The capacity of a mortar mixer is typically about half the volume of the entire drum. Depending on the size and scope of your project, you should choose a product with enough capacity to keep your concrete or mortar wet for six to 12 hours a day.How do you clean and maintain a concrete mixer?Keeping your concrete mixer in good shape simply takes daily cleaning and maintenance. This amount of power enables the mixers to keep 2 to 3 cubic feet of mortar or concrete wet for up to 12 hours.