Solid phases in hcp; transition zone

Solid phases in hydrated cement paste (hcp)

1. Calcium silicate hydrate (C-S-H): 50-60% of the volume of solids. A poorly crystalline gel (fibres, sheets), with a very high surface area; bonds by van der Waals forces and is responsible for the strength and other properties of the paste.
2. Calcium hydroxide (CH, portlandite): 20-25%; large hexagonal crystals; low surface area so contributes little to strength; makes the paste alkaline (protects steel), but is soluble and attacked by acids.
3. Calcium sulphoaluminates: 15-20%; ettringite (needles), later monosulphate; small contribution to strength, and a source of sulphate attack.
4. Unhydrated cement grains: especially at low w/c; they hydrate later if moisture is available (long-term strength). Also present: voids (gel pores, capillary pores, air voids) and water.

Effect of the transition zone on the properties of concrete

The ITZ is a thin layer (10-50 µm) of paste around aggregate surfaces which has higher w/c, larger oriented CH and ettringite crystals, more porosity and microcracks.

• It is the weakest link, so it limits the compressive and especially the tensile and flexural strengths.
• It causes the nonlinear stress-strain curve (bond microcracks form at about 30-40% of ultimate load).
• It lowers the elastic modulus below the value expected from the phases.
• Its porosity gives paths for water, chlorides and sulphates, reducing durability and increasing permeability.
• Improved by silica fume, fly ash, low w/c, superplasticiser and proper curing.