Cement, aggregate, and water are the main ingredients, but it may contain other materials called admixtures. The negative health effects are generally associated with the cement or the admixtures, but in recent years, the aggregate has also been implicated. The aggregate used in concrete can be gravel, crushed stone, or things like crushed volcanic rock, perlite, or fly ash from power plants. The main concern is that the aggregate could release radon. If a concrete house slab contains radon in the aggregate, then there will be radon in the house.
Radon is a common radioactive gas that is given off by soil and rocks.
Radon is found virtually everywhere, but it only becomes a problem when it builds up to high levels indoors. People who live in houses that contain a lot of radon have a better than average chance of developing lung cancer. Some kinds of rock contain naturally occurring radon. This includes some types of granite and shale. Fortunately, most of the aggregate used in the United States contains little or no radon.