10.3.1 Chemical Characterization ofWBFA

As shown in Table 10.1, the WBFA was characterized by a significant presence of heavy metals of particular environmental concern, such as cadmium, chromium, copper, lead, and zinc. Zinc was the predominant heavy metal (2,274 mg/kg), whereas cadmium was the heavy metal with the lowest concentration (9 mg/kg).

With regard to the reuse of WBFA in cement-based materials, it must also be considered that several other chemical species, such as chlorides, sulfates, alkalies, and magnesium oxide contained in this waste (Table 10.1), may exert adverse effects on cement hydration and concrete durability.

It is undoubted that the presence of a significant amount of water-soluble compounds, such as chlorides and alkalies, could promote the formation of a high porosity within the hardened cementitious mixes, thus penalizing mechanical strength development and durability. High contents of water-soluble chlorides can also be deleterious for steel-reinforced concrete, since they will promote the corrosion of iron reinforcing bars. High contents of available alkalies (i. e., the amount of alkalies released into the pore liquid of cementitious matrices) are responsible for the development of deleterious expansion associated with alkali-silica reaction in concretes made with aggregates containing some alkali-reactive forms of silica or silicate, or both. Deleterious expansive phenomena in concrete can also arise from very slow dissolution of significant amounts of sulfates or magnesium oxide, with subsequent precipitation of very expansive phases, such as ettringite (CaSO4-32H2O) and brucite [Mg(OH)2].

In the USA, the current restrictions concern the use of fly ashes originating from combustion of fossil fuels (bituminous and subbituminous coal, peat, and lignite), and these restrictions are specified in the ASTM C 618 method. In Europe, the current restrictions concern both the fly ashes derived from fossil fuels and those resulting from the combustion of biomass and fossil fuel blends (cofiring), with a biomass content not higher than 20 wt%. These restrictions are specified in the EN 450-1 method.

For the above reasons there exists no standard method dealing with the chemical requirements for reuse of fly ash from pure biomass combustion in cementitious mixes. However, it is reasonable that the quality of fly ash from pure biomass burning should follow the same regulation as fly ash from fossil fuel combustion.

Table 10.3 compares the specific chemical characteristics of the WBFA (also reported in Table 10.1 in terms of elemental composition) with the corresponding

limits (chemical requirements) specified by ASTM C 618 and EN 450-1. In this table, the chemical characteristics of the solid residue resulting from the elution of WBFA with deionized water (elution test) are also reported.

The chemical characteristics of this solid residue, also referred to as washed WBFA, were evaluated on the basis of the results of the elution test reported in Fig. 10.2, in terms of percentage removals of alkalies, chlorides and sulfates, and percentage weight loss of WBFA.

As shown in Table 10.3, with exception made for the loss on ignition, the American specifications appear to be less severe than the European specifications. In particular, ASTM C 618 does not restrict chloride, free calcium oxide, magne­sium oxide, and soluble phosphate contents. Moreover, the sulfate limit (5%) is more than the maximum allowable (3%) by European regulation. As far as the alkali content is concerned, the American limit [1.5% (w/w) as available alkalies] appears to be comparable to the European limit [5.0% (w/w) as acid-soluble alkalies] if it is considered that, depending on the type of coal fly ash, the content of available alkalies, as determined by the ASTM C311 test method, may vary from 20 to 50% of total alkalies (acid-soluble alkalies), as determined by the EN 196-2 test method (Berra et al. 1992).

According to the chemical requirements prescribed by EN 450-1, which are mostly more severe than those required by ASTM C 618, and remembering that, to date, these requirments do not apply to pure biomass fly ashes, the WBFA was found to meet all chemical requirements, except for the chloride content (1.07% against the limit of 0.10%) (Table 10.3).

In spite of the high release of water-soluble chlorides (82.2% removal) accom­panied by a relatively low percentage weight loss of fly ash (12%) (Fig. 10.2), even the washed WBFA failed to meet the chemical requirement for chlorides, but the chloride content (0.20%) was only slightly higher than the limit of 0.10% (w/w) (Table 10.3).

As compared with WBFA, washed WBFA was also characterized by much lower contents of alkalies [1.43% (w/w) Na2Oeq against 3.7%] and sulfates [0.75% (w/w) SO3 against 2.9%]. Conversely, washed WBFA was more rich in MgO [3.9% (w/w) against 3.5% for WBFA], but the MgO content remained below the limit of 4.0% (Table 10.3). The washed WBFA could also be richer in heavy metals than WBFA, in consideration of the expected low release of these substances into aqueous solutions.

As far as the use of washed WBFA in cement-based materials is concerned, it is likely that a washing treatment of fly ash with a liquid-to-solid ratio above 10 L/kg could reduce the chloride content below the 0.1% limit (Table 10.3). In that case, the wastewater resulting from the washing of fly ash could contain chloride and sulfate concentrations below the limits established for wastewater disposal. How­ever, this wastewater should be treated for pH correction and, probably, for heavy metal removal. In this regard, the pH of WBFA, defined as the pH of the aqueous suspension of fly ash with an L/S ratio of 10 L/kg, was 12.9. This high pH, which is compatible with the use of WBFA in cement-based materials, was attributable to the release of alkalies and calcium oxide from fly ash.

The electrical conductivity of the eluate (1.53 S/m) evidenced significant release of electrolytic compounds from WBFA.