[PK]

What's the best way to clean a cement mixer drum after use?

[Womble]

We keep a bucket of pebbles beside the mixer, then when we're done, we stick them in with a bucket of water to slosh around for a few minutes. A couple of cycles of that and a bit of a scrub, and you're all good.

[oor wullie]

I chuck in half a dozen rocks (half brick size or sometimes bigger) and some water, let it run a few minutes before tipping out.

[PK]

Thanks, I'll try this.

[Rupert the bear]

Wash it out when finished with each time before you go for tea or use some brick cleaning acid,toolstation. Screwfix (also good for removing lime scale and cleaning alloy wheels   
Rocks stones will damage the drum produce keying points for cement to adhere to an make it harder to clean

[PK]

Just finished a cementing session and tried the stones and water approach. The main concern behind the question was the potential for accumulative accretions over successive uses. Didn't like the residue of water  left behind (which is why I had previously avoided water to clean it out). So, I put in a shovel full of sharp sand and that was effective at drying it out.Only just read your comment, Rupert. (I've always used a wheel barrow and the second hand mixer makes a world of difference).

[Rupert the bear]

Just tip drum forward to let water drain for a few minutes then let it dry, my mixer is 15 years old no dents no cement residue just light surface rust.

[Creagan]

A bucket full of shingle or pea gravel does the job nicely, don't use larger stones or ones with sharp edges. Chuck a load of water in with the stones and tip the whole lot out at the end, quick play round with a hose will pick up any leftover bits of fine material. Tip it all out and let it drip dry. If you rinse through the stones you can reuse them next time.
My Dad's mixer still looks like new (well minus the paint) after 20yrs.
It only takes one idiot to forget to clean it and after that it's downhill all the way though.