On Mon, 09 Nov 2009 10:20:29 -0700, RJ wrote:
In water heaters, this builds up to a "paste"
that can't be flushed out using the water heater drain.
In time, the water heater must be replaced.

I suspect you can still back-flush it, or remove the lower element and
scrape it with something - it might not get it all, but if it's done
every once in a while (annually, say) it can probably be done well enough
to extend the life of the tank considerably - up until the point that
the tank outright fails, I suppose.