Three phase system; role of aggregate phase

Concrete is a three-phase composite consisting of (i) the aggregate phase, (ii) the hydrated cement paste (binding medium) and (iii) the interfacial transition zone (ITZ) between them.

  +--------------------------+
  | paste  ,---------.       |
  |       / aggregate \      |
  |       `---------'  ITZ   |
  +--------------------------+

• Aggregate phase: 70-75% by volume; particles of crushed stone or gravel and sand.
• Paste (matrix): C-S-H, calcium hydroxide, sulphoaluminates and pores; binds the aggregate and gives strength.
• ITZ: a 10-50 µm thick porous shell around the aggregate; the weakest link and the origin of microcracking.

Role of the aggregate phase

1. Volume filler: cheaper than cement; it occupies most of the volume, so it reduces the cost and the heat of hydration.
2. Stiffness and strength: it largely decides the elastic modulus of concrete (E of aggregate 50-100 GPa against about 10-30 GPa for paste). In normal concrete the aggregate is stronger than the paste, so it does not limit strength, but its shape, texture and size affect strength through the bond at the ITZ; in lightweight concrete a weak aggregate can limit strength.
3. Dimensional stability: restrains the shrinkage and creep of the paste, so it reduces cracking.
4. Durability and wear: provides abrasion resistance, soundness, and fire and weather resistance.
5. Unit weight: the density of concrete is mostly decided by the aggregate density (normal, lightweight or heavyweight concrete).
6. Workability: the grading, shape and texture control the water demand and the bond with the paste.