Calcium Hydroxide, also known as slaked lime or pickling lime, is used to make dark beers with a CO32 concentration of at least 150 ppm. It's used to boost the mash pH and increase the brewing water's residual alkalinity. Additions may be added to the brewing water or to the mash directly.

Performing a carbonate reduction involves adding slaked lime or “pickling lime” to the water.  We would highly recommend using BrewFather for your water calculations as Calcium Hydroxide is a mainstay in the water profile analysis section.

The problem with this approach is that you must carefully measure the amount of lime to add, and you must add it gradually while carefully controlling the pH to avoid producing extremely alkaline water from excess hydroxide.