Regulation Of Hydration And Hardening Of Magnesium Phosphate Cement

Magnesium Phosphate Cement（MPC）is an inorgenic cemantitious material made from dead burned magnesium oxide and potassium dihydrogen phosphate in a certain proportion,which is mixed with water and undergoes an acid-base neutralization reaction to produce Mg KPO₄·6H₂O as the main product.MPC has received great attention recently,mainly because of its many unique properties.The most prominent characteristics of MPC are its rapid development of early strength,high late strength,excellent bonding performance（excellent bonding with ordinary cement concrete and steel metal）,good volume stability（small shrinkage）,simple maintenance（natural curing in air）,and hydration products that are not subject to sulfuric acid attack.They have been widely studied and applied.However,MPC is not conducive to construction due to its too fast reaction rate and the release of a large amount of heat during the reaction process.It is hoped that by regulating and controlling magnesium phosphate cement,the laws of hydration,hardening,and heat release of MPC with different proportions will be explored.At the same time,the existing MPC retarders are often difficulty to achieve a balance between working performance and strength.Therefore,finding a retarder that can effectively adjust the setting time and has a small impact on strength is the key to achieving the engineering application of MPC.At present,the analysis of the retarding mechanism of MPC is not clear.During the research process,the retarding mechanism of retarders in MPC was analyzed.This article first studied the effects different prportions on the mechenical and working properties MPC,and then studied its different hydration stages through hydration kinetics to explore its laws.Then,combined with heat of hydration,ion concentration,solid phase composition,and microstructure analysis,the retarding mechanism of boric acid and triethanolamine was analyzed.Finally,the application and validation of new retarders in mortar cement were conducted.The main research results are as follows:（1）Under different M/P conditions,the set time or fluidity of magnesium phosphate increases with the increases of water cement ratio,while strength each age decreases with increase of water cement ratio.Under different water cement ratios,the set time and fluidity magneium phosphate cement first increases and then decreases with the increase of M/P.The strength each age increases and then decrases with the increase of M/P.The optimal M/P is 3.The strength decreases with the increase of boric acid content.The optimal boric acid content is 2%.（2）There are two exothermic peaks in the hydration heat of magnesium phosphate cement,with an increase in water cement ratio,a decrease in the peaks of both peaks,and a delay in the occurrence of the second peak.As the M/P decreases,the time for the first peak of hydration heat to appear will extend and the peak will increase.But for the second peak of hydration heat,as the M/P increases,the time for the peak of hydration heat to appear will be extended,but the peak of hydration heat will decrease slightly.With the addition of boric acid,the peak values of the two peaks of hydration heat decrease and the occurrence time is delayed.（3）Conduct hydration kinetics analysis on magnesium phosphate cement,and measure the reaction resistance constant N and reaction rate K at each hydration reaction stage.The calculation shows that as hydration progresses,the react resistence N shows an upward trend,while the rate K shows a downward trend.In the second stage of cement hydration,with the increase of boric acid content,the hydration resistance constant N decreases and the reaction rate K decreases,indicating that the addition of boric acid inhibits the dissolution of magnesium oxide.The addition of boric acid greatly inhibited the hydration of stage III and the generation of Mg（H₂O）₆²⁺.（4）The addition of triethanolamine will inhibit the dissolution of magnesium oxide and potassium dihydrogen phosphate,and inhibit the generation of hydration products from magnesium phosphate cement.It affects the morphology of the hydrated products of potassium magnesium phosphate cement and hinders the growth of the hydrated product crystals.With the increase of triethanolamine content,the crystal size decreases and the structural compactness decreases.The addition of water cement ratio,M/P,and retarder does not affect the type of magnesium phosphate cement product.Research has found that the addition of triethanolamine can greatly improve the workability of magnesium phosphate cement without causing a decrease in cement strength,and can be applied to engineering.