Chloride Concentrate And Its Relationship With Microstructure And EDL Properties Of Cement-based Materials

Reinforced concrete has been widely applied in different kinds of construction s,while the chloride-induced steel corrosion weakened durability of structure,especially bridge deck slabs potentially covered by de-icing salt and buildings in marine environment.In this study,the phenomenon of“chloride concentrate”was investigated for cement paste.Pore solution expression was applied to extract pore solution and the value of N_c was tested.Measurements on microstructure and EDL properties of cement paste were conducted to explain this phenomenon and figure out the mechanism.Physical and mathematical models were also proposed for characterization of N_c from the theoretical point of view.As the first step of this study,factors which may possibly to affect the value of N_c were studied.The value of N_c of cement pastes with different suppl ementary cementitious materials(SCMs),maturities,water to binder(w/b)ratios,cation ions,chloride concentrations in soaking solution and external applied voltage were measured by pore solution expression and chloride titration.Influences of these internal and external factors on“Chloride concentrate”and N_c were studied.Together with the pore solution expression and chloride titration,microstructure and EDL properties of cement pastes were also investigated.Pore structure(MIP),thermal analysis(TG-DTG),morphology features(SEM)and zeta potential measurement(Electroacoustic method)were conducted to explain the higher chloride concentration in the expressed pore solution.In addition,total chloride content and chemically bound chloride(Fried el's salt)were also tested.Based on these studies,pore structure and chemical composition of hydration products were considered as two main influencial factors for“chloride concentration index”.As the second step,the pore solution expression experime nt was investigated.Pore solution distributions of paste specimens before and after pore solution expression were measured by ~1H NMR.Total chloride content and pore structure were also measured and the variation of these during pore solution expression w ere determined.The porosity of cement paste was calculated from pore solution distribution data,and the consistence with previous studies confirmed the feasibility and practicability of the NMR technique on microstructure characterization of cement-based materials.Based on the variation of pore solution distribution,total chloride content and pore structure during pore solution expression,the chloride concentration in the expressed pore solution was calculated.The calculated and measured results were compared with each other,and reasons that caused the differences between them were also discussed.As the main factors controlling the phenomenon“chloride concentrate”and“chloride concentration index”N_c,the properties and modelling of EDL were studie d in the subsequent step.AC impedance spectroscopy and zeta potential measurements were applied to analyze the properties of EDL.As a non-destructive technique,AC impedance spectroscopy can be used to monitor the microstructure evolution and EDL properties of cement pastes.With the aid of a proposed equivalent circuit model,the continuous and discontinuous pore,solid phase formation and EDL capacitance were analyzed.Moreover,physical and mathematical models of EDL with capacitance or zeta potential of EDL related were established.By idealizing the EDL formed at the solid-liquid interface of cement pastes into a single plane-parallel capacitor,the thickness of EDL with different chloride concentration s and slag replacements were investigated and used to explain the variation of N_c.According to the Stern model on EDL formed at the solid-liquid interface,the chloride distribution was represented with mathematical equation and the value of N_c was calculated with zeta potential and pore structure data.The calculated results showed a good agreement with the measured results as a function of chloride concentration and slag replacement level.In the end,low temperature ~1H NMR measurement was conducted to investigate the pore solution distribution within cement paste during freezing process.The boundary point in the T2 relaxation time distribution plot was determined for absorbed water in EDL and free water in bulk pore.The absorbed water content s with different concentrations in soaking solution were studied and applied to explain the“chloride concentrate”and“chloride concentration index”.