Carbonation is affecting biodegradability testing of fiber cement composites

Fiber cement composites (FCCs) containing natural cellulosic fibres are emerging materials in the building industry. At the end of life, FCCs are often disposed of as part of the C&DW in a landfill. The production of landfill gasses in landfills needs to be kept as low as possible. Generally, leaching of total dissolved organic carbon (DOC) is used as a proxy for the biodegradability of a waste material and the subsequent production of landfill gasses, and is, therefore, used to evaluate biodegradability of waste. In this study, FCC samples with varying average diameter and varying age were subjected to both a batch leaching test (determine DOC leaching) and to a standardized biodegradability test. The batch leaching showed that the DOC leaching ranged between 520 and 1300 mg kg^-1 for the tested samples, and that leaching of DOC decreases with increasing particle diameter and with increasing effects of ageing. Yet, the biodegradability results indicated that the leaching of DOC from FCCs does not result in the release of landfill gasses. This study hypothises that the DOC that leaches from the FCCs is being degraded to CO₂, but that the formed CO₂ is immediately captured by the material itself through the process of carbonation. An inpermeable layer is formed around the material that stops further leaching of DOC. The results of this study therefore suggest that leaching of DOC is a poor indicator for the biodegradability of FCCs.