Experimental Study On Mechanical Properties And Hydration Mechanism Of New Cement-based Composite Cementitious Materials

With the promotion of the"the belt and road initiative"top-level cooperation initiative,China’s construction industry has developed rapidly and has broad prospects.Among them,prestressed concrete pipe pile has become the most widely used concrete component of building pile foundation in China.In order to improve concrete strength,improve concrete performance,reduce environmental pollution and make full use of resources,mineral admixtures or other auxiliary cementitious materials are often mixed into raw materials to form cement-based composite cementitious materials.The research of this project comes from the project"Research and Development of New Material and New Technology of Prestressed Concrete Pipe Pile"entrusted by enterprises and institutions.Taking the cementing materials in prestressed concrete pipe pile concrete as the research object,the mechanical properties of different cement-based multi-component composite cementing materials are studied and the optimal ratio is found.Characterization of hydration process of cement-based composite cementitious materials by electrochemical impedance spectroscopy;Through hydration heat test,XRD and TG-DSC test,the hydration products were analyzed qualitatively and quantitatively,and the hydration mechanism of cement-based composite cementitious materials was explored.The main research contents of this paper are as follows:（1）In order to explore the influence of cement-based composite cementitious materials on mechanical properties,mineral powder,dense silica fume,semi-encrypted silica fume and metakaolin were used to prepare cement-based composite cementitious materials with single additive,double additive and triple additive,and the influences of different kinds and different dosages of auxiliary cementitious materials on the flexural strength and compressive strength at different hydration ages of 3d,7d and 28d were studied.The results show that the encrypted silica fume,semi-encrypted silica fume and metakaolin can improve the flexural strength and compressive strength in different degrees,and the strength of the three-doped group is higher than that of the single-doped group and the double-doped group.Among them,K20P5M5with mineral powder,densified silica fume and metakaolin is the best proportion,which increases the 28-day flexural strength by 60.3%and the compressive strength by 29.5%compared with the pure cement control group.（2）To explore the influencing factors of cement paste compactness,the electrochemical impedance of cement paste with different parameters（age,water-cement ratio）and cement-based multi-component cementitious materials were measured,the Nyquist curve of frequency-impedance characteristic was fitted by using the equivalent circuit diagram,and the resistance R_sof electrolyte solution and the resistance R_ct1generated during charge transfer were analyzed.The results show that with the decrease of water-cement ratio and the increase of age,the Nyquist curve shifts to the right,the arc radius becomes larger,R_sand R_ct1both increase,and the compactness of cement-based materials increases.In addition,it is found that the addition of auxiliary cementitious materials is beneficial to improve the compactness of cement-based materials,and the improvement effect of auxiliary cementitious materials on compactness is more obvious when they are mixed again.The relationship between the parameters R_sand R_ct1and the compressive strength of mortar specimens with the same ratio is established,and the correlation is good.（3）The heat release in the hydration process of cement can characterize its hydration process.In order to explore the influence of different auxiliary cementitious materials on the hydration mechanism of cement,the early hydration heat release rate of cement-based multi-component composite cementitious materials was measured by measuring the hydration heat of cement.The results show that there are two exothermic peaks in the hydration rate curves of cement-based multi-component composite cementitious materials,and the hydration heat increases with time.Among them,the peak value of the second exothermic rate of P5 is the largest,and the exothermic amount is the highest,which indicates that metakaolin has the most significant effect on promoting the early hydration of cement,while the peak value of the second exothermic rate of K30 is the smallest,which indicates that mineral powder can delay the hydration of cement-based composite cementitious materials.（4）In order to explore the similarities and differences of hydration degree of cement-based composite cementing materials,XRD and synchronous thermal analysis tests were carried out on some samples.The test results show that the diffraction peaks of the X-ray diffraction pattern of cement-based composite cementitious materials are different.The CH of K20P5M5 has the lowest diffraction peak and the highest hydration degree.The TG-DSC curve was analyzed,and the total weight loss rate of DZ and K20P5M5 stages increased with the increase of CH content in different groups and different ages,indicating that hydration products increased in later stage.In the same period,the hydration of K20P5M5 produced less CH than that of DZ,indicating that the auxiliary cementing materials can have secondary hydration with CH and consume part of CH.In conclusion,ciphered silica fume,semi-ciphered silica fume and metakaolin can promote cement hydration,improve cement-based strength and improve slurry internal structure to varying degrees.Among them,K20P5M5 is the optimal ratio of this test.