These days the builders are more likely to build poured concrete foundations than foundations made of concrete blocks. Concrete poured has become more popular due to a variety of reasons. Wall forms that are quick to constructed and removed, concrete trucks that can be used almost anywhere, and modern pumps and admixtures which allow "impossible pours' almost common.
Despite the many advantages discussed in the previous paragraphs, concrete block is an appropriate foundation material. If the foundation isn't too big such as a house addition for example-a builder can save money by building the foundation using block instead of subcontracting the work to an expert foundation contractor. In the same way, crawl space foundations can be easily and cost-effectively constructed using blocks of concrete. And for a DIYer with masonry skills the block material offers the opportunity to construct the foundation one block at a time.
Before getting any deeper into this "heavy" topic it's important to understand the basics of terminology. In the world of construction concrete masonry contractor blocks are often known as "CMUs," short for concrete masonry units. In addition to the standard 16x8x8 in. concrete block stocking most home centers, the vast array of CMU sizes, shapes , and surface finishes is truly amazing. Most of this selection is designed for commercial use.
Concrete blocks are made from the same ingredients used in poured concrete walls, which are gravel (called aggregate) as well as sand, Portland cement, and water. The blocks that weigh less than a pound are often referred to"cinder blocks" because the fly-ash (cinders), a waste product from industrial combustion, is used as a substitute for larger quantities of sand or aggregate. The hollow cores of most CMUs make the block much more affordable and easy to work with. Cores may be filled using concrete, foam insulation or sand to increase strength, thermal mass as well as insulation.
Contrary to what many believe, a concrete block foundation isn't necessarily superior to a poured concrete foundation, if it's constructed correctly. Both types of masonry require steel reinforcement and should be laid on a strong firm, solid footing. Good drainage and damp proofing are essential. When expansive soils and other outside forces damage the concrete block foundation, cracks typically appear in the mortar joints which separate individual blocks.