Research On The Salt Attack Performance Of Magnesium Oxychloride Cement

Magnesium oxychloride cement?Mg O-Mg Cl₂-H₂O,MOC?is a type of ternary air hardening cementitious material,which is formed by mixing a certain concentration of magnesium chloride solution with light burned magnesia powder.It has the advantages of early and high strength,fast hardening,light weight,fire prevention,wear resistance,low alkalinity,good decoration effect,corrosion resistance to brine and so on.It is widely used in many fields.However,whether MOC can be used in marine and salt lake environment has always been a hot issue for researchers.Oceans and salt lakes are all over the world,which contain abundant magnesium resources.The comprehensive utilization of marine and salt lake magnesium resources has always been the key point of basic research and production application.If MOC can be used in the marine and salt lakes,the application range of this material will be greatly expanded.The by-product magnesium chloride from marine and salt lake can also be comprehensively utilized,and this will partly solve the problem of large-scale utilization of marine and salt lake magnesium chloride resources.In addition,if MOC can be prepared by mixing seawater and salt lake brine directly,it can realize the nearby utilization of resources,which will greatly save costs and create economic value.Therefore,this paper takes the salt attack performance of MOC as the research subject,and explores the salt attack performance mechanism of MOC by studying the performance decay law of MOC in seawater and salt lake brine.At the same time,the hydration and setting performance,thermal stability,strength development and salt attack performance of MOC prepared by mixing with seawater are analyzed.The main research and research results obtained are as follows:1.The salt attack performance of MOC was studied.The salt attack performance of MOC was characterized by the salt attack coefficient of MOC specimen in a specific salt brine environment,combined with mass loss.Through analyzing the phase composition,micromorphology,pore structure of MOC and p H,conductivity of brine solution after immersion,the salt attack performance mechanism of MOC was discussed.The results show that the salt attack coefficient of neat MOC and mortar MOC are more than 0.80 and 1.00 after being soaked in aging brine and raw brine for180 days.However,the salt attack coefficient of neat MOC and mortar MOC soaked in seawater is similar to that of water,which are less than 0.30 after 180 days.The neat MOC and mortar MOC show different resistance to salt in the three kinds of brine solutions.The main reason is that the concentration of the Mg Cl₂ solution in three kinds of brine solutions is different.The correlation can be verified from the ternary diagram of Mg O-Mg Cl₂-H₂O.2.The strengthening and mechanism of the salt attack to seawater of MOC by adding three kinds of composite modifiers were studied.The results show that the additives of 30%fly ash+1%phosphoric acid,30%fly ash+1%phosphoric acid+1%calcium stearate and 10%composite modifier+1%phosphoric acid have significant improvement on the salt resistance of MOC in seawater.MOC mixed with30%fly ash+1%phosphoric acid,30%fly ash+1%phosphoric acid+1%calcium stearate and 10%composite modifier+1%phosphoric acid maintain very high compressive strength during 180 days seawater immersion.The compressive strength is 86.5 MPa,74.7 MPa and 71.8 MPa,respectively,and the salt attack coefficient is0.98,0.82 and 0.95,respectively.Three kinds of MOC mixed with composite modifiers have excellent seawater resistance because M-S-H,M-Cl-S-H gel,M-Cl-A-S-H gel and 5·1·8 gel are generated in the hydration products,and the insoluble M-S-H,M-Cl-S-H gel and M-Cl-A-S-H gel are wrapped on the surface of5·1·8,blocking the phase transformation of 5·1·8.It also reduces the total porosity and hydration heat release,delays the hydration process,generate few cracks,leading to the denser structure in MOC specimen.The kinetic process of Cl^-dissolution in MOC mixed with three modifiers after seawater immersion is controlled by different diffusion process,which is different from that of MOC in freshwater immersion.Successfully finding three composite admixtures proves that MOC is expected to be used in island construction,which will greatly expand the application scope of MOC.3.The properties of MOC prepared by simulated seawater and real seawater in South China Sea instead of freshwater were studied.Through the analysis of its hydration setting performance,early strength development performance,salt attack performance,thermal stability performance,phase composition and micromorphology,it is found that the hydration kinetics process of simulated seawater,real seawater mixing and freshwater mixing MOC is similar.In the accelerating and decelerating period,the reaction is controlled by autocatalytic process,and in the stable period by diffusion process.The MOC samples mixed with simulated seawater and real seawater have better salt attack performance than the MOC samples mixed with freshwater.The MOC mixed with simulated seawater and real seawater instead of freshwater only has an effect on its phase content,but has no effect on its phase composition,and its thermal stability is similar.The properties of MOC prepared by simulated seawater and real seawater instead of freshwater have not changed obviously.That is to say,it can be used to mix and prepare MOC samples by seawater instead of freshwater,which will reduce cost to a certain extent,and make it hopeful to be used in island field and island reef construction.