# Hyperspectral Imaging Sorting of Refurbishment Plasterboard Waste
## 1 Introduction  
Plasterboard waste is generated during construction, refurbishment and demolition projects. Refurbishment plasterboard waste usually carries small amounts of other construction materials such as concrete, foam, paint, plastics, wood, ceramics, glass and ferrous metals. It may also contain non-construction materials due to cross-contamination or poor on-site segregation practices. Nowadays, gypsum from refurbishment plasterboard wastes can be recycled through several physical separation methods that remove contaminants. These separation methods comprise manual segregation, grinding, sieving, and ferrous and non-ferrous magnetic separators. Several quality specifications are required for recycled gypsum to be used as feedstock in plasterboard manufacturing. The most important quality parameter of recycled gypsum is its purity in terms of calcium sulphate dihydrate content. The European Union, through the Life+ Gypsum to Gypsum (GtoG) project defined a minimum dihydrate content of 80 wt% for recycled gypsum [1]. This is comparable with the minimum dihydrate content of at least 85 wt% recommended by the Eurogypsum Recycling Working Group [2] and above 85 wt% recommended by the British standarddocument BSI PAS 109 [3]. However, recycling of gypsum from refurbishment plasterboard waste for the manufacturing of new plasterboards is still very low in the European Union due to the high degree of contamination. As a result, most of the gypsum from refurbishment plasterboard waste usually ends up in dedicated landfill cells to prevent its decomposition and the release of toxic hydrogen sulphide gas. Recycled gypsum purity depends mainly on the quality of the plasterboard waste received, as current physical recycling processes have limited efficiency and customization possibilities. Furthermore, there are water-soluble chemical impurities present in the gypsum waste, such as chloride, magnesium, sodium, potassium and phosphorus salts, that migrate to the paper-core interface during plasterboard drying and affect paper bonding during plasterboard production [4]. Recently, manual physical segregation combined with a sulfuric acid leaching process has been proven to be an effective technology for producing recycled gypsum from refurbishment plasterboard waste with consistently high chemical purity levels of \(>\)96 wt% [5]. However, physical segregation of the contaminants present in the plasterboard waste must be highly effective to obtain recycled gypsum with \(>\)96 wt% chemical purity after acid leaching. Hyperspectral imaging (HSI) is a technique that combines the properties of digital imaging with those of spectroscopy. Algorithms and procedures for spectral data pre-processing, exploration and classification are usually implemented. HSI systems have been used for the identification of materials coming from construction and demolition waste, waste from electric and electronic equipment, municipal solid waste and end-of-life vehicles [6; 7]. For instance, HSI systems have been used to separate different types of plastics in municipal solid wastes [8; 9; 10]. HSI systems have also been used for the recovery and recycling of concrete, mortar aggregates, bricks, tiles and ceramics present in construction and demolition waste [11], for the separation of concrete, rubber, bricks, wood and plastics in construction waste [12], and to detect gypsum, foam, wood, plastic and brick in demolition waste [13; 14]. Other applications include the classification of polyolefin particles in construction waste, and the detection of asbestos in construction and demolition waste [15; 16]. The main aim of this work was to determine for the first time whether an HSI-based sorting system can effectively segregate plasterboard mixed with contaminants found in refurbishment plasterboard waste.