EATING OUT OF THE GARBAGE CAN—THE U.S. CEMENT INDUSTRY'S CONSUMPTION OF WASTE FUELS AND RAW MATERIALS

Hydraulic (chiefly portland) cement, the binding agent in concrete, is currently being produced at a rate of about 70 million metric tons per year (Mt/yr) in the United States. Portland cement is about 80–95% clinker, which is made in a kiln and is composed mainly of calcium oxide (here noted as C; about 65%), silica (S), alumina (A), and ferric oxide (F), contained in four primary compounds (C₃S, C₂S, C₃A, and C₄AF). Limestone supplies most of the C and makes up about 1.5 of the 1.7 tons of raw materials consumed per ton of clinker. Calcination of limestone yields 0.51 t CO₂ per ton of clinker and the fuel burned to supply the required 3–7 GJ of heat yields a further ~0.4 t CO₂. Cement production is the world’s largest “industrial” source of CO₂ emissions.

A wide variety of waste products, including coal combustion ashes, metallurgical slags, used tires, toxic chemicals, and contaminated soils, are consumed as partial substitutes for primary rock-based raw materials and fossil fuels to make clinker and cement; selection criteria include composition, energy contribution, cost, permitting, and the potential to reduce CO₂ emissions. Certain wastes, especially fly ash and granulated blast furnace slag, can be added to finished cement or concrete as supplementary cementitious materials (SCM); these are activated by the lime released during cement hydration and serve to reduce the clinker fraction in the cement or concrete. Synthetic gypsum from sulfur oxide scrubbers can replace natural gypsum in the finished cement. Currently, the U.S. cement industry’s use of waste materials amounts to 5–6% of total nonfuel raw materials and has reduced calcination emissions of CO₂ by about 3%. Waste fuels contribute 9–14% (>30% if petroleum coke is included) of the total heat consumed. Use of SCM in the cement and concrete industries currently totals 15–20 Mt/yr.