  We use three-dimensional radiation hydrodynamic (RHD) simulations to study
the formation of massive star clusters under the combined effects of direct
ultraviolet (UV) and dust-reprocessed infrared (IR) radiation pressure. We
explore a broad range of mass surface density $\Sigma \sim 10^2$-$10^5 \,
\mathrm{M}_{\odot} \, \mathrm{pc}^{-2}$, spanning values typical of weakly
star-forming galaxies to extreme systems such as clouds forming super-star
clusters, where radiation pressure is expected to be the dominant feedback
mechanism. We find that star formation can only be regulated by radiation
pressure for $\Sigma \lesssim 10^3 \, \mathrm{M}_{\odot} \, \mathrm{pc}^{-2}$,
but that clouds with $\Sigma \lesssim 10^5 \, \mathrm{M}_{\odot} \,
\mathrm{pc}^{-2}$ become super-Eddington once high star formation efficiencies
($\sim 80 \%$) are reached, and therefore launch the remaining gas in a steady
outflow. These outflows achieve mass-weighted radial velocities of $\sim 15$ -
$30 \,\mathrm{km} \, \mathrm{s}^{-1}$, which is $\sim 0.5$ - $2.0$ times the
cloud escape speed. This suggests that radiation pressure is a strong candidate
to explain recently observed molecular outflows found in young super-star
clusters in nearby starburst galaxies. We quantify the relative importance of
UV and IR radiation pressure in different regimes, and deduce that both are
equally important for $\Sigma \sim 10^3 \, \mathrm{M}_{\odot} \,
\mathrm{pc}^{-2}$, whereas clouds with higher (lower) density are increasingly
dominated by the IR (UV) component. Comparison with control runs without either
the UV or IR bands suggests that the outflows are primarily driven by the
impulse provided by the UV component, while IR radiation has the effect of
rendering a larger fraction of gas super-Eddington, and thereby increasing the
outflow mass flux by a factor of $\sim 2$.
