Pores

Summary

Pores - are voids in concrete paste, usually filled with water or air.

Pores can vary in size and cause different concrete properties depending on how they are formed.

Pores are usually created in concrete by:

Air making its way into the concrete during mixing and agitation which is then not properly released from the concrete when compacted.
Excess water in the concrete mix which then leaves voids when it either evaporates or becomes chemically incorporated into cement during hydration.
Air-entraining admixtures.
C-S-H formation.

There are three types of pores in concrete:

Micro voids – the largest of the voids; 1-10mm in size. They are caused by air entering concrete in an uncontrolled manner during mixing and agitation. They can severely damage the strength and durability of the concrete, but can be removed by proper compaction of the wet concrete (usually by vibration).
Capillary pores – voids left by the evaporation of excess water in a concrete mix, reaching 0.01-10 µm in size. A concrete with a large amount of capillary pores can be quite permeable and weak in compression. They can be limited in quantity by reducing the water content of the concrete mix.
Gel pores – exist within the C-S-H, as they’re much smaller than capillary pores, being 0.5-10 nm large. They don’t affect the strength or permeability of the concrete.