Garnet sand for clay plasters?

I wonder if any of you fine folks can help me. I've recently found a free source of 80 grit red 'garnet' sand.  Apparently it was used for water jet cutting.

We're looking to build a small Light Straw Clay building in the new year, on a small budget as I'm sure you can understand. Trying to source free materials where possible. Does anyone know if this sand is usable? Some research points to it being sharp which I understand is what you'd want. Thanks in advance!

It sounds like good material for plaster, though used sand would likely be rounded from abrasion. As long as it is still sharp, it will be useful for sandblasting-type functions.

Glenn Herbert wrote:It sounds like good material for plaster, though used sand would likely be rounded from abrasion. As long as it is still sharp, it will be useful for sandblasting-type functions.

Thanks for your input! I may ask for a sample to do some test with.

In addition to having sharp edges (which you can see with a handheld magnifying lens), the aggregate (sand) in a plaster needs to be "well graded," which means a range of sand particle sizes. It might be helpful to think of a bell curve, with some comparatively largish particles, some smallish, and most in the middle.

"Grit" and "mesh" mean different things but roughly correlate in that aggregate described as “80 grit” will have passed through a similarly sized screen, or mesh.  
Aggregate particles sizes play a role in determining plaster coat thickness. A plaster coat needs to be thicker than its largest aggregate particle sizes, and usually several times thicker. Plasterers appreciate this when a pebble larger than the intended plaster thickness finds its way into the mix; it leaves tracks as it rolls and drags along under the trowel, forcing plasterers to pause as they pick out and discard the annoying “rock.”  

Aggregate particle sizes also play a role in plaster strength. Except for very fine finish plasters (around 1/32” thick) that have a fairly narrow range of fine particle sizes, most other plasters are stronger with a wider range. A thick plaster coat made exclusively with fine aggregate particles will crack much more than the same plaster thickness made with a wider range of particle sizes.

I would describe 80 grit sand as relatively fine when it comes to plasters, suitable as one of several sizes for both base and finish coats.  This size may comprise the bulk of a finish plaster, with larger and smaller particle sizes for balance, e.g., 60 mesh up through 120 mesh and finer.  But it would comprise a small portion of a base coat plaster aggregate, which can have sand particle sizes ranging from No. 3 mesh (1/4” diameter) up to around 90 mesh.  

The sand in base coat plasters can be quite coarse as the plaster layers are thicker—up to 3/8” - ½” for lime plasters, and somewhat thicker for clay plasters.  When I was designing plaster mixes, we often used sand with small pebbles in thick base coats, so long as this size was balanced by finer sands. Here in S. Oregon, the aggregate our local quarry calls “concrete sand” is also the lowest-cost sand available, and we often used it for scratch and brown coats.

So, while you’re checking for sharp edges (not round!) look to see if there’s a range of particle sizes, too; some smaller, some larger, with most in the middle.

Jim
Many Hands Builders

Jim Reiland wrote:In addition to having sharp edges (which you can see with a handheld magnifying lens), the aggregate (sand) in a plaster needs to be "well graded," which means a range of sand particle sizes. It might be helpful to think of a bell curve, with some comparatively largish particles, some smallish, and most in the middle.

"Grit" and "mesh" mean different things but roughly correlate in that aggregate described as “80 grit” will have passed through a similarly sized screen, or mesh.  
Aggregate particles sizes play a role in determining plaster coat thickness. A plaster coat needs to be thicker than its largest aggregate particle sizes, and usually several times thicker. Plasterers appreciate this when a pebble larger than the intended plaster thickness finds its way into the mix; it leaves tracks as it rolls and drags along under the trowel, forcing plasterers to pause as they pick out and discard the annoying “rock.”  

Aggregate particle sizes also play a role in plaster strength. Except for very fine finish plasters (around 1/32” thick) that have a fairly narrow range of fine particle sizes, most other plasters are stronger with a wider range. A thick plaster coat made exclusively with fine aggregate particles will crack much more than the same plaster thickness made with a wider range of particle sizes.

I would describe 80 grit sand as relatively fine when it comes to plasters, suitable as one of several sizes for both base and finish coats.  This size may comprise the bulk of a finish plaster, with larger and smaller particle sizes for balance, e.g., 60 mesh up through 120 mesh and finer.  But it would comprise a small portion of a base coat plaster aggregate, which can have sand particle sizes ranging from No. 3 mesh (1/4” diameter) up to around 90 mesh.  

The sand in base coat plasters can be quite coarse as the plaster layers are thicker—up to 3/8” - ½” for lime plasters, and somewhat thicker for clay plasters.  When I was designing plaster mixes, we often used sand with small pebbles in thick base coats, so long as this size was balanced by finer sands. Here in S. Oregon, the aggregate our local quarry calls “concrete sand” is also the lowest-cost sand available, and we often used it for scratch and brown coats.

So, while you’re checking for sharp edges (not round!) look to see if there’s a range of particle sizes, too; some smaller, some larger, with most in the middle.

Jim
Many Hands Builders

Thank you for that detailed response! I hadn't considered particle size. I have a feeling this sand is all of a similar size so will likely have to mix it with something else.