@NikTheGeek
Hard to say to be honest...my bag was not split, so not sure how the silicate was getting out (perhaps there was a small hole...I didn't see the bag after the mechanic took it out but he said it was not obviously split). That said, if it was left over sand from casting, then I would expect it to take a lot less time to become an issue. My car is at ~77K miles and I would have expected sand that was there from day one to cause an issue much sooner.
The bits I took out of my expansion tank didn't feel like sand either.
And, what I've read from other threads, the bags were clearly split.
I have read about silicate gelation caused by having silicate in coolant systems. What I understand, silicate is used to reduce corrosion particularly in the aluminium components. But with that you risk silicate gelation. There is some info here: https://www.jstor.org/stable/44743009?seq=1 (you can read the preview to give you an idea).
And this basically says that silicate gelation can block radiators, etc: https://www.frontierpower.com/service/use-low-silicate-antifreeze-heavy-duty-diesel-engines/
In my case, I think I had silicate gelation even though the bag did not split. The silicate works by coating the surfaces of the components to protect them...this can then gel up causing blockage (my interpretation/understanding of what happened in my car).