Does anyone know how to remove calcified desposits on shirts?

My husband works outside during the day and in a humid warehouse at night. Since we live in South Florida, he sweats a lot. I noticed that there are hard calcified areas in the armpits of his t-shirts. Does anyone know if there is a way to remove these deposits without destroying the shirt? I tried using Borax and Tide laundry boosters, but they don't help at all.