In ultra-high performance concretes (UHPC), high proportions of powdery fine materials <125 µm (e.g. cements, fly ashes, silica fume) are used to fill the spaces between the aggregates and thus give the concrete a higher packing density. For this reason, and due to an extremely low water content (w/c ratio approx. 0.2), UHPC has an extremely dense structure that is almost free of capillary pores compared with normal concrete, resulting in higher performance and better resistance to corrosion. The aggregate composition of the aggregates contained in concrete is known to influence the denseness and strength of the concrete, but also the rheological properties. In the case of coarse aggregates, this has long been taken into account by the fact that the relevant standards (DIN EN 206 and DIN 1045) distinguish between favorable and less favorable grading curves according to, among other things, the degree of water and cement paste stress. It is also known that the compactibility and strength are influenced by the grain shape of the aggregates. For this purpose, extensive investigations were carried out within the scope of this project using a particle shape analyzer with automated image analysis. The parameters determined for the particle shape of several raw material flours were correlated with rheological measurements of mortars produced from them, from which laws were then derived that were published in the dissertation of Dr. C. Geisenhanslüke. However, it turned out that this behavior below a critical particle size (a few micrometers) was superimposed by the increasing influence of surface forces. In this project, therefore, the influence of the interactions of the particles with each other will continue to be investigated. Especially in the fine particle range, electrostatic, van der Waals forces as well as steric repulsion are shown to determine the flow behavior of the cement paste. Therefore, the raw materials used are investigated for their compatibility with superplasticizers, their surface charges and their behavior in different chemical media. Using scanning probe microscopy and the colloidal probe method, we are able to measure the actual forces that occur between two particles in the fresh concrete slurry by attaching a sphere of silicon dioxide only a few micrometers in size with a very smooth surface to a freely movable spring beam (cantilever), and another to a glass substrate. This experimental setup can then be flooded with various electrolyte and superplasticizer suspensions in a special measuring cell. The electrolyte environment in fresh concrete can be simulated in this way. Using this measuring method, our institute has already been able to directly measure the energy required to separate two particles from each other when different superplasticizers are used after they have been brought into contact. Since the shearing and flowing of concrete is a process in which the cycle of particles colliding and separating from each other occurs in large numbers, this experiment allows conclusions to be drawn about the behavior of fresh concrete. Much more important, however, is the fact that it is possible to determine exactly which molecules or which particles contribute to a possible deterioration or improvement of the flow behavior and to what extent. The behavior of the superplasticizer molecules on the particles, their layer thickness and the rigidity of the layer can also be observed in this way. In addition to this key point of the investigations, the influence of further parameters such as zeta potential, particle size and shape, water content, spec. surface etc. will be evaluated. In the end, a comprehensive picture of the fundamental processes that determine the rheological behavior of UHPC should emerge on the basis of the previous state of research and the newly gained knowledge.