The aim of the present study was to investigate the influence of heavy metals on the polymorph transformation of tricalcium silicate.Heavy metal(0.1wt% to 3.0wt%) of Cr,Zn,Cu,Ni and Pb(in oxides form) was added into the raw mixtures and then sintered together three times at 1450 ℃ for 2 h.The f-CaO content of doped C_3S was determined by the glycerol-ethanol method,and their polymorph transformation was investigated by means of XRD and FTIR.Thermal analysis(DTA/DTG) was conducted to determine the reaction temperatures and mass losses during the sintering process of raw mixtures.The concentration of heavy metal in C_3S after sintering was determined by ICP-AES.The experimental results indicate that heavy metal doping contributes to a higher symmetry of C_3S except for Pb.Addition of up to 3.0wt%,Cr will lead to a decomposition of C_3S into C_2S and CaO;Zn will cause a transformation from T1 to M2 polymorph,and then to R polymorph;Cu and Ni cause a gradual transformation from T1 to T2 and then to M1 and/or M2 polymorph.During the sintering process,all the Pb releases into atmosphere because of evaporation.
The relationships between microstructure and melting temperature of slag containing different heavy metals(Zn, Cu, Pb and Cr) were studied. Furthermore, the corresponding solidification mechanism and rule of heavy metals were analyzed by microscopic tests during melting and reconstructing process. Based on preliminary results, three conclusions were derived. Firstly, pure slag would begin to melt when the temperature reached 1 180 ℃; however, Zn did not play any fluxing action. Secondly, upon adding Cu and Pb, the initial melting temperature of slag decreased by 5-8 ℃ and their fluxing effect was observed. Thirdly, the initial melting temperature and the reaction time for slag decreased by 22 ℃ and 6 s respectively after adding Cr; the fluxing action was significant under Cr. The results of X-ray diffraction(XRD) and Fourier transform infrared spectroscope(FTIR) analyses showed that the above heavy metals had little influence on the reconstruction of slag. Toxicity characteristic leaching procedure(TCLP) leaching tests showed a good solidification effect of the heavy metals with melting slag, fixation rate of Zn, Cu, Pb and Cr was 36.3%, 24.6%, 9.2% and 93.2%, respectively. The leaching toxicity of the heavy metals met the requirements for environmental emission after solidification treatment.
