Durability And Mechanism Of Magnesium Ammonium Phosphate Cement Coatings In Corrosive Environment

Magnesium phosphate cement?MPC?is a kind of inorganic cementing material produced by dead burning magnesium oxide,soluble acid phosphate and additive at a certain ratio under acid conditions through the acid-base chemical reaction and physical interaction,with phosphate as the binding phase.MPC is characterized by dense structure,high early strength,high volumetric stability,strong cohesiveness and favorable adhesiveness.It can be applied in concrete structural repair,base material of composite,and solidification of harmful and radioactive waste.The research on magnesium phosphate cement-based materials at home and abroad mainly focuses on the preparation,properties and hydration of magnesium phosphate cement.The research on the development of magnesium phosphate cement retarder and the effect of retarder on the hydration products and microstructure of cement have also made good progress.In contrast,the research on the technology and performance of high performance magnesium phosphate cement-based coatings prepared by magnesium phosphate cement and the key construction technology is still very scarce,let alone systematic research.Magnesium phosphate cement-based coatings have a rapid setting,which has a negative impact on the operation and performance of the coatings,and limits its application.Magnesium phosphate cement-based coatings have been in contact with water for a long time.Water-soluble corrosion exists and the water stability needs to be improved,which limits its use in water solution corrosion environment.Magnesium phosphate cement-based coatings have been in sulfate corrosion environment for a long time,and their mechanical properties have also declined,which limits their long-term use in sulfate corrosion environment.Therefore,it is of great significance to realize the retarding control of magnesium phosphate cement-based coatings,to study the durability of magnesium ammonium phosphate cement-based coatings in corrosive environments such as water environment and sulfate,and to improve the protection and service performance of concrete.It also provides support and guarantee for energy saving and green development of building materials industry.In this paper,the retarding control of the coating of Magnesium phosphate ammonium cement?MAPC?,the bonding interface of the coating of MAPC and the durability of the coating of MAPC are systematically studied.It is proposed that the setting time of MAPC coatings can be controlled and the construction performance can meet the requirements,and the durability of MAPC coatings in water and sulfate environments can be improved,so as to expand the application scope of MAPC coatings.?1?Retarding control of MAPC coatingsThe diluted suspension of MAPC coatings is prepared.The pH value and hydration temperature are measured by pH meter and hydration heat tester.The reaction mechanism of MAPC coatings system is studied.Combining with phase analysis,the reaction mechanism model of MAPC coatings is proposed.Then,by testing the hydration exothermic properties and pH value of MAPC coatings with different borax content,a new retarding mechanism of borax on MAPC hydration system is proposed.The hydration process of MAPC coating system can be divided into three stages:dissolution of NH₄H₂PO₄,initial crystallization of MgNH₄PO₄·6H₂O and continuous crystallization of MgNH₄PO₄·6H₂O.The addition of borax can only slightly delay the setting time of the MAPC coating system,and no new hydration products are formed in the middle.Compound retarder composed of glacial acetic acid and borax can obviously delay the setting time of MAPC coating system and improve the construction performance,but the strength has little effect.The optimum concentration of glacial acetic acid is 3%.The retarding effect of glacial acetic acid on the MAPC coating system can be attributed to the protective film,cooling and reducing the pH value,as well as the function of functional groups,so that the hydration process of the MAPC coating system does not release ammonia.?2?Regulation and Control of Water Resistance of MAPC CoatingsMAPC coating is prepared and its basic properties are tested.Adding NaCl can improve the water resistance of MAPC coating.When the content is 3%,the thickness,water absorption and tensile strength of the MAPC coating are optimized.The chloride ion participates in the hydration reaction to form a new complex,which makes the structure of the coating more compact in the water environment,and greatly improves the bond strength between the coating and concrete,thus enhancing the water resistance of the coating.?3?Microstructure evolution of bonding interface of MAPC coatings in sulfate corrosion environmentThe bond specimens of MAPC coating and Portland cement are immersed in Ca?OH?₂ solution and Na₂SO₄ solution for a long time.The bond strength of the specimens is tested,the fracture location and fracture surface are observed and analyzed,and the bond interface of the coatings in different immersion periods is analyzed by optical microscopy and SEM.The effects of different environments on the bond strength and interfacial morphology of MAPC are studied.The interfacial properties such as fracture characteristics,interfacial cracks,void morphology and interfacial micro-morphology are obtained,and time-varying models in different environments are established.There are obvious interfacial micro-zones between MAPC and Portland cement.In the micro interface area,the crystals with good crystallinity are formed in gel form and have good adhesion force,which makes MAPC a good repair material.In Na₂SO₄solution,the structure of hydration products is loose and the crystal morphology is improved,but the bonding property remains stable.?4?Sulfate resistance of concrete coated with MAPCBy analyzing the changes of compressive strength,supersonic velocity,apparent morphology and sulfate ion content of concrete coated with MAPC in sulfate corrosion environment,the sulfate resistance of concrete coated with MAPC is studied by means of XRD and SEM,and its internal mechanism is established.The surface of concrete structure under sulfate attack environment is divided into three parts:atmospheric zone,adsorption zone and immersion zone.Corrosion mainly occurs in adsorption zone and immersion zone.The corrosion of concrete in immersion zone is more destroyed by sulfate than that of concrete in adsorption zone.Through the analysis of concrete compressive strength change and supersonic testing,the MAPC coating can effectively prevent the invasion of sulfate solution and play a role in protecting concrete.The hydration product of MAPC can react with sulfate ion to form a new complex,which makes the structure of the coating more compact in sulfate solution,and greatly improves the bond strength between the coating and concrete,thus enhancing the sulfate resistance of concrete.