Evaluation of chloride-penetration resistance of metakaolin concrete by means of a diffusion – Binding model and of the k-value concept

In this paper, the effect of the addition of metakaolin on chloride-penetration resistance of concrete is investigated. Greek kaolin thermally treated at defined conditions and a commercial metakaolin were used. Eight mixture proportions were used to produce high performance concrete, where metakaolin replaced either cement or sand in percentages 10% or 20% by weight of the control cement content. The specimens were placed in a NaCl solution for 90 days and the concentration of total and free (unbound) chloride was determined. The chloride penetration was studied by means of a fundamental model taking into account diffusion and binding (adsorption & desorption) processes. The efficiency factors (k-values) for the studied metakaolins were determined. Both metakaolins present a considerable chloride-penetration resistance, depending on active silica and alumina contents, and their addition significantly decreases chloride penetration leading to improved concrete durability.