Quantitative Analysis Method And Reaction Kinetic Study Of Magnesium Oxychloride Cement

Magnesium oxychloride cement（MOC）is a kind of low carbon,low energy consumption cementitious material,which has many excellent properties such as light weight,high strength and strong adhesion.However,it has unstable properties such moisture absorption,dehalogenation and poor water resistance,which limits its application.Based on“basic salt theory”,the phase transformation in the hardening process of MOC is"slurry-gel phase-magnesium oxychloride crystal".The dynamic process of the transformation from gel to the crystal is the crystallization reaction kinetics of MOC,and the residual gel phase in the hardened slurry is the reason for moisture absorption and dehalogenation.Therefore,quantitative analysis of the gel phase and magnesium oxychloride crystal phase is the basis for studying the reaction kinetics and improving the performance stability of MOC.In this thesis,focusing on the two directions of Cl in the system,the crystalline and amorphous oxychloride phases of MOC were separated by anhydrous ethanol.The method of using conductivity to characterize the alcohol leachable chloride ion content（ALC）and quantitatively analyze the content of the gel phase was explored.The method of measuring the content of crystalline phase by thermogravimetry and quantitatively analyzing crystalline Cl^-was explored.This two were compared and analyzed to verify the accuracy of the quantitative analysis method of crystalline-amorphous phase.On this basis,the reaction kinetics of MOC was analyzed,the rapid evaluation of the moisture absorption and dehalogenation performance of MOC products was explored,so as to further confirm and improve the"basic salt theory".The main conclusions are summarized as follows:1.An"alcohol leaching-conductivity method"was proposed to quantitatively characterize the Cl^-content of amorphous oxychloride in MOC:（1）The feasibility of alcohol leaching in quantitative analysis is confirmed:all amorphous oxychloride in MOC can leached by absolute ethanol without damaging the of magnesium oxychloride crystal phases.（2）The mathematical relationship between the conductivity of MgCl₂ethanol solution and the Cl^-concentration in the range of 0.0010～0.1440mol/L was established,and the key parameters of the conductivity method are optimized..The result of the conductivity method can be as accurate as mercurimetric method,the operation is simple,and the change of the Cl^-concentration in the solution can be monitored in real time.2.The thermogravimetry method was applied to determine the content of various products in MOC.Comparison with XRD showed that thermogravimetry method can accurately determine the crystalline phase（5phase,3phase,Mg（OH）₂,Mg O）and amorphous phase content in MOC,and the experimental operation and data processing are simpler.This method can be applied in unhardened pastes.3.Combined with alcohol leaching-conductivity method and thermogravimetric method,used the Cl crystallization rate as the characterization index of the 5phase crystalline degree,the reaction kinetics of MOC was explored:（1）The reaction process of MOC could be divided into four stages:uncrystallized,acceleration,deceleration and stable.Gel phase was generated in the uncrystallized stage,which make the slurry initially setting.Then the gel phase rapidly turned to crystal at the beginning of the acceleration stage,and slurry was setting finally.Whereafter,the crystallization rate reached its peak value.（2）The crystallization of the 5phase was heterogeneous nucleation.In acceleration stage,crystallization was controlled by the nucleation reaction,the reaction progress and speed were decided by the Mg²⁺concentration and p H value,and the kinetic parameter N was less than 1.In deceleration stage,crystallization was inhibited by interfacial interaction of basis salt hydrates and diffusion of ions,and the N value was greater than 1.（3）The influence of various factors on the reaction kinetics are shown as follows:The larger the H₂O/MgCl₂,the longer the reaction process,and the Mg（OH）₂ was generated along with the 5phase,which makes the greater the impact of the solution boundary condition for the hydration reaction of Mg O during the acceleration stage.The greater the MgO/MgCl₂ molar ratio and Mg O activity,the shorter the reaction progress.However,large Mg O/MgCl₂ has a greater impact on the nucleation reaction,reducing the Cl crystallization ratio,while the Mg O activity has little effect on the phase composition,and more changes the Cl crystallization rate.Phosphoric acid had an inhibitory effect on the nucleation reaction in acceleration stage,and it changed the crystallization from the nucleation reaction to diffusion in deceleration stage,Mg（OH）₂ was no longer generated,and the crystallization ratio and rate of Cl increased.4.The more amorphous oxychloride content,the severer of the moisture absorption and dehalogenation.Conductivity method can quickly evaluate the moisture absorption and dehalogenation property of magnesite products,and the threshold values of ALC and Cl crystallization ratio with obvious moisture absorption and dehalogenation phenomenon were 40.0mg/g and 65.2%,respectively.