Cinder or Ash Mortar

[Jim22]

Hi all,

As a fairly handy set of people i am hoping someone will have come across this before.

I live in a 1930s house that has walls seemingly made 90% of crumbly black mortar stuff. I am fairly sure after some googling that this is ash mortar or ash plaster. It is about an 1 to 1.5" thick and has a very poor and thin white plaster coat over it. Does anyone know if this is a lime mortar type plaster? It seems very soft and crumbly too (and hardly attached to the wall in most places).

Really i want to know if i was to chip this off back to brick, could this be replastered with gypsum plaster, or does it need to be breathable etc?

This stuff is a nightmare when trying to decorate or hang anything even slightly heavy from a wall.

Cheers all,
Jim

 

[blackrodd]

Could be the original cinder block, an attempt at an early thermal block, I was told, we have some here, in Sunny Devon.
You may well find that after drilling with a 6mmmasonry drill, you have a 7, or 8mm hole to fit a plug in
So, drill a 5mm hole on slow revs for a 6, or 6.5mm plug.
You may also find that the cinder blocks are letting the plaster go, shedding the outside layer of cinder
A simple test would be to get a big enough piece plaster, and examine for loads of the grit or agregate, still stuck to the render.
I have never plastered on cinder block so don't know the answer to that part of the post.
If it were my house, I would test a small wall, seal the block work with 2 coats of unibond and use carlite bonding as a backing plaster ,'cos it's light and really sticks well and skim with carlite finish, Not carlite browning is too soft.
If there are many cinderblock houses in you're area, this must have cropped up before, so try and find the men that have worked on them. HTH Regards Rodders

 

[AndyT]

The plaster you describe is what I have in my current and previous Victorian houses. It is lime mortar, mixed with whatever was available to make it cheaper, which can be a higher proportion of ashes than was ever recommended by any building instructor. I believe the builders of the 30s followed quite a few of the Victorian ideas about keeping costs down!
It is an excellent match for the underlying wall construction - which in my case is brick and stone with flexible lime mortar - as it has quite a lot of give in it. But it can be frustrating when trying to drill small neat holes for fixings.

Yes, you could hack it all off and have someone put modern gypsum based plaster in its place. That will give you a smoother, harder wall surface - which might give you problems with cracking if there is any movement in the walls.

The plaster you have at present will be very 'breathable' - so changing it could make problems if you are relying on it being breathable. If you aren't, replacing it with an impermeable plaster will be ok, but if there are hidden damp problems, it might make them worse, or show up somewhere else. I think you can also get efflorescence along the boundary between old and new, unless you strip and replace the whole room.

(Rodders - I think you are at cross purposes - the OP says his walls are brick, not blocks.)

 

[blackrodd]

Thank you Andy, Sorry jim, I missed that bit about brick.
Regards Rodders

 

[Jim22]

Excellent cheers guys. You know it never dawned on me to use a smaller bit...i will use that trick else where. Amazing how my mind jumps over the obvious! hehe.

The two walls i currently have to do are party walls, so damp should not be a problem. So i think i will hack it off, it is mostly coming down when we are taking down the old furniture anyway, and it certainly doesnt offer resistance.

I was concerned about it being breathable, but the render on the outside of the house should be stopping water get in. On the external walls i will be putting insulation, so hopefully all will be okay.

I think i will treat myself to a new mask before i start this job. This dust gets everywhere!

Cheers for the help.
Jim