You might try a new, specialized, 7 1/4 inch, circular electric saw cutting
blade available at Home Depot. I bought one for cutting Hardiplank and it
worked very well. I think the circular saw will have to be thrown away when
the project is done, due to the cement dust that penetrated everywhere.

The blade costs $16.00 and has about 4 or 5 carbide teeth...for the whole
blade! Amazingly, it cut Hardiplank like a charm. A little rough around the
edges, but who cares in your application? You might want to give it a try.
it would be a lot faster than grinding the material away, I'd think.
RichG