Sand and cement mortar

The house I have bought dates from the 1880s, most of the plaster seems to be sand and water and it just falls off the walls if you tap them. But because in those days there were no damp-proof courses the house has had rising damp the whole time. It's impossible to use normal gypsum plaster on bricks that are sopping wet and so instead a sand and cement mortar has been used. It's a nightmare to hack off the wet walls, but it comes away like huge wall fossils and only breaks when it hits the hard concrete floor.
I'm hacking all the old internal plaster off the walls to help them dry out now that I've had a damp-proof course added.