To determine how grain-boundary composition affects the liquid phase sintering of MgO-free Bayer process aluminas, samples were singly or co-doped with up to 1029 ppm Na2O and 603 ppm SiO2 and heated at 1525 degrees C up to 8 h. Na2O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525 degrees C compared to the undoped samples, but similar to the as-received, MgO-free Al2O3, Na2O-doped samples sinter to 98% density with average grain sizes of similar to 3 mu m after 8 h. Increasing SiO2 concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities ( 20-400 Pa.s) of the 0.3 to 1.8 nm thick siliceous grain-boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain-boundary phase. For low Na2O/SiO2 ratios, densification of Bayer Al2O3 at 1525 degrees C is controlled by diffusion of Al3+ through the grain-boundary liquid, whereas for high Na2O/SiO2 ratios, densification can be governed by either the interface reaction ( i.e., dissolution) of Al2O3 or diffusion of Al3+. Increasing Na2O in SiO2-doped samples increases diffusion of Al3+ and Al2O3 solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al2O3 densification can be manipulated by adjusting the Na2O to SiO2 ratio.