Hexagonal water, alkaline ionized water
As Waylander states, calcium and magnesium hydroxides do have some solubility in water.  In the case of magnesium hydroxide you can demonstrate this by simply buying some milk of magnesia and applying red litmus paper to it; the paper turns blue like there were an alkali.

Simply dissolving either calcium or magnesium hydroxide into water would raise the pH beyond the target (you get 10 for magnesium, 12 for calcium).  Additional calcium or magnesium salts may be introduced to shift the dissolution equilibrium for the sparingly soluble hydroxide, and thus control the pH.

Here and here the use of lime and magnesia for water treatment is mentioned.  Both work by making the water slightly alkaline so as to displace heavy metals as less soluble hydroxides.