Lime, quick lime or calcium oxide. This is the highly alkaline substance that makes cement do cement things. It is very caustic and will literally dissolve you if exposed to it for a long time. This so called 'lime' is also what is referred to in the "You put de lime in de coconut, you drink 'em bot' togedder" song by Harry Nilsson. It was used as a medicinal treatment for sour stomach, heartburn or general stomach distress. In small quantities you can safely consume it, but you don't want this shit on your skin in significant quantities, as you found out.