The Influence Of Magnesium Chloride Attack On The Microstructure Of Cement-slag Paste Under Dry-wet Cycling

As the most important material in the infrastructure construction of the ocean and the west,the hydration products and microstructures formed in the hydration process of concrete cementitious materials are the basic units to determine their durability and service performance.Mg²⁺and Cl^-,as the main corrosive ions in the marine and Western brine environment,have significant influence on the microstructures of the cementitious materials,especially under the interaction of erosive ions and dry/wet cycles,the microstructures of the cementitious slurry will undergo significant evolution,the microstructure of concrete is destroyed,the mechanical properties degenerate and the service life is shortened.Relying on"973"project"Formation and regulation of microstructure of long life concrete material under harsh environment"?2015CB655101?,and using modern testing techniques such as ²⁹Si NMR,²⁷Al NMR and SEM-EDS,the composition of hydration products of cement slag cementitious slurry and C-S-H gel by MgCl₂ erosion under the condition of standard curing and dry/wet cycle are studied.It is found that the cement slag slurry C-S-H gel decalcification and dealuminum removal is significant under the interaction of dry/wet cycle and MgCl₂ erosion,but the addition of slag can weaken the decalcification and dealuminum effect.In this paper,the formation and evolution of microstructure of Portland cement paste corroded by MgCl₂ under the condition of standard curing and drying/wetting cycles were studied.With the increasing of erosion concentration of MgCl₂ and the prolongation of erosion age,the production of Friedel's salt and AFt in the Portland cement paste increased,and Al[4]of the silicon chain of the C-?A?-S-H gel changed to Al[6]of AFt and Friedel's salt,Al[4]/Si decreased,and the decalcification and decalcification of the C-?A?-S-H gel is significant.Besides,When the MgCl₂concentration is 15%,the Q³ appeared on the ²⁹Si NMR spectrum.Combining with SEM-EDS energy spectrum analysis,it can be seen that M-S-H gel is generated.And the interaction of MgCl2 erosion in the dry wet cycle promoted the hydration of Portland cement slurry,the MCL of C-?A?-S-H gel increased,the Al[4]in C-?A?-S-H gel silica oxygen bridge site changes to Al[6]in Friedel's salt,and the dealuminum decalcification is more significant.In this paper,the formation and evolution of the microstructure of cement-slag slurry under the interaction of 10%MgCl₂ erosion and dry/wet cycle,and the influence of the corrosive ion transmission inhibitors on its microstructure are investigated.Under the effect of MgCl₂ erosion,with the increase of slag content,the Ca/Si ratio of C-?A?-S-H gel in the cementitious slurry decreased,Al[4]/Si ratio increased,MCL increased,and the change amplitude decreased with the increase of slag content.The slag can weaken the dealumination and decalcification effect of MgCl₂ erosion on C-?A?-S-H gel in the cementitious slurry.With the increase of slag content,the amount of Ca?OH?₂ and AFt decreased,the amount of Mg?OH?₂ and Friedel increased,and the slag incorporation could improve the Cl^-curing ability of the cementite and the ability to resist MgCl₂ erosion.The same erosion age,compared with MgCl2 erosion,under the MgCl2 erosion of dry wet cycle,the diffusion rate of Mg²⁺and Cl^-increased,which promoted the hydration of slag.The slag deagglomerated a large amount of Al³⁺,and some of them are coordinated with Al[4]into the position of the silica gel tetrahedron of the C-S-H gel,resulting in C-?A?-S-H gel MCL and Al[4]/Si ratio were both more aggressive than MgCl₂ at the same age,the other part was combined with Cl^-and Ca²⁺in the pore solution to form Friedel's salt.The incorporated aggressive ion transport inhibitor complexes with Ca²⁺in the pores and plugs the pores,inhibiting the penetration of Mg²⁺into the slurry via water through capillary action,which effectively weakened the erosion of Mg²⁺on the slurry C-S-H gel.The effect of MgCl₂ corrosion on the formation and evolution of C-?A?-S-H gel was studied.Under the action of MgCl₂ erosion,the C-?A?-S-H gel was decalcified and deactivated significantly,and Mg²⁺replaced Ca²⁺to form M-S-H gel without cementation ability.The coordination distribution of Al³⁺in C-?A?-S-H gel is mainly transformed from Al[4]in the bridge silica chain and Al[6]in TAH of the siloxane chain to Al[6]in Friedel's salt.With the increase of Ca/Si ratio,the adsorption capacity of C-?A?-S-H gel to Cl^-increased first and then decreased.And Al doping can enhance the adsorption performance of C-A-S-H gel to Cl^-.