Study On The Mechanism Of Precoating Of Aggregate To Strengthen ITZ And Solidify Chloride Ion In Marine Concrete

Aggregate precoating is a method to improve the interface transition zone（ITZ）of concrete by adding precoating layer to coarse and fine aggregate,so as to achieve the purpose of mechanical properties and durability of concrete.At the same time,it can also effectively solidify harmful chloride ions in marine sand and realize the resource utilization of marine sand,and then use sea sand as fine aggregate for preparing concrete.In this paper,the rheological properties of sulphoaluminate cement（SAC）and phosphoaluminate cement（PAC）,which are respectively used to cover coarse gravel aggregate and fine aggregate of marine sand,and use ordinary Portland cement（OPC）as cementing material to prepare Marine engineering concrete,which makes clear the mechanics and durability of precoated aggregate concrete.At the same time,the micro-morphology and micro-mechanical properties of ITZ were analyzed,and the characteristics of chloride ion curing and the protection of phosphor aluminate cement against electrochemical corrosion of steel were discussed.The microstructure of ITZ and the phase composition of hardened cement slurry were analyzed and characterized by XRD,SEM and EDS.The main conclusions are as follows:（1）According to the relationship between rheological parameters and the thickness of coarse aggregate precoating layer and concrete slump,the yield stress and plastic viscosity range of sulphoaluminate cement-based precoating slurry are 50pa-70pa and 0.08pa·s-0.2pa·s,respectively.Through response surface analysis,the optimized precoated material group is nanometer silica（NS）0.1-0.4%,fly ash（FA）10%-30%,polycarboxylic acid water reducer（SP）0.064%-0.15%.The micro-morphology of precoated concrete ITZ is denser than that of ordinary concrete.Meanwhile,the micro-mechanical properties of ITZ in indentation hardness,indentation depth and dynamic elastic modulus are all due to that of ordinary concrete.（2）After hydration for 28 days,the curing rate of PAC was 40.67%and 48.66%,respectively,in the presence of endogenous and external chloride ions in the corrosive environment.Under the action of various ions in simulated sea water,the curing rate of chlorine ions is 45%and 57.13%,respectively,in the presence of endogenous and external chloride ions,which is better than that of OPC and SAC.According to the relationship between the rheological parameters and the thickness of sea sand precoating,the yield stress and plastic viscosity of phosphoaluminate cement-based precoating slurry are in the range of 8pa-30pa and0.4pa·s-3pa·s,respectively.Through response surface analysis,the optimized precoated material group is NS 0.25%-0.4%,FA 10%-26%,SP 0.108%-0.15%.In practical application,the amount of Friedel salt produced by the reaction of phosphor aluminate cement-based precoating layer on sea sand with chloride ions is considerable,and the amount of Friedel salt produced by the reaction increases from 30.39%to 48.36%in the face of chloride ion erosion from external seawater,which has an excellent ability of chloride ion curing.（3）The compressive strength of precoated aggregate concrete and ordinary concrete at 28days is 62.3MPa and 53.1MPa,respectively.Under sulfate erosion,the compressive strength of the two cements after 180d erosion cycle is 43.7MPa and 36.2 MPa,respectively,and the strength loss is 34.87%and 44.13%.Precoated concrete is not as good as ordinary concrete in freezing resistance.The mass loss of precoated concrete is 5.51%after 300 cycles while that of ordinary concrete is only 1.94%.In terms of chloride ion-related properties,the electric conductivity of sea sand concrete,desalinated sea sand concrete,precoated sea sand concrete and standard sand concrete are 813 C,669 C,504 C and 607 C,respectively,and the chloride ion mobility coefficients are 3.9×10^-12 m²/s,3.1×100^-12 m²/s,2.6×10^-12 m²/s and 2.6×10^-12m²/s,respectively.The content of chloride ion in pore solution of concrete is 0.023%,0.0059%,0.0057%and 0.0024%,respectively.（4）In terms of electrochemical performance,at the frequency of 0.01 Hz,the impedance value of the steel plate coated with phosphate aluminate cement is close to 400Ω·cm²,the impedance value of the steel bar coated with ordinary Portland cement-based and sulphoaluminate cement-based paste is 250Ω·cm² and 142Ω·cm²,respectively,while the impedance value of the blank control steel plate is about 137Ω·cm².In the phase Angle,the phase Angle of the phosphor aluminate cement-based reinforcement coating is 33°,the sulfur aluminate cement-based coating is 30°,the Portland cement-based coating is slightly higher than the ordinary steel plate,the phase Angle of the two is about 20°.The polarization resistances are 2.16×10⁶Ω,3.39×10⁶Ω,6.78×10⁶Ωand 10.19×10⁶Ω,respectively.The results show that the electrochemical corrosion resistance of PAC is much higher than that of other cements.In terms of the time application of precoated steel bars,after 6 cycles,the corrosion depth of ordinary and precoated steel bars is 0.1024mm and 0.3772mm,respectively.