Study On The Effect Of Seawater Mixing On The Performance Of Ultra High Performance Concrete And Regulation Of Autogenous Shrinkage

Ultra-high performance concrete(UHPC)is a new type of cement-based composite material,which has denser microstructure,better mechanical properties and durabilities than ordinary concrete.These outstanding performance makes UHPC more valuable to apply in the Marine environment.However,freshwater is very limited in coastal areas,and it is necessary to transport freshwater or desalinate seawater to prepare UHPC in actual ocean engineering,which directly increases the construction cost and energy consumption.Therefore,using seawater instead of freshwater to prepare UHPC in islands and coastal areas is not only environmentally friendly and sustainable,but also reduces the construction cost.Thus,this paper researches the effect of seawater mixing on the performance of UHPC,systematically analyzes the effect of seawater mixing on the microstructure and macroscopic properties of UHPC matrix and fiber reinforced UHPC,and puts forward a method of preparing seawater mixing UHPC with low autogenous shrinkage.The research contents and main results are as follows:(1)The effect of seawater mixing on UHPC matrix was studied.The results show that seawater mixing accelerats and enhances the hydration reaction of the cementitious materials in UHPC so that increases the hydration products by 12.25%.Besides,seawater increases the polymerization degree of hydration products by increasing the average chain length of C-S-H gel from 3.5068 to 4.1595,and increasing Q~2/Q~1 from0.7533 to 1.0796.Thus,the hydration degree of cement in UHPC is increased by 4.67%.Due to the promotion of hydration,the pore size in UHPC is refined.The total porosity below 10μm decreased by more than 8%,and more capillary pores are converted into gel pores.In addition,the nano-mechanical properties of UHPC are also improved.The indentation modulus and indentation hardness of UHPC paste are increased by 25.25%and 29.10%,respectively,and the interface transition zone(ITZ)between river sand and C-S-H gels is optimized by about 15μm.Because seawater mixing leads to the promotion of hydration and the denser matrix,the flowability of UHPC is significantly reduced,the 28-day flexural strength and compressive strength of UHPC are obviously increased,and the autogenous shrinkage of UHPC is increased by about 300μm/m by seawater mixing.(2)The effect of seawater mixing on Ultra-high performance fiber reinforced concrete(UHPFRC)was studied.The results show that the slight corrosion of steel fibers in UHPFRC is caused by seawater mixing,but the steel fibers cannot be further corroded due to the dense matrix and strong chloride binding ability,which can bind free chlorine ions and resist the invasion of external oxygen.Slight corrosion increases the debonding pulling stress,but decreases the maximum pulling stress,the average bonding strength and the equivalent bonding strength between steel fibers and matrix.Therefore,seawater mixing can strengthen the strain hardening zone but decrease the overall toughness of UHPFRC in the bending test,and make the later compressive strength slightly decrease and the resistance increase.(3)In order to optimize the autogenous shrinkage of seawater mixing UHPC,the effect of zeolite on the performance of UHPC was studied.The results show that the internal curing effect of zeolite reduces the autogenous shrinkage of seawater mixing UHPC by more than 33%,which is obviously better than the optimization of freshwater mixing UHPC.This is because zeolite can absorb chloride ions,so that the formation of hydration ion clusters near the nanopores inhibits water into the nanopores,leading to the reduction of water absorption rate of zeolite.Thus,zeolite begins to release water earlier and releases water for a longer time in seawater mixing UHPC,so that the optimization of autogenous shrinkage effect is better.Moreover,water absorption can cause the decrease of flowability,setting time of UHPC.However,zeolite increases the setting time of seawater mixing UHPC,which further proves that zeolite can absorb chloride ions,so that the acceleration of hydration by chloride ions is weakened.So,the compressive strength of seawater UHPC decreases with the increase of zeolite content.In addition,the internal curing of zeolite can optimize the ITZ,so as to improve the later compressive strength of freshwater mixing UHPC.In a word,seawater mixing can improve the mechanical properties and microstructure of UHPC matrix,but aggravate the autogenous shrinkage of UHPC.Therefore,the autogenous shrinkage of seawater mixing UHPC is optimized by zeolite with no serious degradation of mechanical properties and the optimization of the ITZ.In addition,the compressive strength of seawater mixed UHPFRC does not deteriorate significantly after service in the Marine environment.It can be concluded that seawater mixing UHPC has excellent performance and high application value.