Microstructure of ordinary Portland cement

The microstructure of ordinary Portland cement paste is the structure of the cement paste after hydration, as seen under an electron microscope.

Unhydrated cement

Cement is a fine powder (particle size about 1-100 µm) of the clinker compounds C$_3$S, C$_2$S, C$_3$A and C$_4$AF, with gypsum. The grains have an irregular shape.

Hydrated structure

On mixing with water, the grains dissolve and hydration products precipitate in the water-filled space between the grains:

1. C-S-H gel (50-60% by volume of solids): at first fibrous or honeycomb-like, later massive, with an enormous surface area (about 100-700 m$^2$/g). It forms from the hydration of C$_3$S and C$_2$S and gives the strength.
2. Calcium hydroxide (portlandite) (20-25%): large hexagonal plate crystals that fill spaces.
3. Ettringite (needles, at early ages), which changes into monosulphate plates (15-20% together); formed from C$_3$A (and C$_4$AF) with gypsum.
4. Unhydrated cores of cement grains, especially the large ones.
5. Pores: gel pores (below about 3 nm), capillary pores (about 10 nm to a few µm), and air voids.

 Early (hours)   ->   Later (days)
  o   o   o           oooo C-S-H
 grains in water      fills spaces,
  + needles           CH hexagons

The strength depends on the amount of solids and how well they fill the space (gel/space ratio), so it rises with age as the hydration products grow and the capillary pores shrink. The w/c ratio (a higher w/c gives more capillary pores) and curing decide the porosity, and thus the strength, permeability and durability.