On Tuesday, July 25, 2017 at 7:40:39 AM UTC-4, Ed Pawlowski wrote:
Water sits in the hot water tank and the minerals collect over time. In
your case, they were circulated more than normal. Water heaters are
notorius for mineral buildup.

I don't know the exact chemistry of it but if you get hot water too hot, minerals precipitate out. We've had that problem with boilers at work. If we set the temperature too high, or it got out of control, mixing valves etc would start failing in a day. I would think that temperature is hotter than domestic hot water tanks but maybe there's some effect at a lower temperature.