​The physical interpretation of the flow test is investigated by conducting image-monitored flow tests on mortars prepared with mixtures of natural round sand and crushed angular sand to evaluate the progression of flow and the effect of aggregate shape characteristics on cement mortar rheology. The evolution of flow with the number of drops follows a hyperbolic trend. When analyzed within an energy-based framework, it is found that both the base shear at the mortar-plate interface and the internal energy loss in shear deformation are functions of the shear resistance within the mortar. This is further confirmed with mortars prepared with varying quantities of round particles. It is also shown that particle shape affects packing density and the mobilized friction between aggregates; therefore, particle shape defines the amount of paste required to reduce grain interaction to attain adequate flow.