Not sure why I didn't think of it then, but Calcium Hydroxide, also known as slaked lime, hydrated lime, or caustic soda will form a very basic solution. It is not that soluble in water, but it creates a very basic solution. 0.075g/L will create a solution with a pH of 11.3 and it will be a solution that has 40ppm of Ca. Peak solubility in water is 1.73g/L and will create a solution that has a pH of 12.7, it will be a solution that has 93.5ppm of Ca.
Basically, use it as a pH UP solution to go along with whatever calmag supplement you want to use.