Microstructures of cakes formed on ultrafiltration membrane as a result of packing of colloidal aggregates generated in rapid coagulation have been investigated though small angle neutron scattering experiments. Scattering curves show an extensive collapse of fractal structure of colloidal aggregates due to their packing in the cake. Accordingly experimental values of solid fraction in the cake are higher than expected from gentle packing of fractal aggregates. Different densification features have been detected both at small and large scale depending on the friction forces between the particles and the strength of inter-particles bonds.