Aggregates can be classified as Normal weight or Lightweight.
Normal weight aggregates generally consist of various combinations of naturally occurring sands, gravel and stones, and of different sizes of crushed rock. Their relative densities usually lie between 2.4 and 2.9 and their strength is at least as great as that of normal cement mortar.
Lightweight aggregates are generally manufactured (for example expanded clay or expanded shale) but can also be natural rock (for example pumice). Their relative densities usually lie between 1.2 and 2.0 and their strength is normally less than that of normal cement mortar.