In summary, the total value-adding chain comprising cement manufacture, processing and use begins with resource obtainment and cement production, is followed by the production of cement-bound building materials, the employment of these materials as part of construction work and the utilisation of the corresponding edifice. The value-adding chain is then concluded with the recycling of the waste after the demolition of the edifice – crushed concrete can act as a substitute for the natural resources gravel and sand. Recycled building materials can replace around 5 to 10 percent of the required mineral resources.
Politics and society define the boundary conditions for sustainable construction and habitation. However, actual implementation of the philosophy requires the development of criteria that allow assessment of a project’s compatibility with sustainable development. Methods for such an assessment are the subject of intense discussion among experts. A particularly noteworthy method here are ecobalances. Quantitative criteria and indicators, e.g. for energy efficiency and resource consumption, are largely undisputed and already in use. However, particularly in regard to the assessment of social aspects, qualitative criteria also come into play. Aesthetics make for an especially prominent example. The question as to what role such subjective criteria should play in the assessment of sustainable building is subject to debate. Another distinction that is made is between absolute and relative analyses. While an absolute analysis will examine whether a construction measure complies with a certain set of requirements (e.g. legal limit values), relative analyses are used to compare a number of alternatives (e.g. different building materials) with one another within the context of certain requirements.
Providing complete fractionation, concrete can recycled to 100 percent. Concrete recycling divides into recycling of unset concrete and set concrete. Unset concrete recycling refers to the internal circulation within the concrete works. Return quantities – concrete obtained from mixer and pump cleaning – and unused concrete returned to the plants in truck mixers are separated and reintroduced to production. These return quantities constitute approximately 2.5 percent of a concrete works’ production. The recycled water and the particles contained in the waste concrete are used instead of fresh water and primary aggregates. The use of recycled water, waste concrete and waste mortar for concrete production is regulated in a directive published by the German Committee for Reinforced Concrete (DAfStb).
When recycling set concrete, the broken concrete is treated, i.e. broken down and sieved so that coarse and fine recycled aggregates are produced. Use of pure, high-quality recycled aggregates as an additive for concrete up to strength class C 30/37 is regulated in another of the DAfStb’s directives. For set concrete recycling, employment of cement as a binder is indispensable. Cement further facilitates the utilisation of other mineral construction industry waste, e.g. to manufacture masonry units from slate chippings. Thanks to persistent efforts in the field of construction material recycling, the year 2004 saw more than 61.6 percent of the produced building rubble and more than 93 percent of the rubble from road demolition introduced to recycling. The European targets for the recycling rate are in Germany thus already being met today. To promote construction material recycling, the consortium Kreislaufwirtschaftsträger Bau (construction industry recycling agency) was founded in 1996. The cement industry is associated with this agency through its membership in the German Building Materials Association.