Today the builders are more likely to build poured concrete foundations than foundations made of concrete blocks. Concrete poured has become more popular for a number of reasons. Wall forms that are quick to set up and taken down as well as concrete trucks that are able to travel almost everywhere as well as modern pumps and admixtures that make "impossible pours" almost routine.
Despite all the advantages mentioned above, concrete block can be used as an acceptable foundation material. If the foundation is small for a home addition, for instance, a builder can save money by laying up a block foundation instead of subcontracting the work out to a foundation contractor. Likewise, crawl space foundations are able to be efficiently and quickly built with concrete block. For those who are DIYers with some masonry experience it gives you an opportunity to complete the foundation block by block one time.
Before diving in this "heavy" subject it's helpful to know some terms. In the building industry, concrete blocks are referred to as "CMUs," short for concrete masonry units. Beyond the standard 16 x 8 x 8 in. concrete block stocked by most home centers, the vast universe of CMU sizes, shapes and surfaces is truly astounding. The majority of the selection is designed for commercial use.
Concrete blocks are constructed from the same ingredients used in poured concrete walls -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, can be used to substitute more heavy ingredients such as sand and aggregate. The hollow cores in most CMUs allow the block to be much more affordable and easy to work with. The cores can be filled with concrete foam insulation, or sand to improve their strength, thermal mass, and insulation.
Contrary to popular retaining wall opinion it is true that a block foundation made of concrete isn't necessarily inferior to a concrete foundation, provided that it's properly constructed. Both types of masonry require steel reinforcement and must be placed on a sturdy, stable footing. Proper drainage and damp proofing features are crucial. When expansive soils and other external forces cause damage to a concrete block foundation cracks usually appear in the mortar joints which create the separation between individual blocks.