Effect Of Silica Fume On Water Resistance Of Magnesium Oxychloride Cement

The magnesium resources in domestic are abundant,such as Qinghai Saline Lake magnesium salt excess resources has led to the destruction of the ecological environment in the region.Therefore,the light-burned activity of magnesium oxide(MgO)and magnesium chloride(MgCl2)can be used to produce magnesium oxychloride cement(MOC),which provides an effective way for the utilization of magnesium resources in China.MOC has the advantages of fast hard,high strength,light weight,wear-resistant and so on.The compressive strength can reach 100 Mpa when curing for 28 d.And the MOC calcinated product is MgO,which can be recycled as raw material.However,MOC breaks down with water,resulting in a severe drop in strength,and long-term(28d)immersion strength can only reach less than 1/10 of before.Therefore,improving the water resistance of MOC and keeping it high strength in water become the key to expand its application.The magnesium silicate hydrated(M-S-H)cement is a kind of hydraulic cementing material and has compact microstructure.Using silica fume(silica fume,SF)and MgO to product this cement,and added with sodium hexametaphosphate(SHMP)to improve the workability.Therefore,this topic will modify the MOC through SF.The purpose is that in water environment,SF can react with MgO and water to form M-S-H,fill MOC pores,reduce the decomposition of hydration products,and enhance the MOC immersion strength.In this study,the effect of M-S-H gel on MOC system was verified by adding chemical synthetic M-S-H gel.The results show that the water resistance coefficient of MOC can be increased from 0.4 to over 0.7 by adding M-S-H gel at the curing age of 28 d.Moreover,under TEM electron microscopy,it can be seen that the synthetic M-S-H gels bind the MOC hydration products(Mg3(OH)5Cl·4H2O,5·1·8 or F5 phase),thus to retain the overall structure in water and enhance their immersion strength.Adding SF to MOC and find the optimum ratio(for the purpose of generating M-S-H cement,SF is added by mass ratio SF:MgO =60:40).Because of the introduction of SHMP,it is necessary to consider the influence of SHMP on water resistance of MOC.By designing orthogonal experiment and adopting multiple regression method,mathematical model analysis of dependent variable(softening coefficient)and independent variable(SF and SHMP)was carried out.The results show that the coefficient of softening is the highest when the content of SF is 1 wt.% and the content of SHMP is 2 wt.%,and the coefficient of water resistance of 28 d can reach 1.03.Multiple regression model can judge the factor water resistance of MOC by the mathematical viewpoint.The results show that the SHMP can improve the water resistance obviously,and the SF and SHMP have a synergistic effect to improve the water resistance of MOC.SHMP enhanced the immersion strength by increasing the 5 phase retention rate of MOC hydration products By analyzing XRD spectrum.However,the effect of SF addition on water resistance of MOC is weak,and the small amount(<2 wt.%)can improve its water resistance,but the water resistance of(>15 wt.%)will decrease.In order to investigate the mechanism of water resistance enhancement,the hydration products of SF modified MOC were tested by various means.The results show that XRD is not a good method to characterize gel phase,because the peak of gel phase is easy to be covered by the crystal phase peak;TG/DSC,FT-IR and TEM showed no M-S-H gel phase formation;mercury intrusion Porosity tests show that show a negative correlation between obvious porosity and softening coefficient,small dosage of SF(<2 wt.%)on the porosity of MOC modification is better;the morphology of hydration products were observed by SEM/EDS,showed that the addition of SF not only has the effect of filling pore,but also change the morphology of MOC.Adding SF can change the needle like shape of some hydration products into gel form,and improve the compactness of cement,thus enhancing the water resistance of MOC.