It is really useful to have a tube of ready mixed cement in your tool bag that can be applied with the use of a mastic gun.
Found it particularly useful when I have dug a bit too much wall out to install a metal electrical socket box.
A squirt of this in the back of the hole and pushing the box in causes the cement to spread and set. Put a few raw plugs through the holes in the back of the box to make a secure fitting once the cement has finally hardened off.
When applied thinly it sets quite quickly. If you use it in mass give it a good few hours to set, for best results.