Don't mix antifreezes.

[frank rabbets]

This is what happened to me when I did not disconnect/flush the heater matrixes on changing fluids. Fortunately its just in the expansion tank for some weird reason. Its crumbly plastic stuff. I wondered why I had flat main hoses in the mornings. It had blocked the contraction valve in the rad cap. Thanks Towpac for the flush tip.

 

[Towpack]

That does look like some sort of reaction between types of coolant rather than any sort of corrosion in the system itself Frank. Apparently, mixing silicate (IAT) and non silicate (OAT) types can form a gel like sludge so if it was filled with IAT previously, maybe that's what's happened? Once it's been flushed clear it shouldn't be a problem.