Leaching behavior of hazardous heavy metals from lime fly ash cements.

摘要

The heavy metals leaching potential of lime-fly ash cements was conducted for one low loss on ignition (LOI) and one high LOI ash. These simple cements may find use in highway construction as a component of pavement bases. Three different test methods were used: (1) shaken (stirred) extraction test; (2) modified shaken extraction test; and modified EP-TOX test. Testing was conducted on lime-fly ash samples cured for 7, 28 and 84 d at 40ºC.;The modified shaken extraction test was able to maintain a consistent pH between 5.5 and 6.0 thereby decreasing the effect of leachant pH variation on heavy metal leaching. The modified EP-TOX test, however, had a pH that ranged between 7 and 8 and was more variable. The results mainly indicate that leachability of some heavy metals is greatly dependent on leachant pH.;X-ray diffraction (XRD) was used to investigate the crystallography of the as received ashes and the cured lime-fly ash cements. The major reaction products of hydration include ettringite and an unidentified calcium alumino silicate hydrate (CASH) phase. The XRD peak intensities for these product phases were greater for 84 d samples as expected, indicating product formation with increasing time.;The rate of hydration reaction was also compared using isothermal calorimetry. At 40ºC the low LOI fly ash shows a large hydration peak (dissolution peak) between the time of mixing and about ten hours and another smaller peak (rapid hydration peak) after about 80 h while the high LOI fly ash exhibits only a small initial dissolution peak between the time of mixing and about ten hours. The comparison of calorimetry curves for 40°C and 25°C shows that the LOI fly ash rate of heat evolution increases with increasing temperature but the high LOI fly ash reaction rate is surprisingly sluggish at both 25ºC and 40ºC.