Portland cement powder, ground from much larger clinker particles, has a particle size distribution from about 0.1 to 100 mu m. An important question is then: does particle shape depend on particle size? For the same cement, X-ray computed tomography has been used to examine the 3-D shape of particles in the 20-60 mu m sieve range, and focused ion beam nanotomography has been used to examine the 3-D shape of cement particles found in the 0.4-2.0 mu m sieve range. By comparing various kinds of computed particle shape data for each size class, the conclusion is made that, within experimental uncertainty, both size classes are prolate, but the smaller size class particles, 0.4-2.0 mu m, tend to be somewhat more prolate than the 20-60 mu m size class. The practical effect of this shape difference on the set-point was assessed using the Virtual Cement and Concrete Testing Laboratory to simulate the hydration of five cement powders. Results indicate that nonspherical aspect ratio is more important in determining the set-point than are the actual shape details.