Frequently Asked Questions

What are the different types of building limes?

Building limes are broadly divided into hydraulic and non-hydraulic. ‘Hydraulic’ with regard to mortar refers to setting and hardening when mixed with water.

Non-hydraulic lime mortars are ‘pure’ limes with no hydraulic components, which set solely by carbonation – reaction with carbon dioxide in the air. This is a slow process which occurs from the surface inwards, lasting up to several years depending on the thickness of the mortar and curing conditions. Non-hydraulic limes may also be mixed with pozzolans (additives which introduce hydraulic properties) to produce a mortar with hydraulic properties.

Hydraulic limes contain components which effect a hydraulic set once mixed. This hydraulic set occurs alongside the carbonation process, though at a faster rate. Natural Hydraulic Limes (NHLs) are produced from limestone containing the necessary impurities (silica, aluminium, iron) to produce the reactive hydraulic components when burned in the kiln. No additives can be added to NHLs apart from the source rock and the fuel used to burn. Hydraulic limes (HLs) are limes which contain additives to introduce a hydraulic set. These additives can be pozzolans such as brick dust, metakaolin or trass, or they may be OPC cement.

Non-hydraulic lime will produce the fattest, most workable mortar while having the lowest strength, slowest curing time and least freeze thaw resistance. Natural Hydraulic Limes range from being feebly hydraulic – NHL2 (most similar to non-hydraulic), moderately hydraulic – NHL3.5, to eminently hydraulic - NHL5, which has the highest strength, most freeze thaw resistance while being the leanest and less workable. The most suitable material will depend on the work to be done and the conditions it is being carried out in.

Non-hydraulic lime is available as a dry hydrate powder, lime putty or quicklime. Dry hydrate lime has been slaked to a powder and is ready to mix with sand and water on site. Lime putty is lime soaked in excess water then left for a period of months until it forms a putty texture. It is ideal for fine lime wash or very fine plaster finishes. Quicklime is unslaked lime (calcium oxide) straight from the kiln. When mixed with water this will react aggressively, bulking up and producing large amount of heat (referred to as hot lime or hot mortar). This produces a very rich, workable mortar. Extreme care regarding health and safety must be taken when using quicklime on site due to the aggressive, hot reaction of the mortar.

Hydraulic lime is supplied as a dry hydrate powder ready to mix with sand and water on site.  

What protection from the environment does lime mortar need?

All types of newly applied lime mortar must be protected from adverse environmental conditions both during application and afterwards.

Hot dry weather can cause rapid drying of lime mortar reducing working time, causing shrinkage cracks or failure of the hydraulic set.

Excess moisture from rain on unprotected surfaces can oversaturate fresh lime mortar, slowing curing, inhibiting carbonation and potentially leading to leaching of binder – washout of unset lime. This can result in weaker mortar, patchy finishes, friable sandy mortar and lime staining from leaching onto masonry.

Low temperatures can halt curing of lime mortar and lead to frost damage failure. Non-hydraulic limes – putty mortars, pure quicklime mortars, lime washes, require an effective minimum curing temperature of 10oC to cure. Below this temperature no carbonation will occur, it is also important to remember carbonation of non-hydraulic mortars occurs from the surface in; the surface can set while the mortar behind remains soft. Hydraulic limes such as NHLs require an effective minimum curing temperature of 5oC, below this no set will occur.

Any temperatures below 0oC can cause permanent freeze thaw damage failure to new lime mortar. Moisture in the mortar will freeze and expand, breaking the internal structure of mortar. This will cause cracking, debonding, a weak friable surface or wholesale failure of the mortar.

Irish weather conditions pose potential problems to lime mortar at all times of the year and as such there is no season when fresh lime work can be left exposed without the risk of damage before it is sufficiently cured. During warmer periods fabric such as dampened hessian can control the moisture level while protecting from excessive heat. During cold temperature sufficient protection to prevent temperatures dropping below 0oC is required. On masonry frost blankets can be used, on facades scaffolding should be fully enclosed with enough material to protect from frost, if necessary artificial heating can be introduced inside the scaffolding.

It is also important to remember that lower temperatures slow curing time, increasing the length of time protection required. During the summer months generally at least two weeks are required before a moderate natural hydraulic lime mortar is robust enough to withstand environmental conditions. During winter months this time is increased significantly possibly out to months; if the temperature remains below 5oC no curing can be expected to occur at all.

Here are further detailed guidelines for protection and curing of new lime mortars from St. Astier and guidelines from the Building Limes Forum Ireland regarding frost protection.

How long can lime be stored?

Most bagged, hydrated lime has a shelf life of 12 months once kept dry and sealed. Once opened bags should be used straight away or resealed so as to be airtight. Some premixed lime mortars with additives may have a shorter shelf life.

Lime putty will last indefinitely once kept underwater and airtight. For most uses the older the putty the better the finish will be achieved. Lime putty should however be protected from frost whilst stored.

Further Reading

A full set of comprehensive guidelines for working with lime mortars can be found in the guidelines section of St. Astier's website;