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The transition zone is weaker because it contains larger, more porous crystal structures than the bulk cement paste. When concrete is freshly compacted, water films surround aggregate particles, creating a higher water-cement ratio in this region. This elevated ratio causes compounds like calcium hydroxide and ettringite to crystallize into larger, more porous formations, reducing the zone's strength and durability compared to denser surrounding material.

The water-cement ratio directly controls porosity in the transition zone. Higher water-cement ratios near coarse aggregates promote the formation of larger, more porous crystals of ettringite and calcium hydroxide. These oversized structures, especially plate-like calcium hydroxide layers oriented perpendicular to aggregate surfaces, increase overall porosity and weaken the interfacial region compared to areas with lower water-cement ratios.

Near coarse aggregates, calcium hydroxide crystallizes into oriented, plate-like layers perpendicular to aggregate surfaces, while ettringite forms larger, more porous structures due to the elevated water-cement ratio. Both compounds precipitate from dissolved ions during hydration. In contrast, within denser cement paste regions away from aggregates, these compounds form smaller, more tightly packed crystals, contributing to stronger material properties.

As hydration advances, the transition zone densifies and strengthens. Large initial crystals of ettringite and calcium hydroxide become surrounded by secondary, finely crystallized calcium silicate hydrate (C-S-H), along with smaller crystals of the same compounds. This secondary filling phase creates a denser matrix that gradually improves the interfacial region's strength and durability, reducing the initial weakness caused by high porosity.

When concrete is freshly compacted, water naturally surrounds larger aggregate particles, creating films that increase the local water-cement ratio near these aggregates. This phenomenon occurs because water is attracted to aggregate surfaces during compaction. The resulting higher water-cement ratio in the transition zone leads to more porous crystal formation and contributes to the zone's initial weakness relative to bulk cement paste.

The transition zone's elevated porosity weakens concrete because larger voids reduce load-bearing capacity at the aggregate-paste interface. Initially, this zone is less durable than surrounding bulk mortar due to its larger voids and porous crystal structures. However, as hydration progresses and secondary C-S-H fills the spaces between large crystals, the transition zone densifies, significantly improving concrete's behavior under compressive load and overall structural performance.

Coarse aggregates create more pronounced transition zones because larger particles attract thicker water films during compaction, resulting in higher local water-cement ratios. This elevated ratio promotes the formation of larger, more porous crystals of calcium hydroxide and ettringite near these aggregates. The size and porosity of the transition zone directly correlate with aggregate dimensions, making aggregate selection important for controlling concrete's interfacial strength and durability.