Read the link smokerx gave you. Teks such as q21q21 and amorfati will use calcium hydroxide. Also please read the FAQ, you ask a lot of questions that are very basic and with a bit of reading you can find the answer to.
Kash is incorrect though about calcium hydroxide, its poorly soluble in water so if you want to use it, do a dry tek instead of an a/b, dont just substitute it for lye.