Painting onto new plaster/ concrete

[NewMumSept2014]

Hi,
We recently had our fireplace torn out and a new wood burning stove put in. The guys who did it filled the hole in the wall with plaster "mixed with a bit of cement to make it dry quicker".
We left it to dry for a good 10 days then put a mist coat on (50:50 water and paint) and after 3 coats of Matt emulsion the plaster is still sucking up all the water from the paint and drying a slightly different colour than the rest of the wall.
It just occurred to me that this is probably something to do with the cement they put in it...?
So my question is, PLEASE someone tell me this is fixable!!
I'm thinking maybe I need to put some kind of cement sealant over the top of the paint/ plaster/ cement and start again?