Characterization of MSWI fly ash

Figure 1 shows the size grading curve for the MSWI fly ash. The size grading curves of cement Class C1 (determined by laser granulometry) and sand used in mortar manufacture (determined by sieving) are also shown in comparison. One can observe that the size grading curve of the MSWI fly ash lies between the cement and sand curves and has a larger spread than the other two materials, between 1 µm and 600 µm, approximately.

Figure 1: Size grading curves of cement, MSWI fly ash and sand

Table 3 shows the results of the chemical analysis carried out on the MSWI fly ash. The main compounds of fly ash are (in order of amount): SiO₂, CaO, and Al₂O₃. The loss on ignition at 975 ºC is also very high (13 %). In addition, the ash has high chlorine, sodium and potassium contents. The most abundant heavy metals are zinc and lead. The total quantity of heavy metal represents approximately 2 % of the total mass of the ash. The chemical composition of the ash used is similar to compositions given in the literature [10].

Table 4 shows the main crystalline compounds identified by XRD in the MSWI fly ash. The respective quantities of the different compounds were estimated from the height of the corresponding peaks on the XRD diagrams. The symbol "X" in the column "Quantity" indicates the relative height of the peaks. The most abundant compounds are, in order of size: quartz, sylvite and halite, anhydrite, calcite and lime. These results confirm those obtained by other researchers using similar materials [11, 12].

Figures 2 and 3, respectively, show backscattered electron images of the MSWI fly ash for the two fields of observation, FA1 and FA2. One can see that the MSWI fly ash has various forms, unlike coal fly ash that is composed of spherical particles. Although some MSWI fly ash particles are spherical (full or hollow), a relatively large fraction of the ash appears to have a vitreous form. One can also observe elongated, angular, very porous particles, and clusters of sintered particles.

Figure 2: Observation (FA1) of MSWI fly ash with backscattered electron SEM (approximate size: 500 µm x 400 µm)

Figure 3: Observation (FA2) of MSWI fly ash with backscattered electron SEM (approximate size: 500 µm x 400 µm)

For observation fields FA1 and FA2, X-ray images have also been collected for the following elements: Ca, Si, Al, Fe, Na, K, Cl and S. For each element, the intensity of the response obtained was compared with a threshold intensity (different for each element analysed) above which the element is considered to be present. Superimposing the images collected for the 8 elements analysed then allowed us to reconstruct the different phases of the ash. For more details concerning these imaging techniques, see references [3] and [13].

The different phases determined are:² S (quartz), AS (aluminosilicate), CAS₂ (calcium aluminodisilicate), CaCl₂ (calcium chloride), and CaSO₄ (anhydrite). All remaining, non-identified phases were grouped in a phase called the "inert" phase. Table 5 shows the area fractions for each phase determined for the two fields of observation.

X-ray spectrometric analysis has enabled us to determine the composition of certain phases of the MSWI fly ash. However, this experimental technique, which is regularly used to study the composition of cements, is more difficult to apply to the MSWI ash. Indeed, fly ashes are very heterogeneous and their size grading has a larger spread than cement. Moreover, unlike the main components of cement whose chemical compositions are known, the mineralogical composition of ash is not well known. The analysis of a field of observation of the MSWI ash is not necessarily representative of all ashes given the size of the particles and their heterogeneity. This is why the area fractions of the CaSO₄ and S phases determined for the two observations FA1 and FA2 present significant differences (Table 5). A larger number of observations would be necessary to reduce this deviation by averaging. However, since the aim of our research is not a complete and precise mineralogical characterization of the MSWI fly ash, we will assume that the quantitative composition determined here is a sufficient approximation.

Eight elements in the MSWI ash were analysed by X-ray spectrometry: Ca, Al, Si, Na, K, Fe, Cl and S). These elements represent approximately 80 % of the total mass of ash. However, the elements Fe, Na, and K were not taken into account in the later hydration modeling, for reasons of simplification (for Fe) and because they did not enable us to categorically identify the mineral phases containing them (for Na and K). In addition, a number of minor elements were not analyzed.

Two additional phases were identified by SEM analysis in relation to XRD: aluminosilicate (AS) and calcium aluminodisilicate (CAS₂). The X-ray analysis showed that the MSWI ash particles of these phases contain large proportions of silica, aluminium (and calcium for the CAS₂ phase), but it does not allow us to determine the stoichiometrical coefficients of these elements. In the CaO, Al₂O₃, SiO₂ system, three defined combinations are known: CAS₂, C₂AS and C₃AS [14]. Given the quantity of lime present in MSWI fly ash, the CAS₂ phase appears to be the most probable phase. However, other elements that have not been analyzed could also be a part of the mineralogical composition of these phases.

XRD analysis revealed the presence of halite (NaCl) and sylvite (KCl) in the MSWI fly ash. On the other hand, with X-ray analysis, the response obtained for chlorine corresponds only to that obtained for calcium. The response recorded for the alkalies does not correspond at all to those obtained with chlorine. On the basis of these observations, it would seem that chlorides are present in the form of calcium chloride (CaCl₂). However, the detection of alkalies (especially sodium) by X-ray imaging can be difficult. The presence of halite and sylvite in the MSWI ash is mentioned frequently in the literature. The presence of CaCl₂ has also been reported [15], but in a less systematic way. We think that chlorides are certainly present in the MSWI ash tested here, mainly in the form of halite and sylvite. Their presence in the form of CaCl₂ in small quantities is possible. It is a fact that, below a mass percentage of 5 %, components are difficult to detect with XRD.

² Conventional cement chemistry notation is being used for the fly ash phases, namely C = CaO, S = SiO₂ and A = Al₂O₃.