Hydraulic cement, the binding agent in concrete, is currently being produced at a rate of nearly 100 million metric tons per year (Mt/yr) in the United States and about 2,500 Mt/yr worldwide; world output is enough for about 20,000 Mt/yr of concrete or about 3 tons of concrete per person on the planet. Portland cement, the most common cement, is made by grinding together clinker and gypsum (about 5% by weight). Clinker is made in a kiln and is composed mainly of calcium oxide (here noted as C), silica (S), alumina (A), and ferric oxide (F), manifested as four primary clinker compounds (C₃S, C₂S, C₃A, and C₄AF) in the approximate ratio, respectively, of 6:2:1:1. 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 produced. A wide variety of secondary materials, including coal combustion products, can be used as partial substitutes for primary rock-based raw materials and fossil fuels; selection criteria include composition, energy and other cost savings, and the potential to reduce carbon dioxide emissions. Calcination of limestone to yield C produces nearly 1 ton of carbon dioxide per ton of clinker, and is a major reason why kilns consume about 3–7 GJ of heat energy per ton of clinker, depending on kiln technology.

Some secondary materials are pozzolans—they become cementitious when reacted with lime. Pozzolans can be mixed with portland cement to make blended cements or can replace some of the portland cement in concrete; in either case, the requisite lime is produced by the hydration of C₃S and C₂S. Pozzolan addition generally improves the performance of concrete. Overall, secondary material usage by the U.S. cement industry is increasing and reached nearly 5% of total raw materials consumed in 2005. Coal combustion products accounted for 3% of total raw materials in 2005, including nearly 3 Mt of fly ash and about 1 Mt of bottom ash for clinker, and about 0.5 Mt of synthetic gypsum (about 10% of total gypsum) for cement. Although only about 0.2 Mt of fly ash was used in 2005 as pozzolan in cement, the concrete industry itself used almost 14 Mt of fly ash (mostly as a pozzolan), as well as about 1 Mt of bottom ash as an aggregate. Low-NOx burners at power plants produce a high-carbon fly ash that, unless treated, is unsuitable as a pozzolan, but can still be used in cement kilns.