Pore structure and permeability of concrete with high volume of limestone powder addition

Abstract

Limestone is one of the preferred admixtures used in concrete due to its wide availability and environmental advantages. The durability of concrete is strongly dependent on its pore structure and permeability. To understand the microscopic mechanism underlying this behaviour, mercury intrusion porosimetry (MIP) and ultrasound waveform analysis are employed to evaluate concrete porosity, pore size distribution and the fractal dimension of pores. The results show that the porosity and mean pore diameter of concrete decrease slightly when up to 30 wt% limestone powder is added, though further increase in the limestone content reverses the trend. For the same limestone content, concrete porosity and mean pore diameter increase as the specific surface area of the powder decreases, or as the water-to-binder ratio increases. The results confirm close correlations between concrete permeability, porosity and mean pore diameter. The diffusion coefficient of chloride ions, measured by the rapid chloride migration (RCM) method, is found to decrease with increasing fractal dimension of the pore volume. Based on the percolation theory, the existence of a critical porosity is predicted, below which concrete permeability approaches zero. Three regimes, i.e. subcritical, critical and conventional diffusion regimes, are identified with increasing porosity. The implications of the percolation threshold on concrete permeability and corrosion propagation are discussed.