Study On The Effect Of Coral Powder On The Properties Of Ultra-High-Performance Concrete

Ultra-high performance concrete(UHPC)is an advanced cementitious material that has emerged in recent decades.It has superb mechanical properties and excellent durability,and has a wide range of application prospects in ultra-high rise buildings,municipal facilities,oil,nuclear power,marine and military facilities.However,the preparation of UHPC requires a large amount of cementitious materials,so there are many problems such as high production cost and high energy consumption,which seriously restrict the development and application of UHPC.Therefore,it is an effective way to solve the cost and environmental problems by using resource waste to replace the raw material of UHPC.Coral powder(CP)is one kind of coral waste generated during the development of marine islands,and it can effectively relieve the pressure of coral waste emission by replacing cement or silica fume(SF)in the production of UHPC.In this paper,we use CP to replace cement and SF in the preparation of UHPC,and study its effect on the mechanical properties and shrinkage properties of UHPC,analyze the phase composition of UHPC by X-ray diffraction(XRD),observe the microscopic morphology of UHPC by scanning electron microscope(SEM),and test the pore size distribution of UHPC by nitrogen adsorption(BET).The pore content and unhydrated particles of UHPC are quantified by back scattered electron(BSE).The nano-mechanical properties of UHPC matrix and interface transition zone(ITZ)are analyzed by nanoindentation test.The main studies and conclusions are as follows:(1)The strength test results of UHPC showed that the flexural strength and compressive strength of UHPC increased and then decreased with the increase of CP content when CP replaced cement by equal mass.When CP replaces 5% of cement,the flexural and compressive strengths of UHPC at all ages are higher than those of the reference group,and the flexural and compressive strengths at 28 d are 25.5 MPa and 138.2 MPa,which are 4.9%and 4.1% higher than those of the reference group,respectively.The flexural and compressive strengths of UHPC at all ages were lower than those of the reference group when CP was replaced by SF by equal mass,and the flexural and compressive strengths decreased with the increase of CP replacement.(2)The results of UHPC shrinkage test showed that the total shrinkage of UHPC was reduced by replacing cement or SF with CP,and the shrinkage value decreased with the increase of the replacement amount.In addition,the compressive strength and autogenous shrinkage of UHPC are compared.The linear fitting results between the two show that the development trend of compressive strength of UHPC at different ages is similar to that of autogenous shrinkage,and there is a close correlation between the two.(3)The microscopic test results showed that when CP replaced 15% SF,the amount of active minerals was reduced,resulting in insufficient generation of calcium silicate hydrate(C-S-H)gel,which reduced the compactness of UHPC matrix and ITZ and increased the matrix porosity.When CP replaces 5% cement,the degree of hydration of UHPC increases and the C-S-H content increases,which improves the compactness of UHPC matrix and ITZ,reduces the generation of pores and cracks,and enhances the bond strength of matrix and steel fiber.However,CP replacing 15% cement reduces the cement content of UHPC,which leads to microcracks inside UHPC,reduces the matrix-aggregate ITZ compactness and matrix-steel fiber ITZ bond strength,increases the porosity and increases the macropore content.(4)The results of the nanoindentation tests showed that compared with the reference group,the unhydrated phase of UHPC decreased by 9.8%,the pore volume fraction decreased by 14.3%,and the C-S-H volume fraction increased by 13.6% when CP replaced 5% cement and the performance of matrix-aggregate ITZ and matrix-steel fiber ITZ also improved.With the increase of CP replacing cement content,the unhydrated phase and pore content of UHPC increased,and the C-S-H content decreased.when CP replaced SF,the hydration of UHPC decreased and the pore content increased.(5)The volume fractions of each phase obtained by BSE and nanoindentation were compared and analyzed.It was found that the pore volume obtained by BSE was lower than that of nanoindentation,but the difference between them was not significant;the volume of hydration products obtained by BSE was larger than that of nanoindentation,and the exact hydration products could not be obtained qualitatively;the unhydrated phase obtained by BSE was similar to that of nanoindentation.Therefore,when only the pore volume of the specimen needs to be obtained,the BSE test can be used instead of nanoindentation to reduce the test time and cost.