Study In The Process Of Synthesis Geopolymer Using Circulating Fluidized Bed Fly Ash

Circulating fluidized bed?CFB?fly ash contains a high content of calcium and sulfur,which makes it difficult to achieve large-scale utilization in traditional cement industry.On the basis of chemical composition and mineral composition,CFB fly ash has a potential application in the synthesis of geopolymer.However,the presence of large amount of CaCO₃,CaSO₄ and CaO makes it much more complex for the polymerization process than the metakaolin/fly ash based geopolymer system.In this paper,the mixture of metakaolin and CaCO₃,CaSO₄ or Ca?OH?₂ instead of CFB fly ash was used to investigate.In the SiO₂-Al₂O₃-CaO-Na2O system,the influence of Na2O/Al₂O₃ on the setting time and compressive strength of geopolymer was investigated.The results indicated that the dissolution process was improved with the shortening of the setting time when Na2O/Al₂O₃ increased in the system.After curing for 90 days,there existed Faujasite crystals in the samples with the Na2O/Al₂O₃ ratio range of 0.7-1.1 and zeolite A crystals in the samples with the Na2O/Al₂O₃ ratio range of 0.6-0.8,which promoted the later strength.However,there will exist hydrated garnet in the system if the rate of Na2O/Al₂O₃ is higher than 1.0,which has a negative effect on the compressive strength of the samples.In addition,the compressive strength decreased with the increasing of chlorine aluminate in the samples prepared by the alkali activator containing NaCl.Therefore,it is necessary to ensure a certain concentration of OH-ions existing in the alkali activator for the excellent performance of the sample.The influence of SiO₂/Al₂O₃ on the setting time was higher than that of Na2O/Al₂O₃.In the early time,zeolite crystals promoted the initial strength of low SiO₂/Al₂O₃?SiO₂/Al₂O₃<2.5?samples.While the increase of SiO₂/Al₂O₃ in the system will transform the geopolymer type from Na-PS to Na-PSS,and the latter is more conducive to the improvement of compressive strength.It is worth noting that the sample will harden fast when SiO₂/Al₂O₃ in the sample was too high,resulting in the uneven mixing of the slurry and the difficulty of bubble escape,so that the development of compressive strength of the sample would be affected.Under the condition of the same Na2O/Al₂O₃,replacing water glass with molten glass powder,below 60%,can prolong the sample setting time and improve the mechanical strength of high SiO₂/Al₂O₃ samples.The reactions of CaCO₃,CaSO₄ and Ca?OH?₂ in the alkali activated metakaolin system were investigated.The results indicated that CaCO₃ played a catalytic role in the reaction,which accelerated the setting process.After 28 days curring,CaCO₃ just acted as physical filler with little effect on strength.Although the formation of ettringite in the sample containing CaSO₄ promoted the early strength of the samples,the long-term strength?after 28days?will be affected by micro cracks resulting from the continuous generation of ettringite.The pozzolanic reaction and the geopolymerization reaction are the main reactions in the samples containing Ca?OH?₂.