[Randy Holmes-Farley]

Diatoms need silica to grow, and your cement (both of them) have a whopping amount of it, although it is hard to tell how much gets released to the water column. Nevertheless it isn't surprising that you get diatoms. If the diatom problem goes away, then the silica isn't a concern.

I do not know if the aluminum gets into the water. However, the Portland Cement also contains alumina, so both have it to some extent.