Cementitious building materials like Ultra High Performance Concrete (UHPC), Self-Compacting Concrete (SCC) or concrete with low clinker content possess complex rheological properties. Due to high packing densities and the use of various additives and chemical admixtures, a huge range of non-Newtonian flow characteristics from shear-thinning to shear-thickening, visco-elastic material behaviour and structural build-up can appear. The rheological properties then depend on the time of flow and apparent shear rates in the material, and flow processes become time-dependent. The experimental investigation and subsequent phenomenological description of visco-plastic, viscoelastic and thixotropic cement pastes is not straight-forward. The implementation of these materials into numerical flow simulation is even more advanced. However, description of time-dependent flow in Computational Fluid Dynamics (CFD), can support the analysis of transient processes in complex cementitious suspensions, which, to date, is complicated. Thus, in the presented research, the time-dependent, transient L-Box flow of cement paste was compared with the time-independent, short and steady slump flow test, which has been widely tested for the flow of cement pastes. [1, 2] Roussel’s thixotropy model [3], as applied to CFD modeling in [4] was used to describe cement paste properties and depict their flow in CFD. Experimental flow tests and rheometric analysis was conducted with cement pastes at a high solid volume fraction. The numerical simulation of the flow test was analysed with special regards to transient processes and the evolution of a structural parameter. The effect of thixotropy parameters on flow evolution, i.e. flocculation time and structural breakdown value, was investigated with special regards. It was found that time-dependent phenomena are properly described by thixotropy parameters that were investigated in the lab. Distinct thixotropy modelling thus can help prospectively to grasp time-dependent flow phenomena of cementitious building materials more close to reality. REFERENCES [1] Gram, Annika: Modelling Bingham Suspensional Flow :Influence of Viscosity and Particle Properties Applicable to Cementitious Materials. Stockholm. Dissertation. 2015 [2] Haustein, M. A. ; Eslami Pirharati, M. ; Fataei, S. ; Ivanov, D. ; Jara Heredia, D. ; Kijanski, N. ; Lowke, D. ; Mechtcherine, V. ; Rostan, D. ; Schäfer, T. ; Schilde, C. ; Steeb, H. ; Schwarze, R.: Benchmark Simulations