Heteroaggregation corresponds to an attraction between two different types of particles. It can either lead to stable or instable systems, depending on the conditions. In this paper heteroaggregation in binary mixtures of alumina and silica colloids (size ratio = 8.5) is studied, by coupling an experimental approach and Brownian dynamics simulations. Two main objectives are targeted. The first one is to bridge the gap between coagulation and dispersion, by examining the effect of the relative concentrations of silica and alumina. The second one is to study the effect of the volume fraction of solid, which impacts strongly the aggregation mechanism. The coagulation or the dispersion are evidenced by sedimentation tests, granulometry, and rheological measurements, supported by zeta potential measurements. Heteroaggregates could be observed by transmission electron microscopy. Brownian dynamics simulations give more insight into the very first moments of the process, to predict the adsorption kinetic and the heteroaggregates structure. A very good agreement is obtained between experiments and simulations, which both show adsorption of silica leads to the bridging of alumina particles and thus coagulation at low concentration. This adsorption is rapidly limited by electrostatic interactions, then hindering alumina agglomeration by repulsive interactions. For the higher solid fractions, alumina aggregation is more likely, as two alumina particles can easier encounter before their surface is enough covered by the silica to avoid their aggregation.