Calcium Sulfate - agricultural grade - a.k.a. Gypsum.
Studies by Raaska 1990 and Thongsook and Kongbangkerd 2011 showed calcium sulfate supplements of less than 5% w/w increased mushroom growth.
Raaska, L. (1990). "The growth and productivity of six shiitake lentinula-edodes strains of supplemented sawdust medium." Material und Organismen (Berlin)25(1): 47-62.
Thongsook, T. and T. Kongbangkerd (2011). "Influence of calcium and silicon supplementation into Pleurotus ostreatus substrates on quality of fresh and canned mushrooms." Food Sci Technol Int17(4): 351-365.