Research On Preparation And Properties Of Building Materials From Shale And Sewage Sludge

The great amount of sewage sludge with high water content, which comes from municipal sewage treatment process, has already become one of the critical environmental problems in rapid urbanization. The sewage sludge treatment methods, landfilling, composting, incination, etc., are either unthorough in harmless treatment or high in costs, thus, it is hard to handle all the surging sewage sludge. Therefore, it is critical to develop new ways of utilization. In this paper, the property requirements of raw materials to make shale building materials and their burning characteristics were fully considered. High water content sewage sludge was well mixed with widely available shale, and the water in sewage sludge can be used as the imperative water source of making shale building material. Meanwhile, the organics in sewage sludge can act as extra heat supplier in the manufacture process. Both these factors make it technically, economically and environmently advantageous of using sewage sludge to produce shale building materials.According to the property requirements of raw materials used in manufactures of expanded shale and sintered brick, the pressure dewatered sludges of high water content from different sewage treatment plants were used. The particle size distribution, chemical and mineral compositions of the sludge were investigated and compared with those of shale. The plasiticity index of the mixture prepared with sludge and shale was also studied. In addition, the heat value of the sludge was examined by TG-DTA. The results shown that the sewage sludge has a similar chemical and mineral compositions with shale, and its particle size is very small - most of which are smaller than 0.15mm, and it has a high plasticity index. The plasticity of shale can be significantly improved. When sewage sludge was mixed in, the mix will be suitable for the balling of expanded shale and the preparation of adobe brick. It also shown that the impact resistance of expanded shale can be obviously improved. Meanwhile, the sewage sludge has a large potential of heat release, releasing more than 3000J/g during the heating process from 250℃to 550℃.The investigation of sewage sludge shale sintered brick was performed. The influence of sintering temperature, heating rate and heat preservation time on the performance of the solid and porous bricks were investigated at a constant sewage dosage. The results shown that the solid brick with a bulk density of about 1600 kg/m3 and compressive strength of up to 10MPa can be obtained when 30% sewage sludge was added. The presence of sewage sludge could significantly decrease the bulk density. On the contrary, the sintering contraction will be strengthened as the increase of sludge dosage. The determinations of SEM microstructure analysis and changes in sintering contraction occurred under different sludge dosages and sintering temperatures shown that the densified block structure turned into a loose layer structure. When the temperature was up to 900℃, the microstructure tended to be significantly denser. But when the temperature was extensively high, the solid paticals would be thoroughly trapped by liquid phase, which will result in a high contraction of the product. Therefore, it is very important to fix sintering temperature when sewage sludge is added into shale sintered brick. Further study on the influence of sludge dosage on the property of sintered brick revealed that a high sludge dosage will cause a very severe scumming.The calcining of expanded shale was priorally investigated. The relations of bulk density, particle strength and water absorption ration of expanded shale with factors such as sewage sludge dosage, preheating temperature and preheating time, calcining temperature and calcining time were studied. Results shown that sewage sludge will significantly improve the calcining expantion property of expanded shale. The ultra-light expanded shale with a bulk density of about 500kg/m3 can be obtained at the calcining expantion temperature of 1150℃when 20%~30% of sewage sludge with a water contend of 80% were added. The influence of calcining parameters on properties of expanded shale were further studied, the results revealed that a proper lowing of preheating temperature and preheating time and a moderate raising of calcining temperature and calcining time would benefit for the formation of light sewage sludge expanded shale.To understand the effects of sewage sludge plays in the calcining expansion process of the expanded shale, SEM, XRD, IR and TG-DTA were used to analyze the internal structure, mineral compositions, polymerization degree of Si-O tetrahedron and high temperature sintering/melting changes of sewage sludge expanded shale. It shown that main mineral compositions of the sewage sludge expanded shale wereα-quartz(α-SiO2), kyanite(Al2O3·SiO2), anorthite(CaO·2Al2O3·2SiO2) and so on. With the increase of sewage sludge, the valency of Fe in adobe particle converted from hematite(Fe2O3)to wustite(FeO), which reflected that the reducing atmosphere was strengthened during the calcining process. At this time, the Si-O symmetric stretching vibration wave number and the Si-O tetrahedron polymerization degree were significantly decreased and a large number of bridging oxygen bindings in framework structure was disruptured. All these factors contributed to the lowering of sintering temperature and strengthening of melting degree. The sewage sludge is beneficial for the formation of large quantity of pore structure, while an excessive dosage will result in a connective pore structure and a collapse of the structure. The shell of the expanded shale will become thicker when the preheating temperature was raised; a higher calcining temperature and a longer calcining time will enlarge the size of the inner pore structure of expanded shale, which will eventually form a big pore from the isolated ones.Finally, this paper investigated the volatility and leachability of heavy metal in sewage sludge during the burning and in-situ using processes of expanded shale and sintered brick. Regarding the possible content of heavy metals contained in the sewage sludge, we studied several representive heavy metals, i.e., Zn,Cu,Cr,Pb,Ni, to study the environmental characteristics of manufacture and utilization of building materials from sewage sludge and shale in the most severe condition. And the results shown that the amount of volatilization duiring burning processes of sintered brick and expanded shale made of sewage sludge, compared to that of pure sewage sluge, decreased obviously. Besides, the leaching amount of heavy metals in both materials was very small, and it could meet the corresponding leaching toxicity security standards. Comparatively, expanded shale can solidify heavy metals more efficiently than sintered brick.It is believed that using high water content sewage sluge and shale to produce expanded shale and sintered brick could not only consume sewage sluge sufficiently, but also improve the performance of shale building materials. This is the best techenic method of recyling sewage sludge due to its advantages in effectiveness and environment protection. The advantage is more obvious to produce expanded shale in the presented condition of this paper.