Lime Discussion

[MdB]

My understanding is that a mix with more lime than cement causes the cement to separate which weakens the mix.

 

[Flyfisher]

Well, I'm thoroughly confused now.

I can see how a lot of lime would weaken a cement mix, but I assume that, in this case, that's what was required - it's certainly better than the rock-hard stuff I removed from under many of the ridge tiles (and which ruined the tiles!). I don't see why ridge tiles need to be bedded with a breathable mortar and, as OddJob pointed out, a "harder" mix would be more suitable for roof applications anyway.

I take the earlier point that NHL could have been used but what would be the advantage over 9:3:1, which is really just a weak cement mix? (Isn't it?).

Also, without wishing to open a discussion about the variability of COs, I'm pretty confident that ours knows his stuff - hence the 9:3:1 specification being only for this application and for it to be expressely excluded as a 'general repair mortar'.

 

[Schoolmarm]

I thought that the addition of cement to lime made it not just harder, but made it actually fail.
I always used to say that on here because I read it once but Oddjob linked to the research in greater detail I think. Will go and have a look.

 

[MdB]

http://www.buildingconservation.com/articles/cement/cement.htm

The Smeaton Project, a research programme commenced by English Heritage indicates that a 1:1:6 mix, containing a 50 per cent cement binder, is unlikely to segregate, while a 1:2:9 mix, containing a 33 per cent cement binder, is almost certainly at risk. Until recently it was considered good practice to gauge lime mortars with as little as 5 per cent cement, just enough to impart a chemical set but not enough to make the mortar appreciably harder. However all of the Smeaton Project test samples containing less than 25 per cent failed.

 

[Schoolmarm]

Thanks :lol: :lol: just trawled through all Oddjob's posts to find this one;

6:1:1 mixes are fine if you want a strong mix but 3:1 lime mixes with a couple of handfuls of cement are a bad idea as the cement clumps making the mix weaker than without it. How do I know this? well somebody has tested it and the results are published.

You quoted this article,
http://www.buildingconservation.com/articles/cement/cement.htm
I remember it well and the project,check the date,it was 15 years ago,1995,the year I started in lime,within a year opinions were changing and have changed several times since,and the opinion is now very firmly against any gauging with cement.
Around the same time there were debates about gauging putty mortar with hydraulic or vice versa,now gauging putty and hydraulic is perfectly acceptable,putty as a plasticiser in hydraulic mortar or hydraulic in a putty mortar as a setting agent.
This has come about by people who actually use the stuff day in day out,not people who sit behind a desk working out equasions and producing articles based on assumptions.

NHL limes are classified according to Compressive Strength expressed in N/mm2 measured @ 28 day in mortars prepared with a 1:1 binder/sand ratio as defined by EN 459.
http://www.stastier.co.uk/nhl/info/eunorm.htm


Hydraulic limes compressive strength and setting ability still comes from it's various impurities.
Read the Holmes/Wingate book and you will see how it was used in building major canals.

:wink:

 

[Flyfisher]

What a minefield!

One thing I can't recall reading about is the recommended proportion of pozzolan. In a 3:1 hydrate mix, what amount of brick dust, or other pozzolan, should be added? The same as when guaging with cement, i.e. about 1/3rd?

Also, if using a pure lime mix (non-NHL), what does "and subsequently well tended to ensure correct carbonation" mean in practice. I'm aware of the importance of keeping frost at bay but what else does "well tended" mean?

 

[Gervase]

With Metastar, PFA or brick dust, using a mix of about 1:1:3 pozzolan, lime, aggregate gives a strong, fast-setting mix. It benefits from being knocked up twice, but only make as much as you'll use in half a day, and it certainly won't keep overnight.
Tending means keeping the mortar damp. Lime doesn't directly absorb carbon dioxide from the air, it reacts with it in solution as carbonic acid. In totally arid conditions with zero humidity, lime wouldn't carbonate at all, however much carbon dioxide was around. Keeping the mortar damp with a mist spray and hessian ensures prompt and thorough carbonation.
The CO's recommendation of a 9:3:1 mix indicates that s/he's working from the old English Heritage guidelines from the Eighties; guidelines which the Smeaton Project have pretty well kicked into touch. EH now concedes that no cement should be added to lime mortars. Perhaps the CO needs to engage in a little CPD (and if he or she is a member of the IHBC, as one would hope any conservation officer would be, then CPD to keep up with best practice is mandatory).


ETA: Although as the mix was recommended for new work rather than repair, the EH guidelines aren't really relevant. There are plenty of people still advocating gauging mixes with white OPC for new work. That doesn't make it right in my opinion, however.

 

[Gervase]

Bloody hell, with all those acronyms, I'm beginning to sound like a chairborne tosser!

 

[skier-hughes]

CPD is old hat to, as most people now do CPE, or at least in the financial sector

 

[MdB]

Bloody hell, with all those acronyms, I'm beginning to sound like a chairborne tosser!

I was able to follow your 2LAs, TLAs and FLAs - YMMV. OTOH if you have problems you can JFGI.

 

[Flyfisher]

Thanks for that clarification Gervase. Can I pester you further to clarify what "prompt" means in the context of carbonation? I've used a spray to prevent cracking on very hot days but as far as carbonation is concerned I had rather assumed that this would happen before the mix dries out anyway. I appreciate this time will vary depending on the weather, hence the question about carbonation time relative to drying time (presumably, being a chemical process, this is also dependent on weather, well temperature anyway?).

Would it be a reasonable assumption that if the mortar hasn't dried quickly enough to crack then it has properly carbonated?

JFGI

I've not heard of that one before but I'm assuming it's in the same vein as RTFM :lol:

 

[Gervase]

Ideally the mortar needs to be kept damp even when the weather isn't hot. Carbonation starts from the surface and moves inwards, so provided the first few millimetres have carbonated within a couple of days there will be a degree of weather-resistance in the mortar. Complete carbonation takes much longer, of course, and a lump of compressed mortar bedded an inch or more into the surface may well take several months to carbonate fully.
Carbonation is temperature-dependent to an extent - it slows dramatically at around 5 degrees, but provided the temperature is above that and the mortar is kept damp, it will happen.
Cracking is a physical problem, however, and not really related to carbonation. If the mix is too wet or applied too thickly it will shrink as it dries, and that is what causes cracking. Compacting the mortar as it starts to stiffen will solve that problem - either with an offcut of wooden batten or by beating with a stiff brush (don't whack too hard, however, otherwise you may shake the pointing loose in the joint and lose adhesion).
JFGI - I thought I'd coined that one abut 12 years ago when Google first appeared; Just F***ing Google It!

 

[Ian Bond]

Do not click on the link if offended by strong language. This should get me banned ...

Let me Google that for you

My credo is "No Ordinary Portland Cement, ever.", which is nice and simple.

 

[Schoolmarm]

Your CO should know better,9:3:1 is a mix that was bandied about for years and it's now generally agreed it's wrong,no serious lime practicioner would use it or advise its use.
It's a cementicious mortar not a lime mortar,cement in itself is not strictly a pozzolan,a pozzolanic cement is lime with burnt material such as fly ash etc.
It's completely wrong and unnecessary to use cement anywhere on a lime structure.

What about the white cement that Flyfisher said was specified? Isn't that different?

 

[MdB]

What about the white cement that Flyfisher said was specified? Isn't that different?

Not according to wikipedia: http://en.wikipedia.org/wiki/White_Portland_cement

 

[Flyfisher]

I assumed that white cement was specified for aesthetic purposes.

 

[OddJob]

Just to clarify, the 9:3:1 mix was specified only for new work and not as a repair mortar. It was used to rebuild the top 1m of a chimney and for bedding the ridge tiles. Isn't that consistent with your earlier comments about not using hydrate on a roof?

Ff,looks like all your questions have been answered and I can't add anything to those replies.
But forgive me,rebuilding the top 1m of a chimney constitutes a repair and not new work surely,you're still building on top of the old work,it begs the question what on earth did he specify as a repair mortar?

I don't see why ridge tiles need to be bedded with a breathable mortar and, as OddJob pointed out, a "harder" mix would be more suitable for roof applications anyway.

Ridge tiles may not desperately need a breathable mortar in most cases,it depends what's underneath and I can't claim to be any kind of expert on roofing,but a failed cement mortar allowing water ingress would trap water and could rot the ridge board.
By harder mortar for bedding ridge tiles I mean more durable,hence NHL or a pozzolanic mortar.
Plus what you're not taking into account is the natural movement of the building,again hence NHL or a pozzolanic mortar.
I personally don't use pozzolans very often,for the simple reason most of the work I do doesn't need it.
But the last time I did it was Metastar at something like 3:1:0.5,the 0.5 being the Metastar and that made it plenty hard enough for me,the mix was specified by the supplier who reccomends a much higher ratio for coping stones(which would be subject to similar weather battering as ridge tiles)but I disagree,0.5-1 is more than enough in my opinion.
In case you're trying to pick holes in my arguments
It ain't me that has to get up on the roof and repair them :wink:

 

[Flyfisher]

But forgive me,rebuilding the top 1m of a chimney constitutes a repair and not new work surely,you're still building on top of the old work,it begs the question what on earth did he specify as a repair mortar?

I see your point about the rebuild being a repair. I presume the LBC interpretation was that the rebuild was completely new - a bit like the new work bedding the ridge tiles perhaps? I've added some photos showing the before and after. You're right about it sitting on top of the existing chimney, but any new work has to abut original fabric at some point. A "repair mortar" has not yet been specified for the others works covered by the LBC, but it's a condition of the LBC that agreement is required before work commences. I've not yet started on those bits.

Ridge tiles may not desperately need a breathable mortar in most cases,it depends what's underneath and I can't claim to be any kind of expert on roofing,but a failed cement mortar allowing water ingress would trap water and could rot the ridge board.

. . . which had happened in some places; notably where previous repairs had been done with a rock hard cement mix! When repairing the roof, I wrapped the breather membrane over the ridge board so, hopefully, water ingress should not cause similar problems in the future - well, in my future anyway.

By harder mortar for bedding ridge tiles I mean more durable,hence NHL or a pozzolanic mortar.
Plus what you're not taking into account is the natural movement of the building,again hence NHL or a pozzolanic mortar.

Fair points, which is why I assumed a weak cementitious mix would be suitable.

In case you're trying to pick holes in my arguments
It ain't me that has to get up on the roof and repair them :wink:

No, no, no. I was questioning your points in a genuine sense, to better understand the subject. I apologise if my posts came across as being contrary, that was not the intention at all. Heck, I've only got a few years of this building/repairing lark under my belt and consider myself a rank amateur compared with many of the people here who have worked on old buildings all their lives, but I find that asking loads of questions is a good way to learn - especially with a subject that seems to elicit widely varying answers. I can see why many regard lime as an 'art', which is all very well, but it can be frustrating when I'm in the middle of it and just want to do the best for an old building.

 

[OddJob]

OddJob wrote:
In case you're trying to pick holes in my arguments
It ain't me that has to get up on the roof and repair them

That was said entirely in jest old chap,hence the smilies,meaning literally it ain't me who has to get up there and fix it!
I know you're asking genuine questions mate I just don't have all the answers.
Your LBC and CO seem to have even less though.
I'm alarmed no repair mortar has even been suggested,that tells me your CO and his associates aren't as competent as your confidence in him knowing his stuff suggests.

My interpretation of new work is a new build,what you have is honest and sympathetic repairs.
You should replace like for like and not abut with a more modern and inappropriate material.
Your "new work" should match the original fabric as closely as possible and not just in appearance which leads me to the question what did you use for the red colouring of the mortar on the stack?
Modern stuff like Febtone or Cementone reacts badly with lime,I use brickdust or natural earth pigments and brickdust is a pozzolan.
Don't buy it,go to your local brickworks and ask nicely,they'll give it to you.

I can see why many regard lime as an 'art'

More artisan than art.
I like to think of it as ditching modern techniques and adopting the old building ethics.
But then again I am ever so slightly bonkers :lol:

 

[Flyfisher]

Interesting comments.

I generally agree with 'like-for-like', of course, but what about when something unoriginal and unsympathetic is being rebuilt? It may not be too clear from the photos but the "before" chimney was built with hard bricks and a hard cement mortar. Based on some old photos, I suspect it was rebuilt in the late 30's. I suspect this was also probably about the time that the house was cloaked in cement render - but that's another story. It also capped off a second flue, which I reinstated during the rebuild. Likewise, the pot colour was changed to terracotta to match the other pots on the house, although I was careful to retain the square shape (although goodness knows if that was original).

The 'reddish' mortar must be a trick of the photo, it's not that noticeable outside. There were certainly no colour additives used! The flaunching is more representative of the overall appearance. The brickwork around the top of the chimney was specifically built that way to match the style of the chimney at the other end of the house. The main house is timber-framed and rendered and the gable wall is rendered chalk-lump, so there was no other brickwork that needed to be matched or to take a cue for the new bricks. So, faced with the need to rebuild the chimney in the first place, it seemed more sympathetic to use a soft brick rather than exactly match the existing unsympathetic materials like-for-like. All these details were discussed with the CO and documented in the LBC application, which was subsequently approved with the involvement (I understand) of English Heritage since the house is G2*.

A general repair mortar was not specified because the LBC application included quite a bit of other work, the details of which had not been finalised at the time of application. I had suggested splitting the application but the CO advised to include all necessary works. However, I wanted consent for the roof works as quickly as possible and didn't want to wait for the other works to be fully agreed, so a compromise was reached whereby the LBC was granted with a number of conditions, including the need to obtain approval of all mixes to be used in the works. So, when I started the roof works, I obtained formal approval of the mix for those works only. I'll comply with the other conditions as and when I start on the relevant works. I don't see any need to be alarmed by this - or is that the voice of inexperience?