Effect Of Fine Aggregate Types On The Properties Of Magnesium Phosphate Cement Mortar

With the rapid development of the economy,significant achievements have been made in China’s infrastructure construction.However,a large number of concrete structures and roads have been damaged in advance due to the complexity of the service environment.Therefore,the repair and reinforcement of concrete structures and roads have become a development trend.In order to meet the timeliness and functionality of on-site repair,magnesium phosphate cement(MPC)repair materials have fast solidification and hardening The rapid development of early strength and other advantages are gradually receiving attention from the construction market.At present,most MPC mortar repair materials on the market use river sand as fine aggregate.However,with the increasing depletion of river sand resources and the implementation of the government’s policy of prohibiting river sand mining,artificial sand such as machine made sand has become a trend to replace river sand for MPC mortar.However,the surface of river sand is smooth and nearly spherical,while the surface of machine made sand is rough and has many edges and corners.These factors can affect the performance of MPC mortar to varying degrees.Therefore,based on the actual situation of repair materials and current aggregates,this paper systematically studies the impact of different varieties and different thicknesses of fine aggregates on the performance of magnesium phosphate cement mortar,in order to clarify the feasibility of using machine made sand to configure MPC repair mortar,This is of great significance for improving the performance of MPC mortar and promoting the marketoriented application of non natural sand such as artificial sand and tailings sand.On the basis of determining the basic composition of magnesium phosphate cement,this article mainly studies the effects of aggregate variety and fineness modulus on the performance of freshly mixed and hardened MPC mortar,including the fluidity,plastic viscosity,yield stress,and setting time of freshly mixed MPC mortar,as well as the mechanical properties,volume stability,and durability of hardened MPC mortar.Meanwhile,based on the analysis of the microstructure of MPC mortar,the influence of aggregate variety and fineness modulus on the performance of MPC mortar is explored.The main conclusions are as follows:(1)For fresh MPC mortar,the flowability of machine made sand MPC mortar is better than river sand,increasing by about 28.1 %;As the fineness modulus increases,the flowability of machine-made sand MPC mortar first increases and then decreases,while the flowability of river sand MPC gradually increases;The fluidity of MPC mortar shows a trend of first increasing and then decreasing with the increase of stone powder content,with the highest fluidity at a stone powder content of 15 %.The type,fineness modulus,and stone powder content of aggregates have little effect on the yield stress of MPC mortar.The plastic viscosity of machine made sand MPC mortar is smaller than river sand,and the plastic viscosity of MPC mortar gradually decreases with the increase of fineness modulus.However,the plastic viscosity of machine made sand MPC mortar slightly increases with the increase of stone powder content.In terms of setting time,the setting time of machine made sand MPC mortar is greater than that of river sand mortar,and the setting time of both types of mortar gradually increases with the increase of aggregate fineness modulus;The setting time of machine made sand MPC mortar shows a trend of first increasing and then decreasing with the increase of stone powder content.The setting time is the highest when the stone powder content is 10 %.(2)For hardened MPC mortar,the difference in flexural and compressive strength between machine made sand and river sand MPC mortar at the same fineness modulus is not significant at the same age;The strength of machine made sand MPC mortar increases first and then decreases with the increase of fineness modulus.When Mx=2.8,the compressive strength of machine made sand MPC mortar is the highest;With the increase of stone powder content,the flexural and compressive strength of machine made sand MPC mortar shows a trend of first increasing and then decreasing at different ages,with the highest strength when the stone powder content is 15%.In terms of volume stability,there is little difference between machine-made sand and river sand MPC mortar.The volume stability of machine-made sand and river sand MPC mortar with fineness modulus Mx=2.8 is slightly better than Mx=2.2 and 3.3.In terms of durability,after water immersion,freeze-thaw cycles,and sulfate erosion,the strength loss rate of machine made sand MPC mortar is greater than that of river sand;When the machine-made sand is medium sand(Mx=2.8),the durability of MPC mortar is better than that of fine sand and coarse sand.(3)The microhardness of both machine-made sand and river sand MPC mortar is in the aggregate area>slurry area>interface transition area,and the overall microhardness of machine-made sand MPC mortar is slightly lower than that of river sand MPC mortar,and the microhardness results are consistent with the macroscopic properties;The SEM-EDS results of two types of aggregate mortar show that there are more cracks in the matrix of machine made sand MPC mortar,and the cracks in the interface transition zone are larger,which to some extent affects the macroscopic performance of machine made sand MPC;Under the same fineness modulus,the water absorption rate of machine-made sand MPC mortar is higher than that of river sand MPC mortar at different ages,and both types of aggregate mortar decrease with the increase of fineness modulus;Under the same fineness modulus,the average pore size,most probable pore size,and porosity of machine-made sand MPC mortar are greater than those of river sand MPC mortar.Moreover,as the fineness modulus of fine aggregate increases,the porosity of both types of aggregate MPC mortar gradually increases.