Oobleck! Have you tried cornstarch? Prolly too expensive 
Sorry - added absolutely nothing to the discussion here - pls ignore
Just trying to demonstrate that the fellow could expect more from my side of the email fence than, "Ya bro. I spread it on the wall bro. Pump don't work good nuf bro." We plasterers aren't renowned for our artuculosity.
Potato starch is a common ingredient in gypsum plasters as a thickener, I believe. Its far cheaper than gypsum and I'd be lying if I claimed it never occurred to me to halve my materials expense by increasing the viscosity of a half dosed mix up to something I can keep on the hawk with a judicious teaspoon full. Half dosed would still set solid. Never done it tho.
I've observed differences in rates of mix thickening from different products. It could be polymers hydrating but they are expensive so doubt formulations have much. I put the delayed thickening down to starches.
I probably should have tested a starch in cement mixes by now. I haven't done it only because a more viscous mix, while it will keep aggregate in suspension, calls for a higher PSI to shove it down the hose. I should still give it a shot tho. I predict it imparting a sticky, gumminess to things which would make for a high friction boundary layer in the hose, but maybe it'll behave like cellulose and thicken while staying slick.
Lol, I should probably just stay in my lane. Dry lining. I'm not even that hot shit a renderer and it's a miserable job but I would make A LOT of money if I could shotcrete low cement render mixes.