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Michel Couvreux: An Expert’s Discussion of Natural Hydraulic Lime

Written by Andrew Morrison

Michel CouvreuxI recently asked my friend Michel Couvreux of TransMineral, USA to write a guest piece about lime. He also included input on the many confusing aspects of choosing and/or working with the right material for a straw bale house. I have been using Natural Hydraulic Lime Plaster for years and am a huge fan of the material both in terms of workability and longevity.

I truly believe it is the right material for pretty much any straw bale house; however, I wanted to share with you the reasons why from an expert’s point of view. Please continue reading to hear what Michel Couvreux, an industry leader, has to say and remember to check out the good news about DISCOUNT PRICING at the bottom of the article.

Lime – by Michel Couvreux

The word “lime” encompasses a vast array of products, resulting in certain confusion when choosing the right product for the right application. Which one should you select: Hydrated Lime, Hydraulic Lime, Natural Hydraulic Lime? Why? How?

I’ll first briefly mention Agricultural Lime or “Ag Lime.” This is NOT a lime which can be used as a binder in construction. It is, in reality, similar to a limestone (Ca CO3) which has been crushed and which is used essentially as a fertilizer. It is just a fine sand or aggregate with no bonding ability.

Lime plaster CastleLime, in different forms, has been used in construction since antiquity (plaster or whitewash of ancient Greece, Roman mortars); however, lime has gradually been dethroned by Portland cement (which appeared in 1840), which became widespread during the 20th century, even if its use should have been avoided in numerous applications.

Until the beginning of the 20th century, buildings have mostly been built using lime mortars and generally, they hardly tolerate any intervention using current binders. Indeed, lime mortars have two main advantages that cement mortars do not have: they are flexible and allow the walls to “breathe”.

Natural Hydraulic Lime plasterLime is obtained by burning (calcination) limestone (calcium carbonate – CaCO3). Depending on the composition of the original rock, the lime will develop specific physical qualities which will define its uses. The quicklime (CaO) obtained by calcination is slaked by adding or spraying water (hydration), transforming itself into slaked lime (Ca (OH) 2). This is what we use as a binder in construction.

sand for plasterLime mortars are mainly composed of a mixture of aggregates (sands), binder (lime) and water in varying proportions depending on the intended use. The slaked lime (Ca (OH) 2), with the water acting as a catalyst, recombines with the carbon dioxide contained in the air, allowing the mixture to harden. It is transformed into calcium carbonate (CaCO3) once the water has evaporated.

If the original limestone is very pure, the absorption of CO2 from the air is the only factor involved in the hardening of the mortar. In this case, the slaked lime produced is called Air Lime or Hydrated Lime.

If the original limestone contains clay (between 5 and 22%) or silica, this will give the lime hydraulic properties allowing it to begin its set in the presence of water. This lime is then called Natural Hydraulic Lime or NHL.

Air Lime or Hydrated Lime

In this category, we find two different products with different chemical compositions:


This is the most common lime used in North America. It contains a certain proportion of magnesium oxide. They are classified as Type N (Normal Hydrated Lime) and Type S (Special Hydrated Lime).


More common in Europe, it has a better reputation of purity and durability. They are usually classified by their purity CL 70 to CL 95 for the purest.

These Hydrated Limes are very workable, especially for fine plasters, interior finishes and paints or washes. However, they require a longer curing/drying period (minimum of 2 weeks). During that time, they are very sensitive to frost, heat and humidity. It is rarely recommended for exterior application, especially in a humid environment.

NHL plasterHydraulic Lime or Natural Hydraulic Lime

It is essential to differentiate the simple appellation Hydraulic Lime (HL) from what is called Natural Hydraulic Lime (NHL).


Their hydraulicity (ability to set under water) is artificially created by the addition or combination of products such as Portland cement, pozzolans, etc. Due to proprietary formulations, it is nearly impossible to know their exact composition. Most part of the time, Portland cement is part of the mix and often hidden under terms such as “hydraulic binder”. However, it is important to know that the addition of cement negates the advantages offered by the lime.


Not any addition of any kind by the manufacturer is allowed. This allows for the disclosure of its chemical composition as no proprietary formula is involved. By the European Standards, the NHL has been classified in 3 different categories: 2, 3.5 and 5. Higher the number, quicker the set and harder the mortar.

Natural Hydraulic Limes are very versatile and are used in numerous different applications. They are essentially used on sensitive projects like restoration and preservation and ecologically oriented projects. Their inherent qualities such as elasticity, breathability, self-healing, durability, resistance to salts, suitable compressive strength, etc. make them the primary choice for high quality work.

However, all NHLs are not equal. When confronted with the task of choosing the best product for a lime project, one should:

  • Look at the history of the manufacturer and the history of each product. Newly introduced products should be approached with caution as defects or flaws can take years to develop.
  • Ensure that the lime complies with ASTM C141 and EN-459 by requiring that the manufacturer provide the documentary evidence; complete with test results.
  • Require detailed testing data with real numbers, not only to check the conformity to standards, but also to check the performances of different mixing ratios.
  • Select a manufacturer that will provide technical assistance, guidance, and recommendations. Each project is different and the manufacturer’s contact is essential when looking for the best performance.
  • Verify that the manufacturer or his representative is insured and provides the necessary warranties.

In Summary

Clearly Michel Couvreux has a solid understanding of lime and which ones are right from straw bale construction. I’ve worked with many people over the years with a strong background in lime plaster. For the most part, they all agree that Natural Hydraulic Lime is the way to go. Further, they agree that TransMineral, USA’s NHL provides the highest quality lime available on the market today.

Now for some good news about discount pricing. All you have to do is let Michel Couvreux (www.limes.us) know that you heard about the product from me, he’ll give you my pricing; which is a really good discount. If you’re on the east coast, mention my name and vendor code (95501-NHL) to deGruchy’s Limeworks (www.LimeWorks.us). You will receive a great discount from them as well. They’re located in Pennsylvania. Michel Couvreux is located in California. That said, Michel is the sole US distributor of all St. Astier NHL, so he can often provide you with a good deal no matter where you live. Now you know how amazing the product is, it’s time to get started on your project, grab some discount pricing, and get your home looking incredible!

Want to learn more about straw bale houses and how to build one? Want to do so for FREE? Sign up for our totally free 16 Day Straw Bale eCourse! Find out more HERE.

26 Responses

  1. Hi Andrew and Michel. Thank you for the information and for the available discount! This article provides a lot of details about why lime is so good for straw bales. I was leaning towards using hydraulic lime, and the discount you and Michel are offering has sealed the deal. Have a great day.

  2. Thanks for the article. My question is in regards to humidity: Are strawbale homes a good choice in humid climates (specifically those found in the southeastern US)? If so, I’m guessing an NHL 5 would be better due to its quicker setting time. Right?

    Thanks again

  3. Hello Andrew,
    I have been torn between planning my strawbale build using natural clay/straw on the outside walls or lime plaster. I understand that if I use lime that I should avoid contact with skin, unlike natural clay. Is this correct, and could you tell me any disadvantage of using just natural clay/straw. I think I would prefer clay from the handling point of view, ie it can be reworked and easily touched up.
    I live on Vancouver island which is quite a damp climate. Any advice you can give would be much appreciated.

  4. Thank you for this article. It is full of information that I have wondered about so it’s nice to hear the answers in one place. I live in a cold climate and so plan to use a mix of lime and cement to strengthen the plaster. What do you thin k about that idea?

  5. Can I add color to Natural Hydraulic Lime? What is the color of the finished wall if I can’t add color? OI assume it is white.?.?

  6. You can add color Elaine. In fact, when I work with Michel, I can add almost any color from the Benjamin Moore paint line. the darker colors require too much pigment and so cannot be added to the mix. They make special pigment packs that are added to the finish coat. It’s a lime product so made specifically to work with the plaster. If you don’t add color, the finish product will be white (if you use Ecomortar) or will be shaded by the color of whatever sand you use.

  7. I would recommend against that Jordan. Cement does not breathe as well and it has less flexibility. Further, it’s cure time is much faster than lime so it interrupts the lime cure cycle which lessens the value of the lime in the mix even more. If you live in a cold climate, I would recommend that you use NHL 5 as it is very strong, but without all the down sides of cement.

  8. Hi TK. Nice to hear from you my friend!!! I took this definition from Wikipedia. “A pozzolan is a siliceous or siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties.” In other words, when in combination with lime and water, it becomes a hardening agent that allows the lime to cure under water and makes for a stronger finish.

  9. Hi James. Humidity can be challenging for straw bale homes. I recommend that you include an Energy recovery Ventilator (ERV) in your design so that you can help mitigate the moisture levels in the home. In terms of plaster, I would suggest that 3.5 would be the right choice for you; however, I think following up with Michel would be the best choice as he can tell you for sure. The slower the set of the plaster, the better. This is always true as long as the slow set is not interrupted by freezing temperatures (probably not an issue for you). The higher the number of NHL, the less flexibility it has, so that is a down side of going with a stronger mix. I prefer to stay with 3.5 as often as possible and get great results with it.

  10. Hi Graham. An exterior earthen/clay plaster, especially in your location, will require a TON of maintenance. Being free of any hardening binders, you basically have a plaster made from water soluble materials. When it rains, they hydrate and will wash off the wall without proper maintenance. Lime is a much better choice for your exterior plaster. Natural Hydraulic Lime is a great choice because it does not burn the skin the way hydrated lime can. It will still dry you out, but it’s not terrible. You cannot rework areas the way you can with clay plaster, but that is clay plaster’s downfall as well as it’s advantage as described above.

  11. Andrew, thanks for the response. I’ve done a bit of digging and found that the average humidity levels at a few of the places where you held workshops this year (TX, NE, OR) are right around what I’m dealing with in AR.

    In those conditions is an ERV still the way to go? I only ask because I’m working on a completely off-grid design, and as much as possible, I’d like to do passive heating/cooling.

    Thanks for your help,

  12. Thank you so much for this article! Lots of helpful information. What about humidity inside the structure? I am hoping to build a straw bale chicken coop and I wonder how the NHL would hold up against inside humidity and possible chicken pecks? Also, is it easily washable? Thanks!

  13. Thanks Stacy! NHL is very strong and, once cured, will hold up to chicken pecks. That takes at least 15 days to reach that kind of strength from the final coat application. I would suggest using a metal roofing material as a wainscoting which will be MUCH easier to clean. Having had chickens for years, I know exactly what you are up against. 🙂

  14. Thank you for all of your informative articles. I know you mention in a previous article that you strongly advise against mixing plastering materials, but I came across an article (which I of course now can’t find to reference) that felt that the NHL set to quickly which made it unworkable and advised to mix it with hydrate lime both to improve its work-ability and make it more affordable. They recommended a 1:1 ratio. When reading about NHL I felt intimidated because of the quick set time and the price. Any thoughts on a combination of the 2?

  15. Hi Danielle. I totally disagree about the quick set time. I work with NHL all the time and love it. It has a very workable time line and that can even be improved by mixing the material up to 14 hours in advance of application and then remixing right before use. That provides for excellent workability. I would not bother combining hydrated lime with the NHL. The quality of hydrated lime is so much lower than the NHL and you will end up with an inferior product in the end. Stick with the NHL.

  16. I am working on a house with San Luis Sustainability Group and they claim it’s ok to use cement stucco, with some lime, on straw bale buildings. Are the bad effects only really really long term? They also say using gypsum and cement mix is ok for interior ceilings in a straw bale house – I’m concerned it will make for a hard finish and acoustic echoes. How about using galvanished wire mesh and fixings into straw bale walls? Will it condense or not. This is all as relevant to central coast California. There seem to be no lime plaster or limecrete experts around here, do you have any suggestions how to manage working with such materials in this hot climate? I want the soft finish and flexibility but I don’t want it all to crack off in the heat.

  17. I am very clear about not using cement in my own projects. I have seen too much damage from the material over the life of the home. Most folks who say it’s fine to use are saying that because it’s easier for them, and for no other reason. I would steer clear of cement. I don’t have issues with the mesh and I am a big proponent of using it. any condensation will be drawn into the plaster and removed from the home vie evaporation.

  18. I am living in my strawbale house I built 6 years ago. We have an earth plaster on the interior and a lime plaster on the exterior. I believe it is important to do limewashes occasionally. Is this true? What kind of lime should be used for this? How many coats are needed? What are the best techniques to use? What are some tips and tricks? I keep finding articles that talk about how important it is to do a limewash, but I find very little in regards to HOW to properly do a limewash. Thanks!

  19. Hi Eric. I think t depends on the quality of your lime. If you use NHL (especially a good quality one) then you are less likely to need lime washes. If you do require a lime wash, I would use the same material as the original lime in a watered down version. Be careful though because too much water means that the plaster can “chalk”, get dusty during the curing process. You will want the lime wash to be thin so that it can cure fully and relatively quickly. I would only bother with a lime wash if I a) wanted to change the color of my plaster or b) was seeing cracks that would benefit from the application of more lime. Keep in mind that your plaster WILL CRACK, no matter what. That’s one reason why we do a three coat application: to prevent those cracks from telescoping back into the straw. Some cracks are normal. Large cracks will require more than a lime wash to fix as they likely represent structural mistakes in the plaster or plaster prep. Small spider cracks are the ones that will benefit from a lime wash should you decide to apply one. In terms of the number of layers of lime wash, that depends on your goal. If you are changing the color or filling cracks, then you would apply until you have reached your goal. Hope that helps…in a nut shell.

  20. Thanks for the information. I want to construct some interior walls (non-structural) with a lime/sand/shredded coco mix. I am in a more remote area of Costa Rica. The only thing I have been able to find, so far, is CaCO3 at an ag-store. Can you advise what to do in order to be able to use this? If I heat it will it change it to a form I can use to bind? Thanks for the help!

  21. Hi Michelle. That’s beyond my scope of expertise, so I can’t help with that. Sorry. You might contact Michel directly through http://www.limes.us or read up on some other comments in some of my lime posts here on StrawBale.com. There have been some comments from very strong experts over the years. Buena suerte

  22. Hello, my partner and I are in the process of placing an order for NHL to plaster the light clay straw walls on our tiny home frame. However, when we got our estimate for the NHL we realized we would need to use significantly more than we anticipated, and are now concerned that our frame won’t be able to hold the weight. The footprint of the house is 16x9x10, and sits on concrete pier blocks, below 10×6 wooden beams, connected with metal bolts. The frame itself is made up using 2×4’s, and has a loft and gabled roof. The walls are 10 ft.

    We plan to apply a thicker coat of earthen plaster, followed by the NHL at 7/8ths of an inch.
    Do you think our frame can hold the weight of the plaster?

    Tyler Morning

  23. Hi Tyler. I’m not an engineer, so I can’t answer your question regarding the potential loads of the plaster on the frame. What I can tell you is that adding lime plaster over clay plaster is perhaps the most common failure I am called in to fix as a consultant. It is REALLY hard to do and most scenarios end up in total failure. If I were you, I’d consider NHL for all coats of plaster. It will also cut down on your overall weight as you won’t have the thicker earthen layers at the base.

    On another note, when you purchase your lime from TransMineralUSA, be sure to let them know that you want to use my vendor discount. Just use my name and they will discount your order.

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