Recycling of Air Pollution Control Residues Into Engineered Lightweight Aggregates

The thermal treatment of Municipal Solid Waste (MSW) is continuously increasing in the UK as a sustainable practice to reduce the amount of waste that is sent to landfills and to recover energy in Energy from Waste (EfW) facilities. However, an issue associated with this practice is the volatilisation of hazardous compounds contained in the waste. These contaminants are removed from the gaseous emissions by air pollution control units before the gases are released into the environment. The solid wastes generated during this cleaning process are known as air pollution control residues (APCr) and are classified as hazardous because of their environmental impact associated with their chemical composition and leaching properties. APCr treatment before landfilling can be highly expensive, and therefore, there is a need for recycling these waste materials. The aim of this research was to investigate the viability of incorporating APCr into the manufacture of lightweight aggregates (LWA).

In the first stage, APCr samples collected from different EfW facilities across the UK were characterised for elemental composition, mineralogy and leaching properties. In the second stage, a mix composed of all samples of APCr previously characterised was prepared for the evaluation of APCr treatment. Washing with water and accelerated carbonation were optimised to reduce the leaching properties of APCr. The third stage comprised the manufacture of artificial LWA, through hot bonding and cold bonding processes at a laboratory scale. The physical and technological characteristics of the manufactured LWA were assessed for the fulfilment of the corresponding standards for LWA as well as compared with those of the commercial products Lytag® and Carbon8®. It was demonstrated that the hot-bonded LWA exhibited better technological characteristics and improved leaching properties compared with the cold-bonded LWA.

In the final stage, hot-bonded and cold-bonded LWA were incorporated into lightweight concrete (LWAC) to evaluate their performance. The LWAC made of hot-bonded LWA containing 25% washed APCr, was strong enough for applications in structural concrete exposed to different environmental conditions. Also, this LWAC formulation showed a reduction in the carbon emissions associated with its production, which represents an important environmental benefit. A future scale-up of LWA manufacture from 25% of washed APCr can significantly reduce the amount of APCr that is currently sent to landfills and increase the recycling rates of this waste material.