The present invention relates to high-purity porous gypsum, which is prepared by means of heat and foam (carbon dioxide and steam) generated by mixing and stirring limestone powder and 30% or more of highly concentrated waste sulfuric acid in a sealed reactor, and a preparation method and preparation apparatus thereof, and, more specifically, to a method in which waste sulfuric acid generated in a semiconductor manufacturing process is added to a sealed reactor, limestone powder is added thereto and stirred for a fixed time to give rise to a neutralizing synthetic reaction, moisture is evaporated by means of the reaction heat, and rapid expansion and solidification occur instantaneously due to a dihydrate reaction at the maximum temperature to thus produce solid gypsum containing 25% or less of moisture and carbon dioxide having a purity of 99% or higher.