Calcium deposits? Other minerals in your water? Maybe even certain salts used in hole house water softeners?
My guess is calcium found in your water supply. Your new heater will probably have these also. I have to flush my heater every spring to keep the deposits from building up and shorting the electrical heating element.
Aluminum hydroxide forms when the pH of the water is incompatible with the protective anode rod. The anode rods are magnesium for higher ph (more alkaline) and aluminum for lower Ph. This stuff turns into a gel when wet, so it clumps in the hot water tank, typically.
Aluminum hydroxide is used as a stomach antacid, so it's not a worry to have that stuff in there.
If it's a big issue, test the water PH and switch to a magnesium sacrificial anode, or just flush the tank more often.
A magnesium anode is better at protecting the tank compared with an aluminum anode. Meanwhile an aluminum anode is cheaper, comes with some brands of heaters, and is also suggested if the magnesium anode adds odors (it can) to the water.
Sediment including aluminum hydroxide should be drained from the tank periodically before much accumulates. Particularly for a gas water heater, a layer of sediment at the bottom can result in temperature imbalances that break up the glass lining at the bottom and shorten the life of the tank.
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