Certain characteristics have a propensity to be similar in most ashes. Chemi- cally, CCPs are mainly silicoaluminate glasses, with the presence of other mineral materials such as calcium (Ca), magnesium (Mg) and iron (Fe) (ADAA, 2007). According to the American Society for Testing and Materi- als (ASTM), there are two classes (C and F) of CCPs; class F is produced from the combustion of bituminous coal and class C from sub-bituminous and lignite coals (Manz, 1999). In Australia, the majority of CCPs produced is categorized as Class F—being mainly silica and alumina (80–85%) and
<10% CaO. Class F CCPs are highly pozzolanic and reacts with various cementitious materials. The other type of CCPs produced in countries such as India is the Class C CCPs, which generally contains more than 20% lime (CaO). Hence, they do not require an activator for cementing. Alkali and sulfate (SO4) contents are generally higher in Class C type. Furnace BA can comprise 10–20% of the total CCPs produced and range in grain size from fine sand to coarse lumps. They have chemical compositions similar to FA.
Figure 6.4 Production of CCPs by different technologies and their various applications.
The characteristics of CCPs produced by CCTs (FBC and FGD) differ both physically and chemically from conventional FA, mainly because of the additives and modifications in the combustion technology (Fu, 2010; Li et al., 2006).
2.1. Physical Properties
The main physical characteristics of combustion residues include particle size, particle shape or morphology, hardness and density. These properties are a function of the particle size of the feed coal, and the type of combus- tion and particulate control devices used (Kim, 2002). About 50% of CCPs are glassy spheres, which are mainly composed of silt-sized materials having a diameter from 0.01 to 100 àm (Gupta et al., 2012; Singh and Siddiqui, 2003). The importance of these particles on chemical reactivity is described under “Chemical Properties” (Section 2.2).
When compared with mineral soils, CCPs possess lower values of bulk density, hydraulic conductivity, and specific gravity. Among the CCPs, FA has the lowest bulk density (Table 6.6). Finer particles of FA have larger surface areas, which allows for deposition/absorption of volatile elements on the surface of the CCPs during the combustion process (Goodarzi et al., 2008). The extent of the surface area also determines the water-holding capacity (WHC) and other surface reaction properties (e.g. adsorption) of CCPs. Aitken et al. (1984) found that the surface area alone cannot explain the high WHC. The variation in the proportion of particles with porosity among the ashes may also influence the WHC. Both crystalline (mullite) and amorphous (glass) phases have been identified by X-ray diffraction in FA (Mattigod et al., 1990). The texture is fine for FA and is coarse for the CCT-derived CCPs. The FBC appears dark gray in color which may be attributed to the scrubbing of NOx and SOx. The particle size ranges from 0.01 to 740 àm in the order of FA > FBC > FGD. Bulk density and mois- ture content are high in FBC. Ash content is low in FBC and FGD com- pared to their conventional counterpart (Table 6.6).
2.2. Chemical Properties
The chemical properties of CCPs will largely be determined by the metal oxides (Si, Al, Fe, Ca, Mg, Na, K) that are surface adsorbed during particle formation and the primary minerals that include quartz, mullite, hematite, clays, and feldspars. The electron dispersion spectroscopy (EDS) studies con- ducted by Kutchko and Kim (2006) in an FA sample lists out the predomi- nant elements as Si, Al, Fe, and Ca. They observed lesser amounts of other
elements such as K, Mg, Na, and S. The general association included alu- minosilicates, calcium sulphates, calcium oxides and iron oxides. In the case of FBC and FGD, the Ca- and S-based compounds may be predominantly generated due to the Ca-based additives and stripping of S from the coal.
The maximum temperature and the rate of cooling influence the mor- phology and composition of CCPs (Kim, 2002). The trace elements present in CCPs include As, B, Ba, Cd, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, V and Zn (McDowell, 2005; Punshon et al., 2001; Sajwan et al., 2007; Stehouwer et al., 1999; Wang et al., 1995; Table 6.7). The partitioning of these elements is pri- marily determined by volatilization and condensation processes involved in the combustion of coal. Natusch and Wallace (1974) observed that 5–30% of toxic elements, especially Cd, Cu and Pb, are leachable. Radionuclides of uranium (U) and thorium (Th) series were reported in FA (Tadmore, 1986). Majority of FA are not significantly enriched in radioactive elements or in associated radio- activity compared to common soils or rocks (Zielinski and Finkelman, 1997).
Three groups of solid chemical components have been identified in FA (Adriano et al., 1980; Terman et al., 1978; Table 6.8), which also determines the physical properties of CCPs generated. The CCPs are also composed of two types of silt-sized glassy spheres, (1) some of the spheres are hollow, termed as cenospheres, and (2) spheres filled with smaller spheres named as plerospheres (Ghosal and Self, 1995; Sreenivas et al., 2011). Dur- ing the combustion and subsequent cooling process, many different metal oxides can precipitate and concentrate on the surfaces of these spheres.
Hence, these oxides control the chemical properties of the CCPs generated.
They may also affect the physicochemical properties of some CCPs, espe- cially the pozzalonic (cementitious) reactivity (Stewart and Tyson, 1997).
Table 6.6 Physical Characteristics of CCPs
Physical Properties FA FBC FGD
Texture Fine Fine to coarse
granules
Coarse granules
Color White Dark gray White
Particle size (àm) 0.01–100 0.2–200 1–740
Bulk density
(kg m−3) 960–1600 1520–1680 780–1250
Moisture content
(wt%) 7.8–9 21.5–27.5 8–10
Volatile matter
(wt%) 4.7 33.1 19.6
Ash content (wt%) 79.5 9.1 6.7
Reference Asokan, 2003 Wang et al., 2006 Baligar et al., 2011
CCPs P S Al Ca Fe B As Cd Cr Hg Pb Se References
FA – – – – – 1.69 7.0 0.1 38 1.3 15 2.9 McDowell (2005)
0.956–1.002 – 110–130 5.3–5.7 82–100 – 116–129 2.8–3.5 155–169 0.27–0.45 60–63 12–17 Punshon et al. (2003) 1.388–1.432 1.26–1.3 75–117 13–14 41–73 0.01–0.02 112–125 3.0–5.0 97–155 0.22–0.31 247–263 8.4–11 Adriano et al. (2002)
0.2315 2.738 4.136 8.43 13.51 0.29 – 2.8 26.8 – 2.5 – Sajwan et al. (2007)
FBC – 28 13.0 300 55.0 – – <2 50.0 – 85.0 – Karapanagioti and
Atalay (2001) 0.875 62.0 18.0 254 36.0 1.36 6.5 <0.1 <0.1 – <2.0 <3.0 Wang et al. (1995)
0.1 86.4 27.5 244 20.8 0.21 46.7 3.5 17.5 – 28.0 2.3 Stehouwer et al. (1999)
0.2 38 0.1 391 5.0 1.32 8.1 0.2 33 – 88.0 3.3 Wright et al. (1998)
FGD 0.573 85 25.5 146 85 0.35 107 <0.2 51 – 15 – Ahn et al. (2001)
– 67.1 19.6 260 16.5 0.19 118 <0.1 123 – 139 <6.0 Chen et al. (2005)
– – – 193 51 0.04 68.9 0.35 13.7 0.03 17.1 29.2 Punshon et al. (2001)
0.2 52.1 39.3 175 51.7 0.17 75.0 1.9 36.9 – 16 5.6 Stehouwer et al. (1999)
0.2 182 8.0 201 0.1 0.13 6.3 0.7 15.0 – 100 2.3 Wright et al. (1998)
– 205.4 3.3 269.1 – – 1.08 <0.1 9.70 0.65 4.08 0.84 Wang et al. (2006)
Table 6.8 Groups of Solid Components in FA
Groups Components Properties
Group I Oxides of Si, Al, Fe and Ti Low water reactivity, pos- sess surface charge Group II Metals and metalloids Adsorption to oxides Group III Oxides of Ca, Mg, K, Na, Ba, SO3,
Gypsum
High water reactivity
Adriano et al., 1980; Terman et al., 1978.
The chemical compositions of most CCPs vary according to their par- ent coal materials. In the case of FBC and FGD materials, the major dif- ference comes from the Ca-based additives during/postcombustion, and hence higher amounts of Ca and S can be observed (Table 6.7), compared to conventional CCPs (FA). The Ca concentrations ranged from 146 to 391 g kg−1 for FBC and FGD, compared to <30 g kg−1 for FA. Similarly, S concentration reached up to 105 g kg−1 in FGD, indicating the amount of S scrubbed by the CCTs. The behavior of volatile elements such as Se and Hg is highly dependent upon the burning conditions within the boiler and the sorbents used for capturing S (Punshon et al., 2003). During the combustion and subsequent cooling process, many different metal oxides can precipitate and concentrate on the surfaces of CCPs. The abovemen- tioned physical and chemical properties have driven the coal industries and environment researchers explore the possible applications of CCPs as part of the sustainable utilization strategies of these resources.