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I think the advantage of silica sand is it is guaranteed to withstand the high temperatures the mortar will be exposed to (it's literally glass). I'm not sure what the official arguments are for and against either type of sand, I just think the pure clean white silica sand is more heat resistant...but I really don't know. Maybe someone else will verify my claim.
The finest sieve sand will give you a more workable mortar, particularly in the home-brew refractory mortar where you want really thin joints on the inside. I don't think the color of the sand makes much difference, it's all pretty much silica.
High temp refractory mortars won't contain any sand, because they'll melt at high temps. They're designed for service way above the temps we use. Sand as an aggregate for our application is quite ok. I don't think the type used makes much difference.
I recently acquired some silica sand with the #12 on the bag. It is a very fine grade of sand, the grains a bit finer than what one would expect to find in a two-minute hourglass. I was told by my local building supply yard that the labeled #12 refers to 120 grade silica. My intended use for this sand is for homebrew mortar, but the building supply owner said that while it would work, the chances of hairline cracks would increase.
Anyone have experience with #120 silica sand? I'd love to make up a homebrew mortar with the creamy properties of HeatStop and fill as small a joint as possible.
He is correct. The rule of thumb is to use as large a sand grain as the joint will allow. So for 1/16" joints, you have the correct sand. For 3/8" joints it is too fine. Even more important than the size is the grading. You want from just above dust to the largest size (1/8" for nomal masonry sand), as the cause of the cracking is shrinkage of the cementious paste as it dries. The more the gaps of the mortar are filled with aggregate, the less cracking.
Think of a jar of marbles. The spaces around the marbles are large. Now add in some marbles 1/2 the size, and 1/4 the size, 1/8, etc. The spaces will be much smaller (and uniform) with the blend.
That is what you are trying to achieve with a good mortar or stucco, no matter what it is used for: a blend of aggregate particles coated with a cementious paste.
Thanks Tim, great explanation. Even though I'm planning on really tight internal joints, (no gaps) since I will not be tapering (just beveling) my bricks, there will be a large horizontal gap on the outside of my dome bricks. I'm anxious now to do a test using a mix of 1/8" masonry sand with some of this silica. I'm hoping the refractory properties of the silica enhance the mortar's effectiveness in this application.
If you do not mind having two types of mortar going at once, you can use the fine silica for the face and fill in the wedges on back with a mortar with larger aggregate to reduce cracking on that wide joint.
Don't mind at all, Tom. I was wondering about just that possibility and you've answered it for me. It'll probably take some practice, but I'm sure I can figure out how to do two mortars without tearing my hair out. Thanks again!