Using non-destructive testing tools to assess the pore structure of slag-blended cement paste under steam curing

Abstract

The pore structure is of great concern to the mechanical properties and durability of concrete. This paper investigated the pore structures of slag-blended cement paste under steam curing using two non-destructive testing (NDT) tools: electrochemical impedance spectroscopy and ultrasonic methods. Four different pastes containing 0, 20, 50 and 70% slag were prepared. For the electrochemical impedance spectroscopy method, parameters R1 and n fitted from an equivalent circuit model, Re(CR1(QR2)), were used to evaluate the porosity and the fractal dimension, respectively. For the ultrasonic method, ultrasonic pulse velocity was used to determine the porosity. Moreover, X-ray diffraction and mercury intrusion porosimetry were employed to characterise the properties of the cement paste. The results showed a linear relationship between R1 and ultrasonic pulse velocity. It was found that the porosity of the steam-cured paste was lower than that of the standard-cured paste at 3 d. However, an opposite trend was observed at 28 d. In addition, the fractal dimension of the pore structure dropped as the porosity increased during the hydration process. Although the slag-blended cement paste had a lower porosity than the pure cement paste, the fractal dimension increased with the increasing slag replacement ratio due to the complicated slag hydration products.