LIMESTRONG BUILD™ (LSB) is a Pozzolanic Hydraulic Lime (PHL) plaster, available in two varieties: BINDER and FINISH. LSB Binder is the plaster binding ingredients only, with no aggregate (sand). For job-sized quantities, shipping only the binder provides a significant savings in shipping costs. Sourcing the right type of sand to mix with LSB Binder is the focus of this document.
On the other hand, our FINISH product includes the aggregate—you just add water. As the amount of material needed for the thin finish coat is much less, an all-in-one product is economical to ship and allows complete control of the fine-grained aggregate blend.
IT IS IMPORTANT TO SOURCE the right type of sand to mix with our Limestrong Build Binder. The following guidelines will help you choose the right sand from a local source. You might find a sand supplier that has experience supplying lime plaster sand, but it’s best to visit the yard and identify the right sand grade mixture for yourself.
The best sand grade blend for lime plaster is a bit different from sand for cement-based stuccos (or mortars or concrete)…so just because they tell you it is “plaster sand” doesn’t necessarily mean it will work for a lime plaster, though it might. As detailed below, lime plasters rely on the mechanical bonding provided by the different grain/grit/particle sizes—more so than Portland cement-based plasters and stuccos do.
Bottom line: a good lime plaster sand has three KEY characteristics:
1) the grains/particles are sharp and angular,
2) the particles vary in size, and
3) it’s clean—free of contaminates like dusty silt, powdery clays, and organic matter.
Bottom Line Postscript: These sand-sourcing guidelines point to the ideal, but don’t get too concerned about achieving that exact ideal. Source the best sand you can while using the best-sand-character keys of angular particles, adequate size gradation, and contaminate-free.
All Sand is Not Created Equal
Sand exists in a wide variety. What you want for Limestrong Build is typically found at sand and gravel operations and/or construction supply outfits. Avoid using natural sand from stream beds, beaches or pits, as it tends to be rounded off by years of current and wave action and will not mechanically bind in the lime plaster matrix as is necessary (KEY 1). Think about it this way: what would give you the sturdiest dry-stacked retaining wall—angular field stone or rounded river rocks?
Natural “found” sands also tend to contain contaminants (KEY 3)—silts, clays, and organic matter—that will have an adverse effect on the functional integrity of the lime plaster.
MANUFACTURED SANDS. Construction sands (as opposed to natural sands) are crushed, washed, screened and graded for use in concretes, plasters, and masonry applications. This “made” sand (m-sand) process allows for much more control over the texture, cleanliness, and functional sharpness (mechanical binding properties) of the sand in critical construction-related applications, like plaster.
PUMICE SAND. Pumice sand (like the grade-blended product offered by Limestrong Build), is a functional supplement for replacing part of the sand/aggregate necessary in the plaster. Pumice is a naturally frothy, foamed glass aggregate (of volcanic origin) that provides several advantages to plasters that sand alone cannot. First, the lightweight pumice aggregate lightens the overall weight of the plaster rendering, putting less stress on the bonding substrate and the mechanical bond between layers. Second, the sharp, grippy particle edges boost the plaster’s overall stickiness and mechanical hold. Third and fourth, the vesicular nature of pumice means each particle is riven with tiny pockets and pores, which provide critical curing advantages in terms of both moisture retention and breathability. Finally, those integrated air pockets also provide enhanced freeze-thaw damage resistance to the plaster matrix. Note that if pumice sand is included as a percentage in your aggregate blend, your water requirements will go up, so be consistent in the amount of pumice sand used in each batch, which will then allow you to settle on the right amount of water batch to batch.
Avoid a monogranular sand (KEY 2)—meaning the sand particle size is mainly one or two grades (sizes). Blended grades, especially angular manufactured sands, create a very efficient void structure, which allows for better workablilty and proper vapor exchange (breathability) in the cured plaster. A well-graded sand (pictured above)—one ideal for Limestrong Build Binder—contains at least four (4) blended grades—with the smallest particles making up no more than 15% of the overall blend.
If possible, get a look at the sieve analysis for the supplier’s grade blends—look for a nice bell curve: fewer larger particles, an increasing amount through the medium grades, and back down to a small percentage of fines (no more than 15%). If possible, avoid blends containing particles bigger than what passes through a #12 mesh (including #4, #8, or #10). See annotated photo above.
VOID RATIO. Another way to determine if the sand has the right grade-blend is to check the ratio of voids to sand. The ideal void ratio is 33%, leaving the other 67% as sand. A simple Void Ratio test will tell you what you’re looking at[XR1].
TOP PARTICLE SIZE. As a guiding rule of thumb, the largest sand-particle size in your grade-blend should be no larger than half the thickness of the plaster coat you’re laying down, but no smaller than a quarter of the thickness. This rule of thumb provides the necessary structural grain size range while still allowing a nice finish, as the larger particles are pushed to the back of the layer as you work it.
THE PROPER RATIO of FINES. The sand you use must be clean and free of clay and slit. Contaminating fines demand more water to reach workability, but that causes a host of problems: Too many fines extend the drying period. Too many fines affect the compressive and flexural strength of the plaster. Too many fines promote shrinkage and may cause de-bonding, especially if the suction isn’t ideal.
On the other hand, no fines (or too few—less than 10% of the blend) is not what is wanted either. No fines (sand fines, not powdery silt or clay fines) in the sand blend result in a loose, grainy plaster that does not hold together well, nor do the plaster coats bind well to each other. In extreme cases, the plaster will simply slump off of the wall. So again (KEY 2), pay attention to your grade blend.
Loose, Damp Sand
Plaster sand should arrive damp and loose, and upon arrival, you should be prepared to keep it that way. To protect the sand stock pile, spread a waterproof tarp or durable plastic sheeting on the ground where the sandpile will be. Then cover the newly-arrived clean, damp sand pile with another tarp and securely anchor it. This will keep the moisture in and the contaminates—both chunky and fine—out. The tarp will protect from rain as well—you want damp sand, not wet sand.
Put It to the Test
For peace of mind, make up a small test batch with your sourced sand—first with a sample from the sand supplier before delivery, and then again upon delivery of the stockpile, before you begin application. If you’re going to have issues with the quality and character of the sand, you’ll want to discover them early rather than in medias res.
STRETCH TEST. To performance-test a batch of mixed plaster prior to troweling or spraying on the wall, conduct a stretch test. Pull a hawk-full of plaster across a table or mud-board with your trowel. A great plaster will spread smoothly, with little effort, staying together—and that means the sand’s grade blend is right. If instead you get rips and tears, the sand has too few fines. If it’s too stiff and hard to spread (at recommended water amount), there are too many fines (sand or silts and clays), which tempts you to add more water to soften it up, which introduces other problems.
STICKY 180 TEST. To test for the right consistency and stickiness, scoop a load of prepared plaster onto a small trowel, give it an abrupt, downward shake to set the plaster against the trowel, then quickly turn the little trowel upside down. If the plaster sticks to the trowel, you’re good to go.