Degradation Mechanism Of Cementitious Materials Under The Coupling Effect Of Mg²⁺,SO₄^2- And Cl^- In The Marine Tidal Zone

Cementitious materials are widely used in the construction of various types of marine infrastructure.Cementitious materials in service are often threatened by multiple ion coupled erosion,which reduces the service life of cementitious materials in the marine environment.The complex and harsh tidal zone will result in more severe damage to cementitious materials due to the higher concentration of ions in the tidal zone than in the submerged zone.Therefore,this paper carries out a study on the corrosive mechanism of cementitious materials under the coupling effect of multiple ions(Mg2+,SO42-,Cl-)in the tidal zone based on the above problem.Hardened slurry of cementitious material mixed with 30%fly ash and 30%GGBS alone or 15%fly ash and 15%GGBS in 0.6 mol/L Cl-and different concentrations of Mg2+,SO42-and MgSO4.The dissolution characteristics of cement slurries under coupling were investigated,and the influence of aggressive ions concentration and the type of mineral admixture on the Ca2+ dissolution of cement slurries and the pH of the pore solution under coupling were investigated;the types of products of cement slurry under coupling were studied,the relationship between the dissolution of soluble Ca2+,pH value and products of cementitious materials under coupling were investigated,the effects of ion concentration and mineral admixture type on the products composition of cement slurry were clarified;the microstructure evolution law of cementitious materials under coupling was analysed,the corrosive mechanism of products on cementitious materials under coupling was revealed.The conclusions are as follows:(1)The incorporation of GGBS and fly ash facilitates resistance to Mg2+under the coupling of Cl-and Mg2+,and GGBS chemically bounds more chloride ions than fly ash.As the concentration of Mg2+in the solution increases,Mg2+accelerates the dissolution of Ca(OH)2 and the leaching of Ca2+,while Mg(OH)2 is produced,resulting in a decrease in the alkalinity of the solution.In Cl--Mg2+environment,solutions with high concentrations of Mg2+are not conducive to chemically bounding chloride ions;lower pH values promote the formation of Friedel’s salt when pH>12.3;due to the high concentration Mg2+(≥0.2 mol/L)solutions with pH<12.3,decomposition of the Friedel salt occurs.(2)The incorporation of GGBS and fly ash facilitates resistance under the coupling of Cl-and SO42-,and GGBS chemically bounds more chloride ions than fly ash.The introduction of SO42-in Cl-environment,on the one hand,delays the formation of AFt as Cl-diffuses at a higher rate than SO42-and Cl-reacts with hydration products to form Friedel’s salt,on the other hand SO42-is detrimental to the presence of Friedel’s salt,so that Cl-and SO42-interact with each other and hold each other in check.Friedel’s salt is converted to AFt after the introduction of SO42-in Cl-environment;release of bound chloride ions into the pore solution due to the introduction of SO42-,thereby increasing the amount of free chloride ions in the pore solution;cementitious materials are damaged by the superimposed physical crystallisation and chemical attack of the salt solution at 0.8 mol/L of Na2SO4 solutions.(3)The incorporation of GGBS and fly ash facilitates the resistance to MgSO4 under the coupling of Cl-and MgSO4.The introduction of MgSO4 in Cl-environment,Mg2+ accelerates the dissolution of Ca(OH)2 and the leaching of Ca2+,SO42-reacts with the hydration product Ca(OH)2 to produce gypsum,resulting in a decrease of solution alkalinity;the introduction of MgSO4 in Cl-and MgSO4 environment is not conducive to the presence of Friedel’s salt.The swelling products Mg(OH)2 and gypsum cause the pores to become larger and more porous,which in turn provides a new pathway for Mg2+ and SO42-aggressive ions to invade.In total,37 figures,9 tables and 126 references are presented in this thesis.