Lime Plasters For Straw Bale Building
Natural Hydraulic Lime (NHL) plaster stands out as a commendable choice for straw bale houses, boasting exceptional workability and longevity. Here's why we are huge fans:
User-Friendly Application: NHL plaster is remarkably forgiving during application, offering a seamless and user-friendly experience.
Formidable Durability: Renowned for its robust composition, NHL plaster exhibits impressive durability, ensuring a long-lasting finish for your construction project.
Efficient Cure Time: In construction, time is money. NHL plaster accelerates the curing process, providing a comparatively swift turnaround.
Beginner-Friendly: Even novices in construction find NHL plaster accommodating. Its ease of use makes it an accessible option for those new to the craft.
Safety Priority: Unlike alternatives, NHL plaster is a secure material to work with, reducing health risks associated with caustic options.
This endorsement is not just a casual nod; it comes backed by the expertise of Michel Couvreux. Please continue reading to hear what he, an industry leader, has to say, and remember to check out the resources on strawbale.com community DISCOUNT PRICING at the bottom of the article.
All About Natural Hydraulic Lime
by Michel Couvreux, Guest Writer, Lime Expert, Founder of TransMineral, USA & industry leader
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, or Natural Hydraulic Lime? Why? How?
I'll first briefly mention Agricultural Lime or "Ag Lime." This is NOT a lime that can be used as a binder in construction. It is, in reality, similar to limestone (Ca CO3), which has been crushed and 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.
Sun and Moon - designed and sculpted by Timbo Scursso
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 for 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.
Hydraulic Lime or Natural Hydraulic Lime
It is essential to differentiate the simple appellation Hydraulic Lime (HL) from what is called Natural Hydraulic Lime (NHL).
HYDRAULIC LIMES
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 into 3 different categories: 2, 3.5, and 5. Higher the number, the 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.
Conclusion
Clearly, the plastering of your straw bale home and all the hard work you’ve invested in it is a critical area of decision-making. Most of our workshops use a 3 coat Natural Hydraulic Lime (NHL) system on the exterior as it has excellent durability and is reasonable in application.
However, many options are available, and your ultimate decision will rest upon your unique project priorities. We recommend expanding your knowledge of plastering through hands-on experience at local projects, workshops, books, etc.
Strawbale.com Community Discounts
We receive a small referral commission if you use these discounts. You absolutely don’t have to use them, and we appreciate your support of our ongoing development of resources for straw bale builders!
On the West Coast or in the Central States? We are currently sourcing new partners to share with you.
On the East Coast? Reach out to Limeworks (www.LimeWorks.us) and mention our vendor code 'Strawbale.com' to receive a discount on your initial purchase from them. They're located in Pennsylvania and are very knowledgeable, as well as helpful. (You may have heard of them referred to as 'deGruchy's'.)
You’ll also find in-depth information on plastering technicalities on both websites. We highly recommend spending time exploring their website resources.
Now you know how amazing the NHL product is, and it's time to get started on your project, grab some discount pricing, and get your home looking incredible!
But What About Pozzolanic Hydraulic Lime (PHL)???
Is it the same as NHL? Technically not. See Michel’s note above on hydraulic limes, but there are PHL products on the market that behave like the mined naturally occurring NHL.
We expect the availability of these US-based products to expand. We will provide future updates, such as the exciting advances happening over at EarthHaus Plaster under Ryan & Rose Chivers’ expertise. Read more on EarthHaus’ story and for the latest information on their plaster products here. Currently, they have a full line of finish products.
