Effect Of Lean Kaolin On High Temperature Resistance Of Magnesium Phosphate Cement

Magnesium phosphate cement（MPC）is a commonly used repair material in modern engineering,which has the advantages of high early strength and good bonding performance with old concrete,but due to the high energy consumption and high cost of the production and preparation process of refired magnesium oxide,fast setting time,excessive heat release during hydration and deterioration of mechanical properties at high temperature,its promotion and application are limited to a certain extent.Metakaolin has good modification properties and can be used as a mineral admixture in cement and concrete.In this paper,the effect of metakaolin on the high temperature resistance of magnesium phosphate cement is mainly studied by the preparation of magnesium phosphate cement by adding metakaolin,firstly,metakaolin is incorporated into the raw material of magnesium phosphate cement to explore its effect on the working performance and hydration heat of magnesium phosphate cement,and secondly,the mechanical properties of metakaolin modified magnesium phosphate cement are tested with temperature from static and dynamic aspects,and finally the effect of high temperature changes on the hydration products and microstructure of magnesium phosphate cement is studied.To investigate the deterioration mechanism of metakaolin modified magnesium phosphate cement at high temperature.The main findings are as follows:The effects of metakaolin on the working performance and hydration heat of magnesium phosphate cement were studied,and the results showed that the incorporation of metakaolin had the effect of delaying the setting time of magnesium phosphate cement,which could be increased by 166.7%at the highest,but the fluidity showed a decreasing trend with the increase of metakaolin content,and the fluidity of magnesium phosphate cement slurry decreased by 46.4%when the metakaolin content increased to 40%.The hydration and exothermic exothermic of modified magnesium phosphate cement in kaolin was generally lower than that of unmodified magnesium phosphate cement.Magnesium phosphate cement prepared from metakaolin at various temperature ranges greatly improves its mechanical properties and is optimal at a content of 30%.The mechanical properties of magnesium phosphate cement deteriorate after heating and heating,but the strength will increase to a certain extent at 600°C.Before heating to400°C,10%metakaolin content can improve the high temperature resistance of magnesium phosphate cement,while its high temperature resistance is not improved after400°C;In the process of room temperature to 200°C,the peak stress gradually decreases,while at 600°C,the peak stress of magnesium phosphate cement increases due to high temperature consolidation reaction,and the peak strain of magnesium phosphate cement increases slightly with the increase of metakaolin content at the same temperature.The effects of high temperature changes on the hydration products and microstructure of magnesium phosphate cement were studied.The results show that the temperature will affect the hydration products of magnesium phosphate cement,and as the temperature increases,the compactness of the matrix continues to decrease,and the struvite is partially decomposed into dittmarite and Mg HPO₄ at 200°C,and continues to heat up to 600°C,Mg HPO₄ and dittmarite decompose to form a large amount of Mg₂P₂O₇,and due to the high temperature consolidation of magnesium phosphate cement,its microstructure becomes dense again,and the heating temperature is 800°C.Its hydration products are loose flaky substances（Mg₃（PO₄）₂）;In addition to Mg NH₄PO₄·6H₂O and unreacted Mg O,the main hydration products of magnesium phosphate cement after incorporation into metakaolin will also be formed by aluminum-containing amorphous substance aluminum phosphate salt（APH）,which is flocculent and wrapped around the hydration product,making the structure dense;The change of hydration product with temperature after heating is basically the same as that when metakaolin is not doped,the difference is that as the temperature increases,there will be the formation of amorphous aluminum phosphate（Al PO₄）.