new builds in stone & sandblasting

[cotegreen]

Maybe I am missing something here but we are contemplating an extension to our stone cottage, which obviously will be stone as well (block inner wall with normal membrane DPC), however my question relates to the mortar?

As it will have a normal new build type damp course this will stop any damp rising up however do you still get the same problem with water being absorbed in the face of the stone elsewhere on the wall that cant escape cause of the cement mortar that could in years to come spall the stone faces?

I dont think I have seen any new stone houses with lime mortar so I am guessing probably not however would like to hear anyones thoughts.

Last question is we want to clean up the outside of the house. i have previously posted a question about removing cement mortar but does sanblasting the outside remove the lime mortar ( i am guessing it does?) so what are the other options? I have seen a chemical strip on a house and it seems to look orange now.any suggestions.

Help appreciated again !!!!!!

 

[Gervase]

Yes, damp still needs to be able to escape, so the mortar should be lime, with no cement.
If you are building with sandstone, use a hydraulic lime, as many British fat limes derived from oolitic limestone can cause erosion of sandstone.
Sandblasting isn't a good idea, particularly with softer sandstones and limestones. There are companies that specialist in cleaning buildings using water jets, which are fare less likely to damage the fabric.

 

[MdB]

From what I have read (i.e. I'm not an expert and I'm likely to be corrected by those that are (edit - just seen Gervase's post - he is an expert so do what he says)) you can use lime as mortar with advantages / disadvantages as follows:

(I assume the block layer is the structural layer and you have a cavity between the two layers)
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Block layer built with cement, stone layer built with cement with concrete foundations:

With a strong non porous stone (granite etc) there isn't really a problem as you won't have a weathering problem or moisture ingress. Builder will love it as the cement will set quickly and its what he is used to.

With a weak and/or porous stone (sandstone etc) moisture will get into the stone and will stay there. Frost will cause the stone to shatter and you will end up with a wall that is made from cement mortar and will look like a honeycomb. See photo posted elsewhere here.

With a concrete foundation, there should be little or no movement in the wall but if there is, with a weak stone, the rock will crack rather than the cement mortar.

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Block layer built with cement, stone layer built with lime with concrete foundations:

With a weak stone and/or porous stone, lime mortar will help wick away any moisture and will act as a sacrificial layer in frosts.

If the wall cracks, the lime will crack before the stone and has the advantage it will self heal to a certain extent.

The builder won't like lime as he hasn't used it before and he will only be able to put up a metre or so before having to leave it for a week to set.

Lime is more environmentally friendly than cement due to less energy (40%??) used in its production.

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Block layer built with lime, stone layer built with lime with concrete foundations:

More environmentally friendly than the other options but Building Control might take some convincing. Not sure if lime with blocks is a good idea as it will be weaker than cement but otherwise I don't see a problem.

Builder won't like it due to the weeks drying time per metre. This would probably also have to be extended before any load could be placed on it too.

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Any of the above with limecrete foundations

Can't see a problem but would be weaker than cement so there may be issues.

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Any of the above with no foundations

How walls used to be built but I can't see it getting past Building Control.

 

[Gervase]

I've laid blocks with hydraulic lime and have had no problems getting them past building control. Cement mortar has a compressive strength of some 70 Newtons/mm2, and at the foundation course at the bottom of a single-story extension you'll be looking at a compression force of around three and a half to four Newtons/mm2, so lime is perfecly acceptable (while cement, it can be argued, is a gross overspecification!).
As MdB says, if you're building with a hard stone like granite or dolorite then cement mortars are not a major problem, although they are not as flexible so cracking is an issue should there be any movement.

 

[Roger H]

Gervase, bit worried about your "If you are building with sandstone, use a hydraulic lime, as many British fat limes derived from oolitic limestone can cause erosion of sandstone". How does that square with the usual suggestion on here to use hydrated lime for pointing, rather than hydraulic as you seem to suggest?

Roger

 

[Gervase]

As a general rule, yes, do use fat lime for pointing, but not with soft sandstone That may sound counterintuitive, given the softness of the stone, but fat lime mortar can be injurious to sandstone because of the action of water running over it and leaching out the non-calcium constituents which react with the stone.
As an example, in Edinburgh the sandstone of many buildings is being attacked by the action of the rain on the lime mortar used to construct them. This has resulted in grooves beneath perps and a widening of the gap between the stone of the walling and proud lintels.
In the main, damage is caused where the mortars are derived from oolitic limestone and there are other limestones which might not be as harmful. But as much of the lime mortar available in the UK is made from oolitic limestone, the general advice is not to use them in combination.
I would use one of the French hydraulics - St Astier being one - which are derived from dolomitic limestone and which don't seem to be associated with this problem. You can get a soft NHL2 which would be ideal.

 

[chuckey]

A builder I know hacked of cement render of the front of a stone cottage, then sandblasted it then re-pointed it in lime. I don't know but I guess that the pointing was in a poor state before the render went on. Looks nice now. Interesting technology, the kit is diesel powered and runs at 4000 psi. Once the grit has hit the stone it is itself shattered so there is a vast amount of dust and sweeping up to do. The windows and doors need to be masked with ext. ply to stop the glass getting frosted.
Have you tried brick cleaner?
Frank

 

[cotegreen]

Thanks for the posts, it wont be happening until next year, and it will be sandstone as it has to match the current house as part of planning reqs. With regards to the sandblasting I had not thought about normal brick cleaner so I will give that a go first, failing that will probably go down the water jet route to save any damage .

Thanks for all the advice, I will take it all away and discuss with the builder (who knows lime!)

 

[Nemesis]

Sandblasting is not recommended.

xxx