Study On Preparation Of Reactive Magnesia For Magnesium Oxychloride Cement From Mg-bearing Salt Of Salt Lake

Magnesium oxychloride cement?MOC?is a kind of gas-hard cementing material made by mixing Mg Cl₂ solution of certain concentration with reactive MgO.It has the advantages of high early strength,fast setting speed,low alkalinity,good fire resistance and anti-halide corrosion,etc.It also has the disadvantages of poor water resistance,large heat release,moisture absorption and halogen return and easy deformation.The main raw material used in traditional MOC,reactive MgO,is mostly from calcined magnesite or dolomite.But the regional distribution and preparation technology of magnesite or dolomite has limited the promotion and application of this material in areas lacking magnesite resources.In this paper,the rich magnesium resources in salt lake of Qinghai province were used to replace the raw material of traditional process to prepare reactive materials for MOC.Aiming at the Mg?OH?₂ produced by the three processes of Qinghai salt lake enterprises?calcium carbide slag and Mg Cl₂ prepared by Mg?OH?₂,calcination method and membrane separation method?,The effects of calcination temperature and holding time on phase composition,particle size,specific surface area,hydration activity and setting time of the calcined products were studied.The effects of calcining process and raw material ratio on the compressive strength of MOC specimens were studied by means of orthogonal test and control variable method.And in combination with the setting time and process energy consumption,the optimal calcining process and raw material ratio for the preparation of MOC specimens with relatively high compressive strength were optimized.The X-ray diffractometer?XRD?and scanning electron microscope?SEM?were used to analyze the phase composition and micromorphology of MOC specimens.and the influence of calcining temperature,the molar ratio of MgO to Mg Cl₂ and Baume degree of Mg Cl₂ solution on the phase composition and microstructure were discussed.The main research results are as follows:Mg?OH?₂ prepared from calcium carbide slag and Mg Cl₂ is used as a raw material.After washing process?raw material and water are washed once at a mass ratio of 1:10?,reactive MgO for MOC was prepared by calcining it.The results show that with the increase of the calcination temperature and the extension of the holding time,the Mg²⁺content,and the reactive MgO content in the calcined product gradually increase,while the Cl^-content and specific surface area in the calcined product gradually decrease;The particle size of the calcined products decreases first and then increases with the increase of calcination temperature and the extension of holding time,and the setting time increases.Then,MOC specimens were prepared by means of orthogonal experiments and control variables using reactive MgO prepared by calcining.The results showed that the molar ratio of MgO to Mg Cl₂ and the Baume degree of Mg Cl₂ solution have a significant effect on the compressive strength of MOC specimens of 3-days,and the significance of the molar ratio of MgO to Mg Cl₂ to the compressive strength of MOC specimens is less than that of Mg Cl₂ Baume of solution;At 7-days and 28-days,only Baume of Mg Cl₂ solution had a significant effect on the compressive strength of MOC specimens,and the significance level decreased with age.As the Baume degree of the Mg Cl₂ solution increase,the compressive strength of the MOC specimen increases first and then decreases.When the calcination temperature is 600^oC,the holding time is 30minutes,the molar ratio of MgO to Mg Cl₂ is 6,and the Baume degree of the Mg Cl₂solution is 27,the compressive strength of MOC is higher,and the compressive strength gradually increases with age.Extraction of lithium magnesium slag by calcination method to prepare reactive MgO for MOC.The results show that Mg₃B₂O₆ is difficult to pyrolyze during the calcination process,so it has been retained in the calcined product.With the increase of the calcination temperature,the specific surface area of the calcined product gradually decreases,and the particle size and setting time decrease first and then increase,while the reactive MgO content in the calcined product increases first and then decreases.With the increase of the holding time,the specific surface area of the calcined product gradually decreases,the particle size decreases and then increases,the setting time of the calcined product gradually increases,and the reactive MgO content in the calcined product increases first and then decreases.Then,the reactive MgO prepared by calcination was used as a raw material,and MOC specimens were prepared by orthogonal experiments and controlled variable methods.The results show that the calcination temperature and the Baume degree of Mg Cl₂ solution have significant effects on MOC specimens,but the molar ratio of MgO to Mg Cl₂ has no significant effect on MOC specimens.With the increase of calcination temperature and the molar ratio of MgO to Mg Cl₂,the compressive strength of MOC specimens fluctuated;With the Baume of Mg Cl₂ solution increased,the compressive strength of MOC specimens gradually increased.When the calcining temperature is 600^oC,the molar ratio of MgO to Mg Cl₂ is 10.5,and the Baume degree of Mg Cl₂ solution is 28,the compressive strength of MOC specimens is high,and the compressive strength increases with the age.Extraction of lithium magnesium slag by membrane separation method to prepare reactive MgO for MOC.The results show that as the calcination temperature increases,the Cl^-content in the calcined product first increases and then decreases,and the Mg²⁺content gradually increases,while the reactive MgO content in the calcined product increases first and then decreases;The specific surface area of the calcined product gradually decreases,and the particle size and coagulation time decrease first and then increase,.With the increase of the holding time,the specific surface area of the calcined product gradually increased first and then decreased,and the particle size decreased first and then increased,and the setting time gradually increased,while the reactive MgO content in the calcined product increased first and then decreased.Then,MOC specimens were prepared by means of orthogonal experiments and control variables using reactive MgO prepared by calcining.The results show that the calcination temperature at different ages has the most significant effect on the compressive strength of MOC specimens,and the significance level remains the same at all ages;The significance level of the influence of the Baume degree of Mg Cl₂ solution on the MOC specimens increased with age,and the Baume degree of Mg Cl₂ solution has the same significant level of compressive strength as calcination temperature at 28-days;The molar ratio of MgO to Mg Cl₂ has less influence on the compressive strength of MOC specimens.With the increase of the calcination temperature,the compressive strength of MOC specimens increases first and then decreases;Except that the compressive strength of MOC specimens prepared by the molar ratio of MgO to Mg Cl₂ of 9.5increased steadily,the compressive strength of MOC specimens prepared with other molar ratio of MgO to Mg Cl₂ decreased with age.Except that the compressive strength of MOC specimens prepared with Mg Cl₂ solution Baume degree of 26 increased steadily,the compressive strength of MOC specimens prepared with other Baume degree of Mg Cl₂ solution decreased with age.To sum up,when the calcination temperature is 600^oC,the molar ratio of MgO to Mg Cl₂ is 9.5,and the Baume degree of the Mg Cl₂ solution is 26,the compressive strength of the MOC specimen is higher,and the compressive strength increases with age.According to 28-day age SEM of MOC specimens prepared with different materials can be known that when the calcination temperature is 500^oC,a loose network crystal structure is easily formed in the MOC specimen,and when the calcination temperature is increased from 500^oC to 700^oC,the gel phase in the MOC specimen gradually increases,and the compactness of the specimen increases.With the decrease of the molar ratio of MgO to Mg Cl₂ and the Baume degree of the Mg Cl₂ solution,needle rod-like crystals with relatively large diameters are likely to be formed in MOC specimens.Conversely,the gel phase in the MOC specimen gradually accumulated.