Study On Bonding Properties Of Magnesium Phosphate Cement-based Materials In Water Environment

In the course of service,concrete will be damaged in different degrees due to the complex natural environment.In order to reduce the economic loss and strengthen the sustainable development of concrete structure,it is necessary to repair and reinforce it.As a kind of repairing material,magnesium phosphate cement has the advantages of fast hardening and early strength,good volume stability and excellent bonding performance with old concrete,but the large-scale application of magnesium phosphate cement is hindered to a certain extent because of its poor water resistance.In this paper,the effects of M/P ratio,W/B ratio,S/B ratio,B/M and fly ash content on the compressive strength and bond properties of magnesium phosphate cement-based materials in natural environment and water environment are studied.The loss of compressive strength and bond strength of magnesium phosphate cement-based materials in water environment can be directly reflected by strength retention rate.Scanning electron microscope was used to observe the microscopic morphology of magnesium phosphate cement bonding interface and explore the mechanism of water environment on bonding interface deterioration.The main conclusions are as follows:(1)With the increase of M/P ratio,the compressive strength and bond strength increase firstly and then decrease.When M/P=4,the compressive strength and bond strength reach the maximum.The compressive strength retention rate and bond strength retention rate decrease with the increase of M/P ratio in water environment.(2)With the increase of W/B ratio,the compressive strength and bond strength decrease gradually.The compressive strength and bond strength retention rate decrease with the increase of W/B ratio.(3)With the increase of S/B ratio,the compressive strength increases first and then decreases.When S/B=0.8～1,the compressive strength is higher.The bond strength decreases with the increase of S/B ratio.In the water environment,when S/B=0.4,0.6 and 0.8,cracks appear in magnesium phosphate cement mortar and the compressive strength is low.(4)With the increase of borax content,the compressive strength decreases gradually,and the bond strength increases first and then decreases.In the water environment,the compressive strength loss is the least when borax content is 6%,and the bond strength loss is the least when borax content is 8%.(5)From the microscopic point of view,in the water environment,due to the dissolution of unreacted potassium dihydrogen phosphate and the precipitation of hydration products,pores or micro-cracks appear at the bonding interface.With the further erosion of water,micro-cracks gradually develop,the bonding interface cracks,and the bond strength decreases.