Lime mortar - is my builder's mistake a big problem?

[pandora]

Having personally watch virtually every last little bit of work going on at my cottage for the last 3 months, I had to leave the builders to themselves for 2 days last week. During this time they repointed the chimney stack - it desperately needed doing. We had specified lime mortar, which the builders said they were happy with. I trusted them on the matter, as so far they have structurally repaired the very fragile roof using oak and traditional methods, laid a limecrete floor and happily worked with our ground source heat pump. They seemed happy to accept our reasons and methods, even if they weren't sure why they were doing it.

So when we got back from our 2 days away, we looked at the chimney pointing and were not entirely sure it looked like lime mortarbut it didn't look like cement based mortar either, but being on the ground it was difficult to tell what it was. I asked the builders today what they had used, and they said 'lime mortar.' Oh good, I said. 'Yep - we used 6 parts sand, 2 parts lime, and one part cement just to give it a little bit of strength. It was too soft without it.' :roll: Aaargh!

So, having re-educated them about what 'absolutely no cement' means, what I was wondering is whether that amount of cement in the mortar mix will be a big problem? I know the mortar must be softer than the bricks, but is the bit of cement in this mortar going to make the mortar too hard, yet not tough enough for the job - worst of both worlds, as it were? I don't really want to have to get the builders to re-erect the scaffolding, chip out the mortar they have just put in and re-do it, if it is not really necessary. I realise in an ideal world they would never have done this to start with, but the builders have been otherwise good and I have no wish to get their backs up. It was not a large area, and the rest of the house will be repaired using proper lime mortar.

 

[NT]

Cement:Lime mixes should be 1:1, as other proportions have been found prone to premature failure. So thats not great.

What you've got is close to 1:1:6. It has low enough cement to still be porous, but of course its a fair bit harder than lime mortar. 1:1:6 does get used on historic brickwork, as for how good it is hardness-wise... I dont know. It doesnt self-heal, any movement breaks it. You've got cement mortar effectively.


NT

 

[andrew gayton]

there is lots of previous guidance on compo mixes...... the ashurst works are a good source. a compo mix on a stack, in my opinion, will cause problems not only with permeability, but also sulphate problems............. possibly eventual destabilisation of the stack (imagine that in repointing, only 25mm will be repointed [in a hard mortar] while the remaining 75mm will be obviously more permeable potentially contaminated with sulphates..........) raking out might be possible - dependant on if they have just buttered over - but if the stack (assuming it is 1/2 brick) is that bad a rebuild may well be needed.. any photo's?

 

[andrew gayton]

and

what sort of lime did they use?

 

[Nemesis]

I had builders who did the same - with render and bedding in ridge tiles.

The stuff is now crumbling.

 

[chuckey]

The which handymans book recomends 1:1:6 for walls that have to be weather resistant. I did my 8' high X 100' long victorian garden wall with this mix about 20 years ago and it looks like it I did it yesterday. No flaking/cracking, nothing.
Other then appearances I would leave it. lime inside unlined stacks totally disappears (acidic tar products of combustion) . If the stack is lined it will last the life of the liner + 50/100 years. if unlined life is 30/50 years in either case.
Frank

 

[Nemesis]

Conservationists recommend no cement for chimneys and a weak to moderately hydraulic lime.

The Which book is hardly one I'd recommend for historic buildings really...

http://www.oldhousestore.co.uk/tech_ohs/limeintro.html

http://www.greenspec.co.uk/html/materials/limemortar.html

http://www.wealddown.co.uk/book-shop-building-conservation-craft-revised.htm

 

[chuckey]

The which book is good as far as it goes. What we are talking about here is taking down a chimney stack, trying to clean the cement mortar of the bricks and re-building. If matching bricks are not available, ever brick has to be cleaned off with a disc grinder. What happens if some bricks break? Its a sad job but the best thing to do is to leave it alone.
Frank

 

[Nemesis]

The Which book is maybe not one I would recommend for historic buildings - 'as far as it goes' - not good enough!

It may be possible at this stage to do something before worse damage occurs in the future. Frankly I think it's up to the builders to now put it right.

http://www.cadw.wales.gov.uk/upload/resourcepool/Lime_Mortar_Notes_Eng1695.pdf

 

[Stephen]

Let's consider this practically.

If the mortar has now set hard to try to rake it out could cause more harm than good. It will not start to cause deterioration on day one, but it is an inappropriate mix and some of the problems mentioned could eventually arise (but may not, or not for many years). Every situation is different.

If the mortar is still weak enough to rake out then you could insist that the builders rake out and re-point correctly.

As for the 'correct' lime - this always causes much debate.

Lime putty (or fat lime) comes from pure limestone and is the most flexible and breathable of all limes. However, it is also the least resistant to frost, weathering, etc and has poor (relatively) compressive strength. It has a wide range of uses and is fine externally for many situations (including chimneys), but is not always ideal. Lime sets by carbonation when it is exposed to air and this is why it comes in tubs with alayer of water oer it. If not in contact with air it can last or years in putty form.

Limestone containing impurities creates a naturally hydraulic lime (NHL) that sets by partial carbonation and partial chemical set. Analysis of historic mortars indicates that such materials have been used for centuries and not all historic buildings contain only fat lime in the mixes. In fact in some buildings different limes have been found used in different situations. It is possible to create a hydrualic lime by adding a pozzolan to fat lime.

NHL sets with water, but also by carbonation. It comes in bags and for builders inexperienced with lime it is more like cement to handle (but is NOT like cement in any way otherwise). Older bags may have started to carbonate and may not be good. Try to get fresh. There are suppliers in the UK but limited. It is not cheap. However, it is cheap in France. I would encourage you to use English and that way we will get more produced and other suppliers/manufacturers. However, until that day the booze cruise could be replaced with a lime cruise for some people.

NHL is now produced in 3 grades (the numbers refer to compressve strengths):
NHL 2 is feebly hydraulic
NHL 3.5 is moderately hydraulic
NHL 5 is eminently hydraulic

Each have different characteristics and uses. NHL 2 is not far off fat lime, whereas NHL 5 is far hardier, etc but not quite as breathable or flexible.

If we were looking at a scale of 1 to 10 with Fat Lime at 1, the NHLs would be somewhere just behind Lime whereas cement would be off the scale at something like 50! This shows that relatively the limes differ, but all are substantially different to cement.

In deciding which lime to use you have to consider what you are using it for and where. For areas that do not normally require much breathability or flexibility you could use NHL5 (footings, bedding ridge tiles, flaunching chimney pots, etc). You would also use it if you need a quick set and rapid strength and/or if frost is threatened.

For general work NHL 3.5 is popular and appropriate for pointing, renders, bricklaying, etc.

Ideally fat lime is great for most things but sometimes you should consider NHL. You cannot use fat lime when frost is likely and this limits you to spring-autumn months.

The above only touches the subject. As you can see there are a selection of limes and the choice will be determined by several factors.

With your chimney unless it is a particularly soft bick I would probably use NHL 3.5 for the main pointing and NHL 5 for flaunching. If the bricks are very soft I would have use fat lime for the pointing and NHL 3.5 for the flaunching.

Use of cement in any lime mix is now regarded as inappropriate and potentially damaging. If lime is used correctly there is no need for cement at all.

The mix is always 1:3 (lime:aggregate), because if suitable sharp graded aggregate is used the lime is simply filling the air pockets between the pieces of aggregate and air is usually about 25% of well graded sharp aggregate. The lime is there not to stick things together, but to pack out the gaps and provide a cohesive material through the mortar bed.

To return to your immediate problem: I refer back to the first point - can it be raked out without causing more harm than good?

If not and you have to leave it you may wish to negotiatea reduction in cost with the builder on the basis of him not following the Spec and the fact that future works are more likely, thus leading to earlier and greater costs than otherwise might have been the case.

I hope this brief overview and comments are of some help.

 

[NT]

Cement takes around 28 days to cure, so if you're going to redo it, now is the time.

I think I'd ask if the mortar mix was in writing or verbal. If the latter you're somewhat in limbo legally, but so are they.

These issues happen over and over with builders, yet people still use verbal agreements and get into bother.


NT

 

[Nemesis]

Long, long discussions I recall with Geoffrey at Knedlington over his chimneys - with all and sundry - and although many did not agree re how hydraulic, hydraulic it was - SPAB felt weakly, some lime providers felt moderately... but yes I agree - if the stuff is possible to be removed I'd do it now.

 

[Gervase]

Stephen is right - no cement whatsoever should be used in a lime mortar. Absolutely none. Full stop!
The Smeaton project has shown that gauging with OPC in any ratio compromises the mortar to some extent, and the current advice is not to use any cement at all in a lime mortar (contrary to English Heritiage and National Trust guidelines just five years ago).
In your position I would insist that the builders rake out the joints with hand tools asap and repoint with NHL 3.5.

Apropos other posts, if the stack is sound other than dodgy pointing, it will not have to be taken down, and talk of cleaning individual bricks with a disc grinder is, er, fanciful. Lining, however, may be a good idea if the parging on the inside is damaged and the flue is no longer smoke-tight.
And the 'Which?' guide to DIY is no guide to work on an historic building!

 

[Pford75]

I'm slighlty confused...not a chimney i know...but the SPAB technical pamphlet on flint wall repair, recommends a 6:1:1 mix

Is this wrong?

 

[Nemesis]

My own view is that I'd repair matching the new as far as possible to the old mortar. I think it highly unlikely it had any cement in the mix.

http://www.cadw.wales.gov.uk/upload/resourcepool/Lime_Mortar_Notes_Eng1695.pdf

http://www.cadw.wales.gov.uk/upload/resourcepool/masonry%20notes%20english7511.pdf


In small amounts cement may not be damaging, although may weaken the mix, but really I can't see why it should be necessary. Current thinking is that really it isn't, and the Technical Pamphlet may be a little out of date? 2002

It's still a bit of a controversial subject!

http://www.ihbc.org.uk/context_archive/54/overviewlimes_dir/overviewlimes_s3.htm



If you want to know more read the results of the Smeaton Project?

Also:

http://www.buildinglimesforum.org.uk/index.htm

 

[Nemesis]

I'll add this here:

http://www.chilternsaonb.org/downloads/Adopted_Flint_Note.pdf

http://www.spab.org.uk/html/advice/technical-qas/technical-qa-3-historic-flintwork/?PHPSESSID=4f99b9a1d8199b35bb0ad80b7676bfbf