The effect of mineral admixtures on permeability, porosity and electrical resistivity of concrete

Evaluation of concrete durability is commonly performed by measuring its permeability, porosity and electrical resistivity. Moreover, using admixtures in concrete mix design is one of the methods of enhancing durability. For this reason, in this paper, the effect of pozzolanic admixtures (silica fume, fly ash and zeolite) and limestone powder on permeability, porosity and electrical resistivity of 28-day and 120-day cubic concrete specimens with 15cm dimension is investigated. For concrete preparation, 5, 10, 15 and 20 percent of cement mass were replaced by these admixtures. In this paper, the innovative method of “Cylindrical chamber” was used for measuring concrete permeability. The results obtained revealed that except the 28-day concrete specimen that 20 percent of its cement mass was replaced with limestone powder, other concrete specimens had lower permeability and porosity and higher electrical resistivity than the concrete specimen without any admixtures. In addition, volumetric porosity is a more accurate index for assessing concrete microstructure than surface porosity calculated from the images taken using scanning electron microscope. XRD test results also showed that pozzolanic material produce more calcium silicate hydrates by consuming calcium hydroxide which results in the reduction of permeability and porosity and increase of electrical resistivity, compared with the concrete specimen without any admixtures. Also, a slight reduction in calcium hydroxide and calcium silicate hydrate peaks was observed for the concrete containing limestone powder. This behavior showed that limestone powder decreases permeability and porosity and increase electrical resistivity of concrete, mainly due to the filler effect of its particles.