Friday, 23 July 2010

Making lime putty for building

Guest post by Paulo
(edits where I couldn't resist:)

Today a truck full of bags of quicklime arrived at Casa Felix and we began the process of filling our lime pit to make lime putty. We will use this yoghurt-type stuff to make plaster in a couple of months when (notice I said when not if) the strawbale walls are completed.
For now it needs to be mixed with water and “slaked”. This is where quicklime and water meet, and will bubble and boil and get really hot, and then mellow out. This is where ye old English drinking phrase to 'slake your thirst' comes from. Many of you
who have been to the UK will at some point probably have had a pint or two at an English pub called “The Lime Kiln” – there are hundreds scattered across the country, mainly because every town used to have one as lime was essential for building before cement came along (and ruined everything). And after mixing lime, you really feel like a drink!So what's going on here? Perhaps a quick chemistry lesson is in order.

First of all, limestone or chalk is quarried (in India they even use sea shells, after all limestone is essentially the accumulated corpses of billions of shells). Chemically, limestone is Calcium Carbonate. This material is burned to about 900 degrees C., which drives off carbon dioxide and water (not that there is a lot of water in limestone, but it happens), which creates quicklime – Calcium Oxide.

So we mix this stuff with water, it boils and fizzes, releases its heat and starts to take in the CO2 it lost from the air – when it finally completes this process it will turn back into the limestone it originally came from (or something very similar) – it will become Calcium Carbonate once more.

But this process can take a long time, and starts quickly and violently with the slaking, but then slows down. We keep the lime putty we have made – now like a runny yoghurt –
under a layer of water so that it can't race ahead of us and set before we have put it on our walls. We let it mature like a fine... um... yoghurt... for a couple of months (3 or more is best, but we might need it sooner) and then we can make plaster by mixing it with sand and fibre.

Beware however, quicklime is dangerous stuff to handle. In the old days they used to throw it off the battlements onto invading soldiers climbing up siege ladders in medieval battles, because it will blind you if it gets in your eyes. This is because it is highly reactive when it comes into contact with water – it takes in the H20 and Co2 that was driven off in the burning, and releases the heat it absorbed in that reaction. It get very hot!

Here's a diagram with less chemistry and more pictures for those of you who are more left brain inclined:

We'll use it to plaster the outside of the strawbale walls, where it will then set and turn back into calcium carbonate, giving us a strong, weatherproof, but breatheable protective layer.
OK, so back to the lime pit! The Romanian boys Petrus and Ilie had previously lined an old stone irrigation tank high up on the land with hydraulic lime mortar to seal it and stop any leaks. Paulo then made a cover for it from old roof beams and doors, to stop anyone falling in (although Magnus managed to part way through filling it up).

Magnus and Paulo then began filling the pit with water, ready to add the quicklime (you always add lime to water so it can dilute quickly and not get too hot – the other way around can get very hot and even risk an explosive reaction).

In terms of quantities: we have 1200 kgs of quicklime, and plan to add this to around 3500 kgs of water, giving a total mass of 4500 kgs or 4.5m3, which is the size of the tank. The lime is produced in Portugal and costs around 4.50 euros per 40kg sack (10 cents a kilo!).

Here's a couple of videos of the day's fun. Notice the prophetic moment 10 seconds into the first film where Magnus says “Don't step here because you might fall through” right on the spot where he later stepped and... fell through, when the panel snapped out of an old door and plunged him into the lime pit (with lime in it). Luckily, his ninja reflexes saved him, and he managed to avoid actually touching the lime – got some nasty scrapes on the leg and a banged wrist though. The elfen and safety Elf was not pleased (but managed to keep a straight face). Toast and tea made it better.

“Welcome to the Lime Pit”

“Slaking the Lime!”

or embed:

“More Slaking”


“More Slaking from a safe Distance”


Anonymous said...

Last year, I lime-washed my house (outside) with a mixture of lime with fat. It seems that's how it used to be done in the past, to make the wals waterproof. They look perfectly fine after this terribly wet winter...


Magnus said...

Fat? Like oil or animal fat? Maybe this makes it more waterproof...
Could be an interesting addition.
We want to lime plaster the outside or our walls, then limewash, but with normal lime putty / wash.
Bit of pig fat for good measure??
(does it smell?)

Anonymous said...

From what I learned, any fat is good fat...That is: use what you have at hand. I used bits of fat given for free by the butcher, that's how it used to be done locally. But I've read about using leftovers from oil mills as well (at LNEC - Laboratório Nacional de Engenharia Civil, Lisboa - they have been doing research on these traditional techniques). Also I've learned that romans added figs (!) when limewashing aqueducts...


luís duarte said...

I clicked the lime cycle pic to enlarge it but it looked just like the same. Maybe it's this PC cause it's not working properly. said...

I am doing an art fresco and I need to make lime putty just like you describe, but I will only use one 50 lb bag of hotlime (quicklime). Can I mix it in plastic Home Depot buckets a little at a time, and then cover with a layer of water and the lid? Or do I need to buy a cement washbasin from a re-used building supply store, or can I build a wooden box? What about a metal trough? I obviously don't want to start a fire... Thanks for your input! Cathy

Magnus said...

Hi Cathy,
I would suggest that you can indeed use a bucket for your 50lb bag of quicklime. I remember our teacher doing tiny mixes in old cooking pots, just mixing in the water slowly to the consistency he needed. Just make sure you wear goggles in case any of the powder or liquid jumps out! and gloves, etc. And you can use straight away. The longer it stays in there the better it will be, but it's still usable straight away... Good luck!

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