Study On The Effect Of Mineral Admixture On The Performance Of Magnesium Phosphate Cement

The reason why magnesium phosphate is increasingly used in roads,tunnels,housing and some other fields is that it has the following characteristics: fast setting time?high early strength,high volume stability and good bonding effect with old concrete interface.However,there are some bad characteristics of magnesium phosphatez,like short constrction time ? low workability.Those bad characteristics lead to the more strict requirements for construction technology and application conditions.Its hydration speed and heat evolution of hydration of early stage are fast,such characteristics affect its intensity.Above all,in order to use industrial waste reasonably and improve key problems about fast hydration speed and large hydration heat,the modification of magnesium phosphate cement mineral admixtures performance is studied in this paper.In this article,the effect on cement compressive strength and setting time of magnesium phosphate brought by phosphorus magnesium ratio,water cement ratio,dosage of retarder,age and other factors is studied by orthogonal test.The best ratio of magnesium phosphate cement has been verified by analysis.Then this article studies how fly ash effect the magnesium phosphate cement workability,mechanical properties and volume stability by adding different amounts of fly ash,bentonite and metakaolin to magnesium phosphate cement.And then analyze the magnesium phosphate cement hydration process through hydration heat,XRD,SEM.The results are as follows: The best ratio of magnesium phosphate cement for the waterbinder ratio is 0.16,M/P is 3/1,borax content is 5% and in this case,magnesium phosphate cement intial setting time is 16.61 minutes,3h compressive strength is 24.7Mpa.The results show that adding each of three kinds of minerals can reduce the liquidity and setting time of magnesium phosphate cement.And the reduction degree is more significant along with the increase of mineral admixture.And the compressive strength of magnesium phosphate cement is reduced with the incorporation of fly ash and the impact is greater compared to little impact at early stage.Under certain incorporation,the requirements of rapid repairing materials are still met.Adding betonies will reduce the strength of magnesium phosphate cement,the dosage of it should be less than 6%.Metakaolin will improve the compressive strength of magnesium phosphate and the magnesium phosphate cement will get the best compressive strength when the admixture of metakaolin is 7.5%.Each of these three minerals additions can weaken the chemical shrinkage of magnesium phosphate cement and improve its volume stability.Bentonite has the best improving effect on the volume stability of magnesium phosphate cement.When bentonite content is 12%,14 d chemical shrinkage is reduced by 65%.In addition,these three mineral additions also reduce the heat release rate and the hydration heat of the hydration reaction of magnesium phosphate cement to some extent,among which the reduction affects of fly ash is the most significant and the partial kaolinite is second,improve the matter that the hydration heat of magnesium phosphate cement effectively.