Application Of Organic Acid To Kaolin And Its Application In Alkali Activated Cementitious Materials

Kaolin is an important raw material of Portland,but the course of reaction to form a new structure of matter by breaking the bond or restructuring crystal lattice needs high energies due to the bond energy inside the silicon-oxygen bond and aluminium-oxygen bond are very large.So the specific performance in industrial production is a high temperature environment,which causes a series of problems,such as high temperature of burning,high energy consumption and so on.But the need of energy can be reduced by activation radioactivation.In this paper,we use several organic acids to activate kaolin raw materials and use software to simulate the reaction between the simplified model of organic acid and the simplified model of kaolin in order to explore the activation mechanism of silicate minerals in organic acid environment and provide a new idea for improving the properties of alkali activated cementitious materials.The theoretical results show that the activation energy of aluminium-oxygen bond is lower than the silicon-oxygen bond,so it's easier to be damaged in the course of reaction;when organic acids are used to activate kaolin,the activation energy of the transition state with hexatomic ring is the lowest,but the reaction rate is the highest.The mechanism of formic acid and citric acid to activate kaolin is basically the same,but there is a certain difference in the activation energy when the transition state is formed.The reaction rate between formic acid and Bridge oxygen bonds is much higher than the reaction rate between formic acid and silicon oxygen tetrahedron in the reaction between formic acid and Si-O-Si which is one of the simplified structure of kaolin,and the reaction between formic acid and Bridge oxygen bonds to form the transition state with hexatomic ring needs 68.51 KJ/mol,while the reaction between formic acid and silicon oxygen tetrahedron to form the transition state with hexatomic ring needs 102.36 KJ/mol.The reaction rate between formic acid and aluminium-oxygen bond is much higher than the reaction rate between formic acid and silicon-oxygen bond in the reaction between formic acid and Si-O-Al which is the another simplified structure of kaolin,and the reaction between formic acid and aluminium-oxygen bond to form the transition state with hexatomic ring needs 13.77 KJ/mol,while the reaction between formic acid and silicon-oxygen bond to form the transition state with hexatomic ring needs 96.78 KJ/mol.In the experimental section,we use formic acid and citric acid to activate kaolin,and we take advantage of DSC,XRD,SEM to analysis dissolution rate of silicon and aluminum,activation properties of kaolin.The experimental results show that the best activation system for activating kaolin by organic acids is treating the kaolin ore with formic acid of 0.5mol/L and citric acid of 0.5mol/L for 24days,then some changes in the physical and chemical properties of kaolin occur,such as the content of chemical components of Si02 and A1203 are reduced,the size of powder decreases by 68.75%,the specific surface area is increased by 65.19%,degree of crystallinity becomes poor,the micro morphology of relatively coarse and the destruction of the edge and interlayer of the layered structure is serious.Next,We calcined kaolin ore and modified kaolin to determine the activation effect.The experimental results show that optimum calcinations temperature can be reduced by 100?.By adding kaolin to cement alkali activated cementitious materials can improve its properties remarkably.And the alkali activated cementitious materials hold the best mechanical properties when the content of metakaolin is 18%,content and modulus of water glass are 20%and 1.3,water cement ratio is 0.40.