Mortar is one of the oldest building materials, used to allow large structures to be constructed from small, easy-to-handle components such as bricks, blocks and stone. It is composed from a mixture of a fine aggregate, a binder and water. This combination creates a paste that is used in masonry construction as a bedding and adhesive to bind and fill the gaps between adjacent blocks.
In modern construction, the fine aggregate is typically sand, and the binder cement. This is known as a cement mortar. However, if lime is used as the binder, this is known as lime mortar. Rather confusingly, a small amount of lime may also be also be used in cement mortars, where it acts as a plasticiser, making the mortar slower to harden and more flexible.
Lime mortar has been used in construction since the time of the ancient Egyptians around 4,000 to 6,000 BC. It continued to be used until the 19th century, when the introduction of Portland cement led to the emergence of cement mortars, which were faster setting and had a higher compressive and flexural strength. However, lime mortar is still used today, particularly on older buildings, both as a mortar and a render.
Lime is manufactured from limestone or chalk (calcium carbonate) which is crushed and then heated in a kiln to around 1,000°C. This converts the calcium carbonate into calcium oxide (quicklime) which is reacted with water (slaked) to produce a fine powder.
The word lime is derived from the Old English lim, meaning 'sticky substance' and the root, lei relating to slime or slimy. It obtained this name because it has a very fine particle size, finer than cement, and so is able to penetrate smaller ‘holes’ in the materials it is binding. It then tends to bind them more ‘gently’, as it is more flexible and gives earlier adhesion, but then gains strength more slowly than cement mortars. Lime mortar also remains workable for longer than cement mortar, even when used with absorbent masonry, and so it can be easier to properly fill joints.
Lime mortar is also more ‘breathable’ than cement mortars, that is, it is more vapour permeable, and so is less likely to trap moisture within the masonry construction. It is also better able to accommodate moisture changes, reducing the potential of damage from salts.
|Damage caused by the use of cement mortar rather than lime mortar on a soft sandstone.|
Lime mortar has lower embodied energy than cement mortar, and its binding properties mean it is more possible to re-use masonry. It has a ‘self healing’ capability, and it tends to fail under load before masonry, and is then easier to repair than the masonry would be if it failed.
Lime mortar can me made using hydraulic lime or non-hydraulic lime:
- Hydraulic lime sets by hydration (the addition of water).
- Non-hydraulic lime sets by carbonation (through exposure to the air).
Specifications for lime are set out in BS EN 459-1:2015 Building lime. Definitions, specifications and conformity criteria.
Lime mortar can take a long time to achieve its full strength, which depending on the conditions of moisture and temperature can be many months. Pozzolans can be added to achieve a harder, faster set, and other filler materials may be added to bulk up the mix.
If work is being undertaken to a historic structure, it is important to establish what sort of mortar was used in its original construction (or what sort of mortar predominates in the existing construction) as the use of the wrong mortar can lead to deterioration of the masonry, and the properties of lime mortar and cement mortar make them incompatible and so they should not be used together.
 Related articles on Designing Buildings Wiki
- Binding agent.
- Cement mortar.
- Coal ash.
- Defects in brickwork.
- Defects in stonework.
- Hemp lime construction: A guide to building with hemp lime composites.
- High alumina cement.
- Hot-mixed mortars: the new lime revival.
- Lath and plaster.
- Lime run-off.
- Mortar analysis for specifiers.
- Portland cement.
- Types of mortar.
- Wimpole Gothic Tower conservation.
Organisations with conservation links have been collating resources on COVID-19 impacts, including Built Environment Forum Scotland (BEFS), Historic Environment Forum, The Heritage Alliance (THA), and Historic England, on cleaning surfaces.
Councils are reported to be considering taking up rarely-used executive powers to keep the planning and development system moving during the coronavirus pandemic.
Historic England's 'After a Flood' provides timely advice on how to dry walls properly and avoid further damage to the building fabric.
Context Issue 162 offers a peek into an archive of timber conservation history through the records of the practice of FWB and Mary Charles Chartered Architects.
To meet the government’s target of being carbon neutral by 2050, we must recycle, reuse and responsibly adapt our existing historic buildings, according to this year’s Heritage Counts report, so Historic England and partners are calling for a reduction in VAT rates to incentivise this more sustainable option.
Donald Insall Associates, with the help of Historic England, has completed restoration work of Moseley Road Baths, being converted for use as an arts and culture venue.
Celebrate your local ‘retired members’ and ‘successful learners’ with £500 cash prizes and 2020 Brighton School places!
The Conservation Hierarchy is a new framework developed by the University of Oxford to help construction projects achieve Biodiversity Net Gain.
Jacqueline Hughes, senior risk analyst at Equib, in pbctoday discusses how project managers for town centre developments can get their risk management strategies right.
A new paper from the Adam Smith Institute argues that the problem with the High Street has been totally misunderstood, saying that we need to reform restrictive planning rules and reject a policy of managed decline to reinvigorate our town centres.
The Whole Life Cost of Energy (WLCoE) calculator – issued by government in BETA form – is intended to help building owners and operators to understand the full financial cost of the energy their buildings use, and welcomes feedback
New research published by Historic England (HE) shows the value of heritage to England’s economy as it contributes to economic prosperity and growth through jobs in the heritage and construction sectors and from tourism.