Interfacial transition zone in concrete

Abstract

In fresh concrete a watercement (W:C) ratio gradient develops around the aggregate particles during casting, resulting in a different microstructure of the surrounding hydrated cement paste. This zone around the aggregate is called the interfacial transition zone (ITZ). This review describes the formation mechanisms and the microstructure of the ITZ. The higher W:C implies a diffusion process during hydration, and this zone may be consequently described as a heterogeneous area with a porosity gradient and a complementary gradient of anhydrous and hydrated phases. By using very fine and well-dispersed mineral additions, the initial W:C gradient around the aggregates is lowered and the ITZ is densified. The microstructure of the ITZ may be improved in the vacinity of calcereous aggregate, which reacts with calcium aluminates of Portland cement paste, forming calcium carboaluminates. The overall engineering properties of concrete in relation to the ITZ are beyond the scope of this review. Nevertheless, the local properties of the interfacial zone are reviewed: the mechanical and the transport characteristics of the ITZ are discussed in relation to the porosity and connectivity of pores.