Mechanism of chloride concentrate and its effects on microstructure and electrochemical properties of cement-based materials

Reinforced concrete has been widely applied in different kinds of structures, even while chloride- induced steel corrosion threatens the durability of the structure, especially in case of bridge deck slabs potentially when applying de- icing salts and in case of structures in marine environment. Chloride ions can be introduced into cement-based materials due to their presence in raw materials and mixing water (internal chloride) or from chloride sources in the service environment (intruded chloride). The internal chloride ions affect the hydration process and hydration products of cementitious materials, while the intruded chloride penetrated from surrounding may alter the pore structure and composition of solid/ liquid phase within cement-based materials. Free state chloride in bulk pore solution, physically adsorbed chloride on the surface of hydration products and chemically bound chloride by solid phase are considered as three main types of chlorides in cement-based materials. The chemically bound chloride has received most attention in previous studies due to its effects on microstructure and chemical composition of hydration products. Decrease of porosity and formation of Friedel’s salt are generally occurring with the chemical binding of chloride ions. Besides the chemically bound chloride, other forms of chloride ions are also studied. Pore solution expression is widely used to obtain the pore solution and analyze the free chloride concentration. In previous studies, a higher chloride concentration in expressed pore solution has been obtained in comparison with the environmental soaking solution. This has been defined as “chloride condensation” or “chloride concentrate” in those studies. The formation of electrical double layer (EDL) was considered as a possible reason for this phenomenon. The influences of different factors on the “chloride condensation index” or “c hloride concentration index” Nc have been studied in some previous studies. The “c hloride concentrate” attracts the chloride ions in pore solution onto the surface of solid phase, which may decrease the chloride content in the pore solution and induce the renewed penetration of chloride ion within cement-based materials. In this study, the phenomenon of “c hloride concentrate” was investigated in more detail for cement paste. Pore solution expression was applied and the value of Nc was tested. Measurements on microstructure, electrochemical characteristics and interfacial properties of cement paste were conducted to explain the phenomenon and to figure out the detailed mechanism. Physical and mathematical models were also proposed for the characterization of Nc from a theoretical point of view. As the first step of this study, factors which can possibly affect the value of Nc were studied. The chloride concentration index Nc of cement pastes with different supplementary cementitious materials (SCMs), maturity, water to binder (w/b) ratios, types of cations, 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 Nc were studied. Together with the pore solution expression and chloride titration, microstructure and electrochemical properties of cement pastes were also investigated. Pore structure (MIP), thermogravimetric analysis (TGA), 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 (Friedel’s salt) was also tested. Based on these studies, pore structure and chemical composition of hydration products were considered as two main factors for “chloride concentration index”. As the second step, the pore solution expression experiment was investigated. Pore solution distribution of paste specimens before and after pore solution expression were measured by 1H NMR. Total chloride content and pore structure was also measured and their variations during pore solution expression were determined. The porosity of cement paste was calculated from pore solution distribution data, and the consis tence 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 measured and calculated results were a lso discussed. As the main factor controlling the phenomenon of “chloride concentrate” and “chloride concentration index”, the properties and modelling of EDL were studied 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 interfacial properties of cement pastes. With the aid of a proposed equivalent circuit model, the continuous and discontinuous pores, solid phase formation and EDL capacitance were analyzed. Moreover, physical and mathematical models of EDL with capacitance or zeta potential related to EDL 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 concentrations and slag replacements were investigated and used to explain the variation of Nc. According to the Stern model concerning EDL formed at the solid- liquid interface, the chloride distribution was represented with mathematical equation and the chloride content and value of Nc was calculated based on 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 adsorbed water in EDL and free water in bulk pore. The adsorbed water content with different concentrations in soaking solution was studied and applied to explain the “chloride concentrate” and “chloride concentration index”.