7.9 : Transition Zone

The transition zone in concrete is a critical area where aggregate meets cement paste, marked by a distinct porosity and weakness compared to the surrounding material. The adhesion around the aggregates is primarily due to Van Der Waals forces. The voids within this zone influence its robustness; initially, it is less durable than the surrounding bulk mortar due to larger voids. Initially, when concrete is compacted, a higher water-cement ratio near the aggregates leads to the formation of water films around them, affecting the zone's porosity. This area sees the crystallization of compounds like calcium, sulfate, hydroxyl, and aluminate into ettringite and calcium hydroxide, which tend to form larger and more porous structures near the coarse aggregates due to the elevated water-cement ratio. These structures, especially the calcium hydroxide, crystallize in oriented, plate-like layers, further contributing to the porosity.

Over time, as hydration progresses, the initially large crystals of ettringite and calcium hydroxide begin to be surrounded by a denser matrix of secondary, finely crystallized C-S-H, along with smaller crystals of the same compounds, leading to a densification of the transition zone and an increase in its strength.