My plan was (is) to make a sloppy mix using fine sand, and pour it into the birdbath, so that the channels fill uniformly.
Like almost all people here, I store various bags of sand, gravel, aggregate etc in one of the sheds, in whatever bags come to hand. I took a few scoops of fine sand from one of them, and mixed it thoroughly with cement -- completely dry to start with of course.
Then I added the water. Things started to smell funny.
I poured the mix -- perfect plan for filling the channels btw -- and things smelt even funnier.
Then the mix turned greeny black and some kind of scum formed on the top. I could smell ammonia - very strongly.
Turns out that I had used a bag of lawn sand, which contains "nitrogen, iron, and ferrous sulphate". (That will teach me to hoard. Not.)
The question: What did I make, when I added cement to lawn sand? And is it poisonous?
[3 days later, it hasn't set -- it was merely a VERY smelly sludge. I've had to scrape it all out, and have left the birdbath soaking for a while.]