One cubic yard of concrete weighs around 4,000 pounds. Concrete is measured in cubic yards. Concrete is made out of sand, gravel and cement. Cement is made out of limestone, silica and alumina. Cement is a gray powder that reacts with water to harden the concrete.
When water is added to cement, a chemical reaction locks the materials together. The reaction sets concrete into a solid within hours, but it continues to harden and set up for at least a month. The concrete continues to harden and get stronger until it completely dries out, which can take years.