I don't think that silica in rocks or substrate is necessarily a problem. I've used silica sand before without issues and if you think about it some silica compounds obviously don't dissolve readily or the whole glass thing would be an issue.
These was an article by Randy Holmes Farley a while back, I think in Advanced Aquarist, that looked at using silica based substrates. My memory of it is cloudy but he found that some sands contained higher amounts of silica compounds that could dissolve than others.
It seems reasonable that the same would be true with cement.
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