Three phase system; binding medium phase

Concrete is a three-phase composite: (1) the aggregate phase, (2) the binding medium (hydrated cement paste, hcp) and (3) the transition zone between them.

 +----------------------------+
 | hcp  .------------.  ITZ   |
 |     / aggregate    \       |
 |     '--------------'       |
 +----------------------------+

The aggregate (70-75% of the volume) gives bulk and stiffness; the ITZ (10-50 µm thick) is the weak, porous layer around the aggregate.

Binding medium phase (hydrated cement paste)

Cement reacts with water to produce the paste, which hardens into a stone-like binder.

Solid products of hydration

• C-S-H gel (50-60% of solids): very fine, with a high surface area; gives strength through van der Waals forces; stable.
• Calcium hydroxide (20-25%): crystalline; contributes little to strength; provides alkalinity; weak against acids.
• Calcium sulphoaluminates (15-20%): ettringite and monosulphate; a minor role in strength; vulnerable to sulphate attack.
• Unhydrated cement remaining in the paste.

Voids

• Interlayer (gel) space in C-S-H (below 2.5 nm); capillary pores (10 nm-50 µm), which depend on the w/c and the degree of hydration; air voids (entrapped about 1-3 mm, entrained 50-200 µm).

Water: capillary water, adsorbed water, interlayer water and chemically combined water.

Significance

• Strength depends on the volume of capillary pores (gel-space ratio); a low w/c and good curing give a strong, impermeable paste.
• The paste governs shrinkage and creep (loss of adsorbed water from the C-S-H) and permeability.
• It binds the aggregates by adhesion, holds the steel, and protects it by its high pH.