Cheers, mate. The cement certainly won't harm any plants. Will it hurt fish in your pond? That I'm not so sure about. Fish seem to be very fragile about what's in their water. You'd probably want to do a small-scale experimental test to find out if it would work. Put one of your fish in a tub for a week. Throw a few small chunks of that cement into the tub with the fish. Give it a week. See if the fish is still swimming after a week. That could give you your answer.

I'm not going to pretend to know anything at all, but perhaps my newbie mistake could help answer your question. A couple of years ago, I made a "fish barrel" sunken into one of my flowerbeds. Not knowing any better, I used cinderblocks (which I think are concrete) as risers for my potted water lillies. Had a few Platy fish in there that multiplied like mad. Of course, winter came and the Platy fish weren't warm enough buried in the ground with no heater, so only one survived. She has been living just fine all this time with cinderblocks submerged in her barrel.

My barrel is 100% naturally filtered with plants and oxygenating plants and a pump to move the water and create a little surface disruption. Water is crystal clear and has been all this time. Had a little hair algae this year, so my levels are off somewhere but the water remains clear. I doubt my hair algae is from the cinderblocks because this is the first I have had of it in two years.

Cheers
M