Research On Stability Of Magnetite In Concrete Prepared With Iron Ore Tailings As Fine Aggregate

The iron tailings which containing about 10% of the magnetite would be transformed into hematite in the natural environment,but the speed of the conversion need to be calculated in geological time.The magnetite,in the iron tailings,was mixed into concrete and mortar,the stability of magnetite in cement-alkaline environment was undiscovered.The project deals with the stability of magnetite in cement-based materials to research the effects of magnetite on mechanical properties,chemical durability,compatibility to chemical admixture,and corrosion of reinforcing steel.The project researched the mechanical properties and the fluidity of cement mortar influenced by magnetite.The experiment revealed that the magnetite powder with a specific surface area of 380m2/kg would had a bad influence on the fluidity of cement mortar when be used for fine aggregate and mineral admixture.5% of standard sand replaced by magnetite powder was benefit for the early bending strength,as well as has little bad effects on the compressive strength of cement mortar,mixing 6%~7% of magnetite powder could raise ratio of bending-compressive strength.The limestone powder and quartz powder with almost the same fineness as magnetite powder were added into cement mortar,replacing 20% of cement,the appropriate volume of magnetite powder was 6.7%-10%.When the volume of magnetite powder was over 10%,the existence of magnetite was bad for mechanical property of cement mortar.When the composite admixture of fly ash and magnetitepowder was added into cement,the mechanical property of cement mortar decreased with the amount of magnetite powder aggregate.Then,dihydrate gypsum and magnetite powder were added into cement mortar replacing 10% cement.Fluidity,breaking strength and compressive strength of cement mortar were improved when suitable volume of gypsum and magnetite powder were added into cement.When there were 6% of magnetite powder approximately,the volume of gypsum in cement was 8.5%.The cement contained no more than 15% of magnetite powder could be used for the surface mortar,the colour of surface mortar would not be changed in this case.The chemical stability of magnetite was researched by simulating the alkaline environment in the concrete.The chemical stability of magnetite in alkaline solution was bad,the speed hydroxyl ions expended by magnetite was influenced by particle sizes of magnetite.The magnetite transformed into calcium ferrite by being exposed to oxygen in alkaline environment,which could caused the expansion of concrete and mortar members in the project.The magnetite powder was bad for initial fluidity of cement paste but benefit for decreasing the time-loss of fluidity when 0.4% of polycarboxylate type superplasticize was added.When 20% of cement was replaced by quartz powder and magnetite powder,the ratio of magnetite powder and quartz powder was 1:2 could decrease the time-loss of fluidity substantially but have little bad influence on initial fluidity.When the composite admixture of fly ash and magnetite powder was addedinto cement,increasing the volume of magnetite powder would have bad effects on the compatibility of cementing material and chemical admixture.The cementing material with 8%~8.5% of dihydrate gypsum could obtain optimum compatibility when the volume of magnetite powder was about 6%.The contrast experiment found that the corrosion of reinforcing steel was significantly alleviated by magnetite in a higher concentration of alkaline sodium chloride solution.After dipping in the alkaline solution,the surface of some magnetite produced the red hematite in the air.The project is of technical significance to insure concrete quality in different circumstances in application of iron tailings as fine aggregate and mineral admixture.