Sand, its not all created equal

Hi all;
Wanted to talk about sand... this should be a short post right...
Ever since I was introduced to rocket stoves. I read that ocean sand or sand dune sand was too fine to be used in a rmh build.
No oceans or sand dunes in western Montana...
They also said that creek sand was probably "sharp" enough to be good as a mortar in a RMH.

In the home depot store I always saw the sacks of different kinds of sand... Ha I thought!   We have free sand at home who needs to buy sand!
During the mid winter construction of my shop stove. I broke down and bought a sack of builders sand "medium"  I used a bit finishing construction, but hardly made a dent in the bag.

Fast forward to my studio batch build. I had 3/4 of a sack of sand, may as well use it all up before going down to load up free sand next to the RR tracks.
Working with this sand is much more gritty feeling than when I use the local sand.  
I bedded my barrel with the last of that mortar.  I needed more so it was off to town to resupply. I like to keep half a dozen pails filled for winter time ice sanding.
Returning home with my sand , feeling good that I accomplished another prewinter chore before winter!

I needed more cob. So I mixed up a batch, 3 parts sand to one part dry clay.  Used it to seat the bricks over the roof of my Bbox.
After drying with the two cob batches side by side it was easy to see differences.  The store bought sand - cob was much rougher feeling than the local sand cob.
The local sand was smooth and nice!  So better you might think???  Well as a finish layer on a bench maybe...
The local sand cob is cracking... the store bought has not...  
I have spent seven years patching cob cracks that I thought were just the price to pay for working with mud and high heat...
I do believe that any future cob I mix up will have store bought sand in it...

Why is it so?

John, I'm guessing here but I think the "sharper sand" has better thermal quality's than the super fine sand?
I personally did not think it mattered until now.  

Sand dunes will have the finest sand, "ocean" sands can be fine or coarse, and often both on the same beach you just have to scout about a bit.

Take a bit of both, and look at them through a magnifying glass, or better, a microscope. I bet you find that the "store bought" builders sand is both larger grains, I.E. a lower mesh number, AND is sharper, having the points and valleys of freshly fractured rock. The local sand is most likely much smoother, having had the pointy bits worn down. Builders sand is selected for this sharpness, so that it mechanically locks together in concrete and mortar, and is probably why it lasts longer in your cob. I'm just a hack builder, but I have seen concrete patios made with beach sand fall apart, while an otherwise identical one made with builder sand stayed much more crack free. As someone mentioned, sharp nd worn sand often exist in close proximity, so you might look a bit further afield and find something more suitable on your own land.

I wonder if beach sand has salt on it and that causes problems in concrete? I will look.

Just tonight, I was inspecting my recent mortar mix where I used local sand filtered through a fly screen. Its almost completely dry and when I rub my finger across its surface, I end up getting a chalky residue left on my finger and also see the sand rubbing off.
It does make a nice finish plaster though due to the fineness of the sand but found that if I add some wheat paste to the mix, it eliminates the chalking and makes it more firm.
Not something I want to add to all my mortar mix where it won't be seen so replacing that with a more coarse sharp sand is much more preferable.
Will need to adjust my mix.

I remember Michael Smith from Cob Cottage Co. gave many ways to know if you have a good cob mix. One of the tests he called the "crunch test."
Grab a handful of cob and with your hand next to your ear listen - It should sound crunchy as the sand grains grind together. Although I found that most sand will do this, the sharp sand will do this louder and feel more gritty.
 
The interlocking properties of sharp sand someone else mentioned here I think are also very important to get a superior bond that rounded beach or ocean sand can't do.

Joshua Rimmer wrote:Take a bit of both, and look at them through a magnifying glass, or better, a microscope. I bet you find that the "store bought" builders sand is both larger grains, I.E. a lower mesh number, AND is sharper, having the points and valleys of freshly fractured rock. The local sand is most likely much smoother, having had the pointy bits worn down. Builders sand is selected for this sharpness, so that it mechanically locks together in concrete and mortar, and is probably why it lasts longer in your cob.

I came here to say just this!  The varying grain sizes and angles lock together.  It's sometimes called mason's sand or paver sand.

Beach sand is horrible for mortar and such.  The grains are very fine and very smooth.  It's like stacking marbles.

Just remembered that the Wisners did a DVD video with a clip on sand: