Study On The Early Hydration Process And Rheological Properties Of Potassium Magnesium Phosphate Cement

Potassium magnesium phosphate cement（MKPC）is a new inorganic cementitious material prepared by a certain proportion of dead burned magnesium oxide,potassium dihydrogen phosphate,retarder and additives or mineral admixtures.MKPC forms strength through acid-base neutralization reaction.It has several advantages over ordinary Portland cement,which include short setting time and high early strength,good adaptability to circumstance temperature,good bonding strength with concrete and steel,small volume deformation,corrosion resistance,fire resistance etc.,MKPC is widely used in rapid repair of concrete structure and pavement,solidification of toxic or nuclear waste,and Bio-based building material.In recent years,many scholars in China have studied the use of MKPC as fireproof and anticorrosive coatings for steel structures,and achieved gratifying results.The construction technology of grouting and spraying is crucial to the application and quality control of MKPC,but currently few theoretical works have been done to give quantitative clues and reveal the underlying mechanisms.Based on the early hydration process of MKPC,the rheological properties of suspension and paste with magnesium-to-phosphate（Mg/PO4）molar ratio of 7 were investigated in this paper.The effects of early hydration process of MKPC suspensions with different retarders on viscosity were characterized by pH value and electrical conductivity,hydration temperature,phase evolution and particle morphology.It is found that the hydration of MKPC is a complex process,which is largely affected by the types of retarders.A small amount of MgHPO4·3H2O was formed in 5 min without retarder,and then transformed into MgKPO₄·6H₂O.The hydration was basically completed in 10 min.The suspension mixed with retarders formed a kind of amorphous phase,which wrapped magnesium oxide particles to reduce the dissociation rate of Mg2+.The suspension containing borax did not generate other hydration products within 2h.An unknown crystal with Mg:P:K atomic ratio of 4:2:1 was formed in the suspension with composite retarder after 55 min.The crystal gradually transformed into MgKPO₄·6H₂O after 70 min hydration.The precipitation of the intermediate hydrate affected the viscosity of suspension in varying degrees.Suspension provides convenience for early hydration research of MKPC.Based on the results,the rheological properties of paste with small water-binder ratio were investigated.The research shows that the viscosity of MKPC mixed with borax develops too fast due to the formation of amorphous phase,and the viscosity reaches 61.7 Pa·s in 10 min,which is not suitable for engineering application.The viscosity and yield stress of the paste mixed with composite retarder increase slowly with time,and still have fluidity after 55 min.On this basis,the effects of fly ash,silica fume and quartz powder on rheological properties,early mechanical properties and corresponding microscopic factors of slurry were studied.,which provided theoretical guidance for engineering application and mix design.Figure 38 table 11 references 104...