In order to investigate the mechanism of cohesion of powder particles with water, rheological properties were examined for the system composed of powder and oil. The samples used here were spherical glass powders of three different particle sizes (19-77μ) and spherical copper powder (126μ). They were chosen as typical materials wettable with water (glass) and not so much (copper). The pastes with various fractions of powder and water in castor oil were prepared and the extruder was used to measure the stress under which the paste could pass through the nozzle.
It was found that only for the systems containing water the yield value exsisted in the apparent consistency curve. This yield value was considered to arise from the cohesive force of water bridge between particles. From the comparison of various models of cohesion of particles, it was possible to interpret the change in apparent yield value with water amount by the aggregate size and its porosity which depended on the wettability of particles.
