Lime mortar

Making & using lime mortar, and pozzolanic additives.

Making lime mortar

• Use one part lime putty to four parts sharp sand. It has to be sharp sand because the grains are not smooth like ordinary builders’ sand, but have lots of sharp angles which hold on to the lime and keep the mortar together when set. This is even more important for renders. Angular (sharp) sands are generally formed by winds, and rounded sands are generally formed by water. It’s best to choose sharp sands that have a range of different particle sizes. You can test this by shaking the sand with water in a jar – as it settles it forms different layers of grain size, largest at the bottom and smallest at the top. All the sharp sand that we’ve bought in 25kg bags from builders’ merchants has passed this test easily, so it doesn’t seem that it’s something to worry about unduly.
• It is very difficult to mix the putty and sand on a board (as with cement and sand); we’ve found that the best way to mix small quantitites is in a bucket, using a spade. Put the sand into the bucket first (putty will stick to the bucket), then add the putty, and cut it up with a downward motion of the spade, then twist to mix it with the sand. Continue until you have a uniform mix (which will be a grey/beige colour depending on how dark the sand is). The longer and more thoroughly you mix it the better. You shouldn’t need to add more water, as the putty should be wet enough. In fact it’s a bad idea to add more water, as a wetter mix stands more chance of shrinking and cracking when it dries.
• Be patient when mixing – give it time to mix properly before deciding it’s too dry and needs some water added. It almost definitely doesn’t, as the more it mixes, the wetter it often becomes.
• If you are using hydraulic lime (a powder rather than a putty), mix like cement – with sand and water on a board.
• For larger quantitites use a mixer. Again put the sand in first, so that the putty doesn’t stick to the sides of the mixer.
• If the mix feels a little dry and crumbly, add a little water (but see above).
• Use one part lime putty to three parts sharp sand for pointing.
• Lime mortar can be stored for a long time – if it begins to solidify, it can be ‘knocked up’ with a little water.
• Gets better with age – less chance of shrinkage or cracking.
• Can use wood ash to make it more sticky.
• Never add cement ‘just to be sure’, as this can reduce the carbonation of the lime, and so reduce strength in the long run.
• You can buy quicklime (in 25kg sacks) and lime putty and mortar (in 25kg tubs), and all three are roughly the same price. It probably makes sense to buy lime putty instead of slaking it yourself, but it doesn’t really make sense to buy lime mortar, as it is three parts sharp sand, which is very cheap to buy from any builders’ merchants.
• You can use hydraulic lime if conditions are damp, or walls are particularly thick (hydraulic limes don’t need CO2 from the air to set). Or you can use pozzolanic additives with your non-hydraulic lime (see below).
• If you are used to using sand and cement, then you’ll be used to working quickly so that the cement doesn’t go off. You don’t have to worry about this when working with lime, as it takes so much longer to go off. So if you’re working on your own property, and don’t have a boss breathing down your neck, then relax, enjoy, take your time, and get it right.

lime putty in a 13kg tub…. pic: Mike Wye

… and mortar in a tonne bag

 Pozzolanic additives

• Named after the town of Pozzuoli in Italy where the Romans got their volcanic ash from.
• Non-hydraulic limes need CO2 from the air to set – and if deep inside a wall, it may not set properly. Pozzolans contain silica and alumina, and so the calcium can set by forming calcium silicates and aluminates instead of carbonate.
• Their porosity also allows some air in and so some carbonation can take place.
• Examples of pozzolans are: brick dust; ground bricks or tiles; crushed clay pots; pulverised fly ash (waste from coal-fired power stations); pumice (natural but very expensive). Be careful of any kind of ash though, as it may contain salts that will try to get out and damage either the mortar or the brickwork.
• Add a couple of handfuls of brick dust / pozzolan to a tub of mortar.
• Don’t add pozzolans until you’re ready to use the mortar, or it won’t keep; i.e. don’t store mortar with pozzolanic additives in, as it will set in the tub.
• Don’t need pozzolans for pointing, as it is not going to be deep enough.
• The thickness of walls on domestic buildings probably means that pozzolans are not absolutely necessary, and remember that the more pozzolans are added, the less the lime needs to take CO2 from the air, which negates the most important environmental benefit of using the lime in the first place.

Brick dust pozzolan.
pic: Ty Mawr

Using lime mortars

• This isn’t the place to explain how to lay bricks etc.  – but here are some things specific to lime:
• Damp surfaces and the bricks first – a dry surface will draw the water out of the mortar; it’s best to soak bricks in a bucket first, until the bubbles stop rising.
• Use mortar as dry as can be worked. Too much water encourages shrinkage and cracking. Harder work than cement mortars.
• Avoid frosts. Ideal time is late spring / summer.
• As lime is flexible, you don’t need to use expansion joints.
• Can hammer in ‘galleting’ stones between stones / bricks, to save on mortar.
• Make sure that the joints are not too thick, or the weight of the bricks or stones above may squeeze some mortar out. Just keep an eye on this as you go along.
• If you are adding a section, repairing or pointing an old building, add some (black) cement colour (probably around a tablespoon full in a bucket) to make sure that the new mortar doesn’t dry whiter than the old mortar.
• Rough point as you work, then brush with a rough brush c. 24 hours later to remove blobs and any surface lime. This will avoid staining if it rains (white lines down the walls); then hose down another 24 hours later.
• If it’s a sunny day, cover the wall with damp sacks to allow to dry more slowly; the slower the set the better, to achieve full carbonation.
• When pointing, point flush, and slight shrinkage will take it back a little.
• If pointing an entire building, it’s probably better to leave around 1cm around the edge of the mortar joints to be pointed flush afterwards – then you can ensure that all the pointing will be the same colour.
• Use a round-nosed pointing trowel or even a rounded kitchen knife. Joints don’t need to slope like cement mortar, as they don’t need to shed water.
• Repairing old pointing will be easy if the mortar is lime; but could be difficult if there have been some previous repointing with cement and sand. Try to get as much of it out as possible, with cold chisel and lump hammer (or could drill through the cement with a masonry drill). It’s not the end of the world if it’s not all out, as long as it’s patchy enough for there to be plenty of routes for moisture to escape from the breathable lime mortar below the cement. If the repointing is too shallow, it may fall out. As long as the joint is around twice as deep as it is high, then it should be OK.