What process in gypsum products results from exposure to heat that drives off water?

The correct answer is the calcination of gypsum. This process specifically refers to the thermal treatment of gypsum (calcium sulfate dihydrate) to produce gypsum plaster or plaster of Paris. When gypsum is heated, it undergoes a transformation where water molecules are driven off, resulting in the formation of calcium sulfate hemihydrate. This loss of water through heating is what defines calcination in this context.

In contrast, dehydration generally refers to the general process of removing water from a substance but does not specifically denote the transformation of gypsum. Pyrolysis involves the thermal decomposition of materials at elevated temperatures in the absence of oxygen, which is not the process occurring in gypsum. Combustion is a chemical reaction that typically involves burning in the presence of oxygen and does not apply to the transformation of gypsum when heated, as this does not involve oxidation or burning. Understanding these distinctions helps clarify why calcination is the precise term for the process occurring with gypsum products in the context of heat exposure.