all about building lime

I used to be a girl who knew all about lime; the ones you quarter and stick into the top of a corona beer, the lime that goes in a caipiroska and a caipirinha. The lime that’s indispensable in Thai, Lao and Vietnamese cooking. Even the lime that can be squeezed on oysters, while sipping champagne under a sail of the Opera House and gazing out over Sydney Harbour… sigh…

But now I’m a girl who knows all about the other lime. Building lime. Calcium oxide. Quicklime. Hydraulic and Non-Hydraulic lime. Hydrated lime. Slaked lime, Burnt lime, Fat lime, Lean lime.

Before the advent of portland cement in the mid 19th century, it was lime that held together buildings made of stone and brick. While the wholesale adoption of portland throughout the world practically shelved the use of lime mortars, lime is now making something of a comeback.

These days, lime mortars are primarily used in the restoration of historic buildings; where they are compatible with the existing mortar, they blend in terms of texture and colour to the original, and the flexibility of lime accommodates the softening or weakening of old bricks and stone and does no further damage to them.

Scientific and academic interest in the history of construction history and the efficacy of modern materials has also had an influence on lime’s recovery. We now know a lot more about the chemical properties and reactions at work when using various building materials as a result of studies and testing at a scientific level. For example, due to the worldwide concern about the environment, researchers tell us that lime creates very little CO2 in production compared to portland cement, and this subsequently opens the door to its use as an eco-friendly alternative.

In my case, there are several reasons to consider using lime.

Firstly, portland is too hard for schist-stone and your mortar should never be harder than your building material. Secondly, it’s incompatible with the existing material: You should repoint like for like, if possible. The house was built entirely with a clay mortar. Clay absorbs and releases moisture, it’s highly flexible and highly permeable. Cement is the exact opposite. Using cement on the outside of a clay filled wall would potentially result in cracking of the cement pointing as the rest of the building heaves and shifts. Probably it would trap moisture inside the walls and cause saltpetre and other moisture related problems. But lime, on the other hand, is like clay, flexible and breathable.

So, where do I start? There is not just one type of lime or one standard recipe. So I’ve been trying them all.

At first, due to some translation/linguistic ‘nuances’ (dictionaries can only do so much – check out the list of words for lime in English in the 2nd para, and then times by three to get the Portuguese variations… then you add complications like the similarity of ‘hydraulic’ and ‘hydrated’ and all this results in is a blank stare from the guy at the shop) the only lime I could get my hands on was quicklime. ‘Cal vivo’ in Portuguese.

Quicklime needs to be slaked, i.e. mixed with water and then left to ‘fatten’ (ferment, say) for anything up to 6 months before it is used. Oh bummer. But fortunately there are plenty of ‘skip the rules’ DIYers on the internet offering up all manner of experimental alternatives, so I aligned myself with them to start with.

First I made up ‘hot lime’ and cement mixes; i.e. straight from the bag 1:1:3 lime: portland: sand. At this stage I’m using brickies yellow sand, not ideal for mortar but a nice colour. This is too strong a mix, and it’s a very dodgy way to use the lime, but it results in an aged patina (probably because it’s burning the stone as it reacts) that looks really good. This mix wont work for the engineer though, who’s also the ‘responsavel de obras’, the inspector. It’s too unorthodox and it will probably all fail anyway.

So then I slaked the lime for 24/48 hours. There are a few ways of slaking. Firstly you can use about 1:1 water, and this does a crazy thing of drying to a powder after giving off a lot of heat. That powder is now called hydrated lime (cal apagada). Better still is to use a 2:1-4:1 (depends on the quality of the lime – I find a 2.5:1 a bit stiff and 3.5:1 nice and workable but maybe a bit weak) and this turns into lime putty… after a really fun volcano-like eruption… stand back while watching this movie!

The third slaking method, a very traditional one taken from The Art of the Stonemason by Ian Cramb, is to mix the quicklime with damp sand, 1:3, and turn it over occasionally over 6 weeks. I’ve got this going on as well.

A word of warning though: I had been fiddling about with the quicklime for a few weeks with only casual regard for safety. One day I was turning over the sand-lime mix in the box for just for a couple of minutes wearing a t-shirt. There was no visible powder on my skin, but I washed my arms off with soap anyway. An hour later I was covered in blisters, including my shoulders – it even went through the t-shirt! Not good. But fortunately I’m not quite so stupid not to wear goggles and mask because I could’ve been blinded! Now, several weeks later, every time I go out in the sun, I get blisters. Nasty stuff.

The putty (even slaked for this ridiculously short 24 hr period) makes your mortar really nice and sticky and easy to work with. Even mixed with grey portland, (1:1:6 is the correct ratio – according to CSIRO – very reliable) over several weeks it dries to a bright white. I also did a few mixes with one part clay in the mix, clay that came out of the original wall. This resulted in a better colour, marginally, but more on clay later.

So far all of these mixes contain cement, which for new wall building will be fine, provided the mix isn’t too strong, and I can get the colour right. But for pointing old walls, this needs a pure lime and sand mix. Slaked quicklime/sand mixes are, these days, only recommended for interior pointing because it is so vulnerable to the elements while it takes years to harden. So I moved on to Hydraulic lime.

The term ‘Hydraulic’ (or ‘non-hydraulic’, as quicklime is) refers to the lime’s ability to set under water, or not. Hydraulic lime sets when combined with water, therefore it dries hard quickly, like cement does. Non-hydraulic limes stay soft initially and gradually harden over a longer time.

There are three types of Hydraulic lime, basically weak, medium and strong (or #1, #2, #3.5 and #5.) The strongest is almost as strong as cement, so it’s usually only used for building areas subject to harsh conditions such as by the seaside, or chimneys, or window sills. But what do you know, #5 seems to be all I can get here in middleofnowhereedgeoftheoworld, central portugal. The #5 is a horrible grey (3.5 is white, I believe) so I’ve done some colour experiments: This time I’m using river sand (a greyer sand with larger particles.)

What do you mean you can’t tell the difference? On the left is a straight Hydraulic:Sand mix at 1:3. Horrible colour and horrible to use. None of the nice stickiness of the putties. In the middle is 1:3 plus 5% ferrous oxide for colour. Still horrible but slight improvement. On the right is .5 hydraulic: .5 sand slaked quicklime, : 3 river sand, 10 % ferrous oxide. Getting closer, but still horrible.

So where does that leave me? Hydraulic too strong, too grey; Non-hydraulic too weak, too white. I think I have two more options.

The first one is pozzolans. They are additives that make non-hydraulic lime harden like hydraulic lime. They can be bagged powders like Brick dust, Fly Ash, or fired clays as found in Pozzolan, Italy. There are proprietary mixes like Metastar. But can you get them in Portugal? My hardware supplier has never heard of a pozzolan, and all I get from google is one academic who mentions “nationally produced” pozzolans in a paper. I’d better send her an email. Maybe she’s my new best friend.

My other option, as some of you may have been thinking, is clay. If lime can make a comeback why can’t clay? If my 5 metre tall walls are still standing after 70 years, why can’t I apply the “like for like” rule and repoint with clay? I’m not suggestion that I dig it out of my garden like the old boys did. I don’t mind buying some peace of mind in a bag. And more… there’s actually a company in the Algarve, Construdobe who produce clay mortars… it’s sounding like a solution, isn’t it? If only my engineer can be convinced…

to be continued….