Study On Temperature Rise Deterioration Mechanism Of Magnesium Phosphate Cement Modified By Slag/waste Incineration Ash

Magnesium phosphate cement（MPC）is a kind of cementitious material based on the acid-base neutralization reaction of magnesium oxide and phosphate.It has the advantages of fast setting and high early strength,and has good development prospects in the fields of curing radioactive waste and road repair.There are few studies on the high temperature resistance of MPC at home and abroad,especially on the high temperature resistance and related mechanism of modified magnesium phosphate cement with admixtures.Therefore,it is of great significance to study the high temperature resistance of MPC.By studying the influence of ratio parameters and granulated blast furnace slag（hereinafter referred to as slag）and domestic waste incineration ash（hereinafter referred to as ash）with coarse and fine particle sizes screened by ball milling on the performance of MPC,the mechanical properties and microstructure of MPC before and after high temperature were explored.In view of the above purposes,the effects of magnesium-phosphorus ratio and water-binder ratio on MPC,the effects of single-doped slag,coarse ash slag,fine ash slag and composite-doped slag-coarse/fine ash slag on MPC and mechanical properties after high temperature were studied.The fluidity,setting time,mechanical properties test,volume stability test,high temperature heat resistance test and microscopic characterization were used to explore the performance changes of MPC.The main research results are as follows:The coagulation time and fluidity of MPC are jointly affected by the magnesium-phosphorus ratio and the water-to-adhesive ratio.The coagulation time is greatly affected by the magnesium-phosphorus ratio,the fluidity is greatly affected by the water-adhesive ratio and is positively correlated with the water-adhesive ratio,and it also has good fluidity at low water-adhesive ratio.Due to the dilution effect,the coagulation time of MPC increases with the increase of admixture content,and the addition of 10%slag or coarse bottom ash can improve the fluidity of MPC slurry.For mechanical properties,the early compressive strength and flexural strength of MPC at low magnesium-phosphorus ratio（M/P=4.5/1）developed slowly,and the later strength developed rapidly.Increasing the ratio of magnesium to phosphorus,the intensity of hydration developed faster in the early stage and slower in the later stage.The incorporation of admixtures can improve the compressive strength of MPC,and the compound 30%slag-fine bottom ash has the highest compressive strength.MPC has good integrity after high temperature,and there are no defects such as pulverization,peeling or cracking on the surface of the specimen,and its changes are mainly reflected in mechanical properties and mass loss.When the temperature is105℃～400℃,the quality and strength of MPC specimens decrease significantly;In the range of 600℃～1000℃,the quality of the specimen is basically stable,and the compressive strength has increased slightly.The addition of admixtures can reduce the mass loss of MPC after high temperature,and the mass loss rate decreases with the increase of the amount at the same temperature.After high temperature,the compressive/flexural strength of MPC mixed with bottom ash and slag-coarse/fine ash is basically higher than that of MPC without admixture at the same temperature,showing good high temperature resistance.SEM-EDS,XRD and TG-DTG analysis showed that the mechanical properties and mass changes of MPC after high temperature were closely related to the crystal structure change and phase transition of its hydrated product Mg KPO₄·6H₂O.Through XRD analysis,Mg KPO₄·6H₂O was dehydrated at 105℃to generate MgKPO₄·H₂O,Mg KPO₄·H₂O will continue to appear crystal water shedding phenomenon at 200℃～400℃,KMg PO₄is formed at 600℃～800°C,and the main hydration products at 1000℃are Mg HPO₄·x H₂O（x represents the number of H₂O molecules）.The dehydration reaction of MPC hydrated products in the range of105℃～400℃is the main reason for its mass loss and mechanical property decline.When the temperature exceeds 400℃,the MPC structure is basically stable,and the change of mechanical strength mainly comes from the transformation of crystal structure.The results of NMR showed that the NMR T₂spectrum of MPC pore structure showed three characteristic peaks before and after high temperature.The T₂spectrum area of MPC samples increased with the increase of temperature,which showed that the full-scale pores expanded and evolved after high temperature,and the porosity increased gradually.