Hydration And Hardening Of Magnesium Phosphate Cement And Stability Of Hydration Products

Magnesium phosphate cement?MPC?is a special cement with fast setting,high strength and good volume stability,which is produced by acid-based reaction between phosphate and dead burnt magnesia.Borax is often used in MPC as a retarder to meet the early working requirement.Therefore,the basic composition parameters of magnesium phosphate cement includes the ratio of phosphate to magnesium oxide?P/M?,the ratio of water to cement?W/C?and the ratio of borax to magnesium oxide?B/M?,which have close relationship to the hydration properties of MPC.However,previous research is focused on the influence of a single composition parameter on the hydration properties of MPC,which lacks of systematic study of the interaction between these composition parameters.The retarding mechanism of borax in the hydration process of MPC has also not been clear studied.Therefore,the effect of P/M and W/C on the hydration progress of MPC will be studied,especially considering the effect of borax on hydration process of MPC.Meanwhile the stability of hydration products and hardened MPC in water and facing temperature rising will be studied.It is aimed at making a more comprehensive understanding of the hydration and hardening properties of MPC and offering reference for the optimization of MPC performance.The two common systems of MPC are magnesium ammonium phosphate cement?MAPC?and magnesium potassium phosphate cement?MPPC?,and these two systems were not effectively compared in previous studies.Therefore,comparative study should be made between the two systems,which is helpful to make better use of the two cement systems in practical applications.Firstly,through calculating the theory value of W/C in different P/M when MPC is complete hydrated as Struvite or K-struvite,it has been found that the W/C for completely hydrated MPC increases with the increase of P/M.While,it has also been found that the ratio of water to ammonium dihydrogen phosphate?W/ADP?or the ratio of water to potassium dihydrogen phosphate?W/PDP?is a fixed value when MPC completely hydrated,and the value of W/ADP and W/PDP are 0.782 and 0.661respectively.Through comprehensively analyzing the relationships between the hydration temperature rising and composition change of MPC,the results show that the hydration processes of two kinds of MPC system?MAPC and MPPC?are different.There is one exothermic peak of MAPC at the early hydration stage,and the increase of borax content will lead to the second exothermic peak of MAPC and delay the second exothermic peak of MAPC.While two exothermic peaks are exhibited in the early hydration stage of MPPC,and the increase of borax content will delay the second exothermic peak.The first exothermic peak is the dissolution reaction of phosphate with magnesium oxide,and the second exothermic peak is the rapid crystallization growth of the main hydrated product struvite or K-struvite.The main role of borax is to delay the growth rate of hydration products,not mainly delay the dissolution reaction between phosphate and magnesium oxide.By systematically studying hydration products,microstructure formation,strength development and volume change of MPC with different components,the interaction relationships between basic components on hydration and hardening of MPC was clarified.When the value of W/ADP is less than 0.782,which means the water is insufficient,unreacted ADP will remain in MAPC with low borax content.At high content of borax,ADP will reacted with borax to form severe cracking zone,leading significant volume contraction,when the water is insufficient.For MPPC sysrem,when the value of W/PDP is less than 0.661,meaning the water is insufficient,the crystal growth of the hydration product K-struvite will be inhibited,simultaneously,a severely cracking phase called"K-M_?0¹?-P"will be formed,leading to serious volume contraction.In"K-M_?0¹?-P"phase,the molar content of magnesium was significantly lower than potassium and phosphorus.MPPC has the best strength development when the ratio of water to PDP is 0.661,and it is not good for the strength of development when water is insufficient or excessive.The results also show that the MPC prepared with theoretical water consumption shows less shrinkage,even slight expansion.Therefore,it can be concluded that MPC has the best performance development at theoretical water consumption.Through synthesizing pure Struvite and K-struvite and measuring their solubility constants,the solubility curves of Struvite and K-struvite in different pH value solution were studied.The stability of hardend MPC in water was studied by analyzing the pH value of leaching solution,hydration products,strength and volume change of MPC cement when immersing in water.Meanwhile,thermal stability of hydration products and hardened MPC were studied by using the thermal analysis method.Results show that the solubility of Struvite and K-struvite are small in neutral to low alkaline water,and the dissolution amount of Struvite and K-Struvite will increase by order of magnitude when the pH value is too high or too low.Comparing with struvite,K-Struvite can adapt to a higher alkaline environment.The hydrated MPC with high hydration degree have a small volume expansion in water,and the strength keeps slight increasing or steady.The leaching solution of MPC specimens containing residual phosphate are acidic in early leaching age,resulting in decrease of strength and expension of volume.When MAPC prepared with insufficient water and high content of borax,the hydration product struvite will significantly increase in water,resulting the obvious expension of volume,even resulting breaking of specimens.MPPC prepared with insufficient water also shows obvious volume expansion in water.Struvite and K-struvite will become unstable with temperature rising,and the dehydration rate obviously increases when the temperature is higher than 100 ^oC.There is no obvious difference between the thermal stability of MAPC and MPPC.The degree of hydration will significantly affect the stability of MPC in water.MPC with high hydration degree has good stability in water,so MPC has good water resistance.In summary,the hydration and hardening properties of MPC are influenced by the basic composition parameters.Under the theoretical water consumption?W/ADP of0.782 or W/PDP of 0.661?,the hydration degree of MPC is high,showing good volume stable during the hardening process and keeping good performace in watere.Therefore,it should be considered that choosing the dosage of water nearby theoretical consumption is more suitable for preparing MPC,and the ratio of water to phosphate?W/P?is also an important parameter in the preparation of MPC.As the hydration and hardening characteristics of MAPC and MPPC systems are great different,it should be fully considered the difference between MAPC and MPPC in practical application.