(Ganjian and Pouya, 2009)

Gypsum, calcium sulphate dihyrdrate (CaSO4 2H2O) is a commonly used mineral in the production of plasterboard and plaster-based building products, moulds and cement. When gypsum is heated to 120-180°C, three quarters of its combined water is driven off, resulting in calcium sulphate hemihydrate (CaSO4 1/2H2O), known as gypsum plaster or ‘plaster of paris’. When powdered hemihydrate plaster is mixed with water, it rehydrates and sets back into the rock-like gypsum. The conversion of calcium sulphate between its dihyrdrate and hemidrate is reversible and can be repeated without degradation

Alongside the construction industry, gypsum waste is also generated in smaller quantities by the ceramic industry in the form of spent moulds, and from flue gas desulphurisation processes operating at a smaller scale in other industries such as cement, glass and ceramic manufacturing. Within the construction industry, gypsum waste occurs both during construction from cut boards and excess material, and from demolition and strip out. Since the use of gypsum plasterboard has grown significantly in the past 50 years, an increase in gypsum waste from demolition is expected in the future (Jiménez Rivero et al., 2016).

Due to the high sulphate content of gypsum, it cannot be disposed of in general landfill sites, since the decomposition of the material amongst biodegradable matter can produce the toxic hydrogen sulphide gas. It must therefore either be recycled or disposed of in ‘closed cell’ landfill sites that do not accept biodegradable waste.

Given the reversible nature of gypsum compound conversion, it is one of the few modern building materials that can in theory be recycled in an entirely closed-loop production cycle. Currently only a very small percentage of plasterboard is recycled in Europe and considerable progress is required in waste separation and recycling infrastructure for this to increase (Gypsum to Gypsum, 2016).