| With the accelerated development of energy-efficient building and green building and prefabricated building in China,foamed ceramic with excellent properties such as light-weight,high-strength,thermal insulation,fire resistance,water-tightness and frost resistance,has become a high-profile green energy-saving wall material.However,there is lack of theoretical guidance in composition design of foamed ceramic,and its pore structure and mechanical properties were significant different at the same density,as well as its influence factors and regulation mechanism were unclear.Focusing on these problems,this dissertation relies on the national key R&D plan"Research and Development of Ecological Building Materials in Villages and Towns",had systematically investigated the relationship between composition,pore structure and properties of foamed ceramics,by using coal bottom ash(CBA)as the main raw material.Besides,the formation mechanism and influence factors of pore structure had been revealed,the compression failure behavior along with reinforcement mechanism of foamed ceramic had been clarified.Moreover,the design principle and preparation method of high-strength foamed ceramic had been presented.Finally,the high-strength foamed ceramics were prepared using coal bottom ash and various solid wastes.(1)The influence of chemical components on the preparation and properties of foamed ceramics had been studied by changing oxides contents in CBA powder.The results indicate that the increase of Na2O and K2O content can improve the foaming capacity of CBA mixture.As the content is 6-8wt%,a closed-pore structure can be formed,but brings a significant increase in pore size.However,the increase of Mg O content in CBA mixture initially comprising 6wt%Na2O promotes the formation of spinel grains,which not only improves the foaming capacity of the green body,but also brings the decrease of pore size and the improvement of pore uniformity,thus significantly improves the compressive strength.On the contrary,the increase of Ca O content promotes the formation of anorthite with high melting point,which not only weakens the foaming capacity of the green body,but also leads to the deterioration of pore structure.In addition,the increase of Al2O3/Si O2 ratio helps to broaden the foaming temperature range and improve the specific strength of foamed ceramics.At the same time,the increase of SO3 content improves the foaming capacity of the green body,but leads to an abnormal increase of pore size,even causes the rupture and collapse of the pore wall as its content exceeds 1.5wt%.Thus,the desulfurized ash with high SO3 content is not suitable for the preparation of foamed ceramics.(2)The composition of foamed ceramics prepared with CBA and various solid wastes was clarified through studying the effects of CBA,bauxite residue,feldspar tailings and chromium slag on the pore structure and performance of foamed ceramics.It is found that the increase of CBA content can improve the foaming capacity of the green body,and promote the lightweight and microporous of foaming ceramics.However,the addition of CBA exceeding 70wt%causes an uneven pore size distribution.Bauxite residue can be used as aluminosilicate material to replace slag in foamed ceramics,while feldspar tailing needs to be combined with sodium carbonate as flux material due to its low K/Na content.The best mixture ratio of CBA foamed ceramics is CBA:bauxite residue:feldspar tailing:sodium carbonate:magnesia=50wt%:14.8wt%:26.2wt%:4wt%:5wt%.Besides,chromium slag can be used to provide Mg O in foamed ceramics,and the minimum pore size and the most uniform pore size distribution can be obtained when the chromium slag dosage is 30wt%.(3)The mechanism of pore formation in the green body at high temperature was investigated using HM and XCT,and the relationship between foaming and pore structure was analyzed.Besides,the effects of liquid phase amount at high temperature,crystal phase and foaming agent on foaming behavior and pore structure were also studied.It shows that the deformation behavior of the green body in heating process can be used to judge the foaming capacity,and confirm the best foaming temperature of foamed ceramics.The"spheroidization temperature"of the green body is the typical temperature to obtain uniform microporous structure,and the temperature from spheroidization to hemispherical is the foaming range of the green body.Besides,the amount of liquid phase generated at foaming temperature and the crystalline phase in the green body are intrinsic factors that affect the foaming behavior and pore structure.More than 60%liquid phase generated at high temperature is the basis of foaming for green body,and the granular spinel microcrystalline plays a foam stabilizing role in the melt,which helps to form a uniform and tiny pore structure in foamed ceramics.In addition,both the increase in Si C content and the decrease in Si C particle size can improve the porosity of foamed ceramics.The Si C content should not exceed 0.5wt%and its particle size should not be lower than 5μm,when using Si C as foaming agent.(4)The compressive failure behavior of foamed ceramic was studied,and the effects of porosity and pore structure,crystallinity and crystalline phase on the compressive strength of foamed ceramic were discussed.Besides,the reinforcement mechanism derived from crystallization was also studied.The results show that under one-dimensional compression,low-density foamed ceramic(ρ<400 kg/m~3)exhibits a plastic deformation,while high-density foamed ceramic(ρ>600 kg/m~3)shows a brittle fracture.The porosity(bulk density)is the main factor that determines the compression strength.When the porosity is 65-90%,the compression strength exhibits an exponential relation with the porosity,while a linear relation fitted for the porosity exceeds 90%.In addition,the reduction in pore size and pore size dispersion coefficient can effectively increase the compressive strength of foamed ceramic.Under the condition of similar porosity and pore structure,the compressive strength will be greater with the increase of the crystallinity.The influence of main crystalline phase on the compressive strength is related to the pore structure.Because of the excellent pore structure,foamed ceramic with spinel as the main crystal phase possesses a high compressive strength.Besides,the introduction of 2wt%Cr2O3 can promote the crystallization of the matrix at cooling stage,and thus significantly improve the compressive strength of the foamed ceramic.(5)The composition design principle and preparation method of high-strength foamed ceramics were proposed,based on the relationship between compressive strength,pore structure and crystalline phase,as well as the reinforcement mechanism derived from crystallization.And a series of foamed ceramics with a density of 160-500 kg/m~3 was prepared with CBA,chromium slag,bauxite residual and feldspar tailing.The results indicate that the compressive strength and flexural strength of foamed ceramics prepared by reinforced preparation method is much higher than that of the existing foamed ceramics.What’s more,the prepared CBA foamed ceramic with a density of 500 kg/m~3 have an ultra-high compressive strength of 16.2-17.5 MPa,which is close to the strength index proposed in“China Manufacturing 2025”.At the same time,the solid waste utilization rate in prepared two kinds of CBA foamed ceramics reach 91%and 96%respectively,and the heavy metals in solid wastes are effectively solidified without environmental hazards. |
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