The structure and aggregation kinetics of diffusion-limited cluster-cluster three-dimensional monomeric aggregates and gels is investigated as a function of the molar fraction of two types of impurities. In one case the impurities are allowed to aggregate among themselves whereas in the other the impurities are mobile monomers that remain as such during the whole aggregation process. Computer simulations are performed on a simple cubic lattice for which the functionality of the aggregating particles is effectively 6. The first type of impurity shows a decrease in the fractal dimension when compared to that of a one component system at the same concentration. As a consequence of this decrease, the gelation concentration is lowered. At higher concentrations a gelling to nongelling transition was observed. In the nongelling regime the colloidal aggregates are kept apart by the impurity clusters, developing a local ordering. For the monomeric type of impurity, at large impurity molar fractions, a nonstructured nongelling phase appears at high enough concentration, in which the colloidal aggregates are kept apart by the sea of mobile impurities that inhibits the formation of a gel. Smaller molar fractions of mobile monomeric impurities strongly affect both the fractal dimension and the kinetics of the aggregating colloid.