According to Simpson, Hunderman, and Slaton in “Twentieth Century Building Materials,”
“Concrete block is produced from a mixture of portland cement and aggregates. Commonly manufactured in a nominal 8-by-8-by-16-inch size, concrete blocks, also known as concrete masonry units (CMUs) can be solid or hollow (typically with two or three cores), and block ends can be flat or flanged. Block weights vary depending on the aggregates used in the manufacturing process. Most concrete blocks manufactured today have a core are of 40 to 50 percent and are defined as hollow. The blocks’ configuration determines two key characteristics: compressive strength and fire resistance.
The mass production of hollow concrete blocks is a twentieth-century phenomenon. Throughout the nineteenth century, attempts had been made to cast concrete into building blocks, but the modern industry began with Harmon S. Palmer’s invention of a cast iron block machine, which he patented in 1900.”
- Simpson, Pamela H., Harry J. Hunderman and Deborah Slaton. ”Concrete Block.” In Twentieth Century Building Materials, edited by Thomas C. Jester, 80-85. New York: McGraw-Hill Co., 1995.