One of the worst mistakes installers can make is to try to mix screed in a concrete mixer.
Whether it is a small concrete mixer or an agitator truck, a drum style concrete mixer is never a good idea for mixing screed.
Because screed is a dry/damp product, it doesn't slosh around inside a drum mixer the same way a liquid product such as a concrete slurry does. Instead, in a drum mixer screed will roll around and form balls of poorly mixed sand and cement.
It is not uncommon for a tiler to order a concrete agitator of screed for it to arrive at site from the cement plant and unload 5 cubes of sand balls the size of oranges. This ultimately just get dumped into a skip bin.
Screed requires a force action mixer with paddles that deliver a high shear mixing action to cut through the sand and cement and force the ingredients to mix. At a minimum a planetary mixer should be used and ideally a ScreedPro pump truck mixer that combines force action paddles, with a horizontal mixing action that tumbles the screed and the pumping process which continues the mixing through the delivery hose.
Poor mixing of screed invariably results in low strength, patchy screed with a dusty or sandy and poor performance and reduced longevity.