SO_4^(2-)对复合水泥固化处理Cr(Ⅵ)的影响

INTERFERENCE OF SO_4^(2-) ON SOLIDIFICATION/STABILIZATION OF Cr(Ⅵ) USING COMPOSITE CEMENT

由于CrO42-和SO42-的化学性质很相近,在利用含石膏的水泥固化处理Cr(Ⅵ)时,SO42-会极大地干扰水泥水化物与Cr(Ⅵ)之间的化学作用,进而影响到水泥对Cr(Ⅵ)的固化效果。合成含铬钙矾石的实验表明:在SO42-充足的情况下,CrO42-无法进入钙矾石晶格。用合成的水化硅酸钙(C S H)对CrO42-进行吸附的实验表明:混凝土中的C S H对CrO42-的吸附量不大,说明在水泥固化Cr(Ⅵ)时,C SH对Cr(Ⅵ)的固化不起主要作用。用毒性浸提程序和固化体无侧限抗压实验检测复合型水泥对Cr(Ⅵ)的固化效果。结果发现:在搅拌水中Cr(Ⅵ)的掺入浓度高达4.000 g/L,不含石膏复合水泥的胶砂试件28 d龄期的粉碎样浸提出的Cr(Ⅵ)的浓度已低达0.2 mg/L,在添加Na2CrO4后,胶砂的抗压强度明显提高。掺10%石膏的复合水泥对Cr(Ⅵ)的固化效果不如未掺石膏的复合水泥。

Because the chemical characteristic of SO4^2- is similar to that of CrO4^2- , it would seriously interfere with the chemical reactions between cement and chromate during cement-based solidification/stabilization of haxavalent chromium when gypsum-containing cement is used. Synthetic chromate-containing ettringites experiments indicate that CrO4^2- has little chance to incorporate into ettringite when sulfate is abundant in the synthetic solution. Experiments on chromate binding onto synthetic C-S-H show that the capacity of the adsorption of chromate by C-S- H is very low, suggesting that C- S- H does not play a significant role in cement based solidification/stabilization of soluble haxavalent chromium. Batch leaching test prepared according to the toxicity characteristic leaching procedure and unconfined compressive strength test of waste forms suggest that hexavalent chromium is strongly stahilized in composite cement. Although the concentration of Cr（Ⅵ） in mix water reached 4.000 g/L, it decreased to 0.2 mg/L in leach from 28 d age powdered mortar sand cubes made from composite cement without gypsum. The compressive strength of hard cement mortar-cubes also increased significantly after they contained sodium chromate. Experiments also show that chromate is more effectively solidified by composite cement without gypsum than by composite cement with 10% gypsum.