Solidification Behavior Of Heavy Metal Ions In Portland Cement Hydration Products

In recent years, cement kiln co-processing MSW and solid waste has got widely attention at home and abroad. However, some harmful heavy metal of waste exists in cement, during complex hydration these heavy metal will interaction with it in the form of ion. So the solidification behavior and mechanism of heavy metal ions during cement hydration is necessary to further study. To cure the above problems, this paper studied the solidification behavior of common heavy metal ions Cd2+, Cr3+ and Pb2+ in Portland cement hydration via three systems.A series of C-S-H and doped with Cd2+, Cr3+ and Pb2+ samples were prepared by two chemical methods. Revealed the solidification mechanism and ways of interaction with C-S-H of heavy metal ions in the reaction system by XRD, IR, SEMEDS and 29 Si NMR. All the samples with different molar rarios of Ca to Si have incorporated heavy metal ions, the difference of solidification mechanism was the samples with higher Ca/Si ratios and heavy metal ions occurred precipitations of heavy metallic salt when initial concentration of raw material exceed solubility product constants of corresponding precipitations. Besides, the solidification ways of different heavy metal ions also exsited diversities: Ca2+ both in calcium oxygen layer and interlayer of C-S-H could be substituted by Cd2+. The large shrink of calcium oxygen layer structure which has stronger atomic interaction compared to interlayer structure, which made the interlayer space increase, the replacement rate reached up to 18%. Considering balance charge, the way of Cr3+ substitution in samples should be 2Cr3+→Ca2++Si4+, which caused greater destruction. When the dosage was larger, amorphous silicon oxide phases occurred, and C-S-H became more disorder and finer. BSE-EDS showed the replacement rates of Ca2+ and Si4+ by Cr3+ in Cr-C1.0-0.2 sample were 8.98% and 6.24%, respectively. Pb2+ mainly was solidified into the interlayer of C-S-H, and could connect to the end of C-S-H chain, the replacement rates of Ca2+ by Pb2+ also reached up to 18%, which made C-S-H more amorphous, amorphous silicon oxide was also found when the dosage was high.In the system of C3 S hydration doped with heavy metal ions, as the dosages of heavy metal ions from 0% to 3%(the mass ratios of heavy metal element to C3 S, the same as below), Cd2+ and Pb2+ accelerated the hydration process of C3 S. For Cr3+, the hydration process of C3 S was accelerated when the dosages were less than 1.5%, while hindered when the dosages were more than 1.5%, especially when the dosage reached up to 3%, the production of Ca Cr O4·2H2O crystal phase consumed a large amount of Ca2+, OH- ion and water, and the surface of C3 S particles was covered by portion of this phase, which made the hydration process of C3 S was seriously suffocated, and the proposed electric double layer model of C3 S hydration well explained these phenomena. For hydrated 4 days C3 S dope with 3% heavy metal ions, except the hydration extent of C3 S paste doped with 3% Cr3+ was 23.92%, all the others reached up to 60%. However, C-S-H chain lengths of all samples were less than 2.4 account for only 4 days hydration age. The results of SEM-EDS showed that Cd and Pb uniformly distributed in hydration products, while Cr-rich phase Ca Cr O4·2H2O occurred for C3 S paste doped with 3% Cr3+.Besides, concerning the solidification of heavy metal ions in Type I Portland cement/slag hydration. Under the normal consistency of Type I Portland cement, the setting time was shortened as the dosages of Cd2+ and Cr3+ increasing, while the one was prolonged as Pb2+ increasing. The addition of Pb2+ made early strength of pastes decrease, while 28 d strength increase in Type I Portland cement hydrating system. However, in the hydrating system of Type I Portland cement blend with slag, both early strength and long-term strength of pastes decreased, and this effect enhanced as the addition of slag increasing, which related to the common delayed coagulation of slag and Pb2+, led that pastes were easily subjected to carbonification destruction. The additions of Cd2+ and Cr3+ made strength of pastes with different ages decrease wherever systems. The early compressive strength of pastes with 0.3% Cd2+ and Cr3+ decreased as additions of slag, but 28 d strength occurred maximum at the dosage of slag was 20%. XRD and SEM have confirmed this attributed to the pozzolan effects. The statistical method of EDS Spots analysis effectively measured the contents of heavy metal in different hydration products, the ranks from highest to lowest about the contents of chromium, cadmium and lead were, respectively: AFm/AFt > “outer” C-S-H > “inner” C-S-H > CH; CH > “outer” C-S-H > “inner” C-S-H > AFm and AFm > “outer” C-S-H > “inner” C-S-H >> CH.