By Andrew Morrison
There are a lot of conversations out there about plaster and what options are best. What is often overlooked, or at very least not considered as heavily as it should be, is the choice of sand. This is especially true when using natural plasters such as lime and clay. Cement based plasters rely less on the mechanical bond of the sand particles than natural plasters; however, the choice of sand is still very important. The reason sand is so important is because it is responsible for several jobs in the process of creating a tight, strong, and durable plaster for your straw bale home.
Firstly, the sand acts as the bulk of the material in the plaster. Keep in mind that a scratch coat (the first coat) is typically a 1.5:1 ratio of sand to lime (using a simple lime plaster formula). This means that there is literally one and a half times as much sand in the mixture as there is lime. In the brown coat (the second coat) that ratio goes up to 2:1 sand to lime. How could “just any sand” possibly be an acceptable approach when deciding the majority of the material?
Secondly, the sand provides the structural strength for the plaster by creating a strong mechanical bond.
[restrict userlevel=”subscriber”]Finally, the sand creates the texture needed for final coat of plaster to attach to the wall. If you don’t have the right sand, you won’t have a quality bond. Without a quality bond, your final coat of plaster can delaminate and all your hard work will be for naught.
The good news is that there are only a few major points that you need to remember when working with natural plasters. The bad news is that finding the perfect sand can be harder than you might expect. I am going to focus only on sand for natural plasters because I hold a strong belief that modern, cement based plasters are not a good option for straw bale homes.
The only way I even consider adding cement to a plaster is if it is used solely as a pozzolan. A pozzolan (as described in wikipedia) 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, a pozzolan will give hydrated lime hydraulic properties. The amount of pozzolan needed to make this reaction occur is very small: as little as 5% by volume can do the trick.
I want to be clear that I am not talking about making a “lime/cement” plaster here. I am talking about making a hydraulic lime plaster from a hydrated lime source. Those are two very different things and the topic for another discussion.
Let’s look at the qualities of the sand that are important to keep track of.
Top Three Details for Proper Sand Choice
Grain Size. It is vital that the sand you use have at least 4 different grain sizes within the mix. This allows the smaller, and ever smaller particles to fill the voids in between the larger particles. This makes for a very strong and tight aggregate and, ultimately, plaster coat. Sand grains from 3mm down to 0.075mm are best for scratch and brown coats of plaster and finer sands in the range of roughly 2.25mm down to 0.075mm will help create a smooth final coat. The larger grain sizes are very important in both the scratch and brown coats; however, they are absolutely essential in the brown coat. They provide the rough texture on the wall surface that creates the “tooth” for the final coat to mechanically bond to the brown coat. This is something you don’t want to miss, as the bonding of coats is incredibly important.
Washed Sand. Simply dry screening sand is not acceptable because the fine-grained clay and silt materials (and other contaminants under the 0.075mm screen size) can bond to the larger sand particles and thus not pass through the screen. You might therefore believe that you have grain sizes from 3mm to 0.075mm and nothing below, when in fact you may have significant amounts of smaller particle sizes in the form of bonded clay, silt or other contaminants. It is best to wash and wet screen the sand so that the smallest grains (below 0.075mm) can be completely removed.
The inclusion of such fine grains will have a negative effect on the overall plaster in several ways. Because the fine grains will increase surface area within the mix, more water will be needed to fully hydrate the plaster. The more water added, the more negative effects on flexural and compressive strength, which will lead to potential shrinkage of the plaster in time. This shrinkage could lead to delamination of the plaster and/or excessive cracking. What’s more, the added surface area and water required to saturate the fine grain materials can lengthen dry times of the plaster. This is especially troubling when plastering around specific “weather windows” where freezing temperatures are possible.
Angular Grains. Round sand is not as good as angular sand for your plaster. This is because the angular shapes are irregular and will lock up against one and other much better than round grains will. As a rule of thumb, it is better to work with crushed sand than with river or beach sand. Looking at the sand under magnification will help you see the true nature of its shape. You can see in the pictures how angular sand would be easier to get a tight finish than the round grains.
If you pay attention to these three details when choosing your sand, you will be off to a great start. Keep in mind that the sand we use for natural plaster is slightly different than that used for conventional stuccos. This is because the sand that most stucco companies’ use is much more evenly graded. In other words, the grain sizes are all very close to each other and you don’t get the large range we are looking for.
This last point is very important because when you call a sand yard to ask about their sand options, they will likely ask you what you are using it for. If you say “plastering” they will say “Oh, you need mortar sand;” however, this is that is not the case. You want a well graded, washed, angular sand. Very often that is called “concrete sand” rather than mortar sand; however, the names vary from region to region. Take your time to find the right sand. Your plaster is not only the finish that looks good, but more importantly, is the finish that provides the protection from the weather to everything underneath it. This is not a place to rush and use “good enough” materials (as if there is ANY place in a house where “good enough” is something I would suggest!). Do it right and you should not have to do it again in your lifetime. Now that’s worth the effort!
Andrew has a passion for straw bale construction that is matched only by his desire to teach his knowledge to others. With nearly 20 years of building and contracting experience, he has now moved his practice entirely to consulting and teaching. He shares his knowledge with thousands of people via his DVD series, blog, and hands on workshops. To learn more, please visit www.StrawBale.com.
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