Research On Immobilization Efficiency Of Cr⁶⁺ By Alkali-Activated Slag Cement

As hexavalent chromium has very strong toxicity which isconsidered to be carcinogenic,also has high-solubility and mobility characteristics,the safe disposal of hexavalent chromium wastes has great positive significance to the society.The solidification/stabilization(S/S) technology is one of the most important methods as a treatment technique to isolate hazardous wastes from the environment.Compared to ordinary Portland cement which the most commonly used for solidification/ stabilization,alkali-activated slag cement(AASC) possess the characteristic of low porosity,high strength,small pore and good durability,which must be the more appropriate.Therefore,the investigation of the immobilization efficiency and immobilization mechanism of Cr6+ ions by alkali-activated slag cement has a great significance to promote the application of AASC in the disposal technology of heavy metal-contaminated wastes,also with some academic value and broad application prospect.In this project,compared to ordinary Portland cement,alkali-activated slag cement stone(AASC) was used to investigate the effect of main factors such as alkali activator type,Na2 O equivalent, waterglass module(n), water binder ratio(W/S),solid ingredient on the immobilization efficiency of Cr6+ ions.The products and the microscopic structure of AASS solidification were identified by XRD, SEM, TG-DSC and so on, the pore structure of AASC solidification analysised by using nitrogen absorption measurement.The analysis of hydration kinetics andpore solution also used to investigate the solidification mechanism of Cr6+ ions. The main results of the study are as follows:The immobilization efficiency of Cr6+ ions by AASCwas much better than that of OPC,mainly because of the reducing substances in slag give rise to partial reduction of Cr6+ to Cr3+and the hydration products of AASC(calcium silicate hydrates with a low Ca/Si ratio and a small amount of hydrotalcite-like compounds)hydrotalcite also plays a positive role in the immobilization process by adsorption and isomorphous substitution. At the same time,the micropore of AAS is beneficial to the physical inclusion of Cr6+.There is a relation between Na2 O equivalent and module of Na2O?n Si O2 with immobilization efficiency of Cr6+ by the alkali-activated slag cement.While solidifying 1% Cr6+,the Na2 O equivalent is 3 ~ 6%,the leaching concentration of Cr6+firstly decreases, then increases.While the modulus values of water glass is 1.0~2.48, the leaching concentration of Cr6+from solidification body increases with the improvement of modulus values.When Na2 O equivalent is 5%, modulus values is 1.5,content of Cr6+is 1%, in the range of 0~30%,lowamount of clinker and fly ash make for the leaching concentration of Cr6+ decreasing.While the content of Mg O increased in the range of 0~10%, the leaching concentration of Cr6+ decreases. The replacement of slag with 5% Ca(OH)2 brings out the compressive strength increasing in early age but dropping later,and the leaching concentration of Cr6+ decreasing sharply.