Mortar mix for pointing granite.

[sb1202]

Need to do remove and repoint a granite wall where the previous owner seems to have slapped some postcrete type mortar. Spent the last 2 hours reading/googling on which mortar to use and I'm more confused than when I started. Some say, as its granite and very hard, to use a hydrated lime mortar of 1+1+5 or 1+1+6, other say use NHL5 and my head is bursting. Walls are granite 600m with a rubble in fill.  Pic shows the original good pointing but I have no idea what mortar mix this is.

Any advice appreciated.  

[nod]

4-1 

[sb1202]

4 hours ago, nod said:

4-1 

4-1 what?

[MikeGrahamT21]

Doesn’t look lime based, not light enough.

 

as @nod said, 4-1 sand and cement will be ideal for granite 

[Mike]

If the build is as old as a 600mm thick rubble-filled wall suggests, then the chances are that it's lime mortar, so I'd repoint in that to maintain / restore the vapour permeability & flexibility of the original mortar. You'd need to knock through the existing pointing to see what's behind if you think the original could have been cement-based, but that's only been commonly used since start of the 20th century.

 

1:3 NHL3.5 or NHL5 to sand would be the easy mix; since it's granite the latter would typically be chosen to match the hardness of the stone (NHL5 is usually, though not necessarily, stronger than NHL3.5). Lime putty would be the traditional material with superior properties (more permeable & flexible), but would be harder to use on granite - it would take longer to dry - and isn't suitable if the wall is regularly wet. Having said all that, I'm not a lime expert.

 

Edited January 21 by Mike

[sb1202]

5 hours ago, MikeGrahamT21 said:

Doesn’t look lime based, not light enough

It's definitely lime based. It was renovated in 1982 and the plans stated the pointing and binder were lime mortar - it doesn't specify what though.

[sb1202]

1 hour ago, Mike said:

If the build is as old as a 600mm thick rubble-filled wall suggests, then the chances are that it's lime mortar, so I'd repoint in that to maintain / restore the vapour permeability & flexibility of the original mortar. You'd need to knock through the existing pointing to see what's behind if you think the original could have been cement-based, but that's only been commonly used since start of the 20th century.

 

1:3 NHL3.5 or NHL5 to sand would be the easy mix; since it's granite the latter would typically be chosen to match the hardness of the stone (NHL5 is usually, though not necessarily, stronger than NHL3.5). Lime putty would be the traditional material with superior properties (more permeable & flexible), but would be harder to use on granite - it would take longer to dry - and isn't suitable if the wall is regularly wet. Having said all that, I'm not a lime expert.

 

It's definitely VERY old so it's not cement. It's at least 300 years old according to historical maps and possibly goes back to the 1400s.  Our deeds say it was part of an estate and the castle was built in 1604 on top of an existing watch tower from the 1400s. The walls aren't regularly wet - we're in Aberdeenshire which is broadly similar to the SE of england for rainfall. I've been researching what I can but drawn a blank. I do know that the rubble isn't bound with a mortar - it's just mud/soil. The folks at historic scotland said this was common and this type of property last hundreds of years with the original walls. 

[lookseehear]

I've gone down the rabbit hole multiple times with lime, and the only thing anyone seems to be able to agree on with pointing is that it should be softer than the bricks or stone.

 

The heritage route would be a non hydraulic lime mortar with some pozzolans (natural additives like brick dust, wood ash which act as a source of carbon to help the mortar go off). This would be best given that your walls are constructed with mud, as its soft and will help to keep the joints dry. It's also very slow to go off and can only be done in the milder months.

 

This YouTube channel has a lot of information on lime https://youtube.com/@michaeljamesdesign?feature=shared if you want to do some research.

 

A mild 6:1:1 sand to cement to hydrated (bag) lime is probably the pragmatic answer if you want to get it done in a hurry.

[jfb]

I’d just go with 3.5 nhl (not hydrated) at 3:1 ration sharp sand/nhl

doesn't want to be done if there is  any danger of freezing and needs to be done by somebody who can work at the right pace (not like cement where you can have a section finished in a day - definitely be brushing off a section a day after the mortar has gone in).

 

[Mike]

21 hours ago, sb1202 said:

It's definitely VERY old so it's not cement. It's at least 300 years old according to historical maps and possibly goes back to the 1400s.  Our deeds say it was part of an estate and the castle was built in 1604 on top of an existing watch tower from the 1400s. The walls aren't regularly wet - we're in Aberdeenshire which is broadly similar to the SE of england for rainfall. I've been researching what I can but drawn a blank. I do know that the rubble isn't bound with a mortar - it's just mud/soil. The folks at historic scotland said this was common and this type of property last hundreds of years with the original walls. 

A very interesting history! I've not seen earth mortar before, though in several regions of France they built (as recently as 100 years ago and occasionally still today) 5 or 6 storey buildings from a mix of earth and stones, compressed in-situ (pizé) - so it can be very strong, if protected from the weather - they're usually covered in a lime or clay render. You'd certainly never use cement on them.

 

I've just taken a look online and, if you're not already found it, there's a short pdf on clay mortars here: http://openarchive.icomos.org/1801/1/Historic Environment Scotland_Mortars for Masonry Buildings.pdf which, states that "most clay-bonded walls were externally pointed or harled with lime mortar to provide additional protection from the weather". There is a phone number at the back for Historic Scotland's technical advice department, though it looks like you've already talked to them. But maybe they can help with the identify the mix used, if you can find any original pointing mortar behind the existing.

 

Having just looked it up, the process for what we now call 'natural hydraulic lime' wasn't developed until 1756, so lime putty would no doubt have been used - perhaps burnt & slaked on the estate if there is limestone in the region. Lime putty & sand, probably using a 1:3 mix, would therefore be my fist choice. That would mean repointing it slowly, starting no earlier than Easter so it has the best drying weather. There are courses you can go on to learn more if that interests you.

 

Failing that, I'd go with the 3:1 NHL3.5 to sand, as per @jfb.

 

Edited January 22 by Mike

[Roger440]

Im inclined to agree with Mike and jfb.

 

Id not go NHL5 as its got mud in the wall. NHL5 is nearly as impermeable as what you are taking off.

 

You could of course get some or the original motar out and get it analysed.