The Microstructure Formation And Evolution Mechanism Of C-S-H Gel In Portland Cement Pastes With Granulated Blast Furnace Slag Under The Attack Of Corrosive Ions

Concrete is the main material for marine infrastructure construction,and the service life of the project is determined by its durability.The durability of the concrete is further determined by the compositions and microstructures of hydration products that are the basic units of the concrete.When the concrete structure is served in marine environment,however,the internal microstructure and cementitious behavior of concrete present obvious destruction and decrease as the evolution of these basic units.Therefore,the revelation of the formation and evolution mechanism of hydration products as well as the the design of long-life marine concrete structure based on microstructure-adjusted method is one of the key scientific and technical problems to be solved urgently.Aiming to solve the ambiguous mechanism for the microstructure formation and evolution of C-S-H gel in marine concrete,this thesis systemically studied the microstructural formation and evolution mechanism for C-S-H gel in cementitious pastes under the attack of single or coupled corrosive ions in the seawater by ²⁹Si NMR,²⁷Al NMR,SEM-EDS and other modern analytical methods with the support of"973"project of China,"basic research on the long life of concrete materials and structures under harsh environment-the microstructural formation and regulation of the long-life concrete materials under harsh environment".It was found that SO₄^2-,Mg²⁺and Cl^-attack had a significant effect on the formation and evolution of the microstructure of the C-S-H gel.The molecular structure models of Al-doping C-S-H gel were established and the influence mechanism of Al-doping on the mechanical properties of C-S-H gel was analyzed.The results shown that GBFS could weaken the de-calcification and de-alumination effects of corrosive ions on the C-S-H gel,thus inhibiting the migration of Al[4]on dreierketten chains in C-S-H gel and the transition from AFm to AFt.The decrease of AFt content and higher stability of C-S-H can delay the expansion-destruction of AFt and degradation of mechanical properties of C-S-H gel during corrosive ions attack.Eventually,the durability of concrete was obvious improved and it enriches the interpretation of the existing mechanism.The main work and achievements of the paper are as follows:The formation and evolution of C-S-H gel in Portland cement paste and slag-cement pastes under the standard curing were studied.With the hydration period of PC paste from 28d to 365d,the mean chain length?MCL?of the dreierketten chains increases and the substitution degree of Al for Si in the silicate chain of the C-S-H gel?Al[4]/Si?decreases.The Al[4]s located in bridging sites of dreierketten chains migrate from C-S-H gel and react with AFt to form Al[6]in AFm.With increasing content of slag in the cementitious pastes,the number of Al³⁺ions released with the increase of hydrated slag,thereby more tetra-coordinated Al³⁺ions immigrating into C-S-H,finally leading to the increases of MCL and Al[4]/Si ratio and the decrease of Ca/Si ratio.The increments of MCL and Al[4]/Si ratio and decrement of Ca/Si ratio enlarges with increasing slag replacement.The microstructure formation and evolution of C-S-H gel in cementitious paste with different slag contents under the attack of SO₄^2-,Mg²⁺and Cl^-were systematically studied.The influence mechanism of corrosive ions on the microstructure of C-S-H gel was also revealed.Both SO₄^2-and Mg²⁺have the dealumination and decalcification effect on the C-S-H gel.Accompanied by the increase of ions concentration and the extension of corrosion age,the MCL of C-S-H gel show obvious increase,While Ca/Si and Al[4]/Si reduced significantly.In addition,Cl^-showed a decrease in Al[4]/Si but had no significant effect on MCL and Ca/Si of C-S-H gel.SO₄^2-,Mg²⁺,and Cl^-all led to the migration of Al[4]in bridge site of dreierketten chains,then make it convert into Al[6]in AFm or AFt.At the same molarity of corrosive solution and the same corrosive age?365 days?,Mg²⁺and SO₄^2-respectively showed the most significant influence on decalcification and dealumination of C-S-H gel,and Cl-was the weakest for the dealumination on C-S-H gel.Compared with single ion erosion such as Mg²⁺or SO₄^2-,the MCL of the C-S-H gel in the cement paste was significantly increased,the Ca/Si and Al[4]/Si were significantly decreased and the dealumination and decalcification of C-S-H gel was more serious under the coupling effects of Mg²⁺and SO₄^2-.The effect of Mg²⁺and SO₄^2-on the microstructure of C-S-H gel had mutual promoted effects.The addition of slag could weaken the de-calcification and de-alumination effect of corrosive ions on C-S-H gel and improve the microstructure stability of C-S-H gel.The effect was enhanced with the increase of slag content and curing temperature.Based on the ClayFF force field,the molecular structure models of aluminum-doped C-S-H gel were established by the combination of molecular dynamics and Monte Carlo method.According to the models,the impact mechanism of Al on microstructure and mechanical properties was studied.Al³⁺could exist in the C-S-H gel in the form of Al[4],Al[5]and Al[6].Al[4]existed in the bridge site of dreierketten chains of C-S-H gel in the form of alumina tetrahedron,bridged dreierketten chains and increased the MCL of C-S-H gel;Al[6]existed in the C-S-H gel interlayer,increased the binding force between layers in the C-S-H gel;Al[4]in the dreierketten chains is metastable,and could migrate from the C-S-H gel dreierketten chains into the interlayer or gel pores,converted into Al[6],thereby resulting decreasing Al[4]/Si ratio of C-S-H gel.When C-S-H gel subjected to sulfate attack,interfacial or interlayer Ca²⁺ions were took into solution to form gypsum through the diffusion-adsorption-desorption process,resulting in the decalcification of C-S-H gel.The gypsum then reacted with Al³⁺to form AFt or AFm which consuming the Al³⁺ions in C-S-H.This process promoted the migration of metastable Al[4]in dreierketten chains to convert into Al[6],leading to the decrease of Al[4]/Si.With the increase of slag content,Si-O-Ca connection on aluminum doped C-S-H gel interface become more stable,delayed the desorption of Ca²⁺,at the same time,Al³⁺formed by hydrolysis of slag replenish SO₄^2-consumption,inhibit Al[4]out of dreierketten chains in C-S-H silica gel,weaken the decalcification and dealumination effect of SO₄^2-on C-S-H gel.With increasing aluminum doping amount and decreasing Ca/Si ratio of C-S-H gel,constitution water transformed into a more stable silicon aluminum hydroxy group and dreierketten chains length increases,then the skeleton structure enhanced,finally improved the mechanical properties of C-S-H.The incorporation of GBFS weaken the decalcification and dealumination effect of SO₄^2-on C-S-H gel,thus improving the microstructural stability of C-S-H gel and delaying the degradation of mechanical properties of C-S-H gel.The results of this paper can provide reference for revealing the mechanism of performance degradation for concrete materials and establish the long life concrete designing based on microstructural-adjusted method in marine service environment.