Research On The Early Hydration And Phosphate Modification Mechanism Of Magnesium Oxychloride Cement

Magnesium oxychloride cement（MOC）is an inorganic non-metallic air-hardening cementitious material,which has been widely used due to its advantages of high strength,fast setting speed,strong adhesion,smooth and beautiful surface,and flame retardant thermal insulation.At present,the main application areas are construction materials,non-load-bearing floor materials,decorative materials,refractory products,etc.,and have broad prospects in replacing steel and wood.However,there are also defects such as poor water resistance,prone to warpage,moisture absorption,re-halogenation phenomenon,large heat release and concentration.These defects severely limit the application field of magnesium cement and the development of magnesium cementitious composite materials.In order to overcome the performance defects of MOC and broaden its application fields,this paper takes the preferred three sets of ratio evaluation system as the research object,uses KH₂PO₄ as the modifier,uses mechanical properties as the evaluation method,combines XRD,TG-DTG,DSC,SEM and other micro test methods,also record the early hydration heat.The hydration kinetics and thermodynamics of MOC in the early stage were studied,the hydration process and modification mechanism of KH₂PO₄ modified MOC were discussed,and analyze the strength development law of MOC and the effect of KH₂PO₄ on its strength development and softening coefficient.The main conclusions are as follows:（1）The 5-phase is formed first,and then gradually converted into the 3-phase.The smaller the Mg O/Mg Cl₂ molar ratio,the faster the conversion rate;the higher Mg O/Mg Cl₂molar ratio will increase the hydration rate,shorten the induction period and acceleration period,but It will cause incomplete hydration and reduce the thermal stability of the product;high temperature will affect the degree of hydration and the composition of the hydration product,and increase the proportion of Mg（OH）₂ in the hydration product.（2）KH₂PO₄ does not change the composition of the hydration product,but it will inhibit the phase transition and play the role of retardation,and the retardation effect is more obvious when the molar ratio of Mg O/Mg Cl₂ is low;the greater the dosage,the stronger the retardation effect,but it will increase the cumulative Exothermic heat and destroy the crystal structure of hydration products,the ideal dosage is 1wt.%.（3）The intensity is positively related to the molar ratio of Mg O/Mg Cl₂.In a humid environment,the intensity will continue to decrease due to hydrolysis after a short increase.The violent exothermic behavior or phase transition behavior caused the 3-day strength to decrease temporarily;KH₂PO₄ significantly improved the water resistance but decreased the early strength.When the admixture is too high,the crystal structure of the hydration product will be destroyed,and when the dosage is higher than 2wt.%,the crystal structure of the hydration product will be destroyed.