Preparation Of Radiation Shielding Functional Aggregate And Its Effect On Properties And Microstructure Of UHPC

Radiation shielding concrete is widely used due to its excellent durability,economy,and ease of construction.However,current radiation shielding concrete has the problem of low strength despite its excellent shielding performance,making it difficult to withstand explosive and nuclear impact loads.Ultra-high performance concrete（UHPC）has the characteristics of ultra-high strength,high density,and high durability,making it an ideal material for constructing high-strength,impact-resistant,and radiation-shielding buildings.However,UHPC is mostly made of quartz sand as aggregate and does not contain high-density heavy aggregates,resulting in its radiation shielding ability being lower than that of ordinary radiation shielding concrete.In addition,UHPC requires a high amount of cementitious materials and has a low water-cement ratio,which leads to early shrinkage cracking and a significant reduction or even failure of its shielding performance,limiting its application in nuclear engineering to some extent.Based on the National Natural Science Foundation of China’s"Preparation of Low-Shrinkage Radiation Shielding Ultra-High Performance Concrete and Its Irradiation Damage Mechanism"（52008002）,this article developed a technology for preparing spherical high-strength radiation shielding functional aggregates using alumina,coal gangue,barite powder,and boron oxide as raw materials.The saturated pre-wetted spherical high-strength radiation shielding functional aggregates were used instead of ilmenite to prepare radiation shielding UHPC.The impact resistance,radiation shielding performance,and microstructure of UHPC were studied using a split Hopkinson pressure bar device,¹³⁷Cs gamma source and ²⁴¹Am-Be neutron source radiation device,nanoindentation and mercury intrusion methods to investigate the effect of high-strength radiation shielding functional aggregates on the properties of UHPC.The main research results obtained in this paper are as follows:（1）A porous,high-strength,and excellent radiation shielding performance spherical radiation shielding functional aggregate preparation technology was developed in this study.The prepared high-strength radiation shielding functional aggregate has an apparent density of 3100 kg/m³,a compressive strength of 32.5 MPa,a barrel pressure strength of 12.46 MPa,and a water absorption rate of 10.20%.At the same time,the influence of the raw material composition and firing system of the high-strength radiation shielding functional aggregate on its performance was investigated.The higher the Al₂O₃/Si O₂ratio in the aggregate,the greater the apparent density and strength of the aggregate.Due to the hindered ion migration by the barium sulfate in the barite powder,the greater the amount of barite powder added,the greater the apparent density of the aggregate,but the smaller the compressive strength.As the temperature increases,more liquid phase fills the pores and the structure of the aggregate becomes less loose,resulting in the highest compressive strength and apparent density and the lowest porosity.However,if the temperature is too high and there is too much liquid phase,the aggregate will expand,and the apparent density and compressive strength will decrease,while the water absorption rate will increase.With the prolongation of the firing time,the continuous transfer of materials occurs,and the apparent density and compressive strength of the aggregate increase while the water absorption rate decreases.（2）The influence of the replacement rate of high-strength radiation-shielding functional aggregate（0%,30%,50%,70%and 100%）on the performance,mechanical properties,durability and shielding performance of the radiation-shielding UHPC mixture has been explored.With the increase of the replacement rate of high-strength radiation-shielding aggregate,the slump and expansion of UHPC mixture increase,the apparent density and settlement coefficient decrease,and the setting time increases;With the increase of the replacement rate of high-strength radiation-shielding functional aggregate,the compressive strength,flexural strength,modulus of elasticity and flexural toughness of UHPC decrease,and the compressive strength>114MPa,flexural strength>18.5MPa.The strength of high-strength radiation-shielding functional aggregate is lower than that of ilmenite.When the replacement rate of high-strength radiation-shielding functional aggregate is 70%,the radiation-impact performance of UHPC is the best,presenting the state of cracking and non-scattering required by nuclear engineering.With the increase of the replacement rate of the high-strength radiation-shielding functional aggregate,the curing effect on the UHPC increases,which makes the hydration reaction of the UHPC more complete,the structure more compact,and the UHPC has better chloride ion permeability resistance,carbonization resistance,permeability resistance and volume stability.When the replacement rate of the high-strength radiation-shielding functional aggregate is 100%,the UHPC self-shrinkage is as low as 412uε.The apparent density of high-strength radiation-shielding functional aggregate is lower than that of Ilmenite With the increase of thereplacement rate of high-strength radiation-shielding functional aggregate,the UHPC gamma ray shielding performance gradually decreases,and the UHPC neutron ray shielding performance gradually increases.（3）The effect of high-strength radiation shielding functional aggregate on the hydration product composition and morphology,interface transition zone morphology and micro-mechanical properties,and pore structure of UHPC was revealed.Both UHPC with high-strength radiation shielding functional aggregate and titanium iron sand as additives produce C-S-H gel,calcium hydroxide,and calcium aluminate.As the curing age prolongs,the hydration degree of UHPC increases,the content of calcium hydroxide decreases,and the content of calcium aluminate increases.The high-strength radiation shielding functional aggregate has an internal curing effect on UHPC,promoting the hydration reaction of the interface transition zone,filling the pores with a large amount of hydration products,improve the boundary between the transition zone and the matrix,and increasing the elastic modulus and hardness of the transition zone.The addition of high-strength radiation shielding functional aggregate refines the pore size of UHPC,making the UHPC structure more compact.However,the release of water during the mixing process deteriorates the pore structure of UHPC,resulting in a decrease and subsequent increase in the porosity of UHPC with an increase in the replacement rate of high-strength radiation shielding functional aggregate.