Earth and ash mortar issues

[hughseh]

Hi

I'm looking for advice on a newly discovered issue having started to remove the existing cement render to an early nineteenth century cottage.

It looks to be two mortars, a very soft, soil earth mortar and a very hard, aerated ash mortar. The ash mortar is almost like just chunks of kiln waste and using the white vinegar trick, there is no fizz, so unsure how much lime content.

Should this be repointed in hot lime, or an NHL mix? How far do I take out? The ash mortar is more like stones, it doesn’t come out in a linear manner.

 

[aockenden]

What type of stone are the walls built from? How soft is that stone? The hardness of the stone will help partially answer what type of lime to use on it.

 

[hughseh]

What type of stone are the walls built from? How soft is that stone? The hardness of the stone will help partially answer what type of lime to use on it.

It's sandstone. I wouldn't say soft, but sandstone generally is classed as a softer stone than most

 

[aockenden]

It's sandstone. I wouldn't say soft, but sandstone generally is classed as a softer stone than most

If I had a sandstone house I wouldn’t be mucking about with any NHL at all. Straight non-hydraulic lime with a bit of decent pozzolan mixed with a washed and well graded sharp sand would be my choice. Depth of joints to rake out? We’ll you’ll definitely want to make sure you get all the way back to original mortar, leaving none of the cement in the joints. And then you’d want some reasonable minimum depth ontop of that, probably a couple CM I’d think.

 

[hughseh]

To update, all render now removed and on with the task of raking out (very slow and tedious to do correctly), before repointing. A few Q's, i've had two stone masons look at it, one who was familiar with the local stone, but tbh just wanted money for everything before starting work and another mason who's worked on heritage buildings, but not this specific sandstone. The differences in opinion are quite stark, albeit both with hot lime:

The former thinks the walls shouldn't be pre-wetted and the mix needs to be as dry as possible when being applied, the latter doesnt think it an issue
Former feels the upvc windows need oakum around, the latter thinks expanding foam is fine with upvc sitting against sandstone (i should add the sandstone, where previously covered by cement render and a fillet at the window junction is particularly deteriorated by here)
The former is more than twice as expensive per day than the latter, but appears to be 3x as quick in what he states he is capable of doing in terms of stone repairs. He does advise however than 5sq.m per day is the max for correct repointing.

Any thoughts?

 

[aockenden]

Outside of environmental reasons, the case for using Oakum around a plastic window frame is much less clear than with a wood frame. The fact that one of them is saying that is strange and makes me personally question his reasoning skills (again, unless he’s saying that for environmental reasons which would be very strange to hear coming out of any modern tradesperson’s mouth).

As for whether the walls need to be wetted down before repointing, it is very common practice so I’d be interested what his reasons were for not wanting to do so?

Congrats on the cement render removal.

 

[stuart45]

If you don't damp down first, moisture will be sucked out of the new mortar too quickly.
Ideally masonry should be well damped down, but surface dry. If the surface is too wet it can stain the brick or stonework.

 

[Zebra]

If I had a sandstone house I wouldn’t be mucking about with any NHL at all. Straight non-hydraulic lime with a bit of decent pozzolan mixed with a washed and well graded sharp sand would be my choice. Depth of joints to rake out? We’ll you’ll definitely want to make sure you get all the way back to original mortar, leaving none of the cement in the joints. And then you’d want some reasonable minimum depth ontop of that, probably a couple CM I’d think.

If you're adding pozzolan to a non-hydraulic lime, doesn't that essentially make it hydraulic? Why would you not in preference choose a reliable ready mixed (dry powder to add water on site) mortar from a supplier such as the Old House Store or Mike Wye? I can't see why you'd be wanting to measure pozzolan yourself. Am I wrong about that? Is it not the addition of pozzolan that makes a putty mortar into a NHL mortar?
Also, there's been many mentions of hot lime on here recently, and I'm not sure if this is a question of terminology, whether this just means a putty based mortar?
I would look on the Old House Store or Mike Wye website for advice on which mortar to use. If the sandstone is soft and porous, then a putty-based mortar or NHL2 perhaps. I've used both of those, (and NHL3.5 where there's wind loading) on soft red bricks to good effect, but I'd get some more advice. You don't want to be using the wrong stuff and having to rake it out again (like I did!).
I would think that if some of the harder existing mortar doesn't want to come out, it would do no harm to leave the odd bit behind and work round it. You don't have to get every single last bit out.
I wouldn't rake out the whole wall before putting in any new mortar. I'd do a few courses at a time: rake out, repoint, cover overnight, bash with churn brush... then on to the next few courses. Work from the bottom up so you've got a sturdy base beneath.
Sorry if I'm saying stuff you already know.

 

[Zebra]

To add: more expensive is not necessarily better in terms of choosing tradespeople. It's a tricky task, to choose someone, and other than trusting your gut instinct as to which is better, I don't know what to suggest. 5sqm per day is good going for raking out and repointing, I'd be extremely pleased as a DIYer if I achieved that, although it's probably not that impressive for a professional.

 

[aockenden]

If you're adding pozzolan to a non-hydraulic lime, doesn't that essentially make it hydraulic? Why would you not in preference choose a reliable ready mixed (dry powder to add water on site) mortar from a supplier such as the Old House Store or Mike Wye? I can't see why you'd be wanting to measure pozzolan yourself. Am I wrong about that? Is it not the addition of pozzolan that makes a putty mortar into a NHL mortar?

Yes that’s my understanding as well in theory (although I believe some non-hydraulic purists believe otherwise). However the reasons to opt for a non-hydraulic lime + pozzolan in my mind are:

1: full control so you can add just enough to get some early strength to stabilize the mortar on the wall without over-hardening it
2. Non-hydraulic limes on the market seem to be higher quality on avg than NHLs. Some NHLs on the market seem so grey they almost look like Portland and I think it’s possible they’ve had all the goodness burned out of them
3. If you’re paying someone to do it, you’re more likely to get a meticulous skilled craftsperson if they’re going to use a non-hydraulic lime and add their own pozzolan instead of some bodger whose picked up a bag of NHL down the builders merchant and is cashing-in on the restoration market. Same if they’re buying pre-mixed stuff from Mike Wye.

Just my 2 cents.

 

[stuart45]

hughseh, have you considered doing the pointing yourself? Doing a flush joint with a churn brush finish isn't that complicated. I used to hate pointing when I was younger, but a lot of repointing specialists were people who weren't that good on the trowel.

 

[Zebra]

Yes that’s my understanding as well in theory (although I believe some non-hydraulic purists believe otherwise). However the reasons to opt for a non-hydraulic lime + pozzolan in my mind are:

1: full control so you can add just enough to get some early strength to stabilize the mortar on the wall without over-hardening it
2. Non-hydraulic limes on the market seem to be higher quality on avg than NHLs. Some NHLs on the market seem so grey they almost look like Portland and I think it’s possible they’ve had all the goodness burned out of them
3. If you’re paying someone to do it, you’re more likely to get a meticulous skilled craftsperson if they’re going to use a non-hydraulic lime and add their own pozzolan instead of some bodger whose picked up a bag of NHL down the builders merchant and is cashing-in on the restoration market. Same if they’re buying pre-mixed stuff from Mike Wye.

Just my 2 cents.

Oh. Interesting!

 

[hughseh]

Thanks for the responses, all useful info. A points of reply:

1. Not pre-wetting down - This is reportedly to avoid the risk of lime leaching into the sandstone and preventing alkali-silica reation
2. Adding pozzolan - Adding say 10% by % of the lime i'm fine with. Historic mortars were generally feebly-hydraulic by virtue of being mixed on the ground, it would be very rare to obtain a pure non-hydraulic lime.
3. Lime puttty - based on what i've read, this appears inferior to a hot lime mix. To get to a putty, the lime is saturated and so has a lower free-lime content and higher water content, which eventually evaporates, causing shrinkage
4. Doing it diy - unfortunately the scaffold is on a public highway and so needs a council licences, adding £150 p/w to the cost of the scaffold hire. Plus i''ve already used up leave from work removing the render. I will pay a mason to point the upper half of the building to allow the scaffld to be struck and do the rest myself over the summer

I should add, the original mortar is now indicated as primarily a coal ash mortar face, with a high content, hot lime mortar mix beneath. The thin coal ash layer is brittle, whereas the lime mix beneath is friable and dusty.

Again a difference of opinion between masons, the former stating it will high a high sulphate content and can damage the stone. The latter is not overly concerned by it

 

[hughseh]

A few photos always help

 

[stuart45]

Thanks for the responses, all useful info. A points of reply:

1. Not pre-wetting down - This is reportedly to avoid the risk of lime leaching into the sandstone and preventing alkali-silica reation

I've never heard of that for a reason not to damp down a wall. That's normally an issue with concrete. All the advice you'll find in knowledgeable sources will recommend the walls to be damped down.
However, it's worth checking out. I'd be interested to know if it's correct.
I would think that the lime is more likely to leach into a dry stone or brick.
For example, when acid cleaning brickwork the wall needs to be soaked with water first to keep the acid on the surface.
When bricks are completely soaked before laying, they won't absorb any moisture from the mortar, and won't bed properly and mortar runs down the facework. You need a dry surface to give a small amount of suction, but not so much inside to take all the water out.
However, I'm a bricklayer, not a stone mason, and don't work with sandstone in my area, so I can't claim any great experience with sandstone.

 

[LimeFan]

Hi, I have exactly the same mortar around my whole house - Black ash mortar. House is a mid terrace built 1880s (Sheffield), there has been a pretty bad attempt at repointing in cement by previous owner and i'm hacking it off now. I've posted on multiple facebook lime groups and am also in talks with my local limeworks and have multiple different answers.
The local limeworks insists on making a quicklime mix with wood ash added in. Someone else on a facebook group suggested that wouldn't make a hard enough mix compared to the original mortar and suggested making a mix of fine sand, quicklime and 10% Metakaolin . Behind the newer cement pointing, the black ash mortar is hard as nails, solid like cement. I understand these mortars were made from industrial waste from coal furnaces, high sulphur content.
At the beginning of this journey, I thought I could buy some basic NHL and repoint my house. After doing all the research and asking all the questions, I'm now overwhelmed and stuck with how to proceed. Unlike Hughseh's wall, my home is made of brick.

Can anyone advise on how to proceed with which mortar to use? I've ruled out NHL for repointing early on in my research, it would potentially create a scenario where the original black ash mortar would wick vapour inward because of how different it is to NHL. Maybe a mix similar to Hughseh's or as person on FB suggested, quicklime, fine sand and 10% Metakaolin?
Thanks