The bonding property of cement with aggregate is achieved via Interfacial Transition Zone (ITZ). In this article, the reason for formation of ITZ and its impact in the overall life of concrete will be discussed.
ITZ is a region of the cement paste around the aggregate particles. In ITZ, a gradual transition in the microstructural features occurs.  This zone can be 35 – 50 micrometer wide. The average content of unreacted clinker phase decreases and porosity decreases towards the aggregate surface. Similarly, the content of Ettringite increases in ITZ.  Both cement paste and aggregates show brittle elastic behaviour, that is linear deformation followed by a sudden failure. Concrete however, shows quasi-ductile behaviour i.e. the load bearing capacity increase non-linearly and the damage is also progressive. This kind of behaviour is said to be due to the development of multiple microcracking predominantly in the ITZ. Thus, ITZ is considered as the weak spots in the concrete.
Why ITZ is formed?
Most likely the excess water near the aggregate surface is responsible for the formation of ITZ.
Features of ITZ
During hydration, the C-S-H phase is mostly deposited directly around the cement grains, while the calcium hydroxide is mainly deposited in the open pores. Furthermore, it has been found that the presence of silica inhibits the nucleation of calcium hydroxide, which favours the precipitation of this phase as far from the cement grains as possible. This means more Ca(OH)2 will be present near the aggregate while the C-S-H away from the aggregate.
ITZ has higher porosity than the hardened concrete paste. This affects the transport properties. Porosity increases towards the aggregate surface. It has been reported that the porosity at the interface is 40% higher than in the bulk.
Using the properties of ITZ, meso-scale modelling can be done. Wang (2019) utilized the properties of ITZ to successfully predict the cracking behaviour of ASR and DEF affected concrete using RBSM method.
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