How bad is NHL 3.5?

[Stonehouse]

Feeling a bit deflated after hearing NHL 3.5 can set really hard reducing breathability and causing damage. Is this true? Maybe we should have gone down the plasterboard route!

NHL 3.5 has been put over our walls internally as scratch coat.

 

[Zebra]

NHL is harder than putty-based, but less hard than cement. So it's fine in the right situation.

 

[Lacan07]

Feeling a bit deflated after hearing NHL 3.5 can set really hard reducing breathability and causing damage. Is this true? Maybe we should have gone down the plasterboard route!

NHL 3.5 has been put over our walls internally as scratch coat.

Hello

It'll be fine, as a plaster strength isn't such an issue other than it won't be as flexible if the house moves a lot, which some do and some don't. In terms of breathability its just a bit less than lime putty. Its almost certainly way better than plasterboard which if it gets wet / damp will turn to mush pretty quickly.

For any repointing jobs I would perhaps think about using putty if possible.

 

[stuart45]

I'd agree that NHL is preferable to plasterboard. An air lime is more breathable, but the timescale between coats is much longer.
What was on the wall previously?

 

[Stonehouse]

Phew, thank you. We’ve gone with St Astier ecomortar on top.

The walls previously were a lime, sand mix with animal hair and in random areas very, very thick cement.

 

[stuart45]

Sand/cement patches are common where repairs have been carried out.
NHL is often used when the rooms, especially kitchens and bathrooms, need to go back into use quite quickly.

 

[Stevers]

It's a very long time since I had to learn about the chemical composition of concrete, but I do remember that the processes involved are complex and not necessarily fully understood. The link below should be of interest to those who wonder how NHLs, which have a chemical composition trending towards OPC, are not the same thing to use.
https://www.buildingconservation.com/articles/hydraulicity/hydraulicity.htm

For thirteen years now I've been using NHL 3.5, and before that I used OPC/hydrated lime mixes 1:1:6 followed by 1:2:9. The big difference I found is that cement lime mixes are totally unsuitable for repointing stone walls built with lime mortar, although I have a number of walls rebuilt in 1:1:6 and these have lasted really well. A wall rebuilt in 1:2:9 has been subject to the lime leaching out, but is structurally sound, although the mortar round the coping stones has cracked, probably due to shrinkage plus thermal movement. All freestanding unsheltered masonry in this part of the Mendips goes black over time, whether built with cement or lime mortars and that's just the way it is. One thing I have noticed is that cement mortars do shrink over time, and I haven't had any NHL mortars do that.

 

[Lacan07]

NHL is probably still the most specified lime amongst conservationists, architects, etc. Its been at the forefront of the heritage revival over the last decade. I guess that might change though moving forward.

 

[88v8]

One thing I have noticed is that cement mortars do shrink over time, and I haven't had any NHL mortars do that.

I'm not sure they do shrink.
I think what one sees in that thin boundary, that apparent discontinuity between the stone and the cement, is the result of the stone micro-eroding because it can't breathe. Dust particle by dust particle the stone disintegrates.

I see this in our cottage, inside and out, where the stone has shrunk dustily away from the thin skim of 1970s cement pointing, and inside I see the steady dribble of stone dust onto the floor.
It's on the jobs list...

Aiui all NHL lime mortars, 2, 3.5, 5, ultimately reach the same strength. It's just that NHL 5 is faster.

Ivor

 

[Lacan07]

My porch was added to my house at a much later date and to construct the lower half they used the original soft red bricks (must have had some lying around) with modern cement mortar. You can really see the difference in erosion between the porch bricks and the old house, the porch bricks are totally knackered. I don't know if cement shrinks, others on here have used the material professionally, but it has no flexibility and cracks will occur with an old house that doesn't have modern foundations, and water will soon penetrate only to become trapped again.

 

[Black Timber]

You can really see the difference in erosion between the porch bricks and the old house

What type of lime was used on the old house? Ta

 

[stuart45]

I don't know if cement shrinks, others on here have used the material professionally, but it has no flexibility and cracks will occur with an old house that doesn't have modern foundations, and water will soon penetrate only to become trapped again.

Anything made with OPC as the binder will shrink. This is especially true for large concrete and render work, concrete blocks, and to a lesser extent mortar joints. The stronger the mix, the greater the shrinkage.
With brickwork, the joints are normally small enough for the shrinkage not to be noticeable. New bricks usually expand anyway.

 

[Stevers]

The hard cement renders used to tank my basement have shrinkage cracks. The cracks took a while to appear, so I reckon from the cement matrix shrinking.

This problem is in addition to the damp(?) boundary conditions that cause separation from stone work, that I have also seen with my cement lime pointing that failed.

 

[stuart45]

The render is probably around 3/1, which is considered the point when it becomes impervious with most sands. As an apprentice we were taught to use a really strong mix of around 2/1 for repointing work. I looked it up in one of my old books, and for engineering bricks even stronger. It's taken quite a few years to them to discover the effects of a strong mix.

 

[Stevers]

Correct, 3:1 with sharp sand and latterly SBR. Horrible stuff to put on and take off.

 

[Lacan07]

What type of lime was used on the old house? Ta

It would have been lime putty, slaked on site

 

[Stevers]

Agreed, though the slaked lime putty may have been naturally partially hydraulic from 'contaminants' in the source limestone or chalk. There could also be pozzalanic contaminants from the hole it was mixed in, and perhaps the aggregates used in the mortar. It's a spectrum, but don't ask me how I know about those.

The crazy strength of the mixes in Stuart45's post seem entirely consistent with the various cement mixes I've encountered here from 50-70 years ago that ring like a bell when struck.

 

[Stonehouse]

Hello

It'll be fine, as a plaster strength isn't such an issue other than it won't be as flexible if the house moves a lot, which some do and some don't. In terms of breathability its just a bit less than lime putty. Its almost certainly way better than plasterboard which if it gets wet / damp will turn to mush pretty quickly.

For any repointing jobs I would perhaps think about using putty if possible.

Thank you. We are a mix of granite and soft red brick. Red brick for fireplaces and around windows, doors etc. I do feel worried for the red brick now! If we do any repairs will it damage them like cement if it really hardens?

 

[Lacan07]

Thank you. We are a mix of granite and soft red brick. Red brick for fireplaces and around windows, doors etc. I do feel worried for the red brick now! If we do any repairs will it damage them like cement if it really hardens?

Like I said if its internal plaster on walls then no need to worry about the red bricks, it won't damage them. Its just a little less breathable than a lime putty based plaster.

If however you're planning on doing any repointing of the red bricks, or building work, etc then maybe think about getting a lime putty based 'mortar' for the joints. But even then for smaller jobs a bit of NHL2 won't hurt.

 

[a twig]

In my humble opinion it is very easy to go down the rabbit hole on some of this stuff.

My stance has always been that a quality NHL 3.5 is a great improvement on cement based mortars and renders for any property that isn’t constructed from concrete block work, and then from there you are into diminishing returns on further “betterment” in flexibility and breathability by using NHL 2 and again vs lime putty, against which you have to trade off increased labour, duration of works, potentially less availability of materials/skills etc

If time and money are no object then specialists doing hot lime work are brilliant, but you tend not to see that outside of large scale conservation works for those very reasons