This project deals with the investigation of whether it is possible to reduce the risk of segregation in the fresh concrete through the addition of clay minerals. In the study a locally occuring clay slurry obtained by the washing of aggregates and a commercially available clay product is used. Through experiments it is documented that both clays have a stabilizing effect. Stability is assessed on basis of the sensitivity of the slump flow and the risk of segregation by variations in the water content. The addition of clay minerals necessitate an increase in the paste volume in order to achieve the desired rheological properties. A laser analysis revealed that the grain curve of both clays was similar to the grain curve of fly ash. In light of this experiments was conducted where the clays were replaced by an equivalent amount of fly ash. The experiments showed that it is not the particle size but the shape of the clay minerals and their ability to absorp water that creates stability. In connection with the experiments test specimens of the concrete containing clay minerals were cast. By measuring the compressive strength and Youngs modulus of the test specimens it was established that the addition of clay minerals have no influence on these.