Research On The Performances Of Magnesium Oxychloride Cement Modified By Silica Fume

Magnesium oxychloride cement（MOC）is a cementitious material with a number of outstanding features,such as high strength,repaid setting,low density,marble-like appearance and so on.However,when immersed in water,the main hydration products in MOC are prone to decompose to Mg（OH）₂,leading to a notable strength decrease of MOC.This defect is the main barrier limiting the application and development of MOC.The purpose of this paper is to use silica fume（SF）to modify MOC and investigate how preparing conditions,SF,sodium hexametaphosphate（SHMP）and pH value influence the water resistance of MOC,the main research contents are summarized as fellow:（1）The effects of preparing conditions on the performances of MOC were studied.MOC pastes were prepared under different dispersing methods,different curing conditions and different curing time before their compressive strength,phase composition and microstructure were examined.The results show that ultrasonic dispersing will help improve the dispersibility of raw materials and contribute to the hydration process.The compressive strength of 28 d samples under sonicate dispersing are enhanced by 62%compared with the control group.When cured under 20°C±2°C,RH 70%±5%for 28 d,the compressive of samples are much higher than that of samples cured at natural environment.Moreover,the mechanical properties of samples change obviously before 14 d,after that the hydration process is almost completed at 28 d.（2）The effects of the activity of SiO₂ and the dosage of SF on the performances of MOC were studied.MOC pastes were prepared with the addition of SF or quartz powder before their compressive strength,phase composition and microstructure were examined.The results show that the activity of SiO₂ has an obvious influence on the modification effect,the amorphous SiO₂ in SF can achieve a better result,the strength retention coefficient of 28 d samples with0.5wt.%SF reaches 0.84.This is because SF can not only fill the pores physically but also change the morphology of Mg（OH）₂ generated in water.That can densify the paste and limit water penetration.However,excessive SF will make an adverse impact,since the porosity of MOC rises.As for quartz powder,it only exerts a physically filling effect.（3）The influences of the SHMP on the modification effect of SF were studied.MOC pastes were prepared with the addition of SF and SHMP before their compressive strength,phase composition and microstructure were examined.The results show that SHMP can work with SF together to get a better modification effect.With 1wt.%SHMP and 5wt.%SF incorporated,samples gets a high strength retention coefficient（1.01）.The mechanism is that SHMP can improve the stability of hydration products in MOC in water.（4）The influences of pH value on the modification effect of SF were studied.The pH of mixing water was adjusted to 9.00,10.00,11.00,12.00 and 13.00 separately before they were used to prepare samples.Compressive strength,phase composition and microstructure of samples were then examined.The results show that with the increase of pH value of mixing water,the mechanical strength and the water resistance of ordinary MOC and 0.5wt.%SF added MOC tend to decrease due to the formation of a large amount of Mg（OH）₂.Whereas,the performances of 5wt.%SF added MOC will be improved,since the high alkalinity contributes to the dissociation of SF and more insoluble gels（M-S-H gels and M-Cl-S-H gels）are generated,which make the microstructure more compact and improve the performances of MOC.