Lime or cement mortar?

[meow2222]

Lime or cement mortar?

This is specific to old buildings, and walls of similar construction. These houses typically use soft bricks that break easily, often have little in the way of foundations, sometimes none, and most have no dpc.

Movement

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Skimpy foundations mean minor movement is a normal event for many old houses.

Cement is not movement friendly, and with any wall movement cement typically makes a single clean break. It has no self healing ability. Normal wall movement thus results in broken walls, which compromise the overall house structure, and require extra repair work.

Cement mortar is stronger than soft brick, so when movement occurs it is the bricks that will break rather than the cement. Broken bricks have their core exposed, and without the protection of the fireskin these bricks will usually begin to slowly deteriorate due to wet freeze cycles. Gradual erosion of the brick leaves the wall in need of many bricks being replaced. Its a shame to see walls like this, knowing that just a little more knowledge and no damage would have occurred. Theres a building near here that has about half the bricks near ground level badly decayed, and is now in need of large numbers of bricks replaced.

Lime mortar is weaker than the soft bricks, so when movement occurs it is the lime that cracks, not the bricks. This is the better option. No bricks need replacement.

When moved to breaking point, instead of forming a single break, lime tends to form lots of microcracks. Lime then reacts with the CO2 in the air to grow hard crystals across these microcracks, and thus rebonds itself. It self-heals. Lime mortar is not flexible, but it behaves as if it were in this way. Lime accomodates normal minor movement without incident.

Damp

[Colin Wilson]

Congrats on a cracking post :-)

PS: any ideas whether lime mortar is suitable for repointing a cemented wall ?

[Weatherlawyer]

You can use it but it isn't as likely to stay in place.

The colour of the sand used in cement mortar controls the colour of the mortar and the strength of the mix controls the damage it is likely to do.

If using soft bricks you would use an eight to one mix -which at the price given in the OP (and the quantities of lime needed as three to one) makes it a far cheaper alternative. It is with the overall concern of the project and the consumer's tastes that the costs may seem small.

It isn't in the same category for example as the price of a new car compared to a three year old one. You can of course use any inert admixture to either sand or cement to make mortar. And you can get a pleasant effect with different sands if shortly after the bricks are pointed, the wall is washed.

Maching it all the way through the different courses would require some skill though. Too many would-be bricklayers can't even put a wall up without a great variety of shade in the allegedly same mix.

Errors to watch out for are bricks laid upside down and the poor selection of bricks by the labourer. Each course must be laid with a few bricks from each pallet to prevent the differences in batches showing up.

Also a load of bricks should never be tipped out of a lorry. This isn't so much a problem these days with most lorries transporting them having grabs.

Too frequently the developer will not have all the bricks for an house delivered on site in one or two loads. Often this is a space or security limitation.

And these days most labourers have little or no training, with bricklayers on pricework not caring nor being paid to point out tips in such craftsmanship.

[Dave Fawthrop]

|Lime or cement mortar?

This post deserves a place in the FAQs Maybe after those who understand the problem have discussed it.

[dom]

Tiptop post on lime - and very much my practice.

My process for small quantities of lime putty - buy a new plastic dustbin with tight fitting lid, half fill with water, empty a bag of hydrated (builders) lime into it, leave for as long as possible (ideally months).

I make up lime mortar when needed, half fill bucket with SHARP sand, add a third more to the volume of lime putty by eye - mix with paddle attachment on big drill, then keep bucket in binbag for ready supply- as you say, it keeps so long as it's wet.

Might be worth saying a bit about mixing - as lime mortar is so senstive to water content - the difference between a stiff mixture and soup may be half a cup of water in a bucket. Often the moisture in the sand plus the lime putty is just about the right water content - but if the sand is a bit dry, getting that easy to work with softish butter may take a little added water - go very gently to avoid ending up with soup!

Would the OP like to add a bit about using hydraulic lime?

[The Natural Philosopher]

Yeah, lime is so bloody marvellous that when they built the London sewer they scraped it entirely in favour of Portland cement, which has stood the test of time underground in the damp.

I won;t bother to go through pickling out the things that are plain wrong, and those that are simply exaggerated through prejudice.

Suffice to say there are good reasons why Portland cement is used rather than lime, these days.

[dom]

The great advantage of portland cement is that it undergoes a very predictable/repeatable chemical set, and it's fast - making it more suitable for modern site bricklaying practice. For the same reasons large quantities of concrete can be cast with highly consistent properties.

Oh please do.

Any others than the 2 I've mentioned above?

For the DIY'er with a slower work rate on buildings originally constructed with lime mortar, there really isn't a downside. It's easier to use, it can be kept indefinitely and it's better for the building - and especially with soft bricks or limestone, it's a necessity.

[Tim Lamb]

In message , [e-mail] writes

Snip excellent post....

I think I'm safe from most of the problems. No expected movement, little evaporation and appearance not really an issue. Currently brick waste is sought for crushing as hardcore: avoids the mineral tax:-) However, I can easily shove an extra ration of hydrated lime to the mix as a half measure.

regards

[Martin Bonner]

PLEASE do. Most of the OP seemed very reasonable to me, but I would love to hear opposing views. (I have to say that the paens of praise for the visual appearance of lime mortar struck me as over-egging the pudding).

To be fair, the OP did start:

With a new build, hard-fired bricks, DPC, and extensive foundations, a lot of the arguments for lime go away.

[manatbandq]

What's the "chemical cycle" of cement?

What about the production cycle? How much CO2 is released in processing and transporting v. cement?

MBQ

[Stuart Noble]

As someone has already said, there is no mention of hydraulic lime. For example, brick dust acts as a pozzolan (as does any material that has previously been baked, volcanic ash, iron oxide dyes etc.) and this causes the mortar to set in varying degrees. This aspect seems to be shrouded in mystery (I suspect deliberately). How much of the alleged self healing property is lost, and how does it then differ from cement? I doubt the Victorians could build houses at the rate they did without some form of set taking place.

IME, if you can't protect ordinary lime mortar from the weather, shallow applications such as pointing will be washed away by the first rainfall. Hanging damp sacks all over the place doesn't seem that practical to me.

The other issue is the degree to which modern additives have improved the properties of cement. At a basic level I'm sure we're all familiar with how pva reduces cracking and doubtless there are more sophisticated products around.

There is of course a large element of brown eggs and bicycles about all this too. Right on types who want to turn the whole thing into an art form and congratulate each other on their good taste via various conservation websites.

[dom]

To me that's quite a plus. When repointing, you wait for an initial set, then give it a going over with a stiff brush. Result is an appearance very close to undamaged areas and is very forgiving of skill levels - uniform consistant results for all of us.

[dom]

Do you mean heavy rain washing out new pointing? The initial set of at least the surface should take a few hours. After that I would be surprised if it washed out to a significant degree with normal rainfall. I have heard at (at a limes seminar) of a heavy section of new/repair lathe and plaster ceiling detaching - as a consequence of poor mix and poor application. Potentially quite dangerous - it does take time (weeks/months) for a thick area of lime mortar/plaster to carbonate right through and gain it's full strength.

I don't think you can get away from the fact that cement pointing will trap moisture and lead to soft bricks spalling. It also acts as a sacraficial material around limestone, so it's the mortar that slowly erodes and not the stones.

[dom]

I don't think the lime cycle is quite carbon neutral - as (fossil fuel derived) heat is put into the kiln. However the CO2 driven off in the kiln should be exactly balanced by that absorbed on the mortar taking its final set. The fossil fuel footprint of transportation is probably equal for cement and lime. I think the larger culprit is using large amounts of concrete in construction - quick,convenient, relatively cheap - and a lot of energy consumed in its manufacture/delivery.

[Martin Bonner]

I can't remember the details, but my recollection is that the chemical changes in the production of cement release more CO2 than is taken up during its setting (so there is a net release).

Lime is heated less than cement, so it doesn't take as much fuel to produce.

Lime powder and cement are much of a muchness for transportation. Lime putty of course will be a lot more expensive to transport.

[Weatherlawyer]

What a silly post.

The OP was discusing the use of limes as a compound best suited for soft bricks.

As it happens the Romans who were responsible for the building of sewers in the first century, built with concrete materials. I don't know if that came from Pozzolan but it sounds suspiciously like it may have:

Materials which enable lime mortars to set more rapidly include ash and brick dust. Known as 'pozzolans' after the volcanic additives used by the Romans, these materials are widely found in the lime mortars used in old buildings and monuments.

Where conservation work is required, new mortars ought to match these mortars, not only to ensure continuity with the past, but also to ensure that the new work is both visually and physically compatible with the old. It is therefore important that we know more about the performance of these additives.

A simple everyday definition of 'pozzolan' could be 'a finely powdered material which can be added to lime mortar (or to Portland cement mortar) to increase durability and, in the case of lime mortars, to provide a positive set'.

A more formal definition is given by ASTM C618 as 'a siliceous or siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided form in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties'.

sewers built in Britain were not built until the Victorian era for precisely that reason. Cement is a far better material to use such places. As the bricks were of the very finest available, capable of withstanding traffic in the world's busiest cities, you can see that they chose well.

As for the Victorians building houses quickly; to build the sewers they removed thousands of squalid hosuese and dug trenches by the mile in which the sewers were laid. They then had gangs upon gangs putting the housing on top of them.

I the eras befor modern planning laws it was very easy to use skilled with semiskilled men to get the most out of both.

Such a situation is recurring these days now that there are no apprentices growing into the trades. But that is a topic for alt.politics.thatcherism.

***

One can use lime mortar in modern facework. Just remember to close the gaps up. If you look at some of the fines examples of redbrick face work still standing after a cetury or so, you may notice that they too used lime mortar but that the distances between bricks are a lot smaller than the present standards.

But the OP was not discussing modern standards you damn fool.

[Stuart Noble]

As usual the OP wasn't *discussing* anything, just delivering another sermon.

[John Stumbles]

...

Only in respect of the CO2 used in the reaction: if production takes heat then that will probably involve CO2 release (unless there are solar lime kilns? :-))

I undersdood cement production was particularly energy-intensive. Anyone know how much (or at lease how it compares with lime)?

And what is cement anyway? I thought it was at least related to lime.

I remember finding a trough of mortar outside a building site on a sunday evening. Assuming it had been left there some time I dipped my toe in it to feel how hard it had set. It hadn't: it was soft and workable. Presumably some sort of additive that made it keep for days? (I'm guessing it was a cement mortar: this was in central London, not Briansville[1]

What does fatter mean (in this context)?

Have there ever been cement ceilings? I thought the alternative to lime plaster (and laths) was plasterboard.

Nice ugly?!!

[1] Poundland? Poundbury? The new town/village on HRH's organic eco estate

[<me9]

It can be hundreds of years, I gather a survey was done of the columns in Durham Cathedral some years ago, and some of the Norman mortar in the middle of the columns was not fully cured.

I gather there are two stages to lime setting, the final (very slow) stage being back to the CaCO4 it started off as.

[meow2222]

Phew, lot of feedback on this one! I guess there is a fair bit to discuss. Maybe in time we could put together an FAQ on it, once we've talked about the many points raised.

There is a whole lot to write about lime, and I'm not trying for a book. Just wanted to put the hydrated lime mortar topic up here for us to discuss, as its one thing I havent really seen addressed much here, it is a question that comes up fairly regularly, and we dont seem to have any real concensus so far, and no FAQ on it AFAIK.

So I can add a few more points to the discussion...

Re adding lime to cement mortars, this doesnt give any of the properties of a lime mortar. Also most lime/cement mixes are liable to fail prematurely, so adding a bit of lime to cement mortar or adding a bit of cement to lime mortar are both not recommended. There has been research done on this.

The chemical cycle of cement, I dont know that much about it, only that its made from lime and clay, and takes a lot more heating energy to produce than lime.

Re higher transport costs for lime putty, 2 points. First, transport costs for ready mixed cement mortars will presumably be similar. Second, I'm doubtful that transporting lime putty is likely to become a significant part of building repair practice. As far as I can see it seems to be an excessively priced product with no real need. Bagged hydrated lime is a fraction of the price, and the logical option.

How lime mortar is fatter: Cement mortars are sand plus a thin liquid with very little adhesion while wet. Lime has more stickiness, so the mortar holds together better during working, and is easier to mould into shapes that will keep their position. Hence less chance of snots and waste.

Re lime and cement ceilings, there are plenty of cement ceilings around, either cast in situ or fibre cement board. In time we'll see how long they last, but possibly judging lime life by ceiling life is unfair in that Victorian lime ceilings are normally supported by very thin laths and nails and hung on flexible joists, quite unlike their cast cement counterparts. And it is these lath supports failing that is often the cause of lime ceilings starting to break, and that cant fairly be blamed on the lime itself.

Re rain after application, last time I used lime outdoors it rained when I was done and not a drop was washed away. (This was lime paint not mortar.) I turned the hose on a little bit of it and found only a pressurised jet (finger over hose end) would remove it, so its vulnerable at first to heavy rain, but it was fine in the more common light rain. I think the usual advice is check the forecast, and dont use it before rain. One of its downsides, which I didnt really get into the first time.

There are various lime mortar additives for use when the mortar properties need modification. Examples are pozzolans, stone, hair & other fibres, and linseed. There is also black mortar, and when my brain returns I may remember whats in it. I think there are others that get used too.

You can join in the discussion if you have something to add, or not, upto you. Not much point just whimpering.

NT