Answers to common questions.

Choosing the right mix

There are thousands of different backgrounds and materials to be found in buildings old and new. Finding a compatible product can often be a headache. But if you give us some details, such as background type (eg: hard brick, soft sandstone, timber etc), location, exposure to weather, the finish you want, any previous coatings etc. we will try to help you find the most suitable product. Feel free to e-mail us and we'll try and help.

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What types of paint are suitable for lime plasters?

There is now a much greater choice of suitable paints for lime plasters and renders then in the past. In general, most modern emulsion and masonry paints will seal the wall (even those described as 'microporous'). Limewashes are a good choice and can be used on plaster or render almost immediatley.

More sophisticated paints, such as Aglaia, are also a good choice. With this type of paint fresh plaster should be allowed to dry for up to 12 weeks before painting.

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What makes lime|green’s products eco friendly?

We are committed to using materials which have less impact on the environment and people's health.


Some of the reasons our products are good:

Lime uses less energy to produce and reabsorbs carbon dioxide as it sets. Please contact us to receive our embodied carbon calculations for all our own products and comparative alternatives

Our products are lighter than many alternatives, some renders by as much as 300%! That saves on transport - in the UK we use as much energy transporting construction materials as making them.

The products we sell are non-toxic: they don't contain things such as Chromium VI, nor do our paints give off fumes.

Our plasters use recycled aggregate, with some of them 60% of everything in the bag is recycled.

We design products that not only use less energy to manufacture but also save energy once in place, our mortars and in particular our renders and plaster are thermally effcient.

Our products are durable (excellent freeze thaw and sulphate resistance), yet compatible with old backgrounds, such as soft bricks. That means mortars can last a long time without causing damage such as spalling, or trapping moisture.

Lime green have just completed a project with the BRE aiming to reduce waste during manufacture. The resulting BRE Beaware report is avaible to those who atre intrested.

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How do I prepare the surface for plastering / rendering?

The right preparation and aftercare of all plasters, paints, mortars and renders is essential for their long term durability. Information is available on our datasheets and the St Astier website or you can call or e-mail us.

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Why use lime? What’s wrong with Portland cement?

We don't think there is anything 'wrong' with portland cement, it is a case of using the most suitable material for the job.


The important benefits of the lime we sell and use are numerous. For any particular compressive strength, it is far more 'breathable' than cement and so is far less likely to trap moisture and cause damp and decay. It has excellent sulphate resistance, important on historic masonry which is frequently contaminated with salts. It is more flexible than gypsum or cement, so is less likely to crack when plastered on walls or used for repointing. Lime doesn't contain Chromium VI, which is highly toxic to humans. And finally, lime is burnt at 2/3 of the temprature of portland cement, so its manufacture gives off fewer greenhouses gases.

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Which sand should I use?

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 (upto 3 or 4mm) are beneficial for thick mortar joints or backing coats for rendering or plastering.

Much more detailed information is available on the St Astier website.

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What does breathable actually mean?

Every material has a measurable rate at which water vapour permeates (moves) through it. Materials with high rates of water vapour permeability allow water to evaporate away rapidly, while those with a low figure are likely to trap moisture. It is important to note that water absorption is not the same as water vapour permeability - it is quite possible to have, for example, two plasters with the same water absorption figures but different water vapour permeability rates.

'Breathable' is used as a short way of saying 'a high level of water vapour permeability'. Beware though: there is no legal definition or limit on what may be described as breathable. You will see that most of the products we sell have a figure for their water vapour permeability (sometimes an Sd value is given instead, which measures how much of a barrier the material is to water vapour diffusion: in this case a lower figure is more breathable). We are working towards having a single, easily comparable figure on all our products, so you make a decision based on fact, not marketing hype.

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What are pozzolans?

Pozzolans are fine mineral powders which react with 'free lime' (Calcium Hydroxide: the chemical that makes up lime putty and is present in St Astier Natural Hydraulic Limes) to give a set, even under water. With pozzolans, the lime will set faster and harder then it otherwise would, and will even set under water. Limes which set under water are described as 'hydraulic', so pozzolans turn non-hydraulic limes (such as lime putty) into hydraulic limes.

Pozzolans make lime stronger and more durable, but less breathable and flexible. They give excellent results if used correctly - many Roman structures built using lime and pozzolans are still in good condition 2000 years after they were built.

Natural Hydraulic Limes set by themselves in a similar way to lime putty with pozzolans added. They are an excellent and easy alternative to gauging lime and pozzolans together on site.

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Do Hydraulic Limes Contain Cement?

Products marked NHL2, NHL3.5 or NHL5 must be made according to European Norm / British Standard 459. which means they contain no cement.

However, it is more complicated than that! There are also lime based products classified as NHL-Z which do contain cement, often in the form of GGBS (granulated blast furnace slag a.k.a slag cement). Unfortunately people are often sold NHL5-Z as an alternative to the much higher quality pure NHL5, with the GGBS described as a "pozzolan". In reality, NHL-Z products are cheap, inferior limes, unsuitable for historic buildings. For a useful intoduction on GGBS and its pore blocking effect, see here.

At Lime green we use St Astier Natural Hydraulic Limes which do not to contain any Portland Cement or pozzolans. A few of our products (Monosaintastier and Thermocromex) for new build with concrete blocks do contain cement . This is always clearly stated on the product information.

If in doubt ask your supplier; all reputable suppliers will be up front and clear about the contents of the product and its performance.

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What’s the difference between; hydraulic lime, hydrated lime and lime putty?

All lime comes out of a kiln in the form of 'quicklime' - a dangerous and unstable product. Quicklime needs to have water added to make it suitable for use. If excess water is added, a sticky wet putty is formed. If a precise amount is added, the lime falls to a powder, known as 'hydrate', which can have further water added just before mixing on site. 'Putty' and 'hydrate' describe the physical form of the lime.

Hydraulic is a term which describes the way in which a lime sets by reacting with water (see above). Hydraulic limes are only available as hydrates. Non-hydraulic lime is available as a putty or hydrate. Some people use the terms putty, hydrate and non-hydraulic as if they mean exactly the same thing, which they don't.

The same non-hydraulic quicklime can be made into both putty and hydrate. The putty will be more plastic, finer and retain water better: important characteristics for plasters and limewashes.

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What’s the difference between lime putty and hydraulic lime?

Lime putty (also known as 'Fat Lime' and 'Non-hydraulic lime') sets and hardens by absorbing carbon dioxide from the air giving a slow, soft set.

Natural Hydraulic Lime has an initial set when water is added but then also absorbs carbon dioxide to harden further. Natural Hydraulic limes are available in different strengths: NHL2 (weakest and softest) NHL3.5 and NHL5 (strongest)


The weakest Natural Hydraulic Limes are slightly stronger and faster setting than lime putty, but slightly less breathable and flexible.

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Can lime be plastered over browning or gypsum plasters or vice versa?

In general, the answer is no, the two coats will not be compatible, though it is possible to combine lime and gypsum together in the same mix sometimes.

If you do want to plaster lime on top of gypsum based materials, the only option is to use St Astier Decofond (skim) or Prepbond (bonding coat) which are specially formulated for this job.

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