| The emission of coal fly ash(CFA)in China has reached about 560 million tons per year.Since the utilization rate of CFA is only 70%,its accumulation has exceeded 3 billion tons,bringing huge environmental pressure to China.CFA mainly comprising of SiO2 and Al2O3 has similar chemical and phase compositions with traditional ceramic raw materials.In addition,its particle size is equivalent to that of traditional ceramic raw materials,making it a promising ceramic raw material.This research first analyzed the differences between the physical and chemical properties and sintering behaviors of CFA and traditional ceramic raw materials,and then studied the micro-functional mechanism of CFA in ceramic matrix and alkali activation mechanism.On this basis,the preparation of 100%CFA-based ceramic tiles was carried out.Based on the phase reconstruction effect of alkaline medium,further research and development of new preparation processes of the advanced oxide structural ceramics and advanced non-oxide structural ceramics were carried out,providing new insights for the high-value utilization of CFA.The innovative results are as follows:(1)The action mechanism of different CFA occurrence forms in the ceramic matrix have been revealed by analyzing its microstructure characteristics and element occurrence law.The lignite CFA particles can be divided into three categories according to their chemical compositions and sintering behaviors:(?)large irregular particles with a high acidic oxide(SiO2 and Al2O3)content,acting like common clay;(?)large irregular quartz particles;(?)small spherical particles with a high alkaline oxide(especially CaO,MgO,and Fe2O3)content,acting like common feldspar.(2)The functional mechanism of alkali-activation on CFA impurity phase separation,phase reconstruction,surface structure modification and silicate structure activation has been clarified.In alkali-activation,CFA particles undergo different processes:excess SiO2 in class-? and class-? particles is removed,improving the crystal skeleton effect;class-? particles are coated by freshly generated hydroxysodalite and P zeolite phases,improving the fluxing effect.At the same time,the hydroxyls are grafted onto the surfaces of CFA particles,producing hydrogen-bonding interaction between CFA particles.Therefore,CFA can be transformed to a plastic material.In addition,the silicate network of CFA is gradually depolymerized,and part of Al(?)is converted into Al(?)-which can substitute for Si4+ in the tetrahedral,rendering the silicate structure unstable and improving its reactivity significantly.(3)A preparation technology of novel 100%CFA-based ceramic tiles has been developed.Two types of activated CFA were prepared by controlling the depth of alkali-activation:clay-like activated CFA with a uniform P zeolite coating and feldspar-like activated CFA with a uniform hydroxy sodalite phase.The 100%CFA-based ceramic tiles can be prepared by mixing raw CFA with these two activated CFA.Their performance meets the standard GB/T 4100-2015.The sintering temperature is about 100? lower than the conventional one,and the sintering temperature window is up to 100?.The products have no risks of heavy metals leaching and radioactivity.(4)A novel strategy for preparing advanced oxide structural ceramics(wollastonite and mullite ceramics)by CFA phase reconstruction in alkali medium has been proposed.Nano-fibrous tobermorite whiskers were synthesized in NaOH solution using lignite CFA as the silicon source.The fibers have a diameter less than 200 nm,and the aspect ratio of most fibers is larger than 25,making them high-quality raw materials for wollastonite ceramics.Tobermorite whiskers have high sintering activity,and their micromorphology can be effectively retained in the sintered matrix.The substitution of Al3+for Si4+ can promote the growth and aspect ratio increasement of tobermorite whiskers,which is beneficial to the mechanical properties of wollastonite ceramics.It has also been found that the alkali-activation can make the chemical composition of high-alumina CFA closer to that of mullite and form a P zeolite coating on the surface of high-alumina CFA,promoting ceramic densification and mullite grain growth.The apparent activation energies of the two stages are only 90.39 kJ/mol and 168.86 kJ/mol,respectively,which are lower than those reported in the literature.The optimal sintering temperature of wollastonite ceramics is only 900?,the flexural strength of the product is 52.47 MPa,and the bulk density is 2.15 g/cm3.The optimal sintering temperature of mullite ceramics is only 1300?,the relative density of the product is 90.85%,and the flexural strength is 109.67 MPa.(5)A new method for preparing SiC-based advanced ceramics from water glass-activated lignite CFA was established.In the alkali-activation,the CFA silicate structure and the water glass network are crosslinked by[AlO4]to form a larger network with many defects.During carbothermal reduction,the active Al2O3 and Fe2O3 in CFA induce the formation of mullite mesophase,thereby promoting the growth of SiC whiskers.The whiskers have high crystallinity and purity,with an average aspect ratio of 18.26 and a yield of more than 70%.Based on these,a high-performance SiCw/SiC ceramic composite has been developed.After the alternating H2O2-oxidation/HF-leaching modification,the morphology of the SiCw appears more uniform,and their aspect ratio increases by 39.10%.After the Al2O3 coating,the fibers have a SiC-SiO2-Al2O3 triple-layered structure.The Al2O3 shell and the SiO2 shell form a mullite interface phase during hot-press sintering,which plays a significant toughening effect.The product has a relative density of 93.8%,a flexural strength of 533.30 MPa,a fracture toughness of 13.60 MPa·m1/2,and a Vickers hardness of 20.60 GPa. |
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