Water Erosion Mechanism And Improving Water-Resistance Of Magnesium Oxychloride Cement

Affected by the climate of frigid and drought in alpine regions,Portland cement is prone to cracking and other diseases during the curing period.The magnesium oxychloride cement(MOC)can rapidly hydrate in a low temperature environment and form product with high strength to reduce the influence of temperature stress.Not only the excellent mechanical properties and weather resistance,but corrosion resistance is suitable for the western saline-alkali region.However,as a gas-hardening cementitious material,water transport and erosion in MOC is easy to cause mass loss and strength deterioration.Therefore,there are important significance by investigating the erosion behavior of water in MOC and searching the protection measures to improve the water resistance.Based on the requirements of road Portland cement material index,the influence of active magnesium oxide on the setting time,drying shrinkage characteristics,mechanical properties and water resistance of MOC was analyzed.The reasonable evaluation method of light burned magnesia was clarified.The reaction mechanism of hydrated and the influence of raw materials content on its hydration process were analyzed.The water erosion behaviors such as capillary water absorption process and water diffusion process in MOC were evaluated.The moisture transport behavior and water distribution characteristics of MOC were evaluated by capillary water absorption coefficient and humidity diffusion coefficient.In addition,the dissolution of water on hydration products of MOC was analyzed by dissolution kinetics theory,and the relationship between hydrolysis reaction constant and strength decay of MOC was explored.Based on the dissolution characteristics of MOC,the crystal morphology improvement and moisture isolation technology of MOC were clarified to improve the water resistance.Based on the weakening of mechanical properties of MOC concrete in the water environment and the climatic characteristics of Qinghai Province,the reasonable engineering application range was clarified.The effectiveness of the main research results are as follows:(1)The content of active magnesium oxide in light calcined magnesia suitable for roads should be at 42.8% to 64.5%,and its hydration rate required content of active magnesium oxide at 0.5h should be less than 50% of 24 h.The composition ratio of MOC was active magnesium oxide: magnesium chloride: water = 7:1:15.It was found that the hydration reaction of MOC was based on the process of continuously adsorbing active magnesium oxide with free magnesium ion cluster as the core.(2)The water erosion process showed that there was a material dissolution time point in the process of water transport.Water transport was consistent with the transmission law of Fick's one-dimensional diffusion equation before the point and could acquire the water distribution in MOC.Affected by hydrolysis,water transport was not consistent for partially crystal water converting into free water and participating in moisture transport.The theory of dissolution kinetics indicated that the hydrolysis reaction of MOC was controlled by the adsorption-chemical reaction stage,and the later stage was controlled by the internal diffusion step.The reaction temperature,the concentration of the soaking solution and the reaction contact area were main factors of reaction rate.(3)By improving the morphology of hydrated crystals,reducing water transmission channels,and isolating moisture contact,the hydrolysis reaction area can be reduced.Addition of water reducing agents or retarders can promote the morphology of cement hydrated crystals from leaf-like or fibrous became plate-like or flocculated.Carbonation can form an effective protective layer to wrap the hydration products to prevent water penetration.Water transfer behavior test showed the capillary water absorption coefficient and humidity diffusion coefficient in the saturated state decreased by 53.4% and 77.9%,respectively.Carbonation reaction filled the cement pores,which further reduced the capillary water absorption coefficient by 58.6%.Dissolution kinetics showed the hydrolysis reaction constants decreased by 31.7% and 43.5% after addition of admixtures or carbonation,which slowed down the rate of decay of cement mechanical strength.(4)Liquid water affected the performance of MOC concrete most serious in natural environment,and appropriate gaseous moisture was advantageous for hydration and carbonation reactions.Combined with the climatic characteristics of Qinghai and using the humidity coefficient as the climatic zone index,the water resistance index of MOC was proposed.It was proposed that the MOC concrete can be applied to the over-dry area and the medium-dry area after addition of admixtures,and not only to the over-dry area,middle-dry area,but also to the moist-dry area after carbonation.