Nonaqueous colloidal processing in cyclohexane and slip casting allowed the preparation of green ZrB2 compacts with 67.3% TD (theoretical density), which upon pressureless sintering at 2100 degrees C densified to a final density of 92.4% TD. The effect of controllable sintering parameters (temperature, time, and heating rate) on the sintered properties of ZrB2 compacts prepared using the nonaqueous colloidal processing method was also studied, to maximize the sintered density and tailor the desired microstructure (i.e., dense final object with small grain size). The competition between pore elimination and grain growth mainly controls the final density achievable. ZrB2 compacts with maximum sintered densities of 93% TD could be achieved by controlling the sintering conditions. Finally, the use of carbon and B4C additives to improve the sintered density and microstructure of ZrB2 compacts was also investigated. The additives, compatible with colloidal processing, allowed compacts with greater sintered densities (96-98% TD) and finer grain sizes to be achieved by pressureless sintering, even at a lower temperature of 1900 degrees C.