Experimental Study On Mechanical Properties Of High Ductility Magnesium Phosphate Cement-based Fiber Composite Material With Mineral Admixture

Magnesium phosphate cement(MPC)is a type of inorganic cementitious material,which has high early strength,fast hardening and fast setting,excellent bonding performance,good fire resistance,good volume stability and good temperature adaptability.However,like ordinary Portland cement,MPC belongs to brittle material,which limits its application range.In this paper,MPC-based composites with strain hardening characteristics were prepared by adding PVA fiber and mineral admixtures.The effects of single and double mineral admixtures(fly ash,blast furnace slag and silica fume)on the workability and mechanical properties of MPC and MPC-based composites were studied.The microscopic characteristics were tested by SEM and XRD.The influence mechanism of single and double mineral admixtures on the mechanical properties of MPC was analyzed.The strain hardening mechanism of MPC-based composites was analyzed by using the stress and energy criteria based on fracture mechanics analysis method.The relationship between pseudo strain hardening performance index and ultimate tensile strain were established,and the threshold of pseudo strain hardening performance index was determined.The main research contents and conclusions were as follows:(1)The effects of single and double mineral admixtures(fly ash,blast furnace slag and silica fume)on the setting time,fluidity and compressive strength of MPC mortar were studied.The results showed that adding appropriate amount of fly ash can improve the setting time and fluidity of MPC and improve the compressive strength of MPC mortar.The compressive strength of MPC mortar with 30%fly ash was the highest.The incorporation of blast furnace slag had a negative impact on the setting time of MPC paste and the compressive strength of MPC mortar.The incorporation of silica fume reduced the working performance of MPC.Appropriate incorporation of silica fume can improve the compressive strength of MPC mortar,and the compressive strength of MPC mortar with 5%silica fume was the highest.Compared with single addition,double addition of fly ash and blast furnace slag did not improve the compressive strength of MPC mortar,while double addition of fly ash and silica fume could significantly improve the compressive strength.Through SEM and XRD,it was found that the microstructure of MPC matrix with 30%fly ash,5%silica fume,25%fly ash and 5%silica fume was dense.After adding silica fume,a new hydration product,magnesium silicate cementitious material,was generated,which increased the compactness of the matrix and improved the compressive strength of MPC mortar.(2)The effects of fly ash,silica fume,fly ash-silica fume on the working performance and mechanical properties of MPC-based composites were studied.The stress and energy criteria based on fracture mechanics analysis were used to analyze strain hardening mechanism,which established the relationship between pseudo-strain hardening performance index and ultimate tensile strain.The results presented that the proper amount of mineral admixtures(fly ash and silica fume)could improve the working performance and compressive strength of MPC-based composites.Fly ash,silica fume and fly ash-silica fume had significant impacts on the micromechanical properties,and the pseudo strain hardening performance index was positively correlated with the tensile strain capacity of MPC-based composites.Therefore,fracture toughness and fiber bridging ability could be used to explain the influence of mineral admixtures on the tensile strain ability of MPC-based composites,from the microscopic scale.(3)The relationships between the micro-mechanical index and the macro-mechanical index of the mineral admixture MPC-based composites had been established:when the curing age was 7 days,the comparison was made between fly ash,silica fume,and fly ash-silica fume.The correlation between the micro-mechanical indexes and the macro-mechanical indexes of coal ash-silica fume presented that the pseudo-strain hardening performance indexesP S H_σandP SH_J must reach above 1.8and 17 respectively,at the same time,to have the characteristics of strain hardening and the ultimate tensile strain capacity of more than 3%.The corresponding mineral admixtures were as follows:1.Fly ash of 20%-50%;2.Fly ash-silica fume of 40%,of which 30%fly ash+10%Silica fume or 35%fly ash+5%silica fume.(4)The effects of age on the compressive strength and tensile properties of mineral admixtures and MPC-based composites were studied.With the increase of the curing age(4h to 28d),the compressive strength of MPC-based composites increased.Within7d,the compressive strength increased greatly,and the strength of 7d can reach 90%of28d strength;the ultimate tensile stress and ultimate tensile strain of MPC-based composites increased with the increase of age.