Lime: introduction

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.

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).

Quicklime is different from ordinary ‘bag lime’

Quicklime is different from ordinary ‘bag lime’ that you buy from a builder’s merchants. It is calcium carbonate (limestone or chalk) which is burnt, driving off CO2 to leave calcium oxide.

Chalk is limestone made from very small white particles.

Non-hydraulic lime or quicklime is the purest form, made from rocks containing at least 95% calcium carbonate: limestone is burnt and CO2 is driven off to produce calcium oxide (CaCO3 minus CO2 leaves CaO).

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.

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

Lime rendering a straw-bale children's play house

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

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 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.

Other benefits:

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.

Quicklime being slaked in an old bath

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.

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.

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.

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

Mortar: 1 bucket of lime 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 lime 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 by hand (wearing rubber gloves).

Lime putty can be purchased in 25kg tubs like this one, or in tonne bags

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

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

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

Limewash: 1 part lime 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.

For more detailed information, see here.


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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!”

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