  We present cosmological radiation-hydrodynamic simulations, performed with
the code Ramses-RT, of radiatively-driven outflows in a massive quasar host
halo at $z = 6$. Our simulations include both single- and multi-scattered
radiation pressure on dust from a quasar and are compared against simulations
performed with thermal feedback. For radiation pressure-driving, we show that
there is a critical quasar luminosity above which a galactic outflow is
launched, set by the equilibrium of gravitational and radiation forces. While
this critical luminosity is unrealistically high in the single-scattering limit
for plausible black hole masses, it is in line with a $\approx 3 \times 10^9 \,
\rm M_\odot$ black hole accreting at its Eddington limit, if infrared (IR)
multi-scattering radiation pressure is included. The outflows are fast ($v \,
\gtrsim \, 1000 \, \rm km \, s^{-1}$) and strongly mass-loaded with peak mass
outflow rates $\approx 10^3 - 10^4 \, \rm M_\odot \, yr^{-1}$, but short-lived
($< 10 \, \rm Myr$). Outflowing material is multi-phase, though predominantly
composed of cool gas, forming via a thermal instability in the shocked swept-up
component. Radiation pressure- and thermally-driven outflows both affect their
host galaxies significantly, but in different, complementary ways.
Thermally-driven outflows couple more efficiently to diffuse halo gas,
generating more powerful, hotter and more volume-filling outflows. IR
radiation, through its ability to penetrate dense gas via diffusion, is more
efficient at ejecting gas from the bulge. The combination of gas ejection
through outflows with internal pressurisation by trapped IR radiation leads to
a complete shut down of star formation in the bulge. We hence argue that
radiation pressure-driven feedback may be an important ingredient in regulating
star formation in compact starbursts, especially during the quasar's `obscured'
phase.
