Lime mortar, render and plaster

Advantages over cement

• by being produced at lower temperatures than cement, lime mortar requires less energy, resulting in 20% less CO₂ output.

• lime putty absorbs CO₂ in the curing process. Non-hydraulic lime absorbs nearly its own weight in CO₂, hydraulic lime, around 75% and lower.

• lime mortar can be re-cycled, unlike cement

• bricks using lime mortar can be recycled unlike the cement bonded equivalent which can only be used for hardcore.

• strong, flexible, permeable. Traditional buildings built using lime mortar move and absorb moisture. In comparison with cement mortar which is rigid, lime mortar 'moves' with the structure and so prevents masonry from cracking. By using lime mortar, expansion joints can be avoided. Likewise the imperviousness of cement mortar prevents it from absorbing water from the structure whereas lime mortar acts as a kind of 'wick', absorbing the moisture and allowing it to evaporate. By absorbing moisture, lime mortar is keeping the masonry dryer and lessening the risk of spalling.

What to use where

2 types of lime mortar: Hydraulic and Non-Hydraulic.

• Mortar and pointing in brickwork: non-hydraulic lime mortar ('fat lime') mixed with sand in a 1:3 ratio. However, note that non-hydraulic lime takes time to set. If quick construction is required use hydraulic mortar or non-hydraulic mortar with pozzolan added to speed up carbonation ('lean lime'). Pozzolan is typically brickdust, calcinced clay etc. Adding pozzolan is known as ' gauging'.

• Render: Use either hydraulic lime or non-hydraulic. Non-hydraulic lime requires an element of cement. ( Green note: the CO₂ equation here involves the balancing of the CO₂ produced in the cement making process against non-hydraulic limes greater CO₂ absorption than hydraulic lime.) Hydraulic lime will set much faster than non-hydraulic.

• Plaster: Un-gauged non-hydraulic lime mortar using well-matured lime putty and sharp and well-graded aggregate. This does not require special skills. Use non-hydraulic lime and sand in a 1:3 ratio. Coat with limewash.

The difference between hydraulic and non-hydraulic limes is that non-hydraulic mortar sets with exposure to the air as it absorbs CO₂. Hydraulic lime however relies on a complex chemical reaction between the calcium hydroxide and the impurties which is initiated by contact with water

How Lime mortar is made

To make Lime Putty mortar

1. Limestone - or Calcium Carbonate (CaCO₃)
- burnt in a lime kiln at over 800 degrees, (producing CO₂ as a bi-product)
2. Quicklime or Calcium Oxide (CaO)
- add lots of water
3. Lime putty ('non-hydraulic' or 'fat lime'), a wet mix which improves with maturity - slaked lime, Calcium Hydroxide (Ca(OH)₂)
- add aggregate to the putty
4. 'Coarse stuff' used as brick mortar / render
- sets in situ whilst absorbing CO₂
5. Set lime mortar - or Calcium Carbonate (CaCO₃)

or to make Hydrated lime mortar

- add just enough water to complete the reaction, grind down to form powder
3 'Hydrated' lime or 'Bag' lime available from builders merchants.
- add to water, mix and wait - the longer the better but at least 24 hours
4 Lime putty
- add aggregate to the putty
5. 'Coarse stuff' used as brick mortar / render
- sets in situ
6. Set lime mortar

Hydraulic Lime

Hyraulic Lime has an initial set when water is added to hydrated lime (delivered to site as a dry powder Hydrated Lime), but then absorbs carbon dioxide to harden further. The weakest Hydraulic Limes are slightly stronger and faster setting than lime putty, but slightly less breathable and flexible.

1. Limestone - or Calcium Carbonate (CaCO₃)
- burnt in a lime kiln at over 800 degrees, (producing CO₂ as a bi-product)
2. Quicklime or Calcium Oxide (CaO)
- water is added in just sufficient quantity to produce
3 Calcium hydroxide (Hydraulic Hydrate)
- add to water and aggregate to form mortar
- mortar sets quickly as a result of chemical process
4 Set lime mortar

The hydrates made in the process range in hydraulicity from feebly to eminently hydraulic depending on factors such as kiln temperature and length of time in the kiln, as well as the chemical composition of the limestone. There are three classifications of hydraulic limes, as follows:

Feebly hydraulic lime (NHL2) - less than 12% clay content making it slower to set (about 20 days in water)
Moderately hydraulic lime (NHL3.5) - 12%-18% clay content resulting in a slightly faster set (15-20 days in water)
Eminently hydraulic lime (NHL5) - up to 25% clay content providing a fast set (hours / days). These are currently imported to the UK.

Choosing the right sand

The importance of good quality sand is often overlooked: it is as important as choosing the right lime. Some general guidelines are:
• Ensure that sands are washed
• Choose sands that are not monogranular
• Sharp sands (angular) work better than soft sands.
• Coarser sands (up to 3 or 4mm) are beneficial for thick mortar joints or backing coats for rendering or plastering.