Study On Hydration Phase Regulation And Structure-Activity Relationship During Hydration Of Magnesium Phosphate Cement

Magnesium phosphate cement is a cementitious material with fast hardening and early strength,high bond strength,good volume stability,good wear resistance.It has received wide attention in restoration,structural reinforcement,and curing materials because of its excellent performance.However,the environment’s variability and high durability have put higher and higher requirements on the mechanical properties of materials for repair,reinforcement and curing,etc.Significantly improving the mechanical strength is a direct means to enhance the performance in use,and has always been a research hot spot for magnesium phosphate cement.It is well known that the material’s structure determines its performance,and the current research to improve the mechanical properties of magnesium phosphate cement is mainly focused on magnesium-phosphorus ratio,water-cement ratio,mineral admixtures,etc.There is a lack of research on enhancing mechanical properties by regulating the crystal structure of the hydrated phase of magnesium phosphate cement.Uniform struvite nanowires can be synthesized by controlling the sodium ion concentration and p H,and struvite is the main hydration product of magnesium phosphate cement,indicating that sodium ion has some influence on the crystal structure of the hydration phase of magnesium phosphate cement.Therefore,in this paper,sodium dihydrogen phosphate was chosen to regulate the crystal structure of the hydration phase of magnesium phosphate cement,and the effects of sodium ions on the hydration process and the evolution of the hydration phase structure of magnesium phosphate cement at high water-cement ratio;and the effects of sodium dihydrogen phosphate on the macroscopic mechanical properties of magnesium phosphate cement at low water-cement ratio were investigated respectively.The phase composition,functional group changes,and microscopic morphology of the hydrated phase were systematically analyzed by X-ray diffractometer,infrared spectrometer,scanning electron microscope,and other instruments.The results showed that:Six hydration phases appear in the hydration process of magnesium phosphate cement,and the first formed hydration phase is Mg HPO₄·3H₂O with a long hexagonal lamellar structure,which can exist stably in a weak acid environment（p H=6.5）;the second appeared hydration phase is Mg₂KH（PO₄）₂·15H₂O with long quadrangular lamellar structure,which can exist stably in an environment near p H 7.5;the six prismatic structures of Mg KPO₄·6H₂O is the final hydrated phase of potassium magnesium phosphate cement,which can be stable in an alkaline environment（p H>8.5）;the needle-like structure of Na,K-struvite is similar to K-struvite and can be stable in an alkaline environment（p H>8.5）;amorphous structure of Na-ornithine also occurs in an alkaline environment;hexagonal block structure of Mg₃（PO₄）₂·8H₂O,an intermediate hydrated phase,can be formed at a p H of 8.4.Among them,both K-struvite and Na,K-struvite have good crystallinity,and Na-struvite is an amorphous structure.The effect of sodium ion doping on the intermediate hydrated phase is insignificant,and the sodium ion has a very obvious effect on the crystal structure of the final hydrated phase K-struvite.With the increase of sodium ion doping,the hydrated phase changes from hexagonal columnar to needle-like,and when the sodium ion doping exceeds the potassium ion,the needle-like Na,K-struvite e is always preferentially generated,and the excess sodium ion continues to react to form the amorphous hydrated phase Na-struvite.Among them,when the sodium salt doping is 50%,it can synthesize more uniform needle-like Na,K-struvite.At a low water-cement ratio,adding sodium dihydrogen phosphate will accelerate the exothermic rate of hydration of magnesium phosphate cement hydration and shorten the setting time.Na,K-struvite can be grown in situ in magnesium phosphate cement.Na,K-struvite has good compatibility with magnesium phosphate cement matrix and can be uniformly distributed in magnesium phosphate cement matrix.The mechanical properties of magnesium phosphate cement are significantly improved by refining the pore size of the magnesium phosphate cement matrix and bridging micro-cracks.Among them,the 1d flexural strength of the sample with 50%sodium dihydrogen phosphate doping was up to 12.7 MPa,and 1d compressive strength was up to 78.1 MPa,which increased the 1d flexural strength by65%and 1d compressive strength by 122%compared with the sample without sodium dihydrogen phosphate doping.It was confirmed that sodium ions could modulate the crystal structure of struvite and enhance the mechanical properties of magnesium phosphate cement.The doping amount of sodium dihydrogen phosphate controlled between 40%and 60%has significantly improved the mechanical properties of magnesium phosphate cement,which helps to prepare magnesium phosphate cement-based materials with better mechanical properties and make magnesium phosphate cement better used in repair,reinforcement,and curing materials.