First,in freshly compacted concrete, water films form around the large aggregate
particles. This would account for a higher water-cement ratio closer to the larger
aggregate than away from it (i.e., in the bulk mortar).
Next,as in the bulk paste, calcium, sulfate, hydroxyl, and aluminate ions, produced by the dissolution of calcium sulfate and calcium aluminate compounds, combine to form ettringite and calcium hydroxide. Owing to the high water-cement
ratio, these crystalline products in the vicinity of the coarse aggregate consist of relatively larger crystals, and therefore form a more porous framework than in the bulk
cement paste or mortar matrix. The platelike calcium hydroxide crystals tend to form
in oriented layers, for instance, with the c-axis perpendicular to the aggregate surface.
Finally, with the progress of hydration, poorly crystalline C-S-H and a second generation of smaller crystals of ettringite and calcium hydroxide start filling the empty
space that exists between the framework created by the large ettringite and calcium
hydroxide crystals. This helps to improve the density and hence the strength of the
interfacial transition zone.
A scanning electron micrograph and diagrammatic representati