Study On Properties Of Magnesium Oxysulfate Cement Modified By Metakaolin

Magnesium oxysulfate cement(MOSC)is a new type of green building material,which is different from the hydraulic characteristics of traditional Portland cement.Its hydration property is gas hardness.The hydration principle is the chemical reaction between Mg O and Mg SO4 solution.The reaction of this ternary system is extremely complex and cannot be completely hydrated.At the same time,the hydration product is also unstable.These factors lead to its poor mechanical properties,volume stability and water resistance,which limit its industrial production further expansion.Fly ash(FA)can improve the mechanical properties and water resistance of the cementitious system,so it has become a widely used mineral admixture in magnesium oxysulfate cement.Metakaolin(MK)comes from the calcination of coal-measure kaolin(coal gangue).It is a relatively new type of mineral admixture.It has high pozzolanic activity due to its amorphous structure,which is similar to the nature of FA.In this paper,on the basis of different modifications,the effects of metakaolin alone,fly ash and metakaolin on the mechanical properties,water resistance and volume stability of magnesium oxysulfide cement were investigated.X-ray diffraction(XRD),scanning electron microscopy(SEM)and mercury intrusion porosimetry(MIP)were used to analyze the structure of the sample.The application of metakaolin to magnesium oxysulfate cement is extremely beneficial to the improvement of its own performance and the saving of magnesite resources.At the same time,it provides a feasible way for the existing coal gangue with low utilization rate and ecological harm.In this paper,based on the introduction of citric acid,metakaolin was added to modify magnesium oxysulfate cement.Metakaolin with different fineness has a great influence on the physical properties(fluidity,setting time)and mechanical properties of magnesium oxysulfate cement,and it is possible to explore the optimal particle size of metakaolin suitable for magnesium oxysulfate cement system.The samples mixed with metakaolin and the samples mixed with fly ash(a widely used mineral admixture)were compared in terms of mechanical properties,water resistance,and volume stability.The phase composition,microscopic morphology and pore structure of the two groups of samples were analyzed.The 10% content of metakaolin makes the compressive and flexural strengths of magnesium oxysulfate cement reach 105.77 MPa and 28.62 MPa,which are 16.89% and 42.33% higher than the blank samples.At the same time,within the test dosage range(5%-30%),the softening coefficient fluctuates around 0.923,which is higher than that of the blank sample of 0.852.In addition,the addition of metakaolin reduced the late shrinkage of the magnesium oxysulfate cement samples and improved the volume stability.The effects of metakaolin and fly ash on the fluidity and setting time of magnesium oxysulfate cement slurry are opposite.Metakaolin will reduce the fluidity of cement slurry and shorten the setting time,while fly ash is the opposite,so whether it is possible to adjust the fluidity,setting time and macroscopic properties such as strength.Next,the influence of different proportions of complex admixture on the mechanical properties of the system was explored.The performance is generally better than that of other proportions of compound additions.And when the dosage is 10%,the compressive strength and flexural strength increase to 110.69 MPa and 29.82 MPa,and the softening coefficient reaches 0.939.It can be seen that the modification effect is better than that of metakaolin alone.The addition of mineral admixtures did not change the phase composition of the hydration product,but promoted the formation of more strong phases and reduced the porosity of the hardened matrix,which can be attributed to the particle packing effect,nucleation effect,and particle grading effect.Based on the modification of inorganic acid salt sodium dihydrogen phosphate,metakaolin was introduced to explore the effects of its single admixture and its compound admixture with fly ash on various macroscopic properties of the system.The differences in the influence of the addition of admixtures on the system were compared on the basis of different modifications.The addition of metakaolin to the cement system based on the modification of sodium dihydrogen phosphate can also improve its mechanical properties and water resistance.The optimum dosage is 5%,and the compressive strength is 85.15 MPa,which is slightly higher than that of the blank sample of 80.66 MPa;within the range of 5%-30% substitution,the softening coefficient is around 0.900,which is higher than that of the blank sample of 0.784.The optimal compounding ratio is 2:1.When the dosage is 5%,the strength is 19.40% higher than that of the blank sample.It can be seen that,regardless of single or compound blending,the addition of metakaolin to the citric acid-modified cement matrix is more conducive to the improvement of system performance.