Research On Water Resistance Of Basic Magnesium Sulfate Cement

The influence of molar ratio of raw materials,mixing amount of modifier and mineral additive on compressive strength of basic magnesium sulfate cement before and after immersion in water are studied.The influence rules of material compositions on water resistance of basic magnesium sulfate cement was investigated.The mechanism of water resistance was explored on phase compositions and microstructure change by X-ray diffraction and scanning electron microscope.The results show that the water resistance of basic magnesium sulfate cement is influenced significantly by the material compositions of raw material.When the molar ratio of H2O: Mg SO4 was fixed,the higher the molar ratio of Mg O: Mg SO4,the lower the compressive strength loss was.For instance,for the mixture with fixed molar ratio of H2O: Mg SO4 of 20,the compressive strength loss were 33 percent and17.9 percent for molar ratio Mg O: Mg SO4 of 7 and 9 after immersion in water for 60 days respectively.Besides,the compressive strength loss decreases with the increase of the molar ratio of H2O: Mg SO4 at the same molar ratio of Mg O: Mg SO4.For example,the compressive strength loss increased from 17.9 percent to 48.2 percent as molar ratio of H2O: Mg SO4 increases from 20 to 22 after immersion in water for60 days when the mixture with fixed molar ratio of H2O: Mg SO4 of 20.When the molar ratio of Mg O: Mg SO4: H2 O was fixed,the compressive strength improved with the increase of modifier amount in the range of 0.1%~1.0%.The addition of fly ash,slag and silicon fume also can improved water resistance of basic magnesium sulfate cement.Basic magnesium sulfate cement with an addition of 30% fly ash by weight of light burnt Mg O has the highest softening coefficient up to 0.97 after immersing in water for 70 days.Analysis of hydration product and microstructure of basic magnesium sulfate cement show that higher molar ratio of Mg O: Mg SO4 and lower molar ratio of H2O:Mg SO4 of the mixtures benefit to the stability of the needle shape hydration product(5Mg(OH)2 ·Mg SO4·7H2O)of hardened basic magnesium sulfate cement paste.For this reason,the basic magnesium sulfate cement with higher molar ratio of Mg O:Mg SO4 and lower molar ratio of H2O: Mg SO4 has good water resistance.Addition of fly ash,slag and silicon fume can decrease the porosity and fine the pore size for the function as fillers for pores in the paste,thus improving the water resistance of modified MOS cement further.