Effect Of Modification At High Temperature On The Composition, Structure And Property Of Steel Slag

Steel slag is the residue of steel production process and hasn't been reutilized effectively yet. The annual output of steel slag in 2009 is over 85 million tons in China, while the utilization ratio of which is no more than 10%. Hundred million tons of steel slag are deposited or landfilled, directly leading to the occupation of farm land and pollution of groundwater and soil. In fact, steel slag contains certain amount of cementitious minerals, such as C3S, C2S, C4AF and C2F, but its cementitious activity is relatively lower than that of Portland cement. In addition, steel slag often shows significant variation in chemical and mineralogical compositions, and undesirable volume change due to high f-CaO and f-MgO content, all of which result in its limited application in cement and concrete industry. In the past decades, many approaches, such as fine grinding or ultra-fine grinding, chemical and thermal activation, were applied to improve the activity of steel slag. However, these approaches are referred to as―back-end modification techniques‖, in which additional energy and chemical activators are usually needed, and the modification effect varies with the chemical and mineralogical composition of steel slag. Aided by the National Eleventh Five-Year Research Program of China (2006BAF02A24), a novel method was proposed and conducted in this study, which is to modify the chemical and mineralogical compositions of steel slag by adding adjusting material into molten slag when it is discharged. Effects of the kind and addition of adjusting materials, and modification parameters (such as temperature) on the composition, structure and properties of modified steel slag (MSS) were investigated. Hydration process and hydration properties of MSS were characterized, mineral phase transition and the mechanism of which during modification process at high temperature were illustrated as well. The results of this study are advantageous to the reutilization of steel slag in cement and concrete.Relationships among the chemical and mineralogical composition, micro-structure of minerals and cementitious activity of steel slags produced by some domestic major iron and steel group corporations were investigated. The results show the chemical and mineralogical composition of steel slag varies significantly according to its origin, production time, and steel slag grains from same converter even with different size show various chemical and mineralogical composition as well as micro-structure. Steel slag generally presents high cementitious activity if finer, regularly shaped calcium silicates exist in it.Basic oxygen furnace steel slag (BOFS), from Shaoguan Iron and Steel Company, was modified at different temperature by adding adjusting materials, including lime, electric arc furnace steel slag (EAFS), GGBFS, coal bottom ash, fly ash. The minerals and cementitious activity of BOFS were compared with those of MSS, then proper adjusting materials were selected accordingly. The results indicate that the addition of calcareous material is helpful to increase the amount of alite and belite in MSS. When C/S molar ratio of MSS is higher than 3.0 and modifying temperature is lower than 1300℃f-CaO in MSS is increased. The addition of EAFS and GGBFS results in the formation of more calcium silicates and calcium alumino-ferrite, thus cementitious activity of MSS can be improved evidently. While the addition of alumino-siliceous materials, sucn as coal bottom ash and fly ash, is advantageous to absorbing f-CaO and increasing the amount of calcium aluminium and calcium alumino-ferrite. The effect order of adjusting materials on cementitious activity of MSS is as: (EAFS≈lime≈GGBFS) > (coal ash≈fly ash).Cementitious activity of MSS is improved remarkably when composite adjusting materials are added, e.g. the mixture of GGBFS and lime or that of EAFS and coal bottom ash. After modification, the content of alite with small size is increased by 7.3-12.7%, meanwhile a large amount of ions are doped into alite crystals, the latter induces the formation of more lattice defects. C6AF2 exists as a newly formed cementitious mineral in MSS. Composite adjusting material composed of EAFS-coal bottom ash at a mass ratio of 70:30-80:20 is proposed for economy and property consideration.The phase model of MSS at high temperature was proposed according to thermodynamics calculation, and the evolution regulation of MSS was deduced by analyzing the process of crystallization and ternary diagram. When C/S is higher than 3.0, alite and belite content is increased with increasing of C/S, while RO phase is discomposed into f-MgO. When C/S is in the range of 2.7-3.0, modification process is helpful to the formation of alite and belite, partial RO phase transforms to magnesioferrite and f-MgO. If C/S falls into the range of 2.3-2.7, modification is beneficial to the formation of alite, belite and intermedia phase, meanwhile RO phase transforms to magnesioferrite. For MSS with C/S less than 1.8, f-CaO is absorbed to form belite during modification, while some RO phase transforms to magnesioferrite.The hydration and hardening processes of MSS and cement incorporating MSS were investigated systematically. Only one peak is observed in heat evolution curve of BOFS. In contrast, MSS presents two heat evolution peaks, which is similar to that of Portland cement. The hydration process of MSS can be divided into five stages as: initial period, induction period, acceleration period, deceleration period and steady period. Furthermore, 3 days heat of hydration of MSS is increased by 2.5 times than that of BOFS. MSS was industrially produced by using EAFS and coal bottom ash as composite adjusting material at Shaoguan Iron and Steel Company. The 28 days activity index of ground MSS and the mixture of 30% ground MSS and 70% GGBFS by mass are greater than 85% and 95%, respectively. C30 and C50 grade concretes can be prepared with 30% ground MSS replacement for Portland cement by mass. In addition, C60 grade concrete with good impermeability can be casted using the mixture of 30% ground MSS and 70% GGBFS as a substitution for 30% Portland cement.