There is a huge difference between amorphous silica and crystalline silica as far as your health and your concrete is concerned. The difference between the two is at the atomic level.
Crystalline silica (quartz) is the form of silica that OSHA is writing the new regulations to cover, it is a health hazard. It is not reactive with portland cement, meaning it does not cause ASR (alkali-silica reactivity) regardless of particle size.
Lots of countertop makers use 30 to 40 mesh crystalline silica sand in their GFRC mixes. Crystalline silica flour is silica ground very fine, sometimes as fine as 5 um (microns), much finer than the 45 um (about 325 mesh) typical of portland cement. Stucco guys and even some concrete countertop makers use silica flour in their mixes. It gives the mix stickiness and body, important characteristics when your trowling a cementitious mix on a vertical wall. No matter how small the particle you use in your mix crystalline silica is just inert filler and is not part of your water to cement ratio calculations.
Amorphous silica is characterized as a nuisance dust. This is not to say that there is no health hazard associated with it, just that it's not as severe a health hazard as crystalline silica. Some examples of amorphous silica are silica fume and ground glass used in sandblasting and some countertop mixes.
Amorphous silica is reactive with portland cement — sometimes in good ways, sometimes in not-so-good ways. If the particle of amorphous silica is larger than about 900 um, the particle causes ASR or alkali-silica reactivity. Somewhere smaller than 900 um size the particle is neutral. Somewhere around 600 um the particles react with byproducts of portland cement hydration (calcium hydroxide, free lime) and become pozzolanic; the reactive potential peaks below 5 microns. At submicron levels (less than 1 um - colloidal silica) it tends to lock up a mix and render it unworkable in high dosages. Cement is about 325 mesh or about 45 microns. Please pardon the generalizations.
It’s kind of neat when you think about it: Amorphous silica both causes and prevents alkali silica reactivity.
I’ve heard people in the industry lumping silica fume in with crystalline silica. Hopefully this will clarify things.
Mark Celebuski is a partner in manufacturer Trinic LLC. Reach him at [email protected] This post can also be found on Mark's social media pages.