All About Lime: Hydrated? Hydraulic? Quick or Putty? Do You Know Which is Which?

Lime is a natural substance, but it is caustic and can cause burns. So make sure you are properly protected before using or slaking it. Here's the health and safety guide.
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My 200 year old barn completely repointed with lime mortar by yours truly.

Which Lime is Which?

I’ve used plenty of different kinds of lime, and when I started this gig frankly I had no idea what they were. That happens when you live on a mountain without a car on the edge of the Middle East. You take what you can find and make the best of it.

A decade down the line I’m on a different mountain, one which is situated in more lime-specific Europe where I've used pretty much every kind of lime I could find. I've also had a good long dig around in the jumbled quarry of the white wonder to clarify for those of us without a chemistry PhD what exactly is going on.

Now, I know there are plenty of experts out there just itching to add their increment to the lime knowledge tower. And plenty of non-experts dying to make some noise and confuse everyone too. But before you race to the comments section (which you are welcome to), do bear in mind, this is a post for beginners not a detailed chemistry manual. The point here is to empower people to choose/identify an appropriate lime, explain simply why, and not to kill their confidence or confuse them to the point of paralysis.

So...Which kind of lime do you need and why? And how can you possibly tell which is which?
Let’s take it from the top.
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Limestone before it's heated. Photo by Lamiot https://commons.wikimedia.org/wiki/User:Lamiot
Quicklime – aka Calcium Oxide (CaO)
The original lime. This is pure limestone which has been heated to 8/900 degrees C.

How to use quicklime:
1. Slake the quicklime in a vat of water.
2. Give it a good stir with a spade.
3. Leave it well submerged for at least two weeks. Keep an eye on it to make sure the water isn’t being used up, and refill if necessary. For the best results, slake quicklime for a month or more. THE LONGER YOU SLAKE IT THE BETTER IT GETS.

WARNING: Adding water to quicklime causes a fairly exciting bubbly reaction, and lime can burn, so you need to protect yourself with goggles and cover your skin.

Once you’ve done this, you now have something to work with. It’s called calcium hydroxide or Ca(OH)2, or lime putty.
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Slaked quicklime
Lime Putty (Calcium Hydroxide)
This is what you get when you slake quicklime. It looks like Greek/Turkish yoghurt, or cream cheese, or quark...anyway you get the picture. Lime putty is fabulicious, and you can use it in plaster, as a lime wash, or in lime mortars.

You can also buy lime putty in tubs or bags, depending on where you live. This is the easiest way to use it, because you don't have to do that whole slaking thing.

Aged/Fat/Mature Lime Putty
This is quicklime that has been slaked for a good long time (months). It's commonly agreed to be the best quality lime if you can get it and afford it.
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Lime putty from Cornish Lime UK
Hydrated Lime Powder
As mentioned before, when you slake quicklime, you get hydrated lime (aka calcium hydroxide) which is a soft creamy putty. But you can also buy hydrated lime as a bagged powder. Here the lime has gone through a special process to get it into powder form.

So, if you can't get your hands on the putty, hydrated lime powder is generally considered to be a good second best for lime wash or render. Someone will no doubt quibble with this, but I’ve taken the majority vote on it. There is one big drawback with bagged hydrated lime though: Unlike slaked lime or lime putty, powdered hydrated lime doesn't improve with age. It deteriorates.

Of course it doesn’t end there. That would be way too easy:) There are varying qualities of hydrated lime too. Here they are listed in order; CL 90 being the purest and highest quality.

Calcium Limes (The good stuff)
CL 90 S
CL 80 S
CL 70 S

Dolomitic Limes (The stuff you don't want if you can help it)
DL 85
DL 80

WARNING ABOUT S-TYPE LIME
Folks in the US are often pointed towards S-type hydrated lime. This is not what you want if you can possibly help it. It's in the dolomitic lime category, with a bunch of other crap in as well, and is an inferior product. It's gone through a double hydrating process too, so it's not much different to Portland cement.

NOTE: All the above limes are non-hydraulic, which means they set in air. For this reason, they are also called air limes. As soon as they are applied out in the open, they begin to cure.
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Hydrated lime powder - CL-90S is the best quality
Hydraulic limes are different

Hydraulic limes are made out of limestone that is not pure. To cope with the impurities, the lime is heated to a higher temperature (1000/1200 degrees). In this way, the other material in the lime (often small particles of clay) turns into something called a pozzolan (see below).

Unlike the other limes mentioned above, hydraulic limes set in water, and are more cementitious. They are less environmentally friendly, less able to regulate humidity, and are considered by the purists as a type of devils’ dust (though slightly less devilish than the ultimate pariah of S Type:)) Pragmaticians on the other hand may see them as a last resort.

You can buy hydraulic limes in various levels. NHL 5 is the most hydraulic, NHL 2 is the least. I've never really seen a situation which would warrant anything more cementitious than NHL 2, but it depends what you can find. Some people say hydraulic limes are better for limecrete flooring, others don't:)

Just to confuse people, sellers sometimes refer to hydraulic limes as "hydrated", which is irritating. If in doubt check the chemistry. If it's not PURE Calcium Hydroxide CA(OH)2 or Calcium Oxide (CaO) then it's probably hydraulic.
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Volcanic ash at Mt Etna, by Ji-Elle - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15502947
Pozzolans
These are substances you add to lime putty to make it set faster and harder. You don't in my opinion need to add these in render or lime wash. They are useful for limecretes, say for flooring, which need to be tougher.

Wood ash, brick dust, and volcanic ash are all examples of natural pozzolans.

Note: When you add a pozzolan to your lime it becomes more hydraulic.

Warning: Don’t forget, lime is caustic. It can burn the skin and leave nasty blisters. So protect yourself well when working with it.

Can’t Find a Lime You Trust?
If you’re in the sticks, or in a country that doesn’t regulate particularly carefully, then you may have no clue what you’re buying. I hear ya. I’ve been there. As Leora Light in Costa Rica lamented in our Facebook Group, “Our lime turned up in Cornflour bags, so for all I know it’s flour.” Another group member, Gautam Singh in Mumbai, was so unsure of his lime that he decided to make his own. It’s amazing, and I’ll share that process, as well as a lot more of the incredible lime mortar, tadelakt and plaster of India next time.

Further Reading:
If you want to geek out further on lime, the British Lime Association state it all pretty clearly.
http://www.britishlime.org/technical/lime_in_mortars.php

If you really want to fry your noodle, then delve into the debate on Talking Natural Homes below: Many thanks to experts Wolf Jordon, Rosey Burton, and the various others in that thread for taking the time to answer my question on lime, and inspiring this post. https://www.facebook.com/groups/naturalhomes/permalink/2371455209536372/

I also created an affordable online course which explains the whole thing in more depth, plus how to use lime to get the results you want.

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