Overgangs Zone

The transition zone, found where the aggregate meets the cement paste, is more porous and, as a result, weaker than hydrated paste beyond the transition zone.

First, when concrete is freshly compacted, films of water surround the larger aggregate particles, which can result in a greater water-cement ratio near these aggregates compared to areas further away.

Next, within the concrete's matrix, ions such as calcium, sulfate, hydroxyl, and aluminate from the dissolution of various compounds precipitate to form ettringite and calcium hydroxide.

Near coarse aggregates, where the water-cement ratio is elevated, these compounds crystallize into larger, more porous structures than those in the denser cement paste.

The calcium hydroxide often crystallizes into oriented, plate-like layers, typically perpendicular to the aggregate surfaces, contributing to the overall porosity.

Finally, as hydration advances, the space between the large initial crystals of ettringite and calcium hydroxide begins to fill with a secondary, poorly crystallized calcium silicate hydrate, along with smaller ettringite and calcium hydroxide crystals.

This secondary filling phase contributes to a denser and stronger interfacial transition zone.