In oups.com,
Norm typed:
My house was decorated in the 1970's and had a plaster applied around
a stove to look like bricks. I removed the stove and needed to get
rid of the faux brick. The drywall was easy because I just replaced
it. I tried chiselling the plaster off the concrete floor. After
several evenings of strong arms I searched the Internet. Lots of
people recommended muriatic acid or vinegar. One chemist recommended
baking soda (sodium bicarbonate) in a water solution. I tried that
because he explained the chemical reaction with the sulfur and
sodium. It worked. I let the baking soda solution soak about a half
hour and a putty knife took it off easily. Thank you chemist.

I never had this situation, but I would not have guessed that a baking soda
and water solution would work. I would have guess something acidic.
Interesting. I am surprised that baking soda and water worked, but fro what
you wrote, I guess it did.