Porosity of aggregates is their dominant quality determinant. Porosity governs specific gravity, and determines the physical durability of aggregates. Porosity and permeability often, but not necessarily correlate - i.e., large porosity does not always indicate high permeability. The inter-connectedness or the pore and the pore 'throat' diameters influence permeability. The grain size of the minerals has the major effect on porosity and permeability. But more important then porosity is the dominant pore size and pore size distribution.
Effective porosity of the aggregates is a function of the following:
 | Grain size - does not affect total porosity, only pore size |
| Grain packing - relative stacking pattern of grains |
| Grain size distribution - smaller grains filling voids between larger grains |
| Type and degree of cementation - degree to which interparticle cement fills voids |
| Interconnectedness of pores - isolated or sealed pores do not absorb water |
Porosity is expressed in two ways: as a weight percent of fully water saturated voids to dry rock, and as a void ratio, i.e., ratio of volume of voids to ratio of total volume of rock. The former is applied to aggregates, and the latter usually to soils.
In general, low porosity aggregates are superior to those with high porosity. Low porosity aggregates are denser, have higher strength, and are more durable.
For more information, read this interesting student paper on porosity, porosity determination, and durability implication.
