Chloride penetration and microstructure development of fly ash concrete

In the past decades, supplementary materials have been widely used in cement production. Fly ash (FA) with cheaper in price and with latent hydraulic properties, is one of the widely used supplementary materials. However, the addition of supplementary materials could influence the concrete properties. In general, the durability of concrete is associated with its transport properties. The transport properties, such as permeability, diffusivity and sorptivity are mainly resulted from the development of microstructure. In this study, the chloride migration coefficient measurement of FA concrete and the microstructural observation of cement paste blended with FA were carried out on the samples curing up to one year. Rapid Chloride Migration (RCM) test was used to determine the chloride migration coefficient of concrete. Meanwhile, Mercury Intrusion Porosimetry (MIP) method was conducted to obtain total porosity and pore size distribution of blended cement paste. Test results show that FA concrete has good resistance to chloride ion after the curing age of 28 days compared to ordinary Portland cement concrete. Although cement paste blended with FA has a higher total porosity, less capillary pore exist if compared to cement paste. Except the chloride binding capacity of FA hydrates, the difference in microstructure is probably one of the main reasons why FA concrete has a lower chloride migration coefficient.