The dependency of the rheological properties of highly concentrated suspensions on solid concentration and particle size distributions was investigated using a high pressure capillary rheometer. The maximum packing fractions estimated based on rheological measurements for irregular shaped particles were compared with the maximum packing fractions obtained for spherical particles having similar particle size and size distributions. This was done in order to understand the effect of sphericity on the packing behaviour of irregular particles. The maximum packing fractions of the spherical particulate systems were obtained by a mechanical vibration technique as well as by rheological measurements. The Ouchiyama and Tanaka model for spherical particles was modified to predict the maximum packing fractions of irregularity shaped particulate systems using the concept of packing diameter. The modified model was verified with experimental results obtained for bimodal irregularly shaped as well as spherical particles.