基于加速老化试验的铝酸钙水泥基复合材料固化飞灰长期稳定性研究

Research on long-term stability of fly ash solidified by calcium aluminate cement-based materials through accelerated ageing test

酸雨淋滤是影响飞灰固化体长期稳定性的重要因素。在铝酸钙水泥(CAC)中加入磷酸二氢钠(NaH_(2)PO_(4))和二乙基二硫代氨基甲酸钠(DDTC)得到CAC基飞灰固化/稳定化复合材料,基于加速老化试验研究了飞灰固化体的长期稳定性,通过浸出试验研究了重金属Cd、Pb、Zn的浸出毒性,进行连续提取试验研究了重金属化学形态变化,通过X射线衍射和扫描电镜分析揭示了相关机理。结果表明,固化体中Cd和Pb的浸出质量浓度在模拟老化的第26年达到最低,之后浸出浓度随时间增加,到第78年超出了限值;在模拟的104 a中Zn的浸出质量浓度一直增大。经CAC基复合材料固化/稳定化后,飞灰中Cd、Pb和Zn的有机结合态和残渣态占比增加,铁锰氧化物结合态占比减小;随着老化进行,铁锰氧化物结合态逐渐增多而残渣态减少,导致浸出浓度逐渐增加。CAC的水化产物、磷酸盐沉淀以及络合物使飞灰中团聚体增多、孔隙减小,从而减少了重金属浸出;加速老化试验之后,这些成分减少甚至消失,对重金属的封固效果减弱。本文可为飞灰长期安全处置提供理论依据和技术参考。

The leaching under acid rain is an important factor that affects the long-term stability of solidified fly ash.The sodium dihydrogen phosphate(NaH2PO4)and sodium diethyl dithiocarbamate(DDTC)are added to the calcium aluminate cement(CAC)to obtain CAC-based materials for solidification of fly ash.The long-term stability of the cured fly ash is investigated through the accelerated ageing tests.The leaching toxicities of Cd,Pb and Zn are studied by leaching tests.The sequential extraction tests are conducted to investigate the chemical species of heavy metals.X-ray diffraction and scanning electron microscopy tests are used to analyze the mechanism involved.The results show that the leaching concentrations of Cd and Pb are the lowest at the 26th year.The concentrations increase afterward and exceed the limits at the 78th simulated year.The leaching concentration of Zn increases continuously during the simulated 104 years.After cured by the CAC-based materials,the heavy metal bound to organic and residual increases,and that bound to Fe-Mn oxides decreases.Along with ageing,the heavy metal bound to Fe-Mn oxides increases and the residual decreases,resulting in an increase in the leaching concentration.The CAC hydration products,phosphate precipitation and complexes increase the amount of aggregates and reduce the pores,thus reducing the leachability of heavy metals.After the accelerated ageing tests,these materials decrease or even disappear,resulting in a decrease in the encapsulation effects.This study provides theoretical basis and technical reference for the long-term safe disposal of fly ash.