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chip it out,
You can light a fire inside a metal drum, which will then expand and the concrete will come away.
i have seen idiots smash the drum,which then ruins the mixer,new mortar/concrete then sticks to the dents.
I never use acid on anything steel, it starts rusting,even if cleaned.
ideally never let the drum get dirty, i have had mixers for years, the insides look like they have been polished