We have moved our bathroom and the old bathroom will now become part of the hallway. A small concern about the hole left from the old cast iron drain pipe. The hole is about 10-12 cm in diameter and about 10 cm deep, and I need to fill it in somehow. Is ordinary coarse concrete enough if I press a piece of rebar into the hole? 
If you're going to buy a sack, use fine concrete instead. Coarse concrete also works but may not be the first choice.
Press a wet cloth into the hole and let it sit for an hour or two before casting again. This makes the surfaces thoroughly moist, which is good.
Press a wet cloth into the hole and let it sit for an hour or two before casting again. This makes the surfaces thoroughly moist, which is good.