What Mortar Mix for Red Sandstone?


Postby Greml1n » Wed Sep 05, 2018 11:05 am

The house I live in is an old red sandstone barn, in South Wales, which was converted in the '80s. The old stonework needs repointing with a lime mortar (to allow the stone to breathe). What ratio of sand to lime should I use? Do I use hydraulic lime? and what sort of sand will give a dark grey (old) finish - the sand I used last summer to do a bit made the mortar really white, too white.
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Postby KitchenGuy » Sat Sep 08, 2018 1:55 am

I am from South Wales and I know the type of stones around here (Old Red Sandstone if it is sandstone or it maybe be dolomitic limestone if it looks smooth and breaks glassy or dolomite conglomerate if it looks very course gritty with pebbles contained in it )

The old black mortar ( looks dark blue grey) was made from lime and ash. It wasn't very hardy and most has deteriorated by now. If you are looking to imitate it then use a course sand and use dye to get the colour right. Make sure you do test samples (you don't want a patchwork house) to get the colour right and then you can scale up and work out the gauge of dye you need per mix.. You will need to buy black dye and use less of it, If the mortar has a blue tinge to it then test the dye first as most but not all will look this colour if used lean. I used to find Everbuild dye worked well but that was a while back. Don't use too much water in the mix, lime mortar already has a tendency to glow white and this is made worst by a wet mix.

The usual proportion for pointing is 1;3 or 1;4. The latter will look more in keeping with old lime mortar. Mix 50/50 first is best then slowly add the rest of the sand and mix really well. If you are using it in place of cement then hydraulic lime is the way to go. Try not to get it on the stone and if you do don't wipe it off straight away it will smear and go white. Leave it a few hours to dry out and brush it off. Any white dust left after will wash off after the job after the work has well dried.

Good luck.
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Postby Greml1n » Sun Sep 09, 2018 10:23 pm

Thank you SO much for your reply. I hadn't thought about using dyes to get the right colour. A lot of ours (on the newer part of the house ) looks like someone has used cement, but I understand that cement based mortars don't have the 'give' in them that lime mortar has and it doesn't let the stone 'breathe either.
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Postby KitchenGuy » Mon Sep 17, 2018 10:22 pm

I know they say let the stone breathe but it's really the joints that are let to breathe. Hydraulic lime mortar is a better job as it is slightly pliable and any cracks will re-knit over time due to a reaction with the CO2 in the air. It's just a lot more difficult to work neatly and I know it's very much back in vogue at the moment. I wouldn't entirely rule out sand and cement mixes though. Sand/ cement hydrated lime semi dry mix was pretty much the norm for most of the twentieth century and if it's done properly it will outlive you and me, has some "breathable" qualities, it's also much cheaper, neater, quicker and easier so it does have some advantages.

Good luck whichever way you go.
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Postby Greml1n » Tue Sep 18, 2018 10:49 am

Thank you kitchen guy - what ratios would you use if using sand, cement and lime, please?
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Postby KitchenGuy » Wed Sep 19, 2018 11:53 pm

Usually 1 cement, 1 hydrated lime and 4 parts sharp sand. Don't be tempted to overdo it on the lime or cement. It's already a rich mix and if you make it too strong it is susceptible to shrinkage and cracking. Don't make it too wet or over mix it either. just enough for the dye to mix in well. Imagine wet sand that's wet enough to hold a sand castle but not wet enough to slump down when you remove the bucket.
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Postby Greml1n » Thu Sep 20, 2018 5:54 pm

Thank you very much. I will be ordering everything on Monday. x
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