Concrete blocks are an affordable building material that can be used for a number of construction projects, and are suited very well for construction walls for foundations or utility buildings. Concrete blocks typically have holes in the center to reduce the weight, material needed, and cost. The average concrete block is composed of cement and coal ash, which is why they are often called cinder blocks. Concrete blocks are actually concrete masonry units, or CMUs, but we often refer to them concrete blocks or cinder blocks Learn more about concrete masonry units. A wall typically requires 1 1/8 blocks per square foot.