I recently asked my friend Michel Couvreux of TransMineral, USA to write a guest piece about lime. He also includedinput 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, 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”.
Lime 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.
Lime 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:
DOLOMITIC HYDRATED LIME
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).
HIGH CALCIUM 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.
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.
NATURAL HYDRAULIC LIMES
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.
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. If you let Michel Couvreux (www.limes.us) know that you heard about the product from me, he’ll give you a 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 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.