Study On Sulfate Corrosion Behavior And Mechanism Of Magnesium Potassium Phosphate Cement(MKPC) Mortar

In this paper,the compressive strength,flexural strength,volume deformation,apparent morphology,mass loss,water absorption,osmotic law of sulfate radical(SO₄^2-),microstructure and composition of potassium magnesium phosphate cement（MKPC）mortar of different fine aggregate and hydration ages in the three environments of long-term immersion,freezing and thawing cycle and electric pulse were studied respectively.The corrosion resistance and mechanism of potassium magnesium phosphate cement mortar were studied by three different experimental methods,such as long-term immersion（full immersion）,freezing and thawing cycle and electric pulse.The main contents of this study are as follows:1.The behavior and mechanism of sulfate attack on MKPC mortar were studied by immersion test of potassium magnesium phosphate cement（MKPC）mortar mixed with different fine aggregate（pure river sand,mixed sand（mixed sand and artificial sand）and pure artificial sand）in different sulphate solution（5%Na₂SO₄ solution,5%MgSO₄ solution）with comparison group of specimens immersed in fresh water.Results were compared with those of Portland cement mortar specimens.The results showed that MKPC mortar exhibited better resistance to sulfate attack and water corrosion.The strength of MKPC mortar varied in different environments,that is,the strength of the MKPC mortar was increasing in the natural state,and the strength of the mortar was decreasing in the immersion environment.In the process of 300d immersion,the decrease of the strength of the specimens soaked in each solution was as follows:Na₂SO₄ solution<MgSO₄ solution<clear water,the final strength of the specimen under the soaking state of the solution was less than that of the natural curing state.2.The corrosion tests of MKPC mortar specimens with different hydration ages（respectively curing 5h,1d,3d,28d）in 5%Na₂SO₄ solution and under the coupling action of5%Na₂SO₄ solution and freezing and thawing cycles were carried out to analyze the behavior and mechanism of sulfate attack and sulfate freeze-thaw cycles on MKPC mortar during different hydration ages.The results showed that the early MKPC mortar exhibited better sulfate resistance and salt tolerance.The curing age had significant influence on the resistance to sulfate erosion and salt resistance of MKPC,and the sulfate corrosion resistance of the MKPC mortar specimens with natural curing 3d was better than that of the natural curing of 5h and 1d.The salt tolerant corrosion resistance of MKPC mortar specimens with natural curing 1d was better than that of natural curing 5h and 3d.3.Through the erosion experiment of MKPC mortar with different fine aggregate under the coupling action of 5%Na₂SO₄ solution and freezing and thawing cycles,setting up the contrast group of H₂O and comparing with the Portland cement mortar,the behavior and mechanism of MKPC mortar against sulfate freezing and thawing cycles erosion were studied.The results showed that the freezing and thawing times of MKPC mortar specimens in fresh water and sodium sulfate solution far exceeded the Portland cement mortar.It showed that the frost resistance of the MKPC mortar specimens was much higher than the frost resistance of the silicate mortar specimens,which reflected the good anti freezing and salt freezing properties of the MKPC mortar,and the salt freezing resistance was better than the anti-water freezing resistance.The test results also showed that the strength loss of MKPC mortar specimens in 5%Na₂SO₄ solution after 225 freezing and thawing cycles was significantly higher than the strength loss of 300d in 5%Na₂SO₄ solution,which was mainly attributed to the ice expansion pressure and salt crystallization pressure in the salt freezing cycles.4.The anode of MKPC fine mortar was placed in 5%Na₂SO₄ solution and cathode was placed in 5%MgSO₄ solution to conduct electrical pulse erosion test,comparing with Portland cement mortar to study Sulphate accelerated corrosion resistance of MKPC mortar.The results showed that,under the same age of erosion,the content of SO₄^2-in MKPC mortar specimens under the condition of electric pulse test was much higher than the content of SO₄^2-in MKPC mortar specimens under two experimental conditions of long-term immersion and freeze-thaw cycles.Electrical pulse could accelerate the chemical reaction of SO₄^2-and MKPC based materials,and the main chemical reactions occured in the negative extremes.5.The results of three tests under the conditions of long-term immersion,freeze-thaw cycle and electric pulse accelerated erosion verified that artificial sand could be used to replace part of the river sand,which could optimize the particle size of the cement based material,increase the density of the specimen,reduce the porosity in the specimen and reduce the entry of SO₄^2-;artificial sand could also provide CaCO₃ for cement based materials.The hydration products reacted with sulfate to form new products covering the surface of hydrated products and improved the sulfate corrosion resistance of MKPC mortar.6.The sulfate corrosion behavior and mechanism of MKPC mortar were studied by comparing the three methods of long-term immersion（full immersion）,freeze-thaw cycle and electric pulse accelerated erosion.The results showed that compared with continuous immersion,the content of SO₄^2-in the freeze-thaw cycles test specimens was significantly higher than that in the whole immersion test specimens,and the damage degree of the sulfate freezing thawing coupling was much greater than that under the soaking condition of the sulfate.This was due to the combined effect of sulphate corrosion and freeze-thaw failure.The two factors promoted each other,and the damage was more serious than the single factor of immersion.In general,the effect of SO₄^2-on the internal transmission of the mortar was more prominent than that of the freeze-thaw cycle and continuous immersion.The content of SO₄^2-in the same depth of the mortar would increase significantly,thus more destructive.Electrical pulse could accelerate the migration of SO₄^2-into the cement-based material and the best effect at the cathode of the electric field.It was mainly due to the synergy between the migration of electric field and the free diffusion of sulfate concentration.