Study On Temperature Rise Degradation Mechanism Of Fly Ash/Silica Fume Modified Magnesium Phosphate Cement

The high temperature resistance of building materials is an important topic in the field of civil engineering research.The high temperature effect caused by fire will lead to a large number of high temperature damage in the building material matrix,resulting in a significant decrease in the mechanical properties of building materials,which seriously threatens the safety of building structures.Magnesium phosphate cement(MPC),as a branch of magnesium-based cement,is a new type of green cementitious material that can rapidly coagulate and harden at room temperature,which has the dual characteristics of ceramics and cement.Because of its fast setting and early strength,good adhesion and volume stability,it is mainly used as a fast repair and reinforcement material and is applied to the fields of engineering repair and reinforcement.This paper takes building fire as the research background.Firstly,starting from the basic components of MPC raw materials,through continuous temperature rise test,the effects of different magnesium-phosphorus ratio(M / P),different water-binder ratio(W / C)and different mineral admixture content(FA,SF)on the performance of MPC after high temperature are studied.The high temperature damage and degradation mechanism of MPC were studied by X-ray diffraction(XRD),comprehensive thermal analysis(TG-DTG),scanning electron microscopy(SEM-EDS)and nuclear magnetic resonance(NMR)combined with damage mechanics.The main contents are as follows :(1)The influence mechanism of the basic composition of raw materials on the performance of MPC : The hydration heat,setting time,fluidity,drying shrinkage and mechanical properties of MPC were studied with different M / P,different W / C and different mineral admixture content as variables.(2)High temperature damage evolution characteristics of MPC : The continuous temperature rise test of MPC with different M / P and different mineral admixture content was carried out within 20-1000 °C.The damage evolution law and damage mechanism of MPC after continuous temperature rise were studied.The microstructure of MPC with temperature was studied by XRD,SEM-EDS and TG-DTG,and the high temperature damage characteristics of MPC hydration products were explained.(3)High temperature damage analysis of MPC based on pore structure evolution : There are a large number of irregular and cross-scale pores in MPC.The pore structure parameters such as T2 spectrum,pore size distribution and porosity of MPC after high temperature were measured by NMR,and compared with the pore characteristics at room temperature.Based on the evolution of pore structure of MPC after high temperature,the characteristics of high temperature damage and performance evolution of MPC were clarified.(4)Study on the damage constitutive model of MPC at room temperature and high temperature : The characteristics of stress-strain curves of MPC with different ratios and different temperatures were comprehensively analyzed,and a unified damage constitutive model of MPC including temperature weakening effect factors was proposed.Based on the damage theory and the equivalent stress principle,combined with the Gaussian probability density distribution function,the established compressive damage constitutive model can better predict the stress-strain characteristics of MPC at room temperature and high temperature.