Transport properties in metakaolin blended concrete

One of the approaches in improving the durability of concrete is to use blended cement materials such as fly ash, silica fume, slag and more recently, metakaolin. By changing the chemistry and microstructure of concrete, pozzolans reduce the capillary porosity of the cementitious system and make them less permeable to exterior chemical sources as well as reducing the internal chemical incompatibilities such as alkali–silica reaction. This paper presents the results of a study on the transport properties and durability characteristics of concrete containing different levels of metakaolin. Water penetration, gas permeability, water absorption, electrical resistivity, chloride ingress, and alkali–silica reaction potential were studied and their inter-relationships discussed. Results show that substituting optimum levels of metakaolin improves different aspects of the transport properties and durability performance. At 15% replacement level, compressive strength increased by 20%, while the water penetration, gas permeability, water absorption, electrical resistivity and ionic diffusion had improvements of up to 50%, 37%, 28%, 450%, and 47%, respectively and the 28-day ASR expansion for this mix was reduced as much as 82%. The minimum replacement level required for mitigating ASR and the relationship between physical and chemical effects of metakaolin were discussed.