Lime: introduction

“Ancient structures built using lime, like the Pantheon and the Colosseum, have survived for centuries, often with little to no maintenance.” – Smithsonian Museum

What is lime?

Lime is a traditional and environmentally-friendly building material that was largely replaced by cement during the 20th Century, but is now coming back into fashion.

Quicklime is produced when calcium carbonate (limestone or chalk) is burnt, driving off CO2 to leave calcium oxide.

Various types of lime are used in building as mortars, renders, plasters, slurries and washes. All are made from limestone, which is a sedimentary rock made from the dead bodies of sea creatures that produce calcium carbonate (coral, shellfish, some planktons). Most limestone was laid down in the Cretaceous period (60-150 million years ago).

Chalk is limestone made from very small white particles.

Slaking quicklime to make lime putty.

The lime cycle

  • Limestone is calcium carbonate – CaCO3
  • Limestone is burnt in kilns. CO2 is driven off to produce calcium oxide (CaCO3 minus CO2 leaves CaO, or quicklime)
  • Quicklime is slaked – i.e. water is added. CaO plus H2O produces Ca(OH)2, or calcium hydroxide (lime putty)
  • Lime putty absorbs CO2 from the air to form calcium carbonate (CaCO3) again

If you ask for lime at a builders’ merchants, you’ll probably be given this – hyrdrated lime, or bag lime. It’s pre-slaked with a precise amount of water, and it’s added to cement mortars to increase workability, flexibility, and to increase drying time to avoid cracking.

Types of lime

Non-hydraulic lime or quicklime is the purest form, made from rocks containing at least 95% calcium carbonate.

Hydrated lime or ‘bag lime’ from builders’ merchants is calcium oxide slaked with a precise amount of water, which is driven off by the heat of the reaction, leaving a powder.

Hydraulic lime is produced from limestone containing clay, and has the added benefit of being able to set underwater.

Here’s more detailed information on the different types of lime.

From the Romans to the mid-18th century, cement was lime plus volcanic ash or other additives. Modern Portland cements and other alternatives to lime date from around WW1.

Fascinating demonstration of the lime cycle. Snail shells (calcium carbonate – CaCO3) are burnt and carbon dioxide is driven off, leaving calcium oxide (CaO, or quicklime). Then water is added to ‘slake’ it, which produces calcium hydroxide, Ca(OH)2. Exposed to the air, it sets, or carbonates, by re-absorbing CO2 from the air – i.e. it becomes calcium carbonate again.

What are the benefits of lime?

Environmental benefits

Carbon neutral: Lime, like cement, gives off CO2 (the main greenhouse gas) during its manufacture. However, it re-absorbs CO2 when it sets, and cement does not.

It is recyclable and biodegradable.

Limestone is burnt at around 900°C compared to around 1300°C for cement. This saves on fuel consumption and emissions of pollution and greenhouse gases.

CO2 emissions in the manufacture of lime are 20% less than for cement.

It is less dense than cement, which saves on transport fuel.

Lime rendering a straw-bale children’s play house

Lime rendering a straw-bale children’s play house.

Lime mortars allow bricks to be recycled as you can get the mortar off, unlike cement.

Cements contain heavy metals which are put into the air on burning: lime doesn’t.

It is an important part of any ‘natural house’ – involving timber, straw-bales, lime and earth, all of which are natural, healthy and biodegradable.

Here’s more on the environmental and health benefits of lime.

Slaking quicklime in an old bathtub

Quicklime being slaked in an old bath: the lime must be added to the water and not the other way round, as it could cause an explosion. The ratio of water:lime is 3:1. To ensure that all the lime is slaked, the mixture is raked continuously for 10 minutes. There is a violent reaction, and heat and steam is given off. When it is cool, the ‘lime putty’ can be stored in plastic buckets indefinitely as long as there is a layer of water on top.

Benefits for buildings

It is breathable, so any water that enters a structure through a crack, can escape. This isn’t the case with cement.

It is soft and flexible, so if a building moves slightly it won’t crack like cement, and let water in.

With cement mortars, the only way moisture can escape is through the brick, which can begin to erode away.

Here’s more on the benefits of lime for buildings.

Excellent demonstration of lime plastering.

What can I do?

It’s interesting to go through whole process, and to slake your own quicklime. It can be dangerous though, as a lot of heat is given off in the reaction. Make sure you understand the health & safety implications.

After slaking your quicklime, and allowing it to cool, you’re left with lime putty, which is the basic constituent of lime mortar, render, plaster and limewash.

Lime putty in a 25kg tub

Lime putty can be purchased in 25kg tubs like this one, or in tonne bags, delivered to your site.

Quicklime is a cost-effective way to make lime putty if you slake it yourself – much cheaper than cement.

Mortar: 1 bucket of putty to 4 of sharp sand. The older the mortar the better – it can be kept in airtight bags, and ‘knocked up’ when needed.

Exterior render: 1 part putty to 3 parts sharp sand. Ideally, spray the wall with a weak limewash the day before to provide a key. 2 coats are applied with a trowel or (maybe on a straw-bale wall) by hand (wearing rubber gloves).

How to point a wall using lime mortar.

Interior plaster: first coat 1 part putty, 3 parts sharp sand, plus horse-hair, to bind the plaster. 2nd. coat 1 part putty, 3 parts silver sand (washed and finer), with horse-hair again, cut into 20mm lengths.

Slurry: 1 part putty, 1 part sharp sand. Paint on with a thick paintbrush. Cheap, wonderful texture, will cover anything.

Lime render can also be applied with a spray, like this one.

Limewash: 1 part putty, 2 parts water. Can add pigments. Can apply up to 6 coats (one a day) – coats of limewash can be applied very quickly. All lime products need to be applied to a moist surface.

Here’s a lot more detailed information.

Whilst you’re here, why not take a look at the other 30+ shelter topics available? And don’t forget to visit our main topics page to explore over 200 aspects of low-impact living and our homepage to learn more about why we do what we do.

The specialist(s) below will respond to queries on this topic. Please comment in the box at the bottom of the page.

Phil Christopher is an environmentalist, builder and trainer and he believes in making a low-impact lifestyle an attractive possibility for everyone. Phil runs Huff and Puff Construction, specialising in straw bale and sustainable building, training and design. He’s a passionate advocate of earthen and lime plasters, stating “It’s the part everyone loves doing, even if they didn’t think they would!”

We'd love to hear your comments, tips and advice on this topic, and if you post a query, we'll try to get a specialist in our network to answer it for you.