Concrete is an essential material. With a worldwide estimated consumption of between 21 and 31 billion tonnes of concrete in 2006, concrete is the second most consumed substance on Earth after water*! A world without concrete is almost inconceivable! Concrete is a mixture of sand, gravel and/or other aggregates (the matrix), bound together by a water-based binder, cement. Admixtures (modifying agents) and additives (fine mineral powders) are sometimes introduced to improve the characteristics of the fresh concrete, of the mixing process and/or of the final hardened material. *World Business Council on Sustainable Development, Concrete Recycling - A contribution to Sustainability, 2008.