There is no universal lime mortar specification. There is only the right material for this wall, this stone, this exposure, and this building. There are also the consequences of guessing. The use of hydraulic lime mortars should be limited to use where permanently wet conditions make using more lime-rich, air-lime impossible. Hydraulicity has an inverse relationship with porosity and permeability, therefore hydraulic lime can be utilised in situations where limited porosity and permeability are advantageous. Even then, the choice of binder should often be as weak as practically possible.

Situations might include instances where the construction of pavement adjacent to a historic building exacerbates rainwater splashback. This is common in historic towns. The salts from the roads in winter are then splashed into the porous masonry, causing significant decay. An NHL 3.5 plinth would provide limited porosity to allow the masonry below to dry out to a small extent, but limit the absorption of the salt solution so as to prevent significant decay of the stone. The plinth would function as a sacrificial component, as it will ultimately suffer the same fate as the stonework would have done, but can be replaced more easily.

 

The misuse of hydraulic lime mortars can have a significant impact on the weathering of natural stone. The north Oxfordshire area, for instance, has a substantial degree of Marlestone, a ferrigenous stone (ironstone) with a deep brown-orange colour. Villages founded on the outskirts of this region quarried both grey limestone and the Marlestone and often built in stripes. Even with the original lime mortar, the marlstone is weathers differentially to the grey limestone, highlighting the propensity for the ferrous material to oxidise, expand, and cause increased deterioration of the stone. A generic NHL3.5 specification for external repointing wouldn’t provide a sufficiently porous mortar to provide preferential moisture movement through the mortar joints, keeping the marlstone as dry as possible. Getting this specification wrong would result in moisture being forced into the stonework, thereby accelerating the deterioration.

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Natural hydraulic lime mortars should be avoided for general use in historic buildings. The requirements for lime mortars to perform, particularly in terms of moisture management and flexural strength, mean air lime should always be the first choice for mortars and plasters.

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Hot-mixed lime specification for historic building conservation

This section provides a detailed discussion of the specifications for lime mortars. The information on aggregates and mortar additions applies equally to NHL specifications, except for the use of pozzolans, which are used to impart a hydraulic component to an air lime.

 

Quicklime

For most cases, high-calcium quicklime should be used. This is except in areas of Yorkshire and the Midlands, where Dolomitic limestone is prevalent. A keen knowledge of the building vernacular and bedrock geology is essential. A 4:1 ratio by volume results in a very workable mix, and the lime expansion will provide a ratio closer to 1.6:1. If a richer ratio than 4:1 is used, the resultant mortar will be particularly prone to cracking. Kibbled lime or powdered lime can be used.

 

Sourcing Sand

General: Sands used for pointing mixes, in particular (though the rule applies to all lime mortar uses), should be washed sands. Sharp sand from builders’ merchants is generally always washed. Particular care must be taken to avoid general building sands and other unwashed sands because contaminants can cause colour leaching from the mortar. Sands should be sharp. That is, their particles should be angular and not rounded and smooth. This helps to create a workable mortar. When testing sand, it should be gathered in the hand, slightly damp, and squeezed together. The sand should retain its compressed shape, rather than collapsing and crumbling.

Thought should be given to where the sands are quarried (marine sands are available that are dredged). Whilst it is generally recommended to use local sand, the theory being that your sand selection will be closest to that which was used originally, it relies on the quarry being located in the area on the same bedrock. Some thought should also be given to the bedrock geology, as sands that are rich in clay will impart a hydraulic component to the mix (see pozzolans below).

 

Grading

Sand for a mortar mix should be well-graded, meaning it has a good range of fine and large particles. That is a general rule. The consistency of a mortar mix should reflect the joints it is required to fill. General brickwork requires a fine-course mixture, while stonework typically suits a coarse mix. Each building should be specified individually. This element of specification cannot be stated strongly enough. When using a hot-mixed mortar, in particular, cracking is a far more significant issue due to the high lime content of the mortar. Generally, the finer the sands, the higher the water retention, therefore the more severe the shrinking and cracking. A balance needs to be found between sharps and fines to achieve a workable mortar that is easy to tend to after application. Conversely, a purely sharp-sanded mortar loses its workability, making it challenging to apply.

 

Colour

The colours of the course sand grains will show up clearly if a mortar is brushed to a finish, but carefully in a manner that avoids bristle marks through the joints. In particular, larger black grains will sand out against lime. When matching existing mortar, this is a key consideration. Finer sands will have the most bearing on the mortar’s colour. Inclusion of a small portion of yellow sand, for instance, will soften a white lime mortar to a creamy hue. Great care must be taken when changing mortar colours. It is often desirable to match a replacement mortar with an existing mortar colour. In the case of existing lime mortars, these are often hundreds of years old and have seen a great deal of weathering and pollution to give them their particular hue. Whenever a coloured mixture is desired, it should be remembered that the mortar will soften naturally over the first year of carbonation, and that the mortar will subsequently undergo its own weathering and pick up its own pollution in time.

 

Hair additions

If natural hydraulic lime is specified appropriately, then mortar additions are unlikely to be required in general. In the case of hot-mixing lime mortars, there are several additions which may be required. Animal hair is an invaluable addition to mortar. Little wonder, then, that it exists in just about every historic mortar. It dramatically reduces cracking and gives the mortar added flexural strength. Generally speaking, the wider and deeper the joints to be repointed, the more hair should be added. Familiarity with lime plastering goes a long way to judging the quantity of hair. For repointing, curly hair is generally considered superior; however, straight hair can also be used. Straight hair must be cut for repointing work; its length should be equal to the joint widths to be pointed. Brick joints and other small joints should not require hair, as the mortar cracking is managed sufficiently by the limited joint size and correct grading of sand.

 

Pozzolanic Additions

Pozzolans derive their name from the Italian town Pozzuoli, where the Romans discovered volcanic ash could be added to their mortars to create a hydraulic component to the curing process. Various pozzolans can be added; the most commonly used in the UK is trass, which is indeed a crushed volcanic dust derived from Germany. Great care needs to be taken when specifying the use of a pozzolan, and the hydraulic lime cycle should be considered, as pozzolans introduce silicates and aluminates to create an initial hydraulic cure. Pozzolans work by interacting with the calcium hydroxide (slaked quicklime) to produce calcium silicate hydrates and calcium aluminate hydrates (Refer to the hydraulic lime cycle). The pozzolan is specifically an additive, meaning it is not measured out with the mortar ratios of sand and lime. The pozzolan will combine with up to 45-75% of its mass with the lime. Practically speaking, it’s helpful to assume 100%, which prevents overuse of pozzolan, which would ultimately create the same drawbacks as using an NHL. So, a specification of 5%-20% is a good margin. Assuming the use of a CL90 quicklime with a 20% pozzolan addition (by volume of binder), this would result in a quicklime mortar with 70% free lime. This is still considerably more free lime than the most lime-rich NHL2s on the market, but it has the benefit of an early set when used in wetter or colder conditions.

 

Weather

Extremes of weather are where hydraulic lime mortars have limitations, but where hot-mixed mortars excel. Conservation departments have been recommending lime work be avoided in cold temperatures (below ~10°C) for decades because of the excessive use of hydraulic mortars. A hot-mixed quicklime mortar can be successfully used in temperatures that drop below freezing at night, as long as they are sufficiently covered with dry sheeting and daytime temperatures raise into the plus figures (ideally 3°C and above). Any colder than 3°C, and factors like wind chill can cause mortars to freeze while they are being tended to. The reason hot-mixed quick lime mortar can be successfully applied in these conditions is due to the heat generated during the slaking process. Consistent freezing temperatures, like any other extreme weather condition, are best avoided. Consideration should be given to the need of quicklime mortars to have a sufficiently high humidity to carbonate, therefore if the humidity is consistently below 40%, winter working in more exposed areas should be avoided.

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Hot-mixed mortars are ideally suited for summer use, as they do not require the retention of moisture to achieve a hydraulic set. NHLs need to be kept moist for 48 hours to ensure the hydraulic component of the curing process has been completed. This means sheeting up freshly applied NHL and even misting the mortar regularly with water to keep it from drying out too fast. While hot-mixed mortars do require the presence of moisture to carbonate, this moisture can be taken from the atmosphere in the form of vapour, as long as the humidity is above 40%.

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Lime wash: Lime wash is a building finish that has fallen out of fashion dramatically. Part of this is due to the aesthetics, but part of it is due to the fact that nobody particularly wants to renew a perishable external wall covering every 5 to 10 years if they don't feel they need to. Lime washes are particularly beneficial on wind-driven elevations, as the lime wash, being moisture- and vapour-permeable, allows moisture to shed from the building effectively. It has become common practice to water down lime potty to create a limewash that resembles the thickness of yoghurt; however, lime wash should be created in the same fashion as a hot mixed water. A heat-proof container can be filled with water to approximately a third, and quick lime can then be added until the water boils. The calcium oxide will absorb and combine with the water. Additional water and plenty of mixing will result in lime potty. Lime patty should generally be ‘bedded down for a minimum of three months, which allows the calcium hydroxide crystals to become smaller, more intertwined and more evenly distributed. This results in a much more workable mortar. The lime party should be stored without an excess of water, or else when the putty comes to be used, the mixture will be too wet. The amount of additional water added to lime putty in its storage is variable, depending on the manufacturer. Often, the best putty is produced for oneself and stored. Below are some very typical examples of historic mortar from the Oxfordshire region. Generally, the mortar is applied very flush, ruled and would have subsequently been limewashed. The effect was essentially to create a very flat, flush surface, rule the joints to achieve the effect of finer stone joints, and then the wall would have been finished with a lime wash. Historic line wash can often be found at the tops of walls underneath the rainwater goods or building detailing, where the wall is slightly more protected from the weather.