As the name implies, a concrete block is a building or construction block manufactured of concrete. Concrete is an amalgamation of Portland cement, aggregate and water. Often the aggregate used in the manufacture of concrete block is fly ash, or bottom ash. Both fly ash and bottom ash is the residue, or cinders, resulting from burning coal. Hence, some concrete blocks are known as cinder blocks.
Concrete blocks can be made in nearly any size, but some of the most common are 6 X 8 X 16 inches (15.24 X 20.32 X 40.64 cm) and 8 X 8 X 16 inches (20.32 X 20.32 X 40.64 cm). These blocks weigh about 40 to 45 pounds (18.14 kg to 20.41 kg). Blocks may be solid or contain large holes in the middle. There are also cement blocks, known as concrete bricks, manufactured to the specifications of the common, red brick.
A lighter weight concrete block, manufactured of aerated concrete is also available for construction. This type of block is known aerated concrete (AAC) block, or as an autoclave cellular concrete (ACC) block. This type of concrete block is manufactured from a mixture containing a binder with smaller portions of cement than in the more common block, and an aluminum powder aggregate, along with water for mixing. Though just as strong as regular concrete blocks, AAC blocks are much lighter. The AAC block is, however, much more expensive than the common concrete block, thus is not used extensively.
Most common concrete block is cored, having either two or three holes on the center of the block. This is done for a number of reasons. First, this lessens the weight of the block. Secondly, rebar, or long steel bars, can be inserted through the cores to reinforce a block load-bearing wall. Third, the cores in the block may be filled with sand or gravel in a further reinforcement procedure.
Other sizes and shapes of concrete blocks are also available for different purposes. There are notched blocks for window and door sills, and angled blocks for corners. There are also wedge-shaped blocks on a straight base known as capping units. Additionally, there are header and bull-nosed blocks. Specialty blocks may be obtained suitable to the unique requirements of any sort of masonry structure.
Concrete block, because of its rather bland appearance, is used more frequently in utilitarian structures than for residential construction. Blocks can, however, be painted or coated with a thin, tinted cement veneer to make them more attractive. Blocks are also coated to ensure proper weatherizing, and to provide further insulation for the interior of the structure.