Aggregates

[Nigel Watts]

From what I have read it seems that getting the right aggregates is a critical and often neglected aspect of making good lime plaster or render.

I have tended to buy my sand from ordinary builders' merchants but the last batch of "plastering sand" I bought looked quite dodgy to me as I am sure it was full of silt.

For my Roman cement repairs I dont want to take any chances. Any suggestions? I notice that some specialist lime suppliers also sell sand. Is it best to source from them? My finished stucco needs to have a reddish hue so this is also a factor in the choice of aggregate.

 

[biffvernon]

You first need to decide what you want. Then find a merchant that can supply it. Do it in that order.

How to decide what you want? Hmmmm....

The essential property of sand is the distribution of grain sizes - the proportions of different sizes. Then there's the shape - are the grains mostly rounded or angular. And the colour, which can be modified to some extent by adding pigments.

Your average builders' merchant will just have two heaps called soft and sharp. If that's all the choice there is, for your work you might do best to buy the sharp sand and sieve it yourself through a ~3mm sieve.

Me? I just dig it up from my back garden and send it through one or other of my sieves.

 

[Penners]

I just dig it up from my back garden and send it through one or other of my sieves.

I knew there had to be at least one advantage of living on what is essentially sea-bed.

:wink:

 

[biffvernon]

Yeah, you have to sieve the oyster shells out or the plaster gets very lumpy.

 

[Pford75]

And does the 'washing' which some sand claims, simply remove the very fine silt?

 

[stephenh]

The relevant names and qualities of sands are very confusing and vary all over the country also from one end of a quarry to the other.

For limework avoid such titles as :

rendering, building, plastering, soft

Go for :

washed sharp (coarse / fine), screeding, silica

If your builders merchant comes up with words like ‘Leighton Buzzard’ and ‘Green’, ask what this means and if they are unable to explain then go to another merchant…..

My local Travis Perkins sell washed sharp in both coarse and fine - much cheaper than the specialist lime merchants.

On a local job, just finished, a mix of 3 Leighton B : 2 fine sharp : 2 of 3.5 NHL gave a good mix.

Good luck

 

[biffvernon]

And does the 'washing' which some sand claims, simply remove the very fine silt?

Yes. I can think of no good reason why this is a good idea, unless you are a sand merchant and think you can make a profit because folk think that 'washed' must be better in some way. For lime work, a wide distribution of particle sizes will produce a stronger result. For fine plastering you just want to remove the lumpy bits as they get in the way.

 

[Nigel Watts]

"Sands shall be well graded, washed, and free of clay or silt. They are to be sharp
sands 3 mm or 4 mm down to 0.075 mm with the bulk of the sand in the 1.18 mm/
0.6 / 0.3 / 0.15 range. Fine sands, or monogranular sands are not to be used. In
finishing coats the sand(s) can be finer, but must still be well graded. Samples of
sands should be submitted for agreement."

This is from the guidance for repairing Roman Cement facades in Hammersmith, probably based on if not written by Nicola Ashurst, an acknowledged author on the conservation of lime and plaster.

It would be interesting to know why this washing is thought to be important. In principe I cant see why it should be.

 

[robgil]

did i not read that dirty sand helps lime to set better? or did i pick that up wrong? i seem to remember something about people adding earth to lime mortar and plaster , although like i say i may have picked this up wrong.

 

[BMcD]

some useful info regarding sand:

http://www.naturalhydrauliclime.com/customer/main/download_binary_data.php?id=56

http://www.naturalhydrauliclime.com/customer/main/download_binary_data.php?id=102

http://www.cardigansand.co.uk/charteha.html

http://www.cardigansand.co.uk/chartehb.html

 

[MatthewC]

did i not read that dirty sand helps lime to set better? or did i pick that up wrong? i seem to remember something about people adding earth to lime mortar and plaster , although like i say i may have picked this up wrong.

That sounds like using the dirt as a pozzolan, which seems a bit imprecise.

 

[chuckey]

I have a copy of BSS 1199 and BSS 1200, specs for sharp and soft sand. They show the particle distribution setting the limits as per the Birchwood graphs. Trouble is I have not yet seen anyone stock it. Some sands such as "Medway" are very poor for particle distribution and while trying to get the right sand I actually phoned the manager of a quarry who was supplying sand to my local builders merchant, he told me not to use his product for lime!! I actually found a sand as recommended by a local lime lecturer which my merchant was able to supply, but now he won't
FWIW My rock and rubble cottage was built totally with soft sand dredged from the river Ure, so I don't think that one can get too precious about the grade of the sand as the cottage is more then 200 years old.
Frank

 

[BMcD]

Nigel Watts wrote:

It would be interesting to know why this washing is thought to be important. In principe I cant see why it should be.

The washing of sand is to remove the salts, the cause of efflorescence, this is paticularly important if the sand is from the sea. Also to remove other impurities such as loam (vegetable matter), silt (tiny particles of sand known as fines)and clay. Although a small amount of fines and clay can be useful as it acts as a plasticiser and makes the mix more workable, no more than 4-5% can be tollerated as this can cause problems such as; cracking and blowing because more water is needed to improve the spreadability and so, as the water evaporates, the plaster will shrink and crack. Also the shrinkage of the clay itself on drying out will cause cracking, especially if the clay has formed into balls. Clay and silt particles can also prevent the binder bonding to the aggregate. A simple silt test can be carried out as follows.

1. Mark on the side of a clear jar 50mm from the bottom and 100mm from the bottom, pour 50mm of water into the jar, add 1 teaspoon of salt and gradually add the sand until the sand reaches the 100mm level.

2. Close the lid and shake the jar for 1 minute, then leave to settle for 3 hours.

3. Measure the height of the aggregate and the thickness of the silt layer.

to work out the percentage of silt in the sand the following calculation is done.

Thickness of silt layer % Total height of aggregate and silt. x 100

 

[biffvernon]

We can look at the issue from various angles.

If we are trying to replicate the methods used to build period vernacular buildings, the first criterion is that the aggregate should not come from further than wheelbarrow distance. A bit tricky now that so much of the land has other stuff on it.

Lime mortars and plasters can be made with a wide range of aggregates, from zero lime - a mud mortar - to pure lime with no aggregate - limewash.
Consider how mortars work. The load is transferred by the aggregate particles, which for maximum strength, should just touch each other. The voids between the aggregate particles are filled with fine material, clay and/or lime, to stop the aggregate particles moving. If the fines are 100% clay there will be some flexibility under load as the clay particles slide past each other and individual clay mineral crystals deform. (Clay minerals are related to the mica that comes in large sheets used for stove windows - it's flexible. The same flexibility occurs on a microscopic level.) Lime forms more brittle and stronger crystals of calcite which interlock as they grow in the setting lime mortar, preventing movement of the aggregate. If the voids are filled 100% with calcite the resulting mortar will be stronger than with 100% clay. But it is perfectly satisfactory to have a mortar with any proportion of clay and lime. Assuming that the fines are well mixed, the growing calcite crystals in the setting mortar will lock clay particles into place, the mortar becoming stronger with increasing proportion of lime.

Thus the inclusion of some, or even a lot, of clay in the mortar does not matter in many domestic wall building situations where a high load strength is not needed, or even desirable as flexibility is often an advantage. Unwashed sand (except sea sand or others where salt is an issue) is quite good enough in such circumstances.

For situations where a hard surface is required, such as some plastering jobs, zero clays may give a more desirable finish and if a very smooth surface is wanted one can eliminate the aggregate altogether. Fine plaster-work is hard, brittle and no use for load bearing but good for moulded cornices and such-like. Clay platers are perfectly possible as well and can produce desirable finishes, but they are fragile.

Remember that for maximum load strength, whatever the fines are made up of, there should be just enough fines to fill the aggregate voids and no more. Excess of fines pushes the aggregate particles apart so that the load has to be carried by the much weaker calcite of clay crystals rather than the immensely strong quartz crystals of the aggregate. A deficiency of fines gives a more permeable, breathable mortar, which may in various situations be more or less desirable.

The advantage of a 'sharp' or 'well graded' sand comes from the variation in aggregate grain size with angular rather than rounded grains. The big grains are wedged together by the smaller grains and can be almost self-supporting under load, demanding relatively little work from the binding fines. The same strength of mortar can be achieved with a smaller proportion of the more expensive lime.

Generally, I think we obsess too much about mortars and plasters. For most domestic purposes it's good enough to use what can be dug up for free from within wheelbarrow range plus a bag of lime from the builder's merchant. And never, ever, use Ordinary Portland Cement.

Here endeth the lesson.

 

[Flyfisher]

Interesting stuff, and I entirely agree about the 'wheelbarrow distance' in terms of how builders almost certainly used to operate.

But, there's a big 'if' in the following statement:

If we are trying to replicate the methods used to build period vernacular buildings . . . .

If we are, then fine, use whatever materials are to hand, but be prepared to accept the variability of the results that will undoubtedly follow.

Alternatively, we could try to improve upon the historical methods by using the same basic materials but in a more refined and consistent manner. Hopefully, this will give more reliable results with greater longevity.

We've had many discussions here about the failure of lime-based works, so we can't really deny it's a trickier, less reliable material to use than OPC. Also, lime failure may not have been such a big problem historically when labour costs were far cheaper, relatively, than they are today. So rebuilding a section of failed wall may not have been such a big deal (not that they had much alternative). But these days, labour is the dominate cost and few people want to pay twice because of material failures. Especially when there is the spectre of cheap, reliable OPC peering over their shoulder.

And yes, we all know that OPC is not a functional replacement for lime because of breathability issues, but (unfortunately) it solves so many short-term issues such as speed, reliability and cost, leaving the next generation of owners to pay for the long-term damage - something that so many people on PPUK can surely identify with.

So anything that can make the use of lime quicker, cheaper (think labour costs) and more reliable must surely be a very good thing indeed.

 

[biffvernon]

historically when labour costs were far cheaper, relatively, than they are today.

Ah, but.

Modern methods externalise environmental damage. If the cost of global warming resulting from cement manufacturing was included then OPC would be a fabulously expensive product, good only for the bases of wind turbines.

As we transition to a low-carbon future we may find that labour become relatively cheap again.

 

[LadyArowana]

we may find that labour become relatively cheap again.

Not presumably in the UK where it seems a lot of people would rather collect benefits than work for less than stellar wages.

 

[Flyfisher]

Ooooh - two very tempting diversions to get our teeth into, but I'll go with Biff's point (no offence milady).

I understand your point, Biff, but that only reinforces my point that anything that can make lime use cheaper and more reliable must be a good thing.

As for the embodied energy point:

http://www.navitron.org.uk/forum/index.php?action=printpage;topic=10562.0[/url]"]
Portland cement 4.6 MJ/kg

Lime 5.3 MJ/kg

From Bath University ICE (Inventory of Carbon and Energy) database v1.6a where it appears with this note:

Lime is often chosen as an environmentally friendly material. It was therefore surprising to learn that the embodied energy of lime was slightly higher than for cement. This was observed from the respectable sample size of 39 data records. Lime is fired in the kiln to a lower temperature than cement, which is often misconceived as proof for a lower embodied energy. Yield, density, and time in the kiln are all vital parameters to total energy consumption. This is presented as a possibility for the higher embodied energy.

This, of course, takes no account of the CO2 subsequently absorbed by the curing of lime mortars, but it shows that things are rarely as simple as often presented. Anyway, even if OPC had never been invented, the worldwide building industry would still be a major contributor to carbon emissions and environmentalists would then bemoan the huge environmental costs of lime production.

 

[LadyArowana]

(no offence milady).

None taken, I was just being a grumpy Bunny and poking sticks anyway :lol:

 

[Flyfisher]

Here's a related issue to poke a stick at, if yoiu dare. :wink:
http://www.bbc.co.uk/news/magazine-11382591