Natural Hydraulic Lime Information and Sales

*Secil NHL from £7.20/25 kg*

Prices excludes VAT and delivery

 

Types of building limes
Natural Hydraulic Lime (NHL) The base lime powder to mix with customers own sand
Lime putty (Calcium hydroxide) The original form of lime that sets once dry
Quicklime (Calcium Oxide) Lump lime is the reactive material created when the limestone is heated
Formulated lime What is this?
Hydraulic lime This isn’t the same as a naturally hydraulic lime
Hydrated lime Builders bagged lime that is inferior and rarely used
Working with natural hydraulic limes
Setting times How long will your lime take to set and cure?
Mixing NHL mortars This is straight forward but needs to be got right
When to add hair to lime mortars This can be very important for some work
Pointing with lime mortar Select the correct lime mortar and employ correct techniques
Rendering with hydraulic lime Find an example specification for rendering with NHL mortar
Bedding with NHL Building a wall? Find out the techniques and mixes to use.
Painting hydraulic lime It is very important to use compatible paints with lime products, here are some options
What is lime plaster? How is this defined and which mix to use?
What is limewash? Used for thousands of years, we take the mystery out of limewash
Storing hydraulic limes Hydraulic limes have limited life, if stored well, this limit is extended
Training courses Learn both theory and practical on our one day courses
Limecrete A flexible and breathable floor can be important for traditional buildings
Other stuff
Insulating lime renders Why not consider reducing your heating bills when rendering or plastering?
Why use lime on old buildings? Here are some compelling reasons to use lime
What is wrong with cement mortar? Here are some compelling reasons not to use cement
When should I use lime putty and when should I use NHL? Confused? Read this section
What is slaking? An important process and a common term explained
Pozzolans Making fat limes hydraulic, the Roman way
Harling coat, scat or splatter dash Whatever you call it, this is essential for some applications
How much does hydraulic lime cost? Find out the best value brands and how to purchase it.

Natural hydraulic limes are made from limestone that contains impurities such as clay or silicates. Unlike lime putty which is non-hydraulic lime, NHLs can set in damp conditions, indeed they require water for a minimum period of around 72 hours to gain maximum strength. They also have some free lime available for carbonation. There are three European classifications NHL 2, NHL3.5 and NHL5 based on the compressive strength of laboratory mortars after 28 days. These are often somewhat misleadingly termed feebly hydraulic, moderately hydraulic and eminently hydraulic. We generally recommend the use of NHLs where the need for breathability and lower strength is outweighed by the desire for an earlier and harder set such as working on bedding hard masonry, wall copings, chimneys and slate floors. In some circumstances hydraulic lime mortars can be used for rendering or plastering.

The strength of a hydraulic lime mortar (HLM) varies depending on the manufacturer of the hydraulic lime as some NHL will be high in the band or low. HLM will also vary depending on the ratio of lime binder to aggregate and the type of aggregate or sand used.

Natural Hydraulic Lime (NHL) For prices of NHL click and more information click here

Secil NHL 3.5NHL or Natural Hydraulic Lime, comes from limestone that has natural impurities of clay and other minerals, the amount of impurities within it determines how hard it will set. NHL works by setting in the presence of water, hence the term Hydraulic: Natural hydraulic lime powders come in 3 European grades:

GRADE COMPRESSIVE STRENGTH BANDS (MPa)@28 days EXAMPLES OF USE
NHL2 >2 to <7 Pointing internally or with soft masonry, plastering
NHL3.5 >3.5 to <10 Bedding, pointing
NHL5 >5 to <15 Flooring, below DPC or chimney flaunchings

NHL’s can be used when speed is essential as it sets much quicker than a Lime Putty based mortar. You must be aware that it is less flexible and not as breathable as a Lime Putty Mortar, these attributes are reduced as the NHLs get stronger. For strawbale or historic buildings with soft stone/brickwork then a Lime Putty Mortar is better or a weak mix of NHL.

The ratios of sand : NHL vary depending on what job you are doing, you can generally use a 3:1 mix for most jobs, but for floor screeds you would use a 2:1 mix.

Hydraulic lime

Hydraulic lime (HL); Consists of lime and other materials such as cement, blast furnace slag, limestone filler and other materials that react to harden the mortar. The additives do not have to be declared. This is legally different to a natural hydraulic lime.

Formulated lime

Formulated lime (FL): Formulated lime consists of hydrated lime and/or natural hydraulic lime with added hydraulic or pozzolanic material. Inclusion of any cement or cement clinker must be declared and a limited percentage. This is legally different to a natural hydraulic lime and formulated lime.

Lime putty

Lime PuttyLime Putty is the product that has historically been used as the binder in lime mortars. It has many names, and this sometimes causes issues, for example it can be known as Non Hydraulic Lime, Fat Lime or Air Dried Lime. It is made when quicklime is slaked with enough water to make a liquid and then left to mature for 3 months+ into a cottage cheese like consistency. Lime Putty cures by carbonation with the air once dry, this is why it can be kept for years in an air tight environment.

Lime Putty naturally forces water to the surface of the container that you are maturing it in so it forms its own air tight barrier. Lime putty gets better as it gets older as it has had time to fully slake and force out all of the excess moisture that it has gained from the slaking process. Lime Putty based mortars take longer to set than Natural Hydraulic Limes and cement, this is because it carbonates at 1mm a month, this can be altered by gauging it with pozzolans which we will talk about later.

Hydrated Lime

Often confused with natural hydraulic lime due to similarity of name and also being a powder but is not to be used for the same applications. Hydrated Lime is created when quicklime is added to just enough water to slake it into a powder. It is a lesser form of lime putty, it is usually used as an additive in cement as a plasticiser, and it should never be used as a binder in its own right as it is not strong enough.

Quicklime

Calcium oxide, or quicklime, is also known as lump lime This is the raw material that is used to make lime putty. Quicklime is made by burning limestone or chalk in a kiln. This drives carbon dioxide into the atmosphere, leaving a very reactive material, calcium oxide. It is made in a range of sizes from lumps down to very fine powder.

Hydraulic lime mortars (HLM) and their applications

Please use the following tables to decide on which strength and mix hydraulic lime mortar is appropriate for your project.

Building element
Hydraulic Lime Mortar Designation
Internal walls HLM 0.5
External walls HLM 0.5 – 2.5
Facing to solid construction HLM 1.0 – 2.5
Walls close to/below ground HLM 2.5 – 3.5
Parapets, sills, lintels and cornices HLM 2.5 – 3.5
Copings and cappings HLM 2.5 – 5.0
Chimneys HLM 3.5 – 5.0
Earth retaining walls HLM 3.5 – 5.0
Masonry below water level HLM 5.0

 

HLM Designation
NHL2 lime:sandby volume NHL3.5 lime:sandby volume NHL5 lime:sandby volume
Mean compressivestrength (MPa @ 91 days)
HLM 5
1:2
5.0
HLM 3.5
1:2.5
3.5
HLM 2.5
1:2
1:3
2.5
HLM 1
1:2
1:3
1.0
HLM 0.5
1:3
0.5

Mixing natural hydraulic lime mortars

The Hanson technical department have produced the following useful guide to which we have added our own images:

It is essential that the lime is uniformly dispersed and that any fine agglomerations are broken down. The time of mixing will be controlled by the efficiency of the mixer. Roller-pan mixers and screed mixers have the most efficient action but simple tilting-drum cement mortar mixers can be used if a longer mixing time is allowed. If the job is sufficiently large use a mixer with a capacity for a full bag of lime. The following sequence will be suitable for a tilting-drum mixer.

When mixing wear protective goggles and waterproof gloves.

• Introduce half of the sand and add all of the lime, mix well for 2 to 5 minutes until a uniform colour is achieved.

• Stop the mixer and isolate the drive. Scrape down any material adhering to the back. Add the remaining sand and mix again for 2 to 5 minutes to get uniform dispersion.

• Continue mixing adding water slowly over at least 10 minutes and giving plenty of time for water to be fully incorporated. The mortar should be more like a dough than a slurry and the less water added to achieve this, the better the mortar performance will be.

• The longer the final mixing time the more workable (fatter) the mortar will be. Workability will be improved by allowing mixed mortar to stand for 15 minutes or longer before re-mixing for a further 5 minutes. (In hot weather do not over-mix as water will be lost through evaporation).

Admixtures

Admixtures may be used with natural hydraulic lime mortars, subject to any limitations imposed by the job specification. In particular the use of air-entraining admixture in mortars and renders exposed to severe frost can be particularly beneficial. It is recommended that trial mixes are produced to establish optimum dosage consistent with the required strength.

Additions

Gauging hydraulic limes is not normally required although addition of pozzolanic materials can improve the hydraulic activity and performance in some applications of natural hydraulic lime mortars. Materials such as traditionally used crushed brick, Hanson BS EN 450 Fly Ash, ground granulated blast furnace slag(GGBS) or metakaolin(Argical) may be used to increase the mortar strength designation. Pozzolans are also sometimes added to NHL mortars if there is a concern that the NHL powder may have lost some strength due to being in storage for a long time.

Addition of Hydrated Lime or Lime Putty will improve the mix plastic properties but reduce the mortar strength designation.

It is recommended that trial mixes be produced to establish the optimum properties for a particular application.

Pointing with lime mortar Click here for a lime pointing pdf guide

Whether using a natural hydraulic lime or a lime putty based mortar, the application techniques are identical.Carefully choose which lime is most appropriate for the situation, selecting a weaker mix for a weaker, more porous masonry. If not purchasing a premixed mortar then next select the correct sands for texture and colour as required.

Generally, NHL2 or air dried lime putty mortars are employed internally with either NHL2, 3.5 or lime putty with pozzolan additives tend to be used externally.

Preparation

Any existing defective pointing must be raked out to a depth usually equal to twice the width of the joint, but generally not less than 20mm. The back of the joint should be roughly square in profile. Plugging chisels ensure that the stone or bricks aren’t forced apart. Never use an angle grinder or similar type of equipment that may destroy the masonry.

Damping

The joints must be dampened, with enough time left for the stone or brick faces to dry to prevent smearing. The mortar should be as dry as it is practicable to point with. This allows maximum compaction in the joint, reduces shrinkage cracking and reduces the tendency to smear on the stone faces.

Premixing

All lime mortars benefit from being premixed before being used but with a lime mortar based on a natural hydraulic lime powder it should be around 30 minutes before required and and a lime putty mortar it should be a few days before application. This is unnecessary when purchased premixed When ready to carry out the work, the mortar is then “knocked up” (agitated/mixed) again immediately prior to use to plasticise them – this helps to reduce shrinkage in the mortar. Any pozzolan additive should be mixed in at the final knocking-up stage and not before.

Selecting the mortar

It is very important to select the correct grade of NHL and then mix the correct ratio of sand to NHL in order that the strength and vapour permeability is appropriate for the masonry. Some NHL3.5 mixes have proved too strong for soft sandstone or soft bricks. We would never use NHL5 for pointing and would tend to generally mix a mortar based on 3 parts sharp sand and 1 part NHL3.5 by volume. This would change to NHL2 or a lime putty mix if the masonry is very soft.

Pointing

Start at the top of a wall to allow for cleaning up and spraying to continue. Use a pointing key or metal spatula and force the mortar in from a hawk. Joints deeper than 20mm will need an initial dubbing out as shrinkage can occur otherwise. Finish flush or rebate a little if the joints have widened with age or for personal preference as rebating highlights the stone more.

Brushing

When the mortar is “green hard” (firm enough to brush without smearing but still maleable enough to work), brush or tamp the joints with a churn brush to enhance the aggregate and give a coarser texture to the pointing. During this process, it is common to throw a dry indigenous soil at the soft lime mortar to pre-weather and allow it to blend in to the wall as a fresh, bright lime mortar is not always desired.

Protection

External pointing should be mist sprayed to control drying and protected from direct sun and wind. In winter it should be protected from rain and frost. Hessian cloth is recommended.

Quantities

20kg of lime putty mortar will point 2-3 square metres of average sized stonework or 1- 1.5 square metres of brickwork based on a 10mm joint and 20mm depth.

Safety

Limes are caustic. Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels. Our advice and information are given in good faith. It’s important that users satisfy themselves that they’ve chosen an appropriate product and have a suitably skilled workforce.

Time of year

Please note that great care should be taken not to be applied too late in the year or too soon in spring or else frost damage may occur. It is important to prevent frost crystals forming within the mortar soon after application. The ultimate hardening process takes up to a month for each millimetre of thickness. Therefore it may take 20 months before mortar has carbonated to a depth of 20mm.

It is recommended that trial mixes be produced to establish the optimum properties for a particular application.

Please contact us full more guidance and information.

Rendering with an NHL mortar

The following is an example of a rendering specification that could be suitable for many situations but you should be sure that it is suitable for your own particular project.

1. Ensure that appropriate scaffolding is in place and the worksite safe for workers and public.

2. Take off the existing render, except any existing sound lime mortars, taking care not to damage the structure. Look out for very thick patches of render that are effectively load bearing. It may be preferable to render on top rather than risk rebuilding an area.

3. Dub out any deep holes in the wall with a haired lime mortar, rebuilding defects with cob blocks, bricks or stone as appropriate.

4. Treat wooden lintels with preservative and counter batten with oak lath if rendering over them.

5. Apply one hand harled coat of NHL3.5 mortar mixed to a ratio of 2:1 of lime/sand to provide a key to the wall. This is not always required and depends on the state and size of stone.

6. Apply sufficient coats of haired mortar to smooth the contours of the wall. With a suitable animal hair in the mortar coats can be applied up to 12-15mm thick. The hair reduces any slumping whilst applying and shrinkage cracking whilst curing. Each backing is keyed with a scratch comb. The mortar should be NHL3.5 mixed to a ratio of around 2.5:1 of lime/sand. Each subsequent coat should be a weaker mix or thinner than the previous coat to encourage moisture to be drawn out of the wall.

7. Apply a top coat of floated or hand harled lime mortar as desired. The mix should be NHL3.5 mixed at 2.5:1 or 3:1 with sand and applied at 6-7mm for a float coat or 3mm for a harled coat.

Damping & curing

It is very important to control suction from the wall by light spraying with water half an hour before applying each coat (especially cob and porous brick) and in warm weather it will be necessary to spray each coat afterwards. Whatever the season, protect each render coat during the curing process from all the elements such as hot drying wind, strong sun, rain and don’t apply in temperatures below 5 degrees centigrade or if there’s a risk of frost. A heavy cloth such as hessian sheeting will provide a suitable physical barrier and should be left in place as long as required.

Curing

All coats need to be given at least a few days to harden before subsequent coats are applied. To test whether a coat is ‘green hard’ the surface should be resistant to a fingernail. Many factors will influence the timing such as the season, exposure of wall and the thickness of the coat but it’s normal to expect a couple of days for the harled coat to harden and perhaps 4- 7 days for each of the thicker coats.

Quantities

for the Example Specification per square metre

one 3mm scat coat of NHL3.5 unhaired mortar at 2:1 ~ 6kg/m2

one 15mm scratch coat of NHL3.5 haired mortar at 2.5:1 ~ 30kg/m2

one 7mm floated top coat of NHL3.5 unhaired mortar at 2.5:1 ~ 14kg/m2 or repeat the first scat coat for a rough cast finish.

Time of year

Please note that great care should be taken not to be applied too late in the year or too soon in spring or else frost damage may occur. It is important to prevent frost crystals forming within the mortar soon after application. The ultimate hardening process takes up to a month for each millimetre of thickness. Therefore it may take 20 months before mortar has carbonated to a depth of 20mm.

Please contact us full more guidance and information.

Training

We offer a one day practical course in using lime in renovation and we also sell a training DVD based on the course.

Safety

Limes are caustic. Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels. Our advice and information are given in good faith. It’s important that users satisfy themselves that they’ve chosen an appropriate product and have a suitably skilled workforce.

Bedding/rebuilding in stone with lime mortar

Rebuilding and repairing with lime mortars offers a number of advantages:

  • they will match the existing structure for porosity and density, allowing moisture to move in a similar way
  • they can accommodate general movement better than a hard mortar
  • soluble salts will be less likely to crystallise in the stone or brick faces
  • they will match existing walls aesthetically

Before starting any work, always check that the building is not listed, if it is contact the local authority conservation officer to discuss the project.

Preparation

Try to select stone from a local quarry to match the existing. Take a sample along to the quarry if you’re not sure. You can usually get two size ranges; 4″-6″ and 6″ – 9″. For the bedding mortar select an appropriate strength for your masonry, a regular mix would be NHL3.5 mixed at 1 part lime to 2.5 or 3 parts sharp wash pit sand. The sand should be well graded down to fine. The largest size particle is ideally about 1/3rd the thickness of the bed to be laid, so for a 10mm bed, 3mm is ideal and is also a good maximum size in general to have for an NHL mortar.

Building

If you’re a novice, position a stone dry first to make sure it looks right and you have got the best face showing. Stagger the vertical joints so there isn’t a vertical joint running continuously up the wall

Mixing

See mixing hydraulic limes section on this page. Natural hydraulic limes (NHL) benefit from premixing by an hour then mixing again just prior to use.

Application

Use a mortar bed just thick enough to spread the load evenly, finishing just beyond the front face and then trimming flush with the edge of the gauging trowel. Use a through stone that can tie together the entire thickness of the wall or thereabouts, one every square metre of wall face, pinning the wall together from both faces. The mortar shouldn’t dry out too quickly – protect from sun, wind and rain with damp hessian cloth. Protect from rain if necessary. Build up to a maximum of 1 metre high at a time and then let the lime mortar cure for 2 to 3 days. When ‘green hard’, the joints can be brushed with a stiff brush to expose the aggregate.

Time of year

Please note that great care should be taken not to be applied too late in the year or too soon in spring or else frost damage may occur. It is important to prevent frost crystals forming within the mortar soon after application. The ultimate hardening process takes up to a month for each millimetre of thickness. Therefore it may take 20 months before mortar has carbonated to a depth of 20mm.

Please contact us full more guidance and information.

Safety

Limes are caustic. Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels.

Setting/curing times

Lime putty has to dry through suction from the substrate and evaporation before it can carbonate. Carbonation may take many years before fully completed and fully hardened. Any lime with hydraulic qualities, be it formulated, natural or ‘hydraulic’ will set in the presence of water and doesn’t need to dry before hardening.

All lime mortars benefit from protection from drying rapidly or frost crystals forming in them before fully cured. Hessian is an ideal material to do this.

For curing to complete effectively, it is important that moisture is kept in any lime mix for at least 72 hours before being allowed to dry out slowly. Premature drying will result in a feeble mortar or plaster. Saturated substrate or low temperatures will result in extended curing times.

Repointing an old wall will usually result in rapid hardening due to the likely suction from old dry masonry and mortar and it may be difficult to keep it moist for as long as is ideal and can harden in a day or two. Pointing however, is not structural and will rarely be a major issue if weak and can be a positive asset as it helps to ensure that the pointing is weaker than the masonry, therefore protects the masonry by encouraging moisture egress via the pointing.

Bedding mortars may take a week or more to get significant strength.

Rendering and plastering layers are best applied onto ‘green hard’, recently applied coats as this controls the suction and helps create a good bond. If previous applications are allowed to fully dry, this can make application of new layers difficult to control and work. Our lightweight Secil ecoCORK insulating render can be applied on to previous layers more quickly than normal lime renders or lime plasters.

Assuming applications onto green layers, the following tables shows potential build-ups and example timings:

Internal lime plastering
Timing
Application
day 1 scat, harled coat – if required
day 3 scratch coat
day 10 float coat
day 14 2x skim coats
day 18 1st limewash coat
day 19 2nd limewash coat
day 20 3rd limewash coat
day 21 4th limewash coat

 

External lime rendering
Timing
Application
day 1 scat, harled coat – if required
day 3 scratch coat
day 10 float coat
day 17 1st limewash coat
day 18 2nd limewash coat
day 19 3rd limewash coat
day 20 4th limewash coat

It is possible that the scratch coat and float coat might need around an extra week per coat in cold and damp conditions.

External lime rendering with Secil ecoCORK
Timing
Application
day 1 scat, harled Consolidation mortar coat
day 2 ecoCORK scratch coat with 4mm fibre glass mesh
day 4 or 5 ecoCORK float coat
day 6 or 7 finishing render coat
day 12 1st ‘Whitepeak’ limewash coat
day 13 2nd ‘Whitepeak’ limewash coat
day 14 3rd ‘Whitepeak’ limewash coat
day 15 4th ‘Whitepeak’ limewash coat

N.B. If a silicate mineral paint is to be used instead of limewash, leave 28 days after applying the Finishing Render before painting

Why use lime on old buildings?

Before this century building techniques and materials were very different from those employed today. Traditional properties need to “breathe” to allow moisture inherent in a solid wall construction without a damp proof course to evaporate from the external stonework or render. Many old buildings are constructed from materials such as brick, cob and stone which are relatively porous and often of lower strength. Lime mortars were normally used for bedding and plastering. Lime mortar is a relatively softer mortar and therefore it is able to withstand a certain amount of movement (without cracking) that comes with settlement and seasonal changes in ground conditions. Lime mortar is porous and allows moisture to evaporate, helping to keep a building free of damp.

What is wrong with cement mortar?

Cement_pointing_longwell-green-bristol2-160Apart from the adverse effect it has on the environment in general, cement mortar is usually hard, brittle and less porous than lime mortar. It often contains additives to make it sometimes completely waterproof and is damaging to traditional buildings for several reasons. Cement mortar is often harder than old bricks, cob or some types of stone, therefore when movement occurs it may damage these softer traditional construction materials. Hard cement mortar can trap moisture behind it causing damage to the structure and encourages ground water to rise up a solid wall by capillary action. Trapped water in the wall can cause poor insulation, decay and crumbling. In severe cases a cob wall can fail. The brick, cob or stone can also be subject to frost damage if moisture levels are to high.

When should I use a natural hydraulic lime or non-hydraulic lime?

Natural hydraulic limes set even when wet and come in a range of strengths. They are useful for building with stone or brick where the earlier set may speed up construction. You can also add a pozzolan such as Argical to a lime putty mortar. We suggest that for external bedding and pointing late in the year it would be better to use natural hydraulic lime. It is capable of a faster initial set in cold weather. For most external rendering and internal plastering jobs, the fattiness of lime putty makes a superior mortar that allows coarser sands and thicker coats to be applied without shrinkage.

If the budget is very tight and there are no compelling practical reasons to go for lime putty then a Natural Hydraulic Lime binder mixed with a carefully selected locally sourced sand invariably works out less cost.

If you are a total novice to lime work, it is usually best to use a premixed product to minimise the potential issues that could arise. Premixed lime putty mortars/plasters have an infinite shelf life and need to be kept free from drying out until used. Premixed hydraulic lime mortars (HLM) have a shelf life limited in part to how it is stored and needs to be kept dry until used.

What is slaking?

Slaking-Gary-019-160It is the process for making lime putty. Quick lime is added to water and a chemical reaction occurs resulting in the release of a large amount of heat from the quick lime creating a boiling liquid. The liquid is drained off through a sieve into a settling tank.

Chemically the calcium oxide is converted into calcium hydroxide. The longer the slaked lime is matured in the tank the better the lime putty. We leave ours for around 4 months before mixing with sands to create lime putty plasters and mortars.

What sands are suitable?

For lime putty mortars, the sand used for building, pointing and backing coats of render and plaster should be a washed sharp coarse sand. Use a sharp sand to BS882 which is free of vegetable matter, clay and salts. For finishing coats of internal plaster use a very fine sand with the lime putty or NHL2.

How long can lime mortar be stored?

Premixed lime putty mortars can be stored indefinitely if sealed from the atmosphere and safe from frost in dumpy bags or sealed tubs. Mortars with hair teased in will gradually lose the hair as it dissolves in an alkaline wet mortar and you will end up with an unhaired mortar. Mortar left for many months will take more “knocking up” to plasticise the lime mortar. Hydraulic lime mortars, whether NHLs, Formulated have a shelf life and should be stored dry and sealed from the atmosphere to prolong it’s life. If stored correctly, the bags of dry powder could last anything from 4-12 months from manufacture.

When do I add hair to lime mortar?

Lath_Haired_MortarSome form for fibre always has to be added to a lime mortar when applying it to the first application on to a lath wall or ceiling. For all other applications, the addition of fibres are optional and depend on the build-up and make-up of the plasters. With NHL mortars we would suggest that 8-12mm layers will require fibres and anything above 10mm with lime putty as the binder require fibres. On to solid walls, it is fine to apply thinner coats without fibres.

Traditionally, animal hair such as horse, cow, pig or goat, is added to the backing coats of lime mortar used for external rendering or internal plastering. On rare occasions, white goat hair is used within the final skim coat. Adding hair gives extra strength and minimises shrinking and cracking. It allows thicker coats to be applied to uneven walls and holds the plaster keys in place when plastering onto lath.

Painting hydraulic lime


Somerset_House_Window_Detail_100
A lime mortar should not be painted with a modern paint heavily laden with acrylic additives.

Externally, it is fine to use White Peak limewash or a silicate masonry paint these are very specific paints and should not be confused with others with similar names. Internally, the choice is far greater and as well as the paints listed those listed above can be used inside, here are some more examples of what can be used inside distemper, earthborn emulsion, Clay paint, Mike Wye limewash

What is lime plaster?

This is a finishing plaster used as a final skim internally. We supply three grades of lime plaster – 2/1, 3/2 or 1/1 which are the ratios of a very fine, kiln dried sand to mature lime putty. The 2/1 grade is suitable for plastering onto haired or unhaired coarse float coats, the 3/2 is our most popular and better for feathering into existing plasterwork and the 1/1 is appropriate for matching the finest historic finishes and can also be used a filler for minor cracks.

If making your own lime plaster it is vital to source a suitable washed, kiln dried sand of 1mm or less grain size. It is recommended to mix the lime putty and sand together at least two weeks before application to lessen shrinkage cracks. Sometimes a plaster is created by mixing NHL2 with sand. This needs to be used within a few hours of mixing with sand and water.

What is Limewash?

Limewash at it’s most basic is a traditional, breathable paint made from lime putty which is thinned with water.

Our limewash is approximately 1 part mature lime putty to 1 part water. Other suppliers can make much more watery limewash, if you prefer a more watery limewash, then just add extra water. Limewashes are coloured with pigments and can be used internally or externally on lime plaster, lime render, stone or brick. It works best on porous surfaces and hardens as it absorbs carbon dioxide from the atmosphere to form calcite crystals, giving it a unique appearance. Typically four coats are needed on new external render and three coats on new lime plaster if pigments are used, add an extra coat if using white limewash.

Limewash can be used inside or outside. Before making a decision on what vapour permeable paint to use, it is important to understand that limewash can be a little powdery if not applied well or on to a surface that isn’t suitable. Limewash can also have a non uniform colour due to differences in suction on the surface, this is usually due to differing moisture levels. The most obvious cause of this is rain hitting an external wall. Some people love this authentic appearance, some people loathe it. Be sure that you understand before application. Choose an suitable alternative if you don’t like the effects, in association with your Conservation Officer if the building is listed.

Click here for a full guide to applying limewash.

What is a pozzolan?

This is a powdered additive made from burnt clay. It reacts with the lime in a mortar to create harder chemicals and so is very useful for damp or frost-prone environments. We tend to use this as standard for most of our external work in strictly regulated amounts.

What is harling? – also known as Scat Coat, Thrown Coat or Rough Cast

Harling is a technique for applying a coat of render or plaster. The lime mortar has extra water added to bring it to a runny consistency. It is then cast on the wall using a special tool called a harling trowel which has a curved blade. It provides a rough texture when cured that gives extra key for the next trowelled coat and also controls suction with very thirsty materials such as cob and brick.

Insulating lime renders and plasters

A number of manufacturers now produce insulating renders and plasters. Some are based on lime putty and usually mixed with hemp and pozzolans, some others are based on hydrated lime and cement with various forms of insulation such as hemp or perlite.

Our main offering is a lightweight, dry mixture of hydraulic lime and cork. Being free from cement, it has good eco credentials as well as offering improved thermal performance. ecoCORK is a product formulated exclusively with Secil Natural Hydraulic Lime binder and free from all cementitious content. Mixed with a largely cork aggregate, this offers a lightweight, vapour permeable and environmentally friendly render or plaster. This render is ideally decorated with SecilTEK AD25 primer and SecilTEK PR 01mineral paint or similar.

For more details on application and prices of ecoCORK, click here

Limecrete

Limecrete is a modern interpretation of a traditional solid lime or gypsum floor that can still be seen in many historic or well preserved older homes. It has also been used to create limecrete buildings with limecrete shuttered walls. Limecrete, known as green cement in the USA in the United Kingdom is normally used to create an alternative to a concrete floor with or without a DPM (Damp proof membrane).

It is vital to have an alternative option for a traditionally built house as an impervious, non flexible cement floor may create problems with dampness being forced into the walls at ground level which can result in poor insulation, mould growth with associated health problems and even structural damage. Many conservation officers with responsibility for listed buildings would encourage the use of the limecrete option rather than the less flexible and non-vapour permeable concrete floor buildup required by Building Regulations. The correct materials and techniques should be used on a building irrespective of whether it is listed.

Limeash were generally used on suspended timber floors onto a raft of reed and it is thought used primarily as a fire break. Limeash is sometimes used to these days to describe all lime or gypsum floors whether suspended or on a solid ground floor. These floors were originally made with lime putty and lots of a pozzolanic additives to provide an hydraulic and hard set. Although very effective, this method needs a very long time to harden and carbonate. A Limecrete floor is made using a natural hydraulic lime (NHL) powder that sets much quicker and potentially harder than the limeash floor. A mixture of NHL5 and well graded aggregates can be used to create the limecrete slab but it is also possible to introduce breathable insulation using an insulating aggregate and even underfloor heating (UFH).

For more details about limecrete, please visit our dedicated website

For more information see the information below for the materials, potential u-values and example limecrete build-ups.

 

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