Modification And Properties Of Magnesium Oxysulfate Cement

The modification treatment of magnesium oxysulfate cement refers to the use of admixture doping to change the hydration products and hydration morphology of traditional magnesium oxysulfate cement,so that it has excellent properties such as early strength and high strength,high water resistance and sulfate erosion resistance.In this paper,through the admixture of different admixtures,we study the modification effect of admixtures with different acidity and alkalinity on the traditional magnesium oxysulfate cement,and then study the effect of graphene oxide and carbon nanotube doping on its mechanical properties and hydration process based on the admixture-modified magnesium oxysulfate cement,and then use carbon nanotube doping to study the electrical properties and pressure-sensitive properties of the modified magnesium oxysulfate cement,so as to provide a theoretical basis for the promotion and functional application of magnesium oxysulfate cement.The main research contents and conclusions of this thesis are as follows:Citric acid,sodium tripolyphosphate and trisodium phosphate were compounded to study the effects of different admixtures on the compressive strength and setting time of magnesium oxysulfate cement,and the hydration products and morphology were analyzed by combining X-ray diffraction,scanning electron microscopy and Fourier transform infrared spectrometer.The results showed that when the three admixtures were mixed separately,the citric acid-modified magnesium oxysulfate cement could produce a large number of needle-like5·1·7 phase crystals,and the 28-d compressive strength was 190%of that of the blank specimen.The 3 d and 7 d strengths of the samples prepared by the combination of citric acid and other two kinds of admixtures were stronger than that of the sample modified by admixtures alone,but the 28 d strengths were lower than those of the citric acid modified specimens alone,which may be due to the fact that the acidity of the admixtures had more influence on the hydration of the magnesium oxysulfate cement at the early stage of hydration,and the acidic citric acid could delay the formation of 5·1·7 phase and increase the setting time of the magnesium oxysulfate cement,so that the early strength was lower than that of the different admixture compound modified specimens;while the complexation of citric acid had a stronger effect on the hydration of magnesium oxysulfate cement at 28 d,resulting in higher strength.The water resistance and sulfate erosion resistance of the magnesium oxysulfate cement after admixture compounding were tested,and the results showed that the 56 d water resistance coefficient of the specimen with 2:1 molar ratio of citric acid and sodium tripolyphosphate compounding was 0.88 and the compressive strength was 77.20 MPa;the corrosion resistance coefficient was 0.94 and the compressive strength was 82.75 MPa after soaking in 20%magnesium sulfate solution for 56 d.The water resistance and the best performance of sulfate erosion resistance.In the specimens compounded with citric acid and trisodium phosphate,the molar ratio of 3:1 was 0.87 and the compressive strength was 72.18MPa at 56 d.The corrosion resistance coefficient was 1.06 and the compressive strength was87.89 MPa at 56 d after soaking in 20%magnesium sulfate solution,with the best water resistance and corrosion resistance.Therefore,the results showed that the water resistance of the admixture compound specimens was greater than that of the admixture modified alone,and the corrosion resistance coefficient was greater than that of the citric acid modified magnesium thiosulfate cement alone when the molar ratio of citric acid to sodium tripolyphosphate and trisodium phosphate was 2:1 and 3:1,respectively.The doping of graphene oxide and carbon nanotubes did not affect the type of hydration products of citric acid-modified magnesium oxysulfate cement,and the scanning electron microscopy results showed that graphene oxide could improve the hydration morphology of cement and play the role of template to make the cement matrix more dense,thus enhancing the mechanical properties of cement;carbon nanotubes were more difficult to disperse in cement and prone to agglomeration as a defect in the matrix,thus reducing the strength of cement.The electrical properties experiments were conducted by incorporating carbon nanotubes into citric acid modified magnesium oxysulfate cement,and the results showed that the percolation threshold was reached at 0.50%CNT1 doping,which reduced the resistivity of carbon nanotube composite cement matrix from 1.50×10~5?·cm to 2.63×10~4?·cm in the blank specimen,and greatly improved the electrical conductivity.Cyclic loading experiments were conducted to investigate the effect of carbon nanotubes on the pressure-sensitive properties of modified magnesium oxysulfate cement for cement specimens doped with different amounts of carbon nanotubes,and the results showed that the change in electrical resistance of the composite cement matrix specimens was 0.50%and the strain sensitivity was 15.47 for CNT1doping of 0.50%,which were 1.35 and 3 times that of the blank cement specimens,respectively,greatly improving the pressure-sensitive performance.