Brownian-dynamics simulation is used to investigate the effect of particle volume fraction and interparticle interactions on the structure of colloidal materials formed by irreversible aggregation. Short-range structure of aggregates and sediments is represented by the pair distribution function g(r), and longer-ranged structure by the effective fractal dimension. For unstable DLVO-type particles with a secondary minimum, the simulated short-range aggregate structure resembles that in a stable concentrated dispersion; for particles without a secondary minimum, g(r) is relatively unstructured either in sediments or gels. Hydrodynamic interactions play a crucial role in the formation of dense sediments at high field strengths, but are much less important when Brownian motion is predominant. Studies of small-floc dissociation give insight into the likely consequences of neglecting multi-body hydrodynamic interactions in simulations of this type.