Hydration Behaviors Of Magnesium Potassium Phosphate Cement System And Its Thermodynamic Simulation

Magnesium potassium phosphate cement(MKPC)is a type of phosphate-containing binder,which is formed by a through-solution acid-based reaction between dead burnt magnesia and phosphate.The advantages of the MKPC include rapid setting and hardening,high early strength,low pH and good biocompatibility.It has thus been popularly used in the fields of repair material,conditioning for low-level nuclear wastes,stabilization/solidification agents for immobilizing wastes containing heavy metals and biological material.Hydration behaviors of the MKPC are affected by many factors,in which M/P(magnesia to phosphate salt ratio by mass)and W/B(water to binder ratio by molar)are the two main fators that largely affect its hydration behaviors,like reaction paths,pH evolution,microstructures and macro-performance.Most of studies concentrate on the macro-performance control and on the reaction mechanisms of the MKPC with the common mixing ratios(M/P=1～8 and W/B=0.1-0.2).Studies with a wide range of mixing ratios(M/P<1 and W/B>0.5)are insufficient,which makes it difficult to fully understand the hydration behaviors of the MKPC.In addition,hydration process simulation and calculation method for hydration degree of the MKPC still lack of systematic theoretic basis.Therefore,MKPC systems at ultra high W/B and low W/B were systematically investigated in this work.The effect of W/B and M/P on the hydration behaviors of the MKPC,such as hydration process,hydration products and mechanisms were discussed.Methods for characterizing the quantities of the hydraiton products and calculating the hydration degree of the MKPC were proposed,respectively for the MKPC system with ultra high W/B and low W/B.Additionally,the hydration behaviors of the MKPC was simulated by thermohynamic method,and this work tentatively established relationships between the hydration degree,pore structure and compressive strength of the MKPC.(1)Hydration behaviors of the MKPC at ultra high W/B.Effect of M/P on the hydration process(heat release,pH evolution,conductivity and ion concentrations),hydration products and their evolution of the MKPC at ultra high W/B were discussed.Additionally,a method combined with XRD/Rietveld,TG/DSC and ICP-OES method was proposed to calculate the hydration degree of MgO,KH2PO4 and total hydration at the ultra high W/B condition,in which the hydration behaviors of the MKPC was analyzed from a quantitative standpoint.The results show that no clear evidence was observed of the formation of hydration products of the MKPC at M/P≤0.2 during hydration process,in which the main reaction was the dissolution of MgO.As the M/P ratio was within 0.2～1,the main hydration products were MgHPO4·3H2O and Mg2KH(PO4)2·15H2O,in which process MgHPO4·3H2O precipitated firstly followed by the formation of Mg2KH(PO4)2·15H2O.As the M/P=1,the main hydration products were Mg3(PO4)2·22H2O,MgHPO4·3H2O and Mg2KH(PO4)2·15H2O.As the M/P>1,the main hydration products were MgKPO4·6H2O(k-struvite)and Mg3(PO4)2·22H2O,where Mg3(PO4)2·22H2O precipitated firstly followed by the formation of k-struvite.However,the initially precipitated Mg3(PO4)2·22H2O was unstable as the increase of pH,which destabilized gradually.At low M/P(<4),the initially precipitated hydration products of the MKPC tended to be unstable,and the contents of Mg.P and K elements in the amorphous phase increased gradually.While at high M/P(≥4),the hydration products with crystalline structure of the MKPC was more stable and easily formed.The content of Mg element in the amorphous cement increased gradually,while the content of P and K decreased.With regarding to hydration degree,as M/P≥1,the hydration degree of KH2PO4 was close to that of the total hydration,while the hydration degree of MgO was far below that of the overall hydration,and with the increase of the M/P the hydration degree of MgO decreased.(2)Hydration behaviors of the MKPC at low W/B.Effect of M/P and W/B on the heat release,hydration products and their evolution of the MKPC at low high W/B were discussed.Additionally,a method combined with XRD/Rietveld and TG/DSC method was proposed to calculate the hydration degree of MgO,KH2PO4 and total hydration at the low W/B condition,in which the hydration behaviors of the MKPC was analyzed from a quantitative standpoint.The results show that,as the M/P≤1,the main hydration products of the MKPC were MgHPO4·7H2O and Mg2KH(PO4)2·15H2O.As the M/P ratio increases(within the scope of ≤1),contents of the hydration products MgHPO4·7H2O and Mg2KH(PO4)2·15H2O decrease,and k-struvite becomes to form and increase.As the M/P>1,the main hydration products of the MKPC was k-struvite.Especially,intermediate hydration products like MgHPO4·7H2O,Mg2KH(PO4)2·15H2O and MgHPO4·3H2O were even not observed at M/P≥4 during the hydration process.Comparing to hydration products of the MKPC at low W/B,formation of MgHPO4·3H2O and Mg3(PO4)2·22H2O were not observed.Movever,the crystallinity of the hydration products formed at low W/B was reduced,while more amorphous phase formed at the eary stage.With regarding to hydration degree,the hydraion degree of the MgO was close to that of the overall hydration degree at early stage when M/P≥1.However,the hydration degree of the MgO was lower than the overall hydration degree over time.(3)Thermodynamic simulation on the hydration behaviors of the MKPC.The extented Debye-Huckel model and SQP(Successive Quadratic Programming)method were combined to establish a thermodynamic model of the MKPC.Deconvolution method was used to make a distinction between the hydration stages of the MKPC,and the thermodynamic parameters of each hydration stage were further determined.The results show that,the method to characterize the ion activity in the MKPC by the extented Debye-Huckel model,and to obtain the ions speciation and quantities by a minimum Gibbs free energy of the system that determined by SQP method,could establish a thermodynamic model for MKPC hydration.The hydration stages could be divided into four stages at utra high W/B,namely,dissolution of KH2PO4,dissolution of MgO,formation of amorphous I and formation of phosphate-containing mineral.Instead,at low W/B,formation of amorphous II was included other than the aforementioned four stages at ultra high W/B.The formation of the amorphous I was the predominant factor that restricted the types and quantities of the crystalline MKPC hydration products,no matter the system was at utra high W/B or at low W/B.(4)Coordinated development relationship between hydration,pore structure and mechanical properties of the MKPC.There was a significant linear relationship between the gel/space ratio and the compressive strength of the MKPC.At the same time,the gel/space ratio correlated with the hydration degree,while the hydration degree affected the porosity of the MKPC.Therefore,there were strict and close non-linear mathematical models between the hydration degree,porosity and compressive strength.