Für den Entwurf von Betonzusammensetzungen mit Zusatzstoffen hat sich in Deutschland die k-Wert-Methode bewährt. Dieses Konzept wird auf Betonzusatzstoffe wie Steinkohlenflugasche oder Silikastaub angewendet und ist seit längerem in den entsprechenden Anwendungsvorschriften enthalten. Für Hüttensandmehl liegen dagegen kaum veröffentlichte Erfahrungen zu der Anwendung nach diesem Konzept und der Größe des k-Werts vor. Vor diesem Hintergrund wurden im Rahmen einer Studie Untersuchungsergebnisse aus unterschiedlichen Versuchsprogrammen ausgewertet und daraus k-Werte für Hüttensandmehl abgeleitet. – The k-value concept for ground granulated blast furnace slag as a concrete addition – Cements containing granulated blast furnace slag (gbfs) as a main constituent have already been well established in many markets for a long time. Product properties and application of these cements are described in various codes and standards. In a new version of the European Concrete Standard EN 206-1 which is currently under revision, it is intended to introduce rules for the use of ground granulated blast furnace slag (ggbfs) specified in EN 15167-1 as a separate addition to concrete. As a prescriptive concept for the use of concrete additions, EN 206-1 uses the k-value concept which is applied for fly ash and silica fume. The extension of that concept to ground granulated blast furnace slag is intended. The current paper compiles the principles of the k-value concept. It illustrates the procedure of how k-values are calculated on the basis of concrete test results to define prescriptive rules in the standard. Using the available results of different test programs the effects of different parameters like ggbfs properties (chemical composition, fineness), the amount of ggbfs addition or the influence of Portland cement were investigated. The results confirm the broad range of performance of ggbfs from different sources and the strong influence of the interaction with Portland cement which requires a conservative approach when defining the limits of a prescriptive k-value for ggbfs.