Preparation And Foundational Characteristics Of Magnesium Oxysulfate Cement-based Lightweight Materials

Magnesium oxysulfate(MOS)cement is formulated by the chemical reaction of light-burned magnesium oxide and magnesium sulfate solution.Compared with the production of ordinary Portland cement,the calcination temperature of light-burned magnesium oxide is below 1000 ?,possessing advantages of less energy consumption,lower cost for equipment and related manufacture processes and in comparison with ordinary Portland cement,MOS cement possesses numerous better performances consisting of good fire resistance,good heat resistance,low density,low thermal conductivity,good cohesiveness,etc.Therefore,MOS cement has been considered as an emerging cementitious material for lightweight panels,refractory structures,insulating slabs,etc.MOS cement possess disadvantages consist of low compressive strength,low water resistance,high drying shrinkage.In this paper,three weak acids including citric acid,boric acid and trisodium citrate were added into MOS cement as modifiers.The influences of these weak acids on mechanical strength,water resistance and drying shrinkage of MOS cement were firstly evaluated.Furthermore introducing foam and rice husk into MOS cement paste and preparing three kinds of lightweight magnesium oxysulfate cement,and investigated the mechanical strength,density,thermal conductivity,drying shrinkage and the effects of drying-wetting cycles on the mechanical strength,volume deformation of those three kinds of lightweight MOS cements.The phase composition and microstructure of typical samples were determined by XRD,FTIR,DTA-TG,SEM and super depth of field microscope measurements.The results show that the compressive strength of MOS cement increases with the increase of MgO/MgSO4 molar ratio and decrease of H2O/MgSO4 molar ratio.The addition of citric acid,boric acid or trisodium citrate enhanced compressive strength and water resistance,but reduced the drying shrinkage of MOS cement paste.With the incorporation of weak acid,5·1·7(5Mg(OH)2·MgSO4·7H2O)phase which is a new magnesium subsulfate crystalline product with shape of needle-like crystal whisker forms in cement paste.This substance behaves criss-cross and fills in pores and micro cracks,inducing the better strength gain and volume stability of MOS cement paste.With the optimum dosages of 0.5%,2.5% and 0.5% respectively,the citric acid,trisodium citrate and boric acid exhibited decreasing improvement effect on MOS cement paste.Citric acid has the best effect,which makes the 28 d strength,softening coefficient and dry shrinkage value of magnesium oxysulfate cement increased by 104.8% and 61.09%,and decrease by 32.81%.The lightweight materials with 759~1721 kg/m3,544~1268 kg/m3,450.91~1721.88 kg/m3 volume density and 1.1~47.4MPa,2.9~70.4MPa,1.1~11.2MPa compressive strength at 28 days were prepared by adding foam,rice husk or both of them.The compressive strength,thermal conductivity and dry density of these three kinds of lightweight MOS cements decrease with the increase content of rice husk or foam.The drying shrinkage decreased with the introduction of foam and increased when rice husk are present.For foamed rice husk MOS cement,the drying shrinkage decreased firstly and then increased with the introduction of foam.Drying-wetting cycles caused a sustained drop in compressive strength in matrix cement and all these three kinds of lightweight MOS cements because of the production of Mg(OH)2.The volume of each group of foamed,foamed rice husk MOS cement is shrinkage with the increase times of drying-wetting cycles.The volume of rice husk MOS cement is shrinkage when the volume fraction of rice husk is high,nevertheless,it turned to expansion with low volume fraction of rice husk.