Study On Preparation And Properties Of Steel Slag/Industrial Gypsum Composite Material

As a waste in steel making process, steel slag is generated in increasing emissions butlow utilizations, resulting in resource wastes and occupation of land resources. FGD gypsumand fluorine gypsum are characterized by good purity and high quality, and regarded asgypsum renewable recourses though they are industrial by-product gypsums originating fromflue gas desulfurization purification treatment and hydrogen fluoride production respectively.In this paper, to overcome its poor hydraulicity and strength, basic properties of low-alkalinitysteel slag were studied and further experiments were taken to activate its hydration activity bychemical modification. Composite cementitious materials were prepared by compositingmodified steel slag with calcined FGD gypsum and fluorine gypsum respectively, aiming toconvert steel slag, FGD gypsum and fluorine gypsum into excellent resources in substantialutilization.Chemical and phase constitutions, grain fineness distribution and microscopicmorphology were investigated by X-ray flurescence spectrometer, X-ray diffracmeter,simultaneous thermal analyzer, laser particle size analyzer and field emission scanningelectron microscopy. As is shown, steel slag is composed of CaO, SiO2, Al2O3, Fe2O3, MgO,MnO et al in chemical constitution, and CaCO3, SiO2,2CaO·SiO2,3CaO·MgO·2SiO2, RO,pyribole and olivine minerals, amorphous substance et al in phase constitution. The slag isclassified as low alkalinity dicalcium silicate slag with alkalinity of1.7, and CaCO3accountsfor40.76%in weight. With size raging from0.2μm to200μm, steel slag powders arecharacterized by medium diameter of9.309μm, mean particle diameter of22.77μm andspecific surface of18036cm2/ml. Because of its poor hydraulicity, only3.93MPa of90dcompressive strength was got for steel slag paste.In steel slag chemical modification experiments, CaCl2showed best activating effect onhydration activity of steel slag, followed by Al2(SO4)3·18H2O, KAl(SO4)2·12H2O and Na2SO4,while NaOH and NaAlO2the worst. Hydration products of the slag modified by CaCl2weremainly composed amorphous gel phase and hydrocalumite (3CaO·Al2O3·CaCl2·10H2O), alsolittle Ca(OH)2appeared in early hydration. With steel slag modified by Na2SO4,Al2(SO4)3·18H2O and KAl(SO4)2·12H2O respectively, hydration products mainly consisted of amorphous gel phase and ettringite (3CaO·Al2O3·3CaSO4·32H2O).In chemical modification orthogonal experiments, with0.5-1.5%Na2SO4,0.5-2.5%CaCl2and0.1-0.5%AlCl3·6H2O composed into steel slag, Na2SO4showed much more influences on7d and28d compressive strength, and highly significant effects on60d and90d compressivestrength, compared with CaCl2and AlCl3·6H2O. Optimal activator was obtained with1.5%Na2SO4,0.1%AlCl3·6H2O and1.5%CaCl2. Phases of the hardened body modified by theoptimal activator included CaCO3, SiO2, RO,3CaO·Al2O3·3CaSO4·32H2O, amorphous gel etal, meanwhile amorphous gel and ettringite (3CaO·Al2O3·3CaSO4·32H2O) constituted thecementitious phase.With influences of mass ratio of modified steel slag and calcined FGD gypsum (slag togypsum ratio) on setting time and mechanical strength, and addition of silicone waterproofingagent on mechanical strength, water absorption and softening coefficient studied in advance,steel slag/calcined FGD gypsum composite material was prepared with slag to gypsum ratioof1:1and1.0%silicone waterproofing agent. Properties followed as6min for initial stettingtime and11min for final,2.28MPa for flexural strength and5.32MPa for compressive of2h,5.12MPa for flexural and14.79MPa for compressive of7d,3.37%water absorption for2hand9.05%for24h, and softening coefficient of0.67.Along with influences of mass ratio of modified fluorine gypsum and modified steel slag(gypsum to slag ratio), and addition of silicone waterproofing agent on mechanical strength,water absorption and softening coefficient studied in advance, steel slag/fluorine gypsumcomposite material was prepared with gypsum to slag ratio of1:1and1.0%siliconewaterproofing agent. Properties followed as9min for initial stetting time and45min for final,2.25MPa for flexural strength and5.48MPa for compressive of2h,7.12MPa for flexural and38.96MPa for compressive of7d,3.87%water absorption for2h and6.72%for24h, andsoftening coefficient of0.63.