Concrete as a three phase system

Concrete is a three-phase composite (microstructure view): aggregate, hydrated cement paste and the interfacial transition zone.

  +----------------------------+
  | HCP (matrix)               |
  |    ,-------.               |
  |   / AGGREG- \  <-- ITZ ring|
  |   \  ATE    /              |
  |    `-------'    o pores    |
  +----------------------------+

1. Aggregate phase

About 70-75% by volume. Normally stronger and stiffer than the paste, so in ordinary concrete it does not govern strength; but it controls elastic modulus, density, shrinkage, thermal properties, wear resistance. In lightweight or weak aggregate concrete, aggregate strength limits concrete strength. Shape, size, texture and grading affect workability and bond.

2. Hydrated cement paste phase (matrix)

C-S-H gel (50-60%), $Ca(OH)_2$ (20-25%), sulphoaluminates and unreacted cement, with gel pores and capillary pores. It binds the particles and governs strength (lower w/c, fewer capillary pores, higher strength), shrinkage, creep and permeability.

3. Interfacial transition zone (ITZ)

A 10-50 µm thick shell around each aggregate particle with higher porosity, oriented $Ca(OH)_2$ crystals, and microcracks, because of bleed water films and wall effect. It is the weakest link: microcracking starts there, so it limits strength (especially tensile), modulus of elasticity and durability. It is improved by low w/c, pozzolanic admixtures (silica fume), and good curing.

Role of each phase in strength

• Strength of concrete $\approx$ f(strength of hcp, bond strength at ITZ, aggregate strength) - the weakest of the three fails first.
• In normal strength concrete: governed by hcp and ITZ; in high-strength concrete the ITZ is made denser and aggregate strength starts to matter; in light-weight concrete the aggregate governs.
• Under load the cracks begin at the ITZ, then spread into the paste, finally joining up to cause failure.