Water content estimation of recycled building materials based on near-infrared spectroscopy

Recycling of construction and demolition waste (CDW) is one key component of the circular economy. Reusing construction materials opens the opportunity to preserve finite natural resources and their corresponding landscapes, and also reduces the emission of greenhouse gases. Unfortunately, recycling of CDW is challenging when the new product ought to be used on the same quality level as the demolished one and down-cycling should be avoided. Therefore, the CDW needs to be classified and impurities must be removed. The water content quantification of the recycled aggregates is important because the amount of existing water needs to be known, e.g., when calculating a mixture for recycling concrete. New developments in the classification and the sorting of CDW make use of the information from the spectral range in the visual and infrared region. Therefore, the approach of this study is to examine whether this information can be used to estimate the water content of various CDW materials. Based on reflection measurements in the near-infrared range between 1000 nm and 2300 nm a mathematical relationship between the spectral measurements and the water content is established. Three estimates for the water content are set up. The mean absorption, the water index, and the absorption difference are later tested on CDW material samples covering concrete, autoclaved aerated concrete, lightweight concrete, brick, roof tile, calcium silicate brick and gypsum.