Aluminum (Al) particles are promising fuels for propulsion and energy conversion, sparking decades of research into their combustion. When micron-sized aluminum burns in an oxidizing environment, the process unfolds in two stages: an initial steady, symmetric vapor-phase combustion, followed by unsteady, asymmetric combustion as alumina decomposes, releasing gaseous sub-oxides. These sub-oxides condense and dissolve in the liquid droplet, forming liquid aluminum oxide. However, the multiphase mechanisms and reactions driving this transition remain poorly understood, which undermines the accuracy of the combustion model. To address this, a 3-dimensional Direct Numerical Simulation (DNS) approach based on Navier-Stokes equations is used to model single-particle Al combustion in air. Finally, this model underscores three critical aspects: (1) the development of an accurate and robust numerical framework to elucidate the complex physiochemical processes governing aluminum combustion, (2) detailed quantification of the role of sub-oxides condensation on the burning droplet and its subsequent influence on heterogeneous surface reactions and aluminum vaporization dynamics, and (3) an evaluation of existing vaporization laws (burn rates), contributing valuable insights for macroscopic combustion model refinement.