粉煤灰和粉煤灰水泥石的汞释放规律

Releasing Behavior of Mercury from Fly Ashes and Hydrated Fly Ash Cement Pastes

采用3种不同来源的粉煤灰,与普通硅酸盐水泥按质量比1:1混合制备水灰比为0.4的水泥浆。养护28 d后,采用自制的汞测试系统研究了硬化水泥浆粉末的汞释放行为。结果表明:在90d的测试周期内,零价汞蒸气从粉煤灰及水泥石中持续释放;在常温自然条件下,3种粉煤灰在90 d内的汞释放量最大达到39.7 ng/g,而3种硬化水泥浆粉末的汞释放量最大达到18.5ng/g。硬化水泥浆粉末和粉煤灰的汞释放呈相似的动力学过程,表明水泥石中的C-S-H凝胶对零价汞的固化效果非常小。水泥中的水化产物对零价汞的释放仅仅起到稀释作用和物理包裹作用,并无化学固化作用。汞的释放速率同时受到颗粒内扩散和表面脱附过程的影响。温度和湿度的增加会显著促进汞的释放速率。

Three kinds of fly ashe were selected to prepare cement pastes via mixing ordinary Portland cement(OPC) with the same amount of fly ash. Cement pastes were prepared at a fixed water to the blend cement ratio of 0.4. After curing for 28 d, the hardened cement pastes(HCP) were ground into fine powders. The releasing behavior of elemental mercury(Hg0) from the fly ash and the powdered HCP was analyzed by a homemade Hg measurement system. Detectable Hg0 was found to be released from both fly ashes and hydrated fly ash cement pastes during 90-d period. The Hg release amount of fly ashes at room temperature is 39.7 ng/g fly ash at the maximum during 90-d period. In contrast, the Hg release amount of the HCPs is 18.5 ng/g HCP at the maximum. The Hg release kinetics of HCPs and FAs are similar, indicating that the hydration products of cement present little immobilization effect on Hg0. The Hg releasing behavior from FA and HCP samples indicates that the hydration product of the HCP only has effects on the dilution and physical encapsulation of Hg0. The chemical immobilization of Hg0 by C-S-H does not occur. The Hg0 release rate is affected by the intragranular diffusion process and the desorption process from the granule surface. Elevated temperature and increased moisture content both can accelerate the Hg0 release.