Preparation And Performance Study Of Magnetite Radiation-proof Ultra-high Performance Concrete

As a radiation shielding material with low cost and good structural properties,radiationproof concrete is widely used in the construction of civil as well as military nuclear facilities.High-density aggregates such as magnetite are often used in the preparation of radiation-proof concrete to provide better radiation shielding properties.However,the current design strength of radiation-proof concrete is generally not high,and its structure may be a safety hazard if it is damaged by explosion impact or major disasters such as earthquakes.Therefore,it is of great practical significance to prepare higher-strength radiation-proof concrete.Ultra-High Performance Concrete(UHPC)is a new type of cementitious composite material with ultrahigh strength and excellent durability,which can be applied to radiation protection field not only to improve structural strength,but also to reduce structural dimensions.However,relatively little research has been conducted on radiation-proof UHPC.In addition,UHPC is prone to bursting in high-temperature environments due to its dense structure,which limits its application to a certain extent.Therefore,it is also very necessary to carry out research on the high-temperature performance of radiation-proof UHPC.Based on this,this paper firstly prepared radiation-proof UHPC by replacing river sand with different doping amounts(0%,20%,40%,60%,80%,100%)of magnetite by equal volume,and investigated the effects of magnetite doping on the working properties,mechanical properties,gamma(γ)ray shielding properties,and microstructure of UHPC.Then,in order to investigate the effect of high temperature on the performance of radiation-protected UHPC,radiation-protected UHPC containing blended fibers was prepared based on radiationprotected UHPC with 100% magnetite doping and equal volume replacement of steel fibers with different doping amounts(0%,0.2%,0.3%,0.4%)of polypropylene(PP)fibers,and their working properties and spalling,mass loss,mechanical properties,γ-ray shielding properties and microstructure after exposure to high temperature were investigated.The main conclusions obtained are as follows.(1)The replacement of river sand by magnetite causes a decrease in the fluidity of UHPC mixes,but the UHPC mixes with 100% magnetite admixture still exhibit good fluidity(fluidity233 mm).At the same time,the increase of magnetite dosing shortened the setting time of UHPC mixes.(2)The overall compressive strength and splitting tensile strength of UHPC decreased with the increase of magnetite doping,but the decrease was limited.In addition,the impact test showed that although the dynamic compressive strength of UHPC with 100% magnetite admixture was lower than that of UHPC without magnetite admixture,it was significantly higher than that of ordinary radiation-proof concrete and showed better impact resistance.(3)With the increase of magnetite doping,the γ-ray shielding performance of UHPC was significantly enhanced.Compared with the UHPC without magnetite doping,the linear attenuation coefficient of UHPC with 100% magnetite doping increased by 31.3%,and the thickness of the half-value layer as well as the decimal layer decreased by 23.8%.In addition,the addition of magnetite did not change the type of hydration products,but optimized the pore structure and the magnetite was tightly bound to the matrix,which explained to some extent the reason why the radiation-protected UHPC doped with magnetite maintained better mechanical properties.(4)UHPC bursting and spalling at high temperatures and PP fiber doping and specimen size.Steel fiber alone does not prevent UHPC spalling at high temperatures,where the compressive specimen burst at 800℃,splitting tensile specimen burst at 600℃,while the combined use of steel fibers and PP fibers can effectively enhance the UHPC anti-spalling performance,the higher the amount of PP fiber,the better the anti-spalling performance.(5)At room temperature,the increase of PP fiber dosing led to a slight decrease in the flow of UHPC mixes,but the setting time increased significantly.The compressive strength and splitting tensile strength of UHPC decreased,but the decrease was not significant.With the increase of temperature,the compressive strength and splitting tensile strength of all groups of UHPC showed a tendency to increase first and then decrease.Among them,the strength of UHPC with single-doped steel fibers peaked at 200°C,while the strength of UHPC with mixed fibers peaked at 400°C.In addition,the increase in temperature also led to a decrease in the γ-ray shielding performance of UHPC.(6)Compared to room temperature,the average pore size of UHPC decreased significantly at 200°C as well as 400°C,and the proportion of harmless pores increased significantly,which explained the increase in mechanical properties of UHPC at 200°C as well as 400°C.With further increase in temperature,the porosity as well as the average pore size of UHPC increased significantly,the decomposition of hydration products occurred,the structure of UHPC matrix deteriorated,and the bonding of steel fibers to the matrix decreased,which also led to a significant decrease in the mechanical properties of UHPC.