The Effect Of Rare Earth Fluoride On The Phase Transition Kinetics Of Kaolinite And The Mechanical Properties Of Geopolymers

Kaolin is primary composed of kaolinite,widely used in chemical industry,papermaking,ceramics,paint,and national defense industry,is a very valuable natural minerals,is basic ingredient of synthetic green cementing material geopolymer.Rare earth element has unique catalytic and synergistic effect because of special valence electron layer structure is widely used.In this article,we doped rare earth fluoride into kaolinite.In this paper,comprehensive thermal analysis technology was applied to analyze the process of dehydroxylation dynamics of kaolinite and kaolinite which doped with the rare earth fluoride ErF3?LaF3?NdF3?PrF3 and YF3 of n(Al)/n(Re)=100.The effect of rare earth fluoride on kaolinite phase transformation process is reasearched,the kinetic parameters were calculated by Coats-Redfern integral method and Achar differential method.Kissinger equation,Ozawa equation and Johnson-Mehl-Avrami(JMA)modified equation(I)were used to calculate the kinetic parameters of high temperature phase change process,The changes of phase,structure and chemical bond of the kaolinite and the kaolinite mixed with rare earth fluoride are studied by XRD and FTIR.Measuring the compressive strength of prepared geopolymer,analysing the samples by XRD.Exploring the effects of kinds of rare earth fluoride and content on the mechanical properties of polymer,attaining the following conclusions:(1)The process of Kaolinite dehydroxylation correspond F3 mechanism,while the the kaolinite mixed with rare earth fluoride correspond F2 mechanism.The reaction temperature of dehydroxylation reached the maximum velocity in order of size: kaolinite doped ErF3 > kaolinite doped PrF3 > kaolinite doped NdF3 > kaolinite doped LaF3 > kaolinite doped YF3 > kaolinite doped ErF3 > kaolinite.The activation energy Ea and the pre-exponential factor lnA of the kaolinite thermal decomposition is 281.64 KJ/mol and 44.2531;the kaolinite doped ErF3 is 219.49KJ/mol and 32.7281;the kaolinite doped LaF3: 203.57 KJ/mol and 30.5123;the kaolinite doped NdF3 is 201.33KJ/mol and 30.1208;the kaolinite doped PrF3 is 209.20 KJ/mol and 31.2811;the kaolinite doped YF3 is 199.40KJ/mol and 29.854.The rare earth fluoride help reduce the activation energy and the pre-exponential factor of the thermal decomposition reaction of kaolinite obviously,and facilitate the reaction of the dehydroxylation.(2)Using Kissinger equation,Ozawa equation and JMA correction equation(I)to calculate the activation energy of the high temperature phase transition of kaolinite were 677.32KJ/mol? 698.72 KJ/mol and 688.02KJ/mol,the frequency factor ln? of kaolinite was 62.6299,and the activation energy and frequency factor ln? of doped rare earth fluoride kaolinite of phase transition at high temperature has greatly increased,showed that rare earth fluoride improved the transformation of kaolinite activation energy and frequency factor,The extent of doped YF3 kaolinite phase-change activation are minimum,while the doped of LaF3 are maximum.(3)Using orthogonal test to find out optimal experimental conditions of preparing metakaolin geopolymer: the concentration of NaOH is 10mol/L,liquid to solid ratio is 1.0,water glass accounted for 55% of liquids.Measuring the compressive strength of geopolymer conserving time for 3d?7d and 28 d respectively.We found that different rare earth fluoride and doped content can change the compressive strength of geopolymer,the compressive strength of geopolymer of all the metakaolin doped rare earth fluoride are lower compared with blank metakaolin geopolymer.