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

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, R_e(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.