What do you do between deliveries of concrete on a pumping job? Do you clean the pipeline and waste the concrete in it, or do you save the old concrete in the line and the hopper to use with the next delivery? If you save it you're mixing concrete that has a lower slump or is nearly ready to set with more workable concrete. This is a big nuisance.
You try not to do either one. It is best to plan the job so that the pump works continuously or almost so. This requires a conference and understanding with the concrete producer. If possible there should always be a mixer truck waiting to discharge into the pump hopper, though the mixer truck should not wait too long.