Electrochemical impedance spectroscopy to monitor the hydration of cementitious materials
Taha Abdalgadir, Hussameldin Mohamed
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The electrical properties of Portland cement, and cements containing supplementary cementitious materials (SCM), were obtained over the frequency range 1Hz-10MHz during both the initial 24-hours after gauging with water and up to 1 year. During the initial 24-hours period, the response was measured in terms of conductivity and permittivity with both parameters exhibiting significant temporal changes. It was also evident that whilst the conductivity increased only marginally with increasing frequency of applied electrical field, the permittivity decreased by several orders of magnitude over this range. Moreover, certain features of the permittivity response – which are related to bulk polarization processes – only revealed themselves in the higher frequency range (100kHz-1MHz), and went undetected at lower frequencies. The detailed frequency- and time- domain measurements allowed identification of several stages in the early hydration of cement-based materials and the response can be interpreted in terms of hydration kinetics, physico-chemical processes and microstructural development. It is shown that the methodology can be equally applied to cement-pastes and concretes. In the hardening stage, the conductivity response showed a clear influence of the SCM type, the age of the samples and the used water binder ratio (w/b) in the mixes. The pore solution conductivity has been shown to have a significant effect on the conductivity values particularly at the high replaced mixes. The electrical permittivity showed two different polarization signals depending on the frequency range used, as at frequencies in the range of 100kHz-1MHz, the permittivity response is more related to the samples electrical conductivity, however at higher frequencies (1MHz-10MHz) the permittivity is influenced more by the SCM type and the replacement level in the mixes when w/b is constant. The durability ranking which was obtained from the non-steady-state migration coefficient and the electrical conductivity, showed a strong linear relationship which is in contrast to the relationship between the ranking obtained from the formation factor. This would suggest that both the non-steady-state migration coefficient and the conductivity are affected by the pore solution conductivity of the mixes which, consequently, would give a false indication with regard to the real ranking of the mixes.