I really have no clue, and I don't have the knowledge to accept or reject this theory of chemical plants.

I am not sure that the correlation with Soviet nuclear plants is real. What scares me is that some people will argue that Egyptians mastered nuclear technology seeing this correlation.

For the concrete theory, until on-site analysis gives a result, it's just an advanced perspective. I say advanced because there has been studies by scientists:

https://www.ce.memphis.edu/1101/interesting_stuff/pyramids_in_concrete.html

I stick to this theory, until it is disproved or another one emerge from all other totally impossible ones, even with our technologies.