/ Making lime mortar for repointing stone fireplace

This topic has been archived, and won't accept reply postings.
PeakDJ on 09 Apr 2019

Anyone here who can offer advice on this?  I need to repoint an old stone fireplace and need to mix some lime mortar.  Can I get away with just mixing sand and lime in a 3:2 ratio, then adding water in a big plastic barrel, or do I need to make a lime putty first, then add the sand?  Any tips?  Cheers!

Sl@te Head - on 09 Apr 2019
In reply to PeakDJ:

You'll need to bolt it first before the redpoint ;-) 

1
althesin on 09 Apr 2019
In reply to PeakDJ:

NHL (natural hydraulic lime) 2 or 3.5 works pretty much like normal cement, use sharp sand if you want old style authentic lumpy bits, pointed lime mortar in my kitchen with this stuff, handles ok but don't get it in you eyes!

badgerjockey - on 09 Apr 2019
In reply to PeakDJ:

whack it all in a bucket first as a 3:1 ratio and mix it well dry then slowly add water in small amounts. Cupfuls at a time. Mix it as thoroughly as you can each time. Ideally with a bucket trowel. You want it to be mouldable and tacky like putty and not sloppy or runny nor cakey and dry... it’s actually not as hard as I am making out!

Post edited at 22:36
PeakDJ on 10 Apr 2019
In reply to badgerjockey:

> whack it all in a bucket first as a 3:1 ratio and mix it well dry then slowly add water in small amounts. Cupfuls at a time. Mix it as thoroughly as you can each time. Ideally with a bucket trowel. You want it to be mouldable and tacky like putty and not sloppy or runny nor cakey and dry... it’s actually not as hard as I am making out!

Perfect - thanks!  Will give it a go

Mick B - on 10 Apr 2019
In reply to PeakDJ:

I did the same job this time last year.  One tip I'd give is to clean off the excess which gets on the stone asap.  I was told that it would still come off easily with water after a couple of days, which it didn't. 

I found the best tool for cleaning down the stone was one of the little metolious bouldering brushes.  

Toerag - on 10 Apr 2019
In reply to PeakDJ:

I've repointed 2 chimney stacks, rebuilt an internal wall and various bits of sleeper walls in my victorian house with lime mortar / limecrete.  If it's decorative pointing of brickwork and not really structural then use NHL2 lime in your mortar to stop it ripping the skin off the bricks if you have to remove it in the future. NHL2 is the weakest type of lime and thus best used for plastering and non-structural pointing. It is the whitest though in case colour is important.

Making mortar - make it drier that you think and leave it for some time before trying to use it. It takes time & mixing for the mortar to attain optimum workability, so if you mix it to a workable consistency straight off it will have too much water in it and will become sloppy then crack when it dries. So mix it too stiffly, leave it half an hour, then mix  it again and add water sparingly to get the right consistency. Use a round-ended gauging trowel for your mixing. I'm asuming you're not going to use a mixer, but if you were, be aware that they tend to cause balling unless it's the horizontal type. I'd use a large yoghurt pot as my measure and do about 4 pots' worth of sand/lime (I can't remember ratio, look it up on the St.Astier website) in a bucket, mixed completely dry then add water and so on. It was hard work to get the workability of the mix. I'd do two mixes and leave one 'settle' whilst I used the other, then start another mix whilst using the second. That way I always had a mix settling.

You'll need to spray the bricks with water a lot beforehand to stop them sucking the moisture out the mortar before it's had a chance to chemically harden. I'd still spray the pointed area the next day too. You'll need to rake out the old mortar to at least a finger thickness depth or your new mortar will flake off.  Now, this heavy use of water may cause you problems because your bricks are invariably covered in soot and tar which is quite likely to run and stain your pointing. I don't really know how you're going to get round this other than cleaning them well beforehand and work from the top down. Definitely clean as you go unless you're wanting to acid-wash the lime out of the brick pores.

Edit - just re-read it's stone you're working with, so you'll need to do a lot less wetting (unless it's sandstone, in which case all my brick comments probably apply).

For limecrete I'd essentially make mortar then add aggregate to it until I got to the right consistency/ratio. you're essentially looking for the aggregate packed solidly and mortar filling in the gaps but no more.

Post edited at 16:14
PeakDJ on 11 Apr 2019
In reply to Toerag:

Thanks for that detailed reply - some really useful info there.  I think I'll avoid NHL2 as the stones are quite dark so want to keep the mortar from being too white.  I'll have a think about it though, as it maybe possible to get around this with the sand colour.  

The stones are granite, so I don't think too much wetting will be required.  They are currently quite damp from a few treatments (we were trying to avoid sand-blasting) to get the a bit cleaner.  

The old mortar all sort of fell out when we removed the layer of "crepis" and cement that the fireplace had been covered with...but we have then removed to at least a finger depth, sometimes more.  The stones are huge and the fireplace seems fairly well built and solid so this is more decorative than structural.

Thanks again for the info!  I really appreciate the time you took to give such a good level of detail.

Post edited at 07:28
PeakDJ on 11 Apr 2019
In reply to Mick B:

I think best to wait 2-3 hours or until the brush does not get clogged with mortar - the way they do it here in France is to smear it over the edges of the stones and wait for the perfect time where it can be brushed off to expose the desired amount of stonework.

Toerag - on 11 Apr 2019
In reply to PeakDJ:

Lime basically gets greyer as you progress through the hydraulic grades (NHL3.5 then 5), sand is the key to colour. Happy I could help .

roperat - on 12 Apr 2019
In reply to PeakDJ:

I used to use lime mortar a lot and leaving it to rest after mixing is essential, otherwise it all falls out and stains the stone. The secret is to mix, then make a pot of tea, drink said tea with at least 3 biscuits and then start pointing.

I would definitely avoid using a mixer lime is a sod to clean out of one and its generally not worth the hassle. Even when working on lime kilns and lead workings we always just mixed in a plastic trug with a long-handled shovel as it is easier to gauge the water required and you don't get tempted to mix more than you can reasonably use before it goes horrible.

Keep everything nice and wet and don't be tempted to speed up the curing process, if it's warm cover with damp sacking. Make sure you brush in the mortar, not a little tickle for surface effect but a proper push in to ensure adhesion to the stone, a fence brush is really good for this.Wash the stone down well afterwards to minimise staining a little garden sprayer will do nicely.

If you do any pointing above eyelevel put some glasses on, it hurts like the very devil if it gets in your eye.


This topic has been archived, and won't accept reply postings.