The core-mantle boundary (CMB) shows a complex structure. Seismologists have observed features that are possibly denser than their surroundings. These structures form from a dense layer above the CMB. Therefore typical mantle convection models have assumed an initial dense basal layer. The thickness and density of this prescribed layer are crucial but hardly constrained. Here we investigate core-mantle interaction as one possible origin for this layer and employ 2D Cartesian models of mantle convection that consider a diffusive chemical gradient between the iron-rich core and the silicate mantle. Our simulations show that the diffusive influx is coupled to the convective mantle flow. Convection-assisted diffusion gives a larger influx beneath slabs spreading over the CMB. Additionally, as in the models with a prescribed layer, the rising plumes pull dense material up and form piles. In this study, however, the constant chemical influx leads to piles existing for longer times. The piles are typically smaller but can maybe in combination with a primordial layer explain different seismologically observed structures and the presence of core material in the mantle.