A hot mix asphalt plant produces asphalt paving mix, which may include natural sand, crushed limestone, slag, gravel, oyster shells, decomposed granite, crushed rock, hydrated lime, Portland cement, or other non-plastic mineral matter (collectively called aggregates), depending on the specific grade requirement. After mixing, the hot paving mixture is loaded into trucks, and transported to the paving site for construction. Transportation costs are an important cost factor, and for this reason, the industry is composed of a large number of asphalt plants located close to points of utilization. It is estimated that there are more than 3,500 hot mix asphalt plants in the United States alone.