later strength increase rate is higher,which makes it suitable to be utilized in long-term hydraulic mass concrete;The resistivity of low heat cement is small,and its development rule is different from ordinary cement;The hydration process can be independently characterized by resistivity and hydration heat,and the early hydration behavior of low heat cement can be described better by the resistivity.(2)Hydration heat,compressive strength and resistivity of low heat cement materials with different content of fly ash and slag are determined to analysis their development laws in the hydration process,the feasibility of using resistivity to predict the hydration heat and compressive strength is discussed.Research results show that: With the increase of mineral admixture,the mechanical strength and hydration heat of low heat cement cementitious system decrease,and the reduction is not directly proportional to the amount of admixture;The relativity of hydration heat,compressive strength and resistivity of low heat cement material increase with the age of hydration,and there is a good linear correlation between resistivity in 72 h and compressive strength in 7d,28 d and 90 d,and hydration heat in 3d and 7d.(3)The applicability of traditional hydration heat fitting formula to low heat cement cementitious material is evaluated,and a calculation model of hydration heat for low heat cement cementitious system is established.Research results show that: Mineral component method can only calculate the hydration heat of characteristic age and its results are some different from experimental values;Conversion formula method is accurately fit for the hydration heat calculation of single admixture cementitious material;Numerical fitting method is applicable to the low heat cement cementitious system with different amount of fly ash and slag,whose final hydration heat under the system is 126.6 J/g and 172.4 J/g respectively.(4)A evaluation function is established by linear rule based on the mortar strength and hydration heat of low heat cement cementitious materials.According to which,comprehensive performance satisfaction of cementitious materials is calculated to draw a satisfaction contour map.Research results show that: The contour distribution of comprehensive performance satisfaction of low heat cement cementitious system can be approximated as a series of concentric elliptical lines,and the cementitious material system with0%～10% fly ash and 5%～20% slag has high comprehensive performance for lower hydration heat and higher strength.(5)A prediction model of mechanical and thermal properties of low heat cement cementitious system is established by projection pursuit regression(PPR)software,and the criteria for selecting PPR samples and criterion of model accuracy are presented.Through simulation calculation and the drawing the comprehensive performance contour map,the comprehensive performance optimization is transformed into the optimization of its mechanical and thermal single target properties.Research results show that: The Comprehensive performance optimization method based on PPR model could directly determine the mechanical and thermal index and its mix proportion without subjective weighting and assumed modeling.