| In most developed countries, municipal solid waste (MSW) is usually incinerated to reduce the volume. In recent years, MSW incineration technology has also received a rapid and positive development in the major cities of China. After the incineration,20-30% of the combustion residues such as fly ash and bottom ash are produced. Fly ash is defined as hazardous waste because it contains a large amount of heavy metals and possible organic pollutants such as dioxin. Bottom ash belongs to general waste but toxic and harmful elements are also present. Since improper treatment of MSW incineration (MSWI) ash is hazardous to human health and the surrounding environment, a safe and reasonable treatment is very urgent and desirable. In the past few decades, the use of cement based stabilization techniques is a common and cost-effective method for stabilization of a variety of wastes and even hazardous wastes. Therefore, performing the studies on the immobilization/stabilization of MSWI ash with cement is of important social significance and also economic value.In this study, the influence of MSWI ash addition on cement production process and product performance, as well as the impact of the final product on environment was comprehensively and systematically carried out without altering the present cement industry equipments, processing technology, hopefully providing the theoretical foundation and technical support for the application and promotion of the cement industry cooperative disposal MSWI ash technology. Research work is mainly focused on the several aspects as follows:1) analyze the physical and chemical properties of MSWI ash and propose a possible application method in cement industry; 2) investigate the volatile and condensing process of several elements including alkali, chlorine, and sulfur at high and low temperatures, respectively. On the basis of this study, a model can be developed to simulate the volatile and condensing process. Furthermore, the impact of the process on the operation of cement kiln system is assessed; 3) study the influence of MSWI ash on the cement production process and performance of final product. Besides, bring out technical measures to ensure the solidification process and product performance to meet the requirements of the national standards; 4) study the volatile characteristics of heavy metals during the MSWI ash treatment process as well as the leaching behavior of heavy metals from cement product. Besides, evaluate the impact of this process on the environment; and 5) study the solidification mechanism of heavy metals in cement clinker and hardened cement paste in order to provide a theoretical basis for the long-term utilization of these products. The analysis results of MSWI fly ash and bottom ash from Likeng in Guangzhou show that both ashes are non-radioactive materials and mainly composed of SiO2, CaO, Al2O3 and Fe2O3. They could partially replace raw materials for cement production. However, it is expected that the cement production process could be negatively affected due to the higher content of alkali, chlorine and sulfur in the ashes. The concentration of most heavy metals in both ashes is beyond the limit for typeâ…¢soil in GB15618-1995, "Environmental quality standards for soil" and most of them are present in transferable form, carbonate and Fe-Mn oxides. Therefore, these heavy metals would be easily leached out in low-pH test. It is notable to mention that the leaching toxicity of Pb from fly ash is beyond the limit stipulated in GB5085.3-2007, "Standard for identification of hazardous waste-leaching toxicity" and thus the fly ash is classified as hazardous waste. In addition, the leaching concentration of heavy metals from bottom ash is also beyond the limit in GB/T14848-93, "Standard for underground water" and thus the leaching solution is classified as type V. These results demonstrate that the MSWI ashes would pollute the environment and even dangerous to the surroundings, thus the solidification/stabilization treatment is really desirable.According to the physical and chemical properties of ashes, fly ash and bottom ash are treated in different methods. Fly ash is injected from the head of the cement kiln with calculated prescription, which can not only ensure the tightness of fly ash in the process but also effectively eliminate the possible pollution of dioxin to environment, making the calcination environment of cement kiln in line with the requirement called "3T+E" for decomposition of dioxin. Bottom ash can be used as a replacement for raw materials or admixture in cement. When the bottom ash is mixed with raw materials, the results show that the high volatilization rate of chlorine and alkali will negatively affect the operation of cement kiln. Our volatilization-condensation model presents that build-up caused by alkali, chlorine, and sulfur mainly occurs near the 5th preheated cyclone and thus more frequent cleaning is needed. Since Cd and Pb are volatile elements, their content in the flue gas should be monitored in a much stricter way. Lowering the SIM and AM of raw meal can help decreasing the volatilization rate of alkali, chlorine, sulfur and heavy metals, thus reducing the harm to the kilns and environment. Results also show that the partial replacement of MSWI ash as raw materials can improved the burnability of cement raw meal while exert no large influence on the clinker composition and performance of final products. When bottom ash is used as admixture, it is found that the compressive strength of cement mortar is decreased and the compatibility with other admixture is not affect, i.e. saturation point is not changed, but loss of fluidity with time is increased. Besides, the shrinkage behavior of mortar is better than that of cement with coal fly ash and will not cause steel corrosion. The solidification of heavy metals with cement show good results. For the cement sintered from doped raw material, i.e. 4.6% of fly ash or 5% of bottom ash, and the cement with 15%-80% of bottom ash replaced as admixture, both the 28-d hardened mortar the mortar specimen after carbonation are prepared and tested in pure water or stimulated acid rain leaching. The leaching behavior of heavy metals of the two samples in either broken-mortar leaching test or surface leaching test can totally meet the requirements of typeâ…¢water according to GB/T14848-93. These results provide theoretical foundation and technological supports for MSWI ash treatment in cement industry under present cement industrial equipment and production process.In this study, several different methods including Continuous Acid Extraction (CAE), XRD,DSC,SEM and EDS were used to analyze the solidification and distribution state of heavy metals in the co-processed cement clinker and hardened cement paste. Based on the results, the relationship between the state of heavy metals and various cement hydration products including Ca(OH)2,sulfur/aluminate,C-S-H and also silicate,aluminate, ferrite minerals in cement clinker were revealed. The main results include:(1) The elements of As, Ba, Cd, Cu, Pb, and Zn in fly ash are mainly dissolved in silicate minerals while Cr and Mn in the silicate minerals and intermediates evenly; (2) In hardened cement paste, the heavy metals carried in by clinker are mainly solidified in the C-S-H. Part of Ba and Cr are present in the form of hydroxide precipitation. Some amount of Cd is combined in Ca(OH)2 while some of Cr in the sulfur/aluminate hydrates; 3) In the heavy metals carried in by bottom ash replaced admixture, As, Cu, Mn, Zn, and most of the Pb are mainly maintained in the ash while most Ba, Cd, Cr, and Pb migrates and are solidified in C-S-H and some Ba and a small amount of Cr still exist in the form of hydroxide precipitation; and 4) Hardened cement paste can well solidify the heavy metals carried in by either ash-substituted clinker or admixture. At the age of 90d, negligible heavy metals can be detected in the pore solution. A mass of heavy metals cannot dissolve until at a pH of less than 11 and collapse of paste. When the pH is around 11 but the paste is still intact, the leaching of heavy metals can still meet the requirement for type IV underground water. The results clarify the solidification/stabilization mechanism of heavy metals by cement and its hydration products and also provide a theoretical basis for the long-term use of co-processed cement products.The Continuous Acid Extraction (CAE) method developed in this study improves the traditional acid neutralization capacity (ANC) test. Through CAE, separation of different minerals in clinker from hydration products in silicate cement or admixture-added cements can be realized because different materials decompose at different pH ranges:Ca(OH)2 at pH of around 12; sulfur/aluminate hydrates at pH ranging from 11 to 12; C-S-H at pH of 7 to 11; silicate minerals at pH higher than 7; aluminate hydrate at pH of 3.5 to 5; and ferrite at pH lower than 3.5. The results demonstrate that CAE method is valid for the analysis of solidification and distribution of heavy metals in cement-based materials. This method provides an alternative technical method for cement chemistry research.The two solutions proposed in this study to dispose MSWI ash have already been adopted in pilot scale in the Guangzhou Zhujiang cement factory with clinker production of 5000 ton/day. The actual operation results show that when 2.2% bottom ash added as raw materials or 5% as admixture, the production process has been altered significantly. Even though the volatilization of alkali and chlorine would aggravate the "build-ups" at 5th preheated cyclone, production process will not be affected after appropriate cleaning. The concentration of dioxin, Pb, Cd, Hg, SO2, NOx and dust emitted from cement kiln is under the national standard limits and thus will not bring any harm to environment. Leaching of heavy metals whether from broken sample or intact sample can meet the requirement of typeâ…¢underground water in GB/T14848-93 and it will not bring any water pollution.In this study, the MSWI ashes were appropriately treated in cement industry:fly ash is injected into the kilns after calculation and bottom ash is used as partial replacement of cement raw material or admixture without altering the present cement industry equipments, processing technology. The results demonstrate that not only the performance of final product can be guaranteed but also the meet the national standards including production, environment and safety of product. The solidification and distribution of heavy metals in clinker and hardened cement paste doped with MSWI ashes are preliminarily studied which can serve to provide a theoretical basis for the long-term use of these products. Research of this paper and successful implementation of the pilot practice can offer cement factories a model framework and a useful reference to co-process different kinds of waste (especially the dangerous wastes) from various aspects, such as planning, implementation and environmental safety evaluation.
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