Study On The Internal Laws Of Composition,Structure And Properties Of Converter Steel Slag And Its Application

The complex composition,poor stability,difficult grinding and low cementitious activity of steel slag are the main reasons that restrict its large amount of utilization as a valuable resource.Meanwhile,most of the existing researches are from the technical point of view of improving a certain performance of steel slag,and the fundamental research on the internal relationship among the composition,mineral phase structure and performance of steel slag is not systematic and in-depth.As a result,various problems arose in the development of massive resource utilization technology.Therefore,clarifying the internal relationship among the composition,structure and performance of converter slag,controlling the composition of steel slag,and subsequently controlling the performance of steel slag,are the key points to realize the large amount of resource utilization for the converter slag.At the same time,the experimental results of the theoretical researcheswere also needed as scientific guidance in resourceutilization for steel slag.(1)The changingregularity between the basicity of slag(4.0-1.8),the phase structure and some related properties was presented as follows:With the decrease of basicity,the contents of C3S,C2F and periclase gradually decrease,RO phase gradually increases,and C2S increases firstlyand then decreases;when thebasicity drops to 2.5,C3MS2 begins to precipitate and the content increases with the decrease of the basicity;when the basicity drops to 2.0,the C3S disappears.With the decrease of the basicity,C3S phase and free calcium oxide(f-CaO)decrease,the stability of converter slag increases;the concent of RO phase increases,and the grindability of steel slag decreases;the hydration minerals of C3S phase and C2F phase decrease,and thereby the cementitious activity of steel slag decreases.When the basicity of steel slag reaches 2.5,the cementitious activity achieves the lowest value.As the basicity of slag continues to decrease,the activity increases with the increase of the C3MS2 phase.Therefore,the cementitious activity decreases firstly and then increases with the decrease of the basicity.(2)The changing regularity among the Fe2O3 content(10～30%),the phase structure and some related propertieswas presented as follows:When the basicity reaches 4.0,with the decrease of Fe2O3 content,C3S in slag increases gradually,and precipitated lime,C2S,C2F,periclase and RO phases decrease subsequently;when the basicityreaches 2.0,with the decrease of Fe2O3 content,C2S and periclase increase gradually,C2F and RO phases decrease gradually.The stability of steel slag decreases with the decrease of Fe2O3.The grindability of steel slag increases with the decrease of Fe2O3.The changing regularity of cementitious activity of steel slag change with Fe2O3in different agree:When the basicity of the slag is determined to be 4.0,the cementitious activity increasesfirstly and then decreases with the decrease of the Fe2O3content;when basicity is determined to be 2.0,the cementitious activitydecreases overallly,while some local pointsfluctuate whichmight be related to the increase of C2F,and the hydration rate of C2S,C3MS2,C2F etc.(3)The changing regularity among P2O5 content(0.5～3.0%),mineral phase structure and properties of steel slag:When the basicity reaches 4.0,with the decrease of P2O5,C3S gradually increases,C2S and RO phases decrease,while C2F and periclase have no obvious changes;when the basicity reaches 2.0,C2S and RO phases decrease,C3MS2 and C2F increase.Therefore,the stability of steel slag decreases with the decrease of P2O5,whilethe grindability of steel slag increases.Especially,it should be noted that when the basicity reaches 2.0,with the decrease of P2Ocontent in slag,the grindability of steel slag can be improved,while the cementitious activity of steel slag will decreaseat the same time.The 7-day activity index of high and low basicity steel slag increased with the decrease of P2O5,the 28 day activity index of low basicity slag increased with P2O5,while that of high basicity slag decreased.(4)The changing regularity among MgO content(2～10%),mineral phase structure and properties of slag:when the basicity reaches 4.0,with the MgO content decreases,C3S and RO phase increase,the C2S and periclase content decrease,while the C2F content basically remains unchanged.When basicity reaches 2.0,the C2S and C2F content increase,the periclase,C3MS2 and RO phase decrease.The stability of steel slag increases with the decrease of MgO content.The relative grindability of steel slag decreased slightly with the decrease of MgO.The cementitious activity of steel slag increased with the decrease of MgO.Based on the obtained results,it can be found that the change of steel slag composition leads to the change of mineral structure,the variation rules of cementitious activity,grindability and stability of steel slag are not consistent,but mutually restricted.Therefore,based on the analysis of the slag composition possessing the best single performance,such as stability,grindability and cementitious activity,combined with the effects of composition on the mineralogical structure and properties of steel slag,it can be considered that the basicity should be controlled at 2.0～3.0,and Fe2O3 content in slag should be controlled at about 20%.At this time,the better cementitious activity and grindability of steel slag can be achieved,and the destruction of the stability can be reduced to the minimum.In addition,the content of P2O5 in slag can be increased in a certain composition range,which can improve the stability of steel slag;the MgO content should be as low as possible,which is conducive to improving the stability of steel slag.The converter slag with chemical composition similar to the synthetic slag was used for the upgrading test.By adding quartz sand and pulverized coal to the converter slag,the actual basicity of steel slag was controlledwithih 2.0～3.0 and Fe2O3 content was about 20%.The experimental results show that the stability,grindability and cementitious activity of the modified steel slag are better than those of the original steel slag.Compared with the original steel slag,the f-CaO content of the modified steel slag decreased by 39.6%,the relative grindability index increased by 11%,the 7-day activity index increased by 3%,and the 28 day activity index increased by 4.8%.All the above meet the standards for the f-CaO content and activity index of grade I productsfor Steel Slag Powder for Cement and Concrete in GB/T 20491-2017.