Thermodynamic Modeling Of Hydration, Borax-Induced Retarding And Chloride Salts Attack Of Magnesium Ammonium Phosphate Cement System

Magnesium ammonium phosphate cement?MAPC?is an air-hardening cementitious material formed through an acid-base neutralization reaction between dead burned magnesia and soluble acid phosphates?i.e ammonium dihydrogen phosphate or diammonium hydrogen phosphate?.MAPC possess high early strength,short setting time and strong bonding strength,and can be used in fast-repairing of construction structure,restorations of dental and bone and radioactive waste solidfication.The performance of MAPC mainly depends on molar ratio of raw materials,type and amount of the retarders,type of the additives,and curing temperature.So far,the previous research mainly exmployed experimental approach to investigate the hydration behavior of MAPC system.No consensus has been reached on both the hydration mechanism and borax-induced retardaring mechanism of MAPC system.Few work can be found on the hydration behavior of MAPC subjected to brine environment.This paper empolyed hydrogeochemical simulation software PHREEQC to study the pH evolution and hydration phase diagram of MAPC system at 25?.The results demonstrate that the formation of intermediate phase-schertelite and struvite is affected by both the pH value of the solution and the concentrations of ions.The concentration of Mg²⁺ions required for the formation of struvite is reduced when the concentrations of NH₄⁺and PO₄^3-ions is high.Increasing the activity of water promoted formation of struvite.It is more conducive to the formation of schertelite when the concentration of NH₄⁺and HPO₄^2-ions in the solution is high.With the hydrolysis of MgO,the pH of the solution increased,and schertelite gradually transformed into struvite.To explore the retardation mechanism induced by borax in the MAPC system,both of the hydration pH value evolution and hydration phase diagram under different borax content were simulated.The results reveal that incorporation of borax increases the pH value of the MAPC system in the initial stage,inhibite the dissolution of magnesium oxide particles,reduced temperature of the system,and consequently decreases the reaction rate.The dissolved B?OH?₄^-ions by borax react more easily with the Mg²⁺ions in the solution to form MgB?OH?₄⁺complex ions.This ionic complexation reaction competes with the formation reaction of schertelite to get Mg²⁺ions in the solution,and consequently reduces the amount of schertelite phase.The increase of the pH value of the solution in the initial reaction stage also promotes the ionization of H₂PO₄^-and HPO₄^2-ions,which increases the concentration of PO₄^3-ions in the solution and reacts with MgB?OH?₄⁺complex ions to preferentially generate luneburgite and delay the formation of struvite phase.To further investigate the attack of chloride?NaCl,KCl and CaCl₂?on the MAPC system,the evolution of both the pH value and hydration phase diagram of MAPC system incorporated with various dasages of different types of chloride salts are simulated.It is found that incorporation of NaCl delay the development of the pH value.The pH value evolution of MAPC solution slowed as the NaCl content increased.The addition of NaCl reduces the dosage of schertelite and struvite in the hydration product of MAPC solution,resulting in the formation of a new hydration phase of Na-struvite?NaMgPO₄·7H₂O?.The addition of KCl neither affects the type and conversion of MAPC hydration products,nor produces new phases.The incorporation of CaCl₂ reduces pH value of the MAPC system.The addition of CaCl₂reduces the formation of the schertelite phase and leads to the emergence of hydroxyapatite in MAPC solution.Formation of hydroxyapatite consumes PO₄^3-ions in solution,results in the increaseing M/P molar ratio,and delay the formation of struvite.