We have a cast iron soil pipe which connects into a clay drain pipe at ground level in the basement, there are some racks on the collar of the clay pipe, no leaking. But concerned it may produce some sewage gas since the basement has some odor smell.
How can I seal the crack ? Can I fill the crack with cement ? A plumber suggested us to pour a pile of cement on it to cover all the collar ? Does cement can seal the smell?
Please advise.
Many thanks ahead.

Go get some concrete caulk, HD, Lowe's etc. Much more flexible than concrete.