I have removed all coolant, removed thermostat and ran the engine with plain water, drained into a white bucket to see a very very small amount of the material settle at the bottom.
I see a very similar picture on google which shows it as "silicate dropout" - the engine did shoot hot exhaust into the coolant when the threads let go in the block - the coolant may have cooked it to the point of such mineral being precipitated out of it. - OR its just plain stupid stop leak giving me grief.
prepping for a citric acid flush now on the weekend and now looking for a better quality pump than the GMB brand, NPW Japan make good pumps, used them and also GMB on a lot of other toyotas/nissan/honda etc. - I have had good experience with NAPA rebuilt pumps too on other cars (mostly american) - but this one is a sure PITA - two pumps within 2000 miles is not good - doing the pump on this engine is not very hard but thats no reason for such work over and again.
Pump failures of this engine are littered all across the web, I think it partly maybe the routing of the belt too - the belt wraps around the pump this way
that tensioner is a hard SOB to unload - in comparison the MB tensioner is a piece of cake.
If that flush dont work - Im going to remove the radiator and heater core and get a radiator shop to bubble them out.