This study is a part of a long-term collaboration between Telge Återvinning AB at Södertälje in South Sweden and Lulea University of Technology (LTU) in the Northern part of Sweden. Ashes and other industrial wastes used for landfill cover construction have been studied for several years. However, there is a need for further investigations with regard to the long-term mechanical and chemical stability of ash liners in landfill cover constructions. Long-term changes of ashes are investigated by laboratory studies on accelerated weathering (ageing) using experimental design. With regard to weathering, several stages can be identified: hydration and carbonation are well known processes while the processes surrounding the conversion of ash to clay minerals are less well known. There are a number of studies showing that the process of mineral transformation during the ageing of coal or MSWI ashes is quite similar to that of volcanic ashes in nature. Yet, the time frames are quite different: while volcanic ashes need several thousands of years for clay mineral development, there are evidences as well that e.g. clay illite is formed from glass phases in MSWI bottom ash after only 12 y or that clay like amorphous material can be formed in micro-scale throughout the surfaces of coal ash particles after 8 y of natural weathering (Zevenbergen et al., 1999; Zevenbergen et al., 1998). There are a lot of studies performed on rapid fly ash conversion into zeolites by hydrothermal alkaline treatment, the success of which strongly depends on alkaline conditions and the silica-alumina composition of the fly ash source (Inada et al., 2005). These results provide further support to the hypothesis that the observed rapid clay like mineral formation arose as a result of the initially high pH of ash, which promotes rapid dissolution of certain components of aluminosilicate glasses. Furthermore, in a long term perspective these aluminosilicates can transform into zeolites, smectites or halloysites dependent on the solution pH and leaching rate. Based on these studies on volcanic, coal or MSWI ashes we presume that refuse derived fuel (RDF) ashes, like those that are used in the Tveta landfill cover, will be subject to analogical weathering and mineral transformation processes.In order to investigate the mineral transformation in RDF fly ashes, a designed laboratory experiment was performed. A reduced factorial experimental design for accelerated ageing has been applied to evaluate the influence of five factors: carbon dioxide (CO2), temperature, relative air humidity, time and, quality of added water (Table 1). Table 1 Factors and levels tested in the reduced multivariate factorial design for the study of accelerated ageing of RFD fly ashesFactorLowMiddleHighCarbon dioxide, CO2 (%)Atmosphere (0.038)20*100Temperature, ºC5 3060Relative air humidity, Rh (%)3065100Time, months31022Water qualityDistilled -LeachateThe influence of these factors on mineralogical composition, leaching behaviour and acid neutralization capacity (ANC) is analysed and evaluated with the aid of multivariate data analysis. The MVDA modelling was performed with SIMCA-P+ 11.5 version program developed by Umetrics AB (Eriksson and Umetrics Academy, 2006). Principle component analysis (PCA) technique was used and presented in this paper. PCA is an interdependence model where all variables are analysed simultaneously as a single set in a data matrix X. Triplicates were tested for each factor combination. Sampling was performed after 3, 10 and 22 months of accelerated ageing. Mineral composition was analysed by X-Ray Diffraction (XRD). Acid neutralisation capacity was performed at 8.3 and 4.5 pH with 0.1 M HCl solution. The experimental set-up of accelerated ageing of RDF fly ashes is showed in Fig. 1. Preliminary evaluation of the mineral transformations in aged RDF fly ashes revealed that the carbonation process was not yet completed in the some of the specimens (Fig.2). This still caused high pH (pH=12.7) in the solution even though a calcite phase was found in all aged fly ashes. Multivariate data analysis confirmed that carbon dioxide affects the pH and ANC of fly ashes during ageing of RDF fly ashes. The specimens prepared with leachate water had higher ANC than the specimens with distilled water. The ANC8.3 was most influenced by 30 ºC temperature and 65 % relative humidity (ANC8.3 = 0.05 mmol/g) and this well corresponds to the results found in the literature. The ageing time factor has the highest influence on ANC4.5. A more detailed analysis of other mineral phases including clay-like minerals in aged fly ashes will be performed later.The results of this study will contribute to the better understanding of ash formation processes and improved possibilities to make beneficial use of ashes as an alternative to landfilling.Figure 1. Experimental set-up for investigations of the long-term behaviour of the ashes under different environmental conditions. Figure 2. XRD patterns of RDF fly ashes at different ageing conditions. a) N33, b) N71, c) N15, d) N85, and e) N51. The peaks are labelled A (anhydrite), C (calcite), E (ettringite), F (Friedel's Salt), Ge (gehlenite), H (halite), He (hematite), P (portlandite), Q (quartz), S (sylvite), V (vaterite).
Cagliari: CISA, Environmental Sanitary Engineering Centre , 2009. Vol. 1
International Waste Management and Landfill Symposium : 05/10/2009 - 09/10/2009