Thermodynamic Modeling Of Hydration Behavior And Stability Of Hydration Products For Magnesium Phosphate Cement System

Magnesium phosphate cement(Magnesium potassium phosphate cement,MPC)is a new type of cement which is widely applied in fast repairing of constructions and pavement,artificial bone and solidification of nuclear waste.There are several types of magnesium phosphate cements,in which magnesium ammonium phosphate cement(Magnesium ammonium phosphate cement,MAPC)and magnesium potassium phosphate cement(Magnesium potassium phosphate cement,MKPC)are most commonly used.Currently,many researches have focused on the experimental exploration of magnesium phosphate cement,few theoretical explanations have been proposed about hydration mechanism and composition of hydration products.The final hydrates composition is influenced by many factors,such as molar ratio of raw material,dosage of additives and reaction temperature.To improve the properties of magnesium phosphate cement,it is essential to investigate the composition of hydration products.Therefore,this study explored the mechanism of hydration process and composition of hydration products through thermodynamic modeling.Firstly,all thermodynamic data about MAPC system andMKPC system has been collected and evaluated.Coupled with PITZER model,a thermodynamic database is established for MPC system.The accuracy of the database is evaluated by thermodynamic software PHREEQC.Afterwards,the pH value of solution for MAPC and MKPC system at 25 ℃,0.1 MPa is simulated based on the established database.The effect of M/P ratio on compositionof hydration products are studied.The precipitation and dissolution condition of hydration products are also given in this study.It can be found that the composition of hydration products for MAPC system is affected by M/P ratio:(1)When M/P<0.49,the hydration product is(NH4)2Mg(HPO4)2·4H2O;(2)When 0.49<M/P<1.00,the hydration products are(NH4)2Mg(HP04)2·4H2O,MgHP04·3H20 and MgNH4P04·6H20;(3)When M/P>1.00,the hydration products are MgNH4PO4·6H2O and excessive magnesia.For MKPC system:(1)When M/P<0.475,the hydration product is MgHPO4·3H2O;(2)When 0.475<M/P<0.64,the hydration products are MgHPO4·3H20 and Mg2KH(PO4)2·15H2O;(3)When 0.655<M/P<1.00,the hydration products are Mg2KH(PO4)2·15H20 and KMgPO4·6H2O;(4)When M/P>1.00,the hydration products are KMgPO4·6H2O and excessive magnesia.Effect of some additives on pH value of solution are studied,such as H3PO4,(NH4)2HPO4,(NH4)3PO4,K2HPO4,K3PO4,boric acid and borax.It reveals that the addition of H3PO4 and boric acid can reduce the pH value of solution,while the addition of(NH4)2HPO4,(NH4)3PO4,K2HPO4,K3PO4 and borax can improve the pH value of solution.Composition of hydration products effected by additives are also investigated in this research.Results showed that in MAPC system,the precipitation of(NH4)2Mg(HPO4)2·4H2O and MgHPO4·3H2O can be easier and the content of(NH4)2Mg(HPO4)2·4H2O and MgHPO4·3H2O decreased,when more dosage of(NH4)2HPO4 or(NH4)3PO4 is added.In MKPC system,the precipitation of MgHPO4·3H2O and Mg2KH(PO4)2·15H2O can be easier and molar content of MgHPO4·3H2O and Mg2KH(PO4)2·15H2O decreased,when more dosage of K2HPO4 or K3PO4 is added.The addition of H3PO4 can reduce the amount of hydration products in both MAPC and MKPC system.To understand the effect of temperature on composition of hydration products in MPC system,thermodynamic simulation have been carried out in the temperature range of 5-45 ℃.Results demonstra that,with the increase of temperature,the concentration levels for hydrates to precipitate increase,and the molar content of final hydrates stays the same.