Leaching Behavior of Hazardous Heavy Metals from Lime Fly Ash Cements

Abstract

The heavy metals leaching potential of lime fly ash cements was evaluated for one low loss on ignition (LLOI) and one high loss on ignition ash (HLOI). Three different test methods were used: (1) a standard shaken (stirred) extraction test, (2) a modified shaken extraction test, and (3) a modified EP-TOX test. Testing was conducted on the as-receive ashes and lime fly ash samples cured for 0, 7, 28, and 84 days at 40°C. The results show that such curing time sequences can be used to study the rate of sequestration by the hydration product relative to the rate of leaching from the ash. X-ray diffraction (XRD) was used to characterize the reaction products of cured lime fly ash cements and isothermal calorimetry to study the rates of hydration at 25 and 40°C. The leaching tests results indicate that leachability of some heavy metals is greatly dependent on leachant pH and likewise, the test method used. Furthermore, formation of hydration products throughout the leaching does appear to partially sequester some heavy metals. The XRD shows that major hydration products include ettringite and an unidentified calcium aluminosilicate hydrate (CASH) phase, depending on the ash source. Isothermal calorimetry results indicate that hydrations of LLOI fly ash is measureable at 40°C but slow at 25°C, whereas HLOI fly ash hydrates with much slower yet detectable rates.