Influence Of Synthesis And Dissolving Factors Of Potassium Magnesium Phosphate And Its Mechanism

Potassium magnesium phosphate cement?MKPC?is composed of over burnt MgO,potassium dihydrogen phosphate?KH₂PO₄?,retarding components and mineral admixtures in certain ratio and is able to produce phosphate hydrates as binder by the neutralization reactions between acid and base in acidic condition.It is a new inorganic cementitious material,which include high early strength,short setting time,low drying shrinkage,good adaptability to circumstance temperature,good bonding strength etc,which has broad application prospects in the rapid repairing of concrete pavements,airports,the repairing and reinforcing concrete structures and the solidification of harmful substances.However,it is found that the MKPC had a fast coagulation setting and poor water resistance,which limits its use in other areas,therefore,it is of great significance to study the synthesis and dissolution mechanism and mechanism of potassium magnesium phosphate for the popularization and application of MKPC.In this paper,a series of experiments were conducted to study the pH,temperature,and rotational speed on the synthesis of potassium magnesium phosphate,then prepared high-purity potassium magnesium phosphate by orthogonal experiments.The concentrations of potassium magnesium phosphate in pure water,different pH,and in MgO,KH₂PO₄,KH₂PO₄ and MgO solutions were tested,X-ray diffraction analysis?XRD?,scanning electron microscopy-energy dispersive spectroscopy?SEM-EDS?and thermogravimetric analysis?TG-DTG?qualitative analysis were used to further reveal the dissolution mechanism and mechanism.The important conclusion as follow:?1?The factors affecting the purity of potassium magnesium phosphate were as follows:temperature,rotation speed and pH value;the corresponding optimal conditions were 25°C,16 r/s,and 8.0;the analysis of the content of three elements in the synthetic powder showed that the purity of the synthesized high-purity potassium magnesium phosphate was 99.36%;?2?The dissolution of high purity potassium magnesium phosphate in pure water was as follow:When the pure aqueous solution reaches the equilibrium,the concentrations of three elements were c?K?>c?P?>c?Mg?,the maximum dissolution amount was 3.9974×10^-2 g,and the dissolution rate was 1.9987%;the powders were mainly Mg₃?PO₄?₂·22H₂O and MgKPO₄·6H₂O at equilibrium;by SEM-EDS,it can be seen that the large-sized and irregular spherical densely packed structure became flaky,smooth structure after dissolution;?3?The dissolution of High-purity potassium magnesium phosphate at different pH was as follow:When equilibrium was reached in acidic solutions?pH=4.5,5.5,6.5?,the three element concentrations were c?K?>c?Mg?>c?P?;with the increase of the acidity of the solution,the maximum dissolution rate and dissolution amount increased in turn,were 8.7940×10^-2g,8.6875×10^-2g,8.4481×10^-2g and 4.3977%,4.4377%and 4.2242%;equilibrium was reached in alkaline solutions?pH=8.0,9.0,10.0,11.0,12.0?,alkaline solutions?pH=8.0,9.0,10.0,11.0,12.0?,the three element concentrations were c?K?>c?P?>c?Mg?,the alkalinity of the solution was enhanced,and the maximum dissolution rate and elution amount showed fluctuant changes,the maximum dissolution rate and dissolution amount reached 7.9374×10-2g and 3.9687%at pH=12.0,the powders were mainly Mg₃?PO₄?₂·22H₂O and MgKPO₄·6H₂O at equilibrium,the dissolved crystal morphology has undergone major changes by SEM-EDS,the crystal was large-size,smooth plate-like crystals in the acidic environment,and crystal was small-size,loosely-packed in the alkaline environment;?4?The remaining powders of MgO,KH₂PO₄,KH₂PO₄ and MgO solutions in the equilibrium state was Mg₃?PO₄?₂·22H₂O and Mg₃?PO₄?₂·8H₂O,the microscopic morphology showed small size,loose stacking structure;?5?Adding solid waste powder?fly ash,steel slag,nickel slag?in MKPC materials,the three kinds of solid waste incorporation are beneficial to the hardened paste,in which the best effect is 20%,comparing with the control group the early strength and volume Deformation were increased by 18.49%,32.86%,23.15%and199.81%,21.97%and 7.327%,respectively in 1 d,the analysis of XRD and SEM-EDS showed that the type of hydration products increased and the pore structure was dense,revealing the water resistance modification,the essence is the change of the microstructure structure.Based on the mechanism of water resistance modification of potassium magnesium phosphate cement hardened paste,the application of potassium magnesium phosphate cement was accelerated.The purity loss of MgKPO₄·6H₂O is lower under high temperature and high alkali conditions,which provides a reference for the application of fireproof coatings.