Three phase system and structure of hcp

Concrete as a three-phase system

At the microscopic level, concrete consists of:

1. Aggregate phase (70-75% of volume): gravel or crushed stone and sand; provides volume, stiffness and dimensional stability.
2. Hydrated cement paste (hcp): the binder phase.
3. Transition zone (ITZ): a thin (10-50 µm) layer of paste next to the aggregate surfaces; the weakest part.

 +---------------------------+
 | paste  ___ITZ___  paste   |
 |       / AGGREG- \         |
 |       \  ATE    /         |
 |        ---------          |
 | ( ) air voids    . . pores|
 +---------------------------+

Structure of hydrated cement paste

Solid phases

• Calcium silicate hydrate (C-S-H): 50-60% of the solids; a poorly crystalline gel with a huge surface area (100-700 m$^2$/g); the main source of strength, through van der Waals forces.
• Calcium hydroxide (CH, portlandite): 20-25%; large hexagonal crystals; low strength and surface area; soluble; gives alkalinity (pH about 12.5) but reduces chemical resistance.
• Calcium sulphoaluminates: 15-20%; ettringite (needle-like) and monosulphate; play a minor role in strength, and monosulphate makes the paste vulnerable to sulphate attack.
• Unhydrated cement grains: remain in the paste, especially when the w/c is low.

Voids in hcp

• Interlayer space in C-S-H: about 0.5-2.5 nm; too small to affect strength.
• Capillary pores: 10 nm to 50 µm; spaces left by water not filled by hydration products. Their volume depends on the w/c and the degree of hydration, and they control strength and permeability.
• Air voids: entrained air bubbles of about 50-200 µm; entrapped air voids from about 1 mm up to 3 mm.

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

Effect on concrete properties

• Strength is inversely related to the capillary porosity (Powers: $f = f_0 x^3$, where $x$ is the gel-space ratio).
• A lower w/c and longer curing reduce capillary pores, so strength and durability rise.
• Drying shrinkage and creep come from the loss of adsorbed and interlayer water from the C-S-H.
• Permeability and durability depend on how continuous the capillary pores are; CH is vulnerable to acids and leaching, and the sulphoaluminates to sulphate attack.