  Colloidal gels formed from small attractive particles are commonly used in
formulations to keep larger components in suspension. However, despite
extensive work characterizing unfilled gels, little is known about how larger
inclusions alter the phase behavior and microstructure of the colloidal system.
Here we use numerical simulations to examine how larger `granular' particles
can alter the gel transition phase boundaries. We find two distinct regimes
depending on both the filler size and native gel structure: a `passive' regime
where the filler fits into already-present voids, giving little change in the
transition, and an `active' regime where the filler no longer fits in these
voids and instead perturbs the native structure. In this second regime the
phase boundary is controlled by an effective colloidal volume fraction given by
the available free volume.
