How do you remove old concrete from your concrete mixer? Depending on whether it is a plastic or metal drum, you may be able to tap the sides with a hammer to pop the concrete loose, provided it isn’t too thick or stuck to corrosion so that it has a good bit. Plastic drums have a good bit of give, so the flex helps allow the concrete to break up and pop loose, but the drum can be cracked if too much enthusiasm is employed. Metal drums will dent or bend, this shouldn’t cause much trouble as long as you don’t beat it to the point of warping the drum.
There are chemical products that concrete batch plants use to remove hardened concrete from the mixer drums on their trucks, one example is a product called “KreteOff” KO Manufacturing – Krete Off 117 – Industrial Chemical Cleaner . You spray the material on the hardened concrete and it causes it to soften, to a consistency something like oatmeal, then spray it, and the concrete off with water. It works well for build-up, but thick layers will take several applications.
If you are talking about a rotating bucket mixer that has a little bit of concrete accumulated on the sides and mixing surfaces, pour a bit of muriatic acid in there, diluted about 50% with water, and let it run for about an hour. Most of it should come off with minimal effort afterwards.
You may benefit from rolling a bag of pea gravel through the mixer with the acid.
If, on the other hand, you mean that you have a big chunk of concrete that hardened in the mixer, the best way to fix that is probably to go to the hardware store and buy a new mixer.
NOTE: If you need to leave excess concrete unattended in your mixer for whatever reason, dump some plain sugar in there. It will never set. You will need to thoroughly clean it to be certain that you don’t contaminate any subsequent batches, though.
On building sites in the nineteen seventies it was common practice every so often to part fill the drum with wood and some paper and set fire to it . This heated the metal up so it expanded and threw off some of the cement , if it was the gently tapped much of the rest would come away from the sides and the paddles . We then tipped it out and poured in some coarse gravel and some water and let the mixer run for about half an hour . This cleaned up much of the remaining stubborn cement and polished the inside again .