<a href=showthread.php?s=&postid=10171470#post10171470 target=_blank>Originally posted</a> by goldmaniac
Wait - so the smaller the sand particles, the less it'll increase strength? to a point?
I'm using used refugium sand in my mix. Kent's BioSubstrate. sugar sized particles. is this helping the strength?
G.
In concrete construction, the larger the aggregate, the stronger the cement. Fine sand is only used in grout and mortar where the cement will not be greater than 1/2" thick; however, maximum strength isn't the key goal with DIY rock, so you can cheat a little.
Using fine sand, or too little sand will allow for more shrinkage and subsequent cracking. Slowing the cure process by keeping the rock damp (wet towels or misting) will minimize shrinkage cracks.
If you are making large rocks, larger aggregate is more important. I find crushed oyster shell to be a good size for our purpose, and it's the cheapest alternative (farm supply store). Oyster shell has a grey colour that matches live rock and portland. If you are using white portland, then aragonite or dolomite will match closer.
Oyster shell takes phosphate, silicate and DOC (dissolved organic carbon) out of solution, due to its unique matrix. This isn't a huge benefit to water quality, unless you take the rock out and clean it with acid to recharge it. You would lose beneficial bacteria in the process, and it's very impractical, so you should stick to carbon, polymeric adsorbents and ion exchange media and resins for that duty.
