Portland cement is a fine gray powder mixed with sand, gravel and water to form concrete. Cement is the binding agent, or glue, that holds concrete together. The process to manufacture it is complex and precise. Essentially, Portland cement is made by heating raw materials to 2,700 degrees Fahrenheit in a rotary kiln to form clinker nuggets which are ground and mixed with gypsum.

Calcium is a basic ingredient in cement normally supplied by limestone. Other ingredients are silica, alumina and iron, usually supplied by clay or shale. These materials are converted by heat primarily into calcium and aluminum silicates to form clinker.

A Complex, Energy-Intensive Process

Raw materials are introduced into one end of a kiln. Kilns are typically 270 to 440 feet long and 12 to 14 feet in diameter. The kiln is slightly elevated at the end where the materials are introduced. The materials move slowly toward the other end and the heat source as the kiln rotates. In the heating process, the materials are transformed into clinker.

Testing is conducted to ensure that the right proportion of materials goes into the raw mix to produce high quality Portland cement. The normal "recipe" for clinker calls for about 67 percent calcium oxide, 22 percent silicon dioxide, 5 percent aluminum oxide, 3 percent iron oxide and 3 percent other materials such as magnesium, sulfur, sodium and potassium.

Large amounts of fuel are required to heat materials to 2,700 degrees F. For example, it takes about 400 pounds of coal to make one ton of cement. The typical plant produces several hundred thousand tons of cement annually. Petroleum coke is another source of traditional kiln fuel. These are all nonrenewable fossil fuels.

Fuel-Quality Wastes

Fifteen cement plants in the U.S. supplement fossil fuel with fuel-quality hazardous waste fuels. These wastes are typically spent solvents, printing inks, resins and other substances that supply the proper heat value to the kiln. Some plants also use non hazardous waste oils.

In addition, 35 cement plants use non hazardous scrap tires as a fuel supplement. This helps reduce the unwanted inventory of two billion scrap tires in the U.S.