Experimental Study On Bond Durability Between MKPC Paste And Ordinary Portland Cement Concrete

Concrete has been widely used in various practical projects since its inception.However,concrete has also withstood the test of different natural environments.Under harsh environments such as salt corrosion and freeze-thaw cycles,concrete is often destroyed.Therefore,the secondary repair of concrete structures has become a research hotspot.As for the selection of concrete repair materials,it is necessary to satisfy the indexes of bonding performance,shrinkage strain,thermal expansion coefficient,Poisson's ratio,elastic modulus and tensile strength.In order to meet the above indexes,many scholars at home and abroad have done a lot of research and developed a number of special cement,among which MPC has been widely concerned in recent years due to its excellent properties.MPC?Mgnesium phosphate cement,MPC?is a new type of inorganic cementitious material.It has the characteristics of fast hardening,early strength,good volume stability,strong environmental temperature adaptability,good compatibility with concrete and good durability.It is precisely because MPC has the advantages mentioned above that it has been applied in the rapid repair of concrete structures.As early as the end of the 20th century,MPC has been applied to the airport runway in foreign countries.The early mature technology is to prepare MPC from MgO and NH₄H₂PO₄ and main product of hydration reaction is bird droppings?NH₄MgPO₄·6H₂O?.However,the large amount of ammonia produced by the reaction caused damage to the equipment.In 1998,Singh et al.took potassium phosphate as an acid component,and then prepared potassium magnesium phosphate cement?MKPC?in a certain proportion with reburned magnesia and retarder.There was no harmful gas emitted during hydration reaction,so potassium magnesium phosphate cement became a more ideal material.In this paper,the bond durability between MKPC paste and concrete is studied by related experiments:?1?In this paper,the change of bond strength between MKPC paste and concrete in fresh water,recycled water and 5%Na₂SO₄ solution was studied.The degradation mechanism of bond strength between MKPC paste and concrete after repeated freeze-thaw cycles was also studied.The effects of fresh water and 5%Na₂SO₄solution on bond durability were compared.The results show that the residual bond strength of MKPC paste is the highest in the environment of 5%Na₂SO₄ solution erosion,and the bond strength decreases the most under the action of circulating water.The strength of blank group A sample decreases 91.56%during 120 days,and freshwater erosion is between the two.The bond strength of MKPC in fresh water and 5%Na₂SO₄ solution both declined obviously,but after 120 freeze-thaw cycles,the bond strength of blank group A in 5%Na₂SO₄ solution remained above 1.5 MPa,which was 1.5 times higher than that in fresh water.?2?In this paper,the compressive strength,flexural strength and deformation of MKPC matrix in fresh water,recycled water,5%Na₂SO₄ solution and freeze-thaw environment are studied,and the durability of MKPC matrix itself is explored.The experimental results show that the compressive strength of MKPC matrix increases first and then decreases in fresh water,worn-out water and 5%Na₂SO₄ solution.The damage of MKPC matrix is the most serious and the compressive strength decreases the most.In the freeze-thaw cycle of 5%Na₂SO₄ solution,the loss of flexural strength of MKPC matrix is less than 15%,but it is as high as 30%in freshwater environment.In both water and freeze-thaw environments,MKPC matrix exhibits expansion deformation in varying degrees.Expansion deformation in 5%Na₂SO₄ solution is higher than that in fresh water.Expansion deformation value of each age in circulating water is the highest among the three erosion environments.?3?The diffusion of SO₄^2-in the matrix of MKPC under the action of long-term water immersion,freeze-thaw cycle and electric pulse was studied,and the effects of two mineral admixtures of metakaolin and fly ash on the resistance to sulfate ion diffusion were discussed.The test results show that MKPC has better sulfate resistance as a repair material.After soaking in blank group A for 300 days,the maximum diffusion depth of SO₄^2-is not more than 30 mm;after 120 freeze-thaw cycles,it is not more than 20 mm;and under the action of 14 days electric pulse,more than 90%of SO₄^2-in group A is concentrated in the range of 0-40 mm in the cathode area.With the addition of modified mineral admixtures,the maximum diffusion depth of SO₄^2-decreases significantly,especially for group D specimens mixed with 10%metakaolin and 10%fly ash.The maximum diffusion depth of SO₄^2-under 14-day-old electric pulse is about 30 mm.?4?In this paper,the change of bond strength,compressive strength and deformation of MKPC in long-term water immersion and freeze-thaw environment was studied by mixing metakaolin and fly ash in MKPC matrix.With the addition of metakaolin,the hydration products of MKPC specimens are more abundant.The bond strength and compressive strength of MKPC specimens in normal indoor environment are greatly improved,but their water resistance and freeze-thaw resistance are poor.After mixing10%metakaolin and 10%fly ash,the bond water resistance and freeze resistance of the specimens have been greatly improved due to the good particle size distribution and the related effects of fly ash.In addition,the matrix of MKPC with fly ash has micro-expansion effect in the early stage,and the shrinkage value of 28 days in normal indoor environment is the smallest;in water environment and freeze-thaw environment,the expansion value is the smallest.