Table 10.4 gives the results of the monolith leaching test on cubic specimens of cement pastes (water-to-binder weight ratio 0.50) made with blended cement [70% (w/w) Portland cement and 30% (w/w) WBFA]. In this table, the concentrations of selected heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) in each of eight leachates are reported as the average values of three replicate leaching tests.

With respect to the other heavy metals investigated, the higher concentrations of copper, lead, and zinc measured for most leachates were directly related to their higher contents in the original fly ash (Table 10.1).

Using the data in Table 10.4, we calculated the leaching rate of each of the selected heavy metals as an average within each leaching period, and these rates are reported in Fig. 10.3 for each of the eight leachant renewals.

For copper, lead, and zinc, the leaching rates dramatically reduced after the first leachant renewal (the first two renewals for Zn), thus revealing the existence of two different mechanisms governing the leaching process of such heavy metals. At early leaching times (first two renewals), the controlling mechanism appeared to be the release of heavy metal from the outer surface of the monolith specimen by dissolution into the leaching solution or by wash-off, or both. At longer leaching times, the release was probably controlled by diffusion, and the heavy metal ions had to migrate within the pore liquid of the cementitious matrix of the test specimen prior to reaching the liquid bulk. As a result, this leaching phase was characterized by a much lower rate as compared with the initial leaching phase. In the case of
cadmium, chromium, and nickel leaching, no dissolution/wash-off phenomenon was detected during the early release phase.

As shown in Fig. 10.3, after the first or the second leachant renewal, the release rate of each heavy metal did not significantly vary with increasing leaching time. Thus, the high concentrations of heavy metals measured for the seventh and eighth leachates (Table 10.4) were attributable to the much higher contact times between the specimen and the leachant (20 and 28 days for the seventh and eighth renewals, respectively).

With use of the results in Table 10.4, the cumulative mass of each heavy metal released per unit exposed surface area of specimen, Mt (mg/m2), was also calculated and is plotted in Fig. 10.4 as a function of the square root of the cumulative leach time, t (h1/2).

For the leaching of cadmium, chromium, and nickel, there existed straight line relationships between Mt and t1/2, with no intercept on the coordinate axis. This is typical of leaching processes controlled by the diffusion mechanism. Conversely, for copper, lead, and zinc leaching, linear relationships with positive intercepts on the ordinate axis were obtained. This is typical of leaching processes controlled initially by dissolution or wash-off phenomena, or both.

To predict the long-term release of copper, lead, and zinc from monolithic speci­mens, the leaching data in Table 10.4, relative to these metals, were considered over the leach time interval for which diffusion was the release-controlling mechanism. In other words, the first two leachant renewals were considered as preconditioning steps for the subsequent leaching test. In this way, straight line relationships between Mt and t1/2 were obtained for the release of copper, lead, and zinc, as shown in Fig. 10.5.

With use of the linear regression equations resulting from the data in Figs. 10.4 and 10.5, the releases of heavy metals after 100 years of leaching were estimated

and compared with the standard limits (Category I applications) as specified in the Dutch Building Materials Decree (1995). These specifications are commonly taken as a reference for evaluating the environmental quality of cement-based materials incorporating hazardous wastes. Figure 10.6 compares the estimated releases of the selected heavy metals with the Dutch standard limits.

As can be noted, all the releases were well below the corresponding regulatory limits and this proved the good immobilization capacity of heavy metals by the cementitious matrix investigated and, consequently, the good environmental qual­ity of the blended cement formulated with 30% (w/w) WBFA.

10.2 Conclusions

The WBFA is characterized by a significant content of heavy metals of particular environmental concern, such as cadmium, chromium, copper, nickel, lead, and zinc, and by a remarkable amount of water-soluble compounds, such as alkalies, chlorides, and sulfates.

According to the European chemical requirements established for reuse of coal fly ash as a mineral admixture in cement-based materials, the biomass fly ash studied appears to be suitable for the formulation of blended cements, provided that its chloride content be preliminarily reduced.

As indicated by the results of the water elution test on WBFA, a single-stage washing treatment of this ash with deionized water might be sufficient to reduce the chloride content to acceptable levels.

As evidenced by the results of the monolith leaching test on hardened pastes of blended cement [70% (w/w) Portland cement-30% (w/w) WBFA], in spite of the high content of water-soluble compounds of WBFA and the acid pH conditions of the leachant throughout the test (pH 6.0), very low releases of heavy metals were always obtained, thus revealing a high metal immobilization capacity by the cementitious matrix and, consequently, a good environmental quality of the blended cement investigated.

For some heavy metals such as copper, lead, and zinc, the release from a monolithic specimen appears to be governed by two different leaching mechan­isms: dissolution/wash-off at earlier leach times and diffusion at longer leaching times. Conversely, in the case of cadmium, chromium, and nickel leaching, no dissolution/wash-off phenomenon was detected.