The potential role of energy-from-waste air pollution control residues in the industrial ecology of cement

Industrial ecology draws an analogy between industrial activity and natural ecosystems with the inherent implication that, in its ideal form, an industrial ecosystem cycles resources efficiently, with minimization of waste. Industrial symbiosis between the cement and other industries can make a substantial contribution to sustainability. About 3.6 Gt of cement was produced globally in 2011, consuming more than 5 Gt of raw materials and about 11% of total industrial energy, and leading to about 7% of global CO2 emissions. In the same year, global generation of municipal solid waste was estimated at 1.3 Gt, of which about 16% was thermally processed, usually by combustion to generate energy-from-waste (EfW). This equates to about 2.1% of total industrial energy consumption, but would generate about 6 Mtpa of air pollution control (APC) residues. Use of these EfW APC residues in blended cements has been suggested, but they contain soluble toxic elements and are classified as hazardous wastes in most jurisdictions. This paper discusses the effects of incorporating EfW APC residues on the technical performance of blended cements, and on several characteristics of importance for the environmental acceptability of this practice, including (1) total inorganic pollutant concentrations in blended cements, (2) pH-dependent leachability of toxic metal pollutants, e.g. according to BS EN 12457-2, and DD CEN/TS 15364, and (3) diffusion-controlled leaching of monolithic blended cement pastes, e. g. according to EA NEN 7375. Potential pollutants, especially Pb and Zn, but also Cd, Hg, Sb, Sn, and Se, were found to be enriched in EfW APC residues relative to cements. Apart from their potential to pollute the environment, metals in APC residues can affect cement hydration and hardening, and this has been observed. EfW APC residues also contain high levels of several elements that are problematic for quality control of blended cements, notably chloride, which causes steel reinforcement corrosion, and alkalis (Na and K) implicated in the destructive alkali-silica reaction. Although leaching data for both granular and monolithic samples suggest that the mobility of low concentrations of metal pollutants is reduced in cement-based matrices, leachability of chloride remains high and leaching of soluble constituents in APC residues can also be expected to increase paste porosity over time. Furthermore, the literature indicates that pozzolanic or cementing properties of EfW APC residues are not sufficiently strong or reliable to justify their use as a cement replacement, and there is also the potential for components of APC residues to cause other deleterious expansion reactions in cement-based materials. Therefore, industrial ecology cannot solve the problem of management of EfW APC residues through incorporating them in blended cements without radically re-thinking the principle that 'dilution is not the solution to pollution,' which underlies much waste management policy and legislation. Relaxation of these principles would also have undesirable implications for numerous other hazardous wastes. Management of such wastes by blending into cements might seem convenient, but could ultimately lead to wide uncontrolled dispersal of pollutants, especially in cement product recycling, and undermine the quality of blended cements. Originality: This paper considers the technical benefits and environmental acceptability of using EfW APC residues in blended cement production, with a focus on the fate and behavior of the toxic metal concentrations in the APC residues. It summarizes the available information on this subject, and provides a critical analysis and discussion aimed at the cement and waste management industries, as well as policy-makers, to assist in decision-making.