The Formation And Evolution Mechanism Of Concrete Paste Microstructure Subjected To Seawater Erosion

Erosion ions such as Cl^-,SO₄^2-,Mg²⁺in seawater can cause deterioration of concrete performance and even structural damage,shortening the service life of concrete.However,the destruction of concrete by erosive ions begins with the microstructural damage of the gelled slurry.Therefore,the formation and evolution of different gelling slurry microstructures and the influence mechanism of erosion ions on the microstructure evolution of gelling slurry in seawater can provide a theoretical basis for the design of marine concrete.This paper relies on the national"973"project"Basic research on the longevity of concrete materials and structures under severe environments"?2015CB655100?,researching Portland cement paste,cement-powder mortar,cement-fly ash-silicon mortar Under the mixing and curing of body,cement-fly ash-slag slurry,and the mixing and curing of seawater,the evolution of hydration product composition,C-S-H polymerization degree and Al phase product transformation of each slurry,and on this basis,under the seawater erosion,the influence mechanism of different initial erosion time on the microstructure evolution of the above four gelling slurry.The main results are as follows:?1?The formation regularity of the micro-structure of Portland cement slurry under clear water mixing and standard curing conditions was determined.With the prolongation of hydration age,the silica tetrahedron in the C-S-H gel changes to dimer and multimer,the degree of polymerization increases,and the MCL increases.The Al produced by the hydration of C₃A and C₄AF in the Portland cement migrated into the bridged silicon tetrahedral position on the C-S-H gel to form a C-A-S-H gel,resulting in an increase in the C-S-H gel Al[4]/Si.With the continuous development of hydration process,Al[4]in the metastable bridge siloxane tetrahedron migrated a little and combined with AFt to form AFm;with the extension of age,AFt increased first and then decreased,AFm increased gradually.?2?Revealed the influence of mineral admixture on the microstructure of gelled slurry.The single doping of fly ash can increase the Al[4]/Si and MCL of C-S-H gel,reduce the specific surface area,cumulative pore volume and average pore diameter of the slurry,and the variation range increases with the extension of age;silica fume and pulverized fly ash blending can reduce Ca/Si of C-S-H and increase MCL and Al[4]/Si;slag and fly ash can further increase Al[4]/Si and MCL of C-S-H,reduce the specific surface area and accumulation of slurry pores.Pore volume and average pore size.With the extension of age,the degree of polymerization of C-S-H in the three composite slurries increased,MCL and Al[4]/Si increased,and AFt changed to AFm.The formation of the microstructure of the gelled slurry under seawater mixing and curing conditions was determined.Seawater mixing and curing will solidify Al³⁺formed by C₃A in Portland cement,produce Friedel's salt and AFt,reduce Al[4]/Si of CSH in the slurry,increase its polymerization degree and MCL,and the variation range with hydration age The period is extended and increased.With the extension of age,Al in C-A-S-H is transferred to AFt,AFm and Friedel's salts;the incorporation of fly ash can weaken the dealuminization effect of seawater mixing and curing on gelling slurry.With the extension of age,the polymerization degree and MCL of C-S-H gel silicon oxide chain increased,and Al[4]/Si increased first and then decreased.The migration of Al from fly ash and C-A-S-H to AFt,AFm and Friedel's salt;the combination of fly ash and silica fume,and the combination of fly ash and slag will further weaken the mixing and curing of seawater to the gelled slurry.Dealuminization,and the weakening effect is more significant than that of fly ash.The effect of the initial time on the dealuminization of seawater erosive gelling slurry was ascertained.After the initial erosion time is delayed,the dealuminization effect of seawater erosion on the Portland cement paste C-?A?-S-H is gradually weakened;when seawater erodes cement-powder ash slurry and cement-fly ash-silica ash slurry The dealuminization effect of C-?A?-S-H is the weakest at the initial erosion time at 7d and 14d respectively;when the seawater erodes the cement-fly ash-slag slurry,it is delayed with the initial erosion time.The deamination of C-?A?-S-H is weakened,but the dealuminization of C-?A?-S-H is similar to that of seawater erosion at the early onset time?1d and 7d?.