Preparation

• Turn your concrete mixer on and allow it to spin for two to three minutes. Add 2 gallons of water and leave the mechanism to spin. The moisture from the water should loosen less concentrated deposits of older cements and bring them into the center of the drum. Use a rubber mallet to tap the outside of the drum as it spins. This will help knock stuck fragments of cement loose. Leave the mixer to run for 10 minutes.
  
  

Hammer and Chisel

• Open the release catch so the water and the cement deposits can drain out. Examine the inside of the drum and knock off any areas of cement still present by chipping them using a hammer and chisel. Chip along the surface of the drum or away from it when using the chisel. Driving the chisel directly into the drum may cause dents.
  
  

Chemicals

• The final step in removing old cement from your mixer is to add a chemical solution. Muriatic or hydrochloric acid are two effective solutions for removing dried cement from the mixer drum. They should get rid of any deposits that can't be remove through natural means. Add one part hydrochloric acid to a bucket containing two parts water. Carefully pour the mixture into the drum of the cement mixer and turn it on again. Tilt the mixer as it runs to ensure the chemical solution reaches all areas inside the drum.
  
  

Finishing

• Tip the acid mixture back into the bucket. Hydrochloric acid can be disposed of down drains. However, make sure it is heavily diluted with water before disposal to ensure its corrosive powers are restricted. Spray down the inside of the mixer with a hose to remove the chemical residue and any fragments of concrete.