How Concrete Blocks Are Made

Shape/Style

Concrete blocks are precast and hardened masonry concrete products normally used in the construction of block foundation and retaining walls. Most styles of concrete blocks are designed with hollow cavities; however, some blocks are made with smooth sides. While concrete blocks are manufactured in different sizes and shapes, the most common type of concrete block used in construction measures 8 inches high by 8 inches deep by 16 inches wide. During the manufacturing process, chemicals are sometimes added to improve the strength and durability of the blocks, as well as shorten the curing time. Pigments are frequently added to the concrete mixture to give the concrete blocks an even color throughout. The surface of some concrete block products are coated with a glaze that is then baked on. When used to construct a wall or foundation, concrete blocks are stacked one at a time to the length and height needed. The blocks are held together with concrete mortar.

Mixture

Concrete blocks are usually made from a mixture of sand, gravel, water and Portland cement. The manufacturing process involves a total of four stages including the mixing, molding, curing and cubing processes. Typically, the mixture used to form concrete blocks contains more sand and less gravel and water than concrete mixtures used for other construction purposes. A stiffer, drier mixture is required in order for a concrete block to hold its shape once it is removed from the mold. Concrete blocks are light gray in color. A single concrete block weighs 40 lbs on average. Lower density blocks are made by using cinders such as coal ash in the concrete mixture instead of sand and gravel, giving the blocks a coarser texture. These cinder blocks are dark gray in color and weigh 25 to 33 lbs. Cinder blocks are known for having a higher insulating value and better soundproofing qualities than other concrete block products.

Lighter Weight Blocks

Lightweight concrete blocks use crushed expanded clay, slate or shale as the raw materials instead of sand and gravel for making the block. Pumice and volcanic rock known as scoria are sometimes used as well. In the process of forming these blocks, the raw materials are heated to a temperature of 2000 degrees F. These lighter weight concrete blocks usually weigh about 25 lbs and are most often used to construct partitions or non-load-bearing walls.

Manufacturing Process

The dry materials are blended together for several minutes before water is put into the mixture. Blades attached to a rotating shaft are used in this mixing process. Any chemicals or color pigments are added before the concrete is mixed, generally for about 8 minutes. Afterward, the mixed concrete is forced down into molds, where as many as 15 concrete blocks can be molded at one time. The concrete mixture filling the molds is then compacted by the weight of the upper mold head. Some manufacturers use hydraulic pressure cylinders to aid in the compacting process. Compacted blocks are pushed down and out of the molds, where they are placed onto a curing rack and moved to a curing room where the blocks sit at room temperature for a couple of hours. This allows the blocks to harden slightly before being cured with low-pressure steam at a temperature of 185 degrees F. Once the curing temperature has been reached, the steam is shut off. The blocks are left to soak in hot, moist air for another 12 to 18 hours after which time they are air dried. The entire process takes about 1 day from start to finish.

Resources

• How to Lay Concrete Blocks