He probably thinks that all sand is made out of silica, and in alot of cases sand is silica. But, not all sand is silica based, some are granite, just crushed by nature, into a very fine powder. Because silica dust has been shown to cause irritation in the lungs, including cancer, due to sharp glass getting into the lung cavities and eventually cutting into the flesh there, causing all kinds of problems, you can't buy sand for things like children's sand boxes etc. Same with ceramic clay. It is all sold as silica free for the same reason...clay dust is much finer than most sand dust and has to be silica free.
Calcium carbonate, which is what most calcium based sands are made, are said to be digestible but they are not. The grains are too big and are not in the stomach long enough to even start to break down. In fact, it tends to clump together, neutralizes stomach acids and has a high risk of causing impactions. One person did an experiment awhile ago, he measures a very exact amount of calcium based sands (I couple of types, including that reptilite stuff) put it into hydrochloric acid about the same strength as found in the digestive tract of dragons and left it in there for a week or two....then he drained all the acid, dried out the sand and weighed it. He was using a pretty acurate scale for it too, and he noticed no appreciable decrease in the amount of sand. If it was digestible, it would have broken down and adsorbed into the acid...but it didn't, it just stayed there.
Calcium powder we dust insects with is about a million times finer than calcium based sands, hence it does absorb much easier. Its like dropping a tiny granule of sugar into water and dropping a sugar cube into water...which one dissolves first?
All that being said, it doesn't mean every single dragon housed on calcium based sand will be impacted. It just means they have a higher chance of being impacted. Other factors can affect the chances impaction occurs. Insufficient water for one can make it that much harder to 'flush' out the sand, temps not hot enough etc. Regular children's play sand is not 100% safe either, it can cause impactions as well, but has a less of a risk than calcium based sands or crushed walnuts.
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PHLdyPayne