Structure Modulation And Properties Characterization Of Silica Fume/Metakaolin Geopolymer Based Porous Materials

The emission of industrial solid waste will not only damage the ecological environment,but also lead to the waste of resources.The utilization of solid waste to curb the environmental pollution is one of the effective way to achieve high value-added recycling of industrial solid waste,and geopolymers can be used as a bridge to achieve this goal.Geopolymers are new types of green cementitious materials with zeolite like three-dimensional network structure,which is composed of[Si O₄]^4-and[Al O₄]^5-tetrahedral alternately connected by bridging oxygen.Compared with traditional cement,geopolymers can be prepared directly from industrial solid waste without grinding or burning,with low energy consumption and no greenhouse gas emission.At present,geopolymers as cementing materials have been well studied and applied in the field of building materials.In recent years,the zeolite-like properties of geopolymers have also been widely concerned.Geopolymers and zeolites have the same structure in the atomic scale,so geopolymers also exhibit catalytic,adsorption or ion exchange capacities.Commercial zeolites are usually nano or micron sized powders,which require to be shaped by adhesives in practical industrial applications to give them certain mechanical properties to deal with the scouring of high-speed gas or liquid and complex reaction environment.Compared with zeolite,geopolymer,as cementing material possess excellent mechanical properties.However,due to its amorphous or quasicrystal structure,geopolymer has limited pore content and relatively low specific surface area,so its performance still lags behind that of zeolite.Therefore,to regulate the pore structure of geopolymer and simultaneously ensure its mechanical properties and high adsorption or catalytic performance as zeolite is a major challenge for the development of geopolymer materials.In this study,silica fume waste and metakaolin mineral are utilized as the main raw materials,which are representative in geopolymer synthesis.In addition,the Si/Al molar ratio of the system can be adjusted by varying the mixture ratio of the two compounds owing to their special purity and high reactivity.The pore structure of geopolymer was regulated by seed induced in-situ transformation of geopolymer into monolithic zeolite and activated carbon embedded polycondensation and hydrothermal crystallization.The main contents and results of the present work are list as follows:(1)Before the study of seed induced in-situ transformation of geopolymer into monolithic zeolite,the feasibility of seed induced organic template free one-step hydrothermal synthesis of ZSM-5 was investigated using silica fume and metakaolin mixtures with different proportions.The results showed that silica fume and metakaolin could be converted into ZSM-5 zeolite which exhibited similar characteristics to those obtained from conventional chemical reagents under the following optimal conditions:crystallization time of 72 h,hydrothermal temperature of 180?,seed dosage of 5 wt.%,metakaolin adjusted initial Si/Al molar ratio of 15,and Na₂O/H₂O ratio of 0.028.The as-synthesized Na/ZSM-5 powders with the highest crystallinity was used to prepare Ni/ZSM-5,Cu/ZSM-5,and Zn/ZSM-5 catalysts by means of ion exchange and calcination treatments,and the catalysts showed high denitration efficiency of 92%,100%,and 95%,respectively.(2)Seed induced in-situ hydrothermal conversion of geopolymer block into monolithic zeolite to improve the conversion efficiency of zeolite and the pore structure of geopolymer.The results show that this strategy follows similar laws with the conventional hydrothermal synthesis method and has universality.The CO₂ adsorption capacity of the monolithic Na/ZSM-5 at 35?reaches 1.79 mmol/g,which is much higher than that of geopolymer(0.01 mmol/g).The adsorption capacity of monolithic Na/ZSM-5 for Ni²⁺ions in aqueous solution is 13.22 mg/g,and the adsorption kinetics and thermodynamics analysis show that the adsorption of Ni²⁺ions onto monolithic Na/ZSM-5 belongs to ion exchange process.The Ni²⁺ion replaces the Na⁺ion in monolithic ZSM-5 and balance the negative charge of[AlO₄]^5-in the form of extra-framework cation.DFT calculation shows that Ni²⁺ion in ZSM-5 exhibits stronger CO₂ affinity than that of Na⁺ion.Therefore,the CO₂ adsorption capacity of monolithic ZSM-5 enhanced to 2.38mmol/g after Ni²⁺adsorption.Moreover,the as-synthesized ECR-1 self-supporting membrane can effectively reject the Na⁺,K⁺,Ca²⁺,Mg²⁺and Cl^-ions in simulated seawater.The rejection rates are 99.99%,97.32%,100%,100%and 96.82%,respectively,with water flux of 0.75 kg/(m²·h).(3)Preparation of activated carbon/geopolymer composites with high mechanical strength and adsorption properties by surface modification of activated carbon by utilization of the strong alkaline environment of geopolymerization process.The results show that the introduction of activated carbon into the geopolymer matrix creates the interface between the geopolymer and activated carbon,which reduces the mechanical strength of the matrix.However,increasing the alkalinity of alkali activator can effectively alleviate the mechanical loss and significantly promote the formation of oxygen groups on the activated carbon surface.When the mass ratio of KOH,silica fume,metakaolin and activated carbon is 22.4:8:12:6,the compressive strength of the obtained activated carbon/geopolymer composite is 22.3 MPa,and the CO₂ adsorption capacity reaches 1.49 mmol/g.DFT calculation shows that the oxygen groups on the surface of activated carbon exhibit high affinity for CO₂,which promotes the CO₂ adsorption capacity.In addition,the hydrothermal treatment of the activated carbon/geopolymer composites can improve the zeolite conversion efficiency owing to the intentional interface,and further enhance the CO₂ adsorption capacity to 2.68 mmol/g.(4)The iron based heterostructure activated carbon/geopolymer composite catalyst was prepared by impregnation method using activated carbon/geopolymer composite as carrier and ferric nitrate as precursor,and its SCR performance was also evaluated.The results show that when the iron loading amount is 10 wt.%,the NO conversion is greater than 96.5%with the N₂ selectivity above 94%in the temperature range of 350-500?.The excellent NO conversion efficiency and N₂ selectivity of the catalyst are mainly attributed to the synergistic effect of isolated iron ions in geopolymer and highly dispersed Fe O_x clusters on the surface of activated carbon.The former provides active sites for low temperature SCR,while the latter has reactivity at higher temperatures.The distribution characteristic of iron is attributed to the special heterostructure of activated carbon/geopolymer composites.(5)Geopolymer based inorganic membrane was prepared using silica fume and metakaolin as raw materials,and the membrane separation characteristics for Cr(?)ions were systematically studied.Then the geopolymer membrane with intercepted and enriched Cr(?)ions were calcined to in situ convert the Cr(?)ions into Cr₂O₃semiconductor to afford dye wastewater treatment by hybrid photocatalysis and membrane separation.The results show that Cr(?)ions on the surface of geopolymer membrane do not migrate during calcination,and they are all transformed into nano-Cr₂O₃ oxide particles and highly dispersed on the surface of geopolymer membrane.The photocatalytic performances under different downstream pressures confirmed that the degradation efficiency increased with increasing downstream pressure(below atmospheric pressure):100 min was required for 100%degradation of basic green under static conditions,whereas only 50 min was required at 0.09 MPa owing to suitable membrane-induced concentration of the dyes.