[ckelloug]

Here's another one (before you buy the aggregate):

High grade quartz contributes to great wear resistance but also lowers elastic modulus. In another words, buy stuff other than quartz to get the best Young's modulus.
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Gotta hand it to ya Walter! You tend to call more stuff right by common sense and research than I do by esoteric theory.

During my post on factors influencing aggregate, I started researching fracture toughness which is the determining factor in the strength of brittle aggregate. I found this paper http://www.minsocam.org/ammin/AM67/AM67_1065.pdf on a method for analyzing fracture toughness by smashing a sample of material and examining the sizes of the pieces it breaks into. They analyze quartz and several other things.

One interesting point is that there are two classes of quartz, a weak one and a strong one. Synthetic fused quartz has a crystal structure that is much less strong than some types of natural quartz. Also, they tabulate the fracture toughness values of several materials. It looks like alumina (Al2O3) is about 6 times the fracture toughness which means it will likely perform substantially better than quartz.

This is borne out by a data sheet from Coors Tech, a spinoff from the Coors beer company that makes technical ceramics. (Beer used to be packged in ceramic jars in the late 1800's. . .) The data sheets point out using alumina as an epoxy filler. http://www.coorstek.com/products/grindingmedia.asp

It's probably on the insanely expensive side but Zirconia has over 10 times the fracture toughness of the low strength quartz variant.

Of course all of this is moot if we are failing in the epoxy or in the bond between the aggregate and the epoxy although walter's last test with the large home depot special aggregate showed fracture in the aggregate.

Finally, this post is mainly academic until we determine whether our epoxy is sticking as well as it ought to be but I thought others might be interested.

--Cameron