Influences of chloride immersion on zeta potential and chloride in concentration of cement-based materials

In this paper, the zeta potential of freshly mixed cement paste and hardened cement pastes, as well as the concentration index, was measured. The influences of chloride concentration in mixing water and slag content on zeta potential of freshly mixed pastes were studied. A proposed model was expressed to explain the relationship of zeta potential and concentration index of hardened cement pastes immersed in chloride solution. The results showed that the increase of chloride concentration in mixing water and slag replacement improved the zeta potential of freshly mixed cement, the hydration rate and concentration of ions in mixed water affects the zeta potential. With the increase of chloride concentration in soaking solution, the chloride concentration index and zeta potential of hardened cement paste all gradually decreased. The addition of slag gave some changes on chloride in concentration and zeta potential. The relationship among chloride concentration index, chloride concentration in soaking solution and slag replacement revealed by Gouy-Chapman model was in good agreement with the measured results

AC impedance spectroscopy characteristics of chloride-exposed cement pastes

In this paper, the characteristics of AC impedance spectroscopy of cement paste immersed in chloride solution were measured and analyzed with a proposed equivalent circuit model. The elements in the proposed equivalent circuit, including the resistance of interface between electrode and specimens, resistance of continuous and discontinuous pore, capacitance of solid phase and electrical double layer (EDL) were discussed. The results showed that the resistance of interface between electrode and testing specimen was much lower than that of cement paste. With the increase of chloride concentration in the soaking solution, the resistance of continuous gradually decreased due to the higher conductivity of chloride solution. Stripped out the impacts of concentration of pore solution on resistance of pores, the resistance of continuous pore increased firstly due to the decrease of continuous pore volume from the formation of Friedel's salt. However, the resistance of discontinuous pores gradually decreased with the increase of soaking solution concentration due to the transformation of continuous pores to discontinuous pores. The reaction between chloride ions and hydration products and formation of Friedel's salt decreased the porosity of cement pastes and led to higher capacitance of solid phase. Based on an idealized two-plate capacitor model for EDL, the thickness of EDL was calculated from the measured capacitance. The decrease of EDL thickness with chloride concentration in soaking solution was in agreement with the results of chloride contents in EDL obtained from pore solution expression test. (C) 2019 Elsevier Ltd. All rights reserved

Measurement Of Power-law Creep Parameters By Instrumented Indentation Methods

New experimental methods have been developed to measure the uniaxial power-law creep parameters α [alpha] and n in the relation έ[epsilon dot]=α[alpha]σn[sigma] (έ [epsilon dot] is the creep strain rate and σ [sigma] is the creep stress) from the load, time, displacement and stiffness data recorded during an instrumented indentation experiment performed with a conical or pyramidal indenter. The methods are based on an analysis of Bower et al., which relates the indentation creep rate to the uniaxial creep parameters based on simple assumptions about the constitutive behavior (Bower et al., 1993). Using finite element simulations to establish the influences of finite deformation and transients caused by elasticity, the proposed methods are explored experimentally using amorphous selenium as a model material. Cylindrical specimens compressed at high temperatures and low strain rates deformed stably into barrel-like shapes, while tests at low temperatures and high strain rates caused the sample to catastrophically shatter. These observations are consistent with the stress exponent and kinetic activation parameters extracted from the nanoindentation creep tests. With a few notable exceptions, the values of both α [alpha] and n derived from the indentation data at 35°C [Celsius] are generally in good agreement with those measured in uniaxial compression, thus demonstrating the validity of the approach. Another technique, based on the measured elastic contact stiffness and assumed values of Young’s modulus and Poisson’s ratio, is proposed as an alternative method to extract α [alpha] and n when the affect of thermal drift is such that the measured displacement in the indentation test is no longer reliable. Experimental verification of the stiffness method is provided through constant load and hold indentation creep experiments performed with a Berkovich indenter on amorphous selenium at 35°C [Celsius], where the affect of drift is extremely small, and high purity polycrystalline aluminum at 250°C [Celsius], where the affect of drift is so strong that the measured displacement is completely useless. Results show that the stiffness method can accurately predict the projected contact area and α [alpha] and n for both materials, thus demonstrating the validity of the proposed stiffness method

Effects of chloride concentration on microstructure of cement pastes by AC impedance spectroscopy HU

Alternating current (AC) impedance spectroscopy has been applied in characterizing microstructural evolution and electrochemical properties of cement-based systems. In the present paper, an equivalent circuit model was proposed to study the influences of chloride binding on microstructure and solid-liquid interfacial properties of cement paste. Chloride concentration index of pore solution was measured to correlate to the parameters in equivalent circuit model corresponding to electrical double layer at solid-liquid interface. The results showed that the parameters of equivalent circuit model can properly indicate the microstructure and interfacial properties of cement paste. Resistance of continuous pores was gradually decreased with chloride concentration in soaking solution due to the higher conductivity of pore solution. The capacitance of electrical double layer was increased with chloride concentration in pore solution due to more content of chloride ions in electrical double layer. While the thickness of electrical double layer was decreased as chloride concentration increased, which is in agreement with mathematical calculation