I use a mix of water and muriatic acid, it eats right through that calcium, in no time. Use the MA carefully, and with rubber gloves. Mix 1 part MA to about 10 parts water, in a ventilated area. Rinse well afterward.
Muriatic acid is the stuff they shock swimming pools with, and it can be found in the outdoor/pool area of Home Depot, $7 for 2 gallons.