dg wrote:
In practical terms, it equates to a barrow full of mortar or concrete
per mix.

That is a site mixer.

The small ones from hire shops for occasional jobbers will tip over if
you put 15 shovels of sand, 3 of cement and a bucket of water in them.
Use 10 shovels max. And don't put half bricks in to clean them nor
smack the bowl with anything. They are too tinny to take any but the
gentlest treatment.

Once you start whacking them you will have to do so every time you use
them as the smoothness of the bowl will have been lost. Just rinse them
with coarse gravel and water if left to stand longer than a half hour
or so. Or if you are using dry mixes, scoop them empty with a small
gauge.

When they do eventually clog, dry them out, put some newspaper in them
and set it alight.
The heat cracks the cement and the iron will stay smooth for years.