calcium hydroxide (cal)
Food-grade slaked lime, a white, strongly alkaline powder in which dried corn is soaked and cooked for tortillas and tamales.
Caution: Use only food-grade product, never builders' lime, and keep the caustic powder away from your eyes, skin and lungs.
Calcium hydroxide, called simply cal in Mexico, is slaked lime and is used mainly for nixtamalization, in which dried corn is cooked in limewater and left to soak overnight. The alkaline water loosens the tough hulls, turns the corn into something that can be ground into a cohesive dough, and gives tortillas their characteristic, slightly earthy flavour; without this step corn dough will not hold together. On its own it tastes chalky and bitter, which is why the corn is rinsed thoroughly after soaking. It is sold in Latin American shops, by mail order and, in the US and UK, often as pickling lime in the canning section; in German-speaking countries pharmacies can supply food-grade calcium hydroxide.
Substitute:
There is no good substitute: baking soda is not alkaline enough to loosen the hulls reliably and will not give a cohesive dough. If you cannot get cal, use ready-made masa harina, which has already been nixtamalized.