Betonbrechsande stellen in der derzeitigen Abfallerfassung und -aufbereitung einen wenig beachteten Stoffstrom dar. Dabei besitzen sie erhebliches Potenzial, um nicht nur feine natürliche Gesteinskörnungen, sondern auch einen Teil des Bindemittels bei der Betonherstellung zu ersetzen. Dieser Beitrag stellt zunächst baupraktisch relevante Betonbrechsandeigenschaften vor, um anschließend die derzeit aktuellen technischen Herausforderungen und normative Randbedingungen für die Wiederverwendung von Betonbrechsanden als feine rezyklierte Gesteinskörnung zu beschreiben. Fortgeschrittene Aufbereitungsverfahren lassen jedoch perspektivisch eine bessere Kreislaufführung von Betonbrechsanden erwarten, wenn diese gezielt fraktioniert und einzeln behandelt werden, um sowohl feine Gesteinskornungen als auch (reaktive) Bindemittel zu erhalten.
Concrete in the material cycle: recycling of crushed concrete sand
Crushed concrete sand represents a material flow that has not been tracked in waste statistics to date and has not been sufficiently considered in recycling. Although the estimated annual quantities of crushed concrete sand and the potential for recycling are considerable, processing with the aim of a closed material cycle hardly ever takes place. Unlike coarse concrete chippings, the reuse of crushed sand as recycled aggregate poses technical and normative challenges, as crushed concrete sand contains a higher proportion of hydrated cement. Suitable processing methods can reduce their negative impact and reactivate the cement they contain. Methods for this are currently under development and are being tested on a large scale in some cases. This means that crushed concrete sand could potentially be used to replace both natural sand and Portland cement, which is particularly harmful to the climate, in concrete production. However, this requires suitable material flow management in order to first separate crushed concrete sand into fractions rich in cement and aggregate and then process them according to their intended use as cement substitutes or fine recycled aggregates. For ecologically sound processing and reuse, the entire process chain, including dismantling and transport routes, must also be taken into account. In addition to pure crushed concrete sand, mixtures of mineral construction and demolition waste also have high potential for processing as aggregate and binding agents. However, the available material flow data is even worse than for crushed concrete sand, especially since the possibilities and effectiveness of processing are influenced by the material composition. While high quantities of clay-containing components (e.g. broken bricks) rule out individual processing methods such as carbonation, low clay content can be expected to have positive effects on the binding agent properties.