Cementitious properties of ladle slag fines under autoclave curing conditions

Ladle slag fines consist mainly of γ-C₂S, which does not show cementitious property in water, but can exhibit significant cementitious property in the presence of alkaline activators at room temperatures. This study deals with the hydraulic reactivity of ladle slag fines under autoclaving conditions. The results indicate that ladle slag fines cannot be used as cementing material alone because of the presence of free lime in ladle slag. The combination of a ladle slag fine and a siliceous material, such as silica flour (ground quartz), can eliminate the soundness problem and give very high strength. The introduction of a small amount of Portland cement or hydrated lime into ladle slag fine-silica flour system can increase strength significantly. Lime is more effective than Portland cement due to the presence of Al in Portland cement. The autoclaving temperature should be higher than 175 °C and the time for constant temperature does not need to be more than 4 h to achieve satisfactory strength. Finally, seven batches of experiments are designed to plot isostrength contours for ladle slag fine-cement-silica flour and ladle slag fine-hydrated lime-silica flour ternary systems.     

Structural characteristics and hydration kinetics of modified steel slag

This study investigates the structural characteristics and hydration kinetics of modified basic oxygen furnace steel slag. The basic oxygen furnace steel slag (BOFS) was mixed with electric arc furnace steel slag (EAFS) in appropriate ratios and heated again at high temperature in the laboratory. The mineralogical and structural characteristics of both BOFS and modified steel slag (MSS) were characterized by X-ray diffraction, optical microscopy, scanning electron microscopy, Raman and Fourier transform infrared spectroscopies. The results show that modification increases alite content in MSS and decreases alite crystal size with the formation of C₆AF₂. One more obvious heat evolution peak appears in MSS's heat-flow rate curves in comparison to BOFS, becoming similar to that of typical Portland cement paste. As a result, its cementitious activity is much improved.     

The gasification reactivity of unburned carbon present in gasification slag from entrained-flow gasifier

CO₂-gasification reactivity of unburned carbon in both coarse and fine slags was studied in a pressurized thermogravimetric analyzer (TGA) and compared with a char obtained from a drop tube furnace (DTF) at 1400 °C from the same original coal. Results show that the reactivity of the unburned carbon in the coarse slag is always higher than that in the fine slag, around 1.11 to 1.88 times. The DTF char is less reactive than the unburned carbon in both fine and coarse slags. In order to understand the effects of minerals on the CO₂-gasification of unburned carbon, reactivity of the unburned carbon in demineralized slag was also investigated. Results show that the minerals in the coarse slag catalyze the CO₂-gasification of unburned carbon, but that in the fine slag inhibit the CO₂-gasification. An elemental analyzer was adopted to analyze C, H, N and S in the original coal, the DTF char and the slags. The main reasons for the differences in the gasification reactivity of unburned carbon in the slags are the morphology of slags, the degree of graphitization of unburned carbon and the components of crystalline phase, which have been investigated by SEM/EDX and XRD. It is found that a higher ordering of carbon layers and a lower content of catalytic components are the main reasons for the lower reactivity of the unburned carbon in the fine slag compared to that in the coarse slag.     

Slag erosion-resistant coating for periclase-magnesia-aluminum spinel brick

The magnesia-carbon bricks are used at the ladle slag line adds carbon to molten steel and thus has an adverse effect on the smelting process. Steel production enterprises hope to remove carbon from the ladle slag line. This study presents a Silica sol (SS)–magnesium aluminate spinel(MA) coating that can be applied to the surfaces of periclase–MA spinel brick (spinel brick) to improve their slag erosion resistance, facilitating the elimination of carbon from the ladle slag line. The slag corrosion resistance mechanism of a spinel brick covered with the SS–MA coating was analyzed through scanning electron microscopy and simulation using FactSage software. The results show that increasing the SS content (C_SS) led to the formation of fine cracks during the coating-drying process and was detrimental to the slag erosion resistance of the coating. The optimum C_SS was 20%. In the high-temperature slag erosion experiment, the MA in the coating was capable of forming solid solutions with Fe and Mn in the slag, and the nano-SiO₂ in the coating interacted with the slag, increasing the viscosity of the slag and reducing its penetration. In addition, the presence of the coating extended the penetration path of the slag and reduced the contact area between the slag and the spinel brick and their chances to undergo chemical reactions, thereby protecting the spinel brick from slag erosion. The coated spinel bricks outperformed the magnesia-carbon brick in terms of slag erosion resistance, providing a basis for selecting carbon-free refractory materials for the ladle slag line.     

Effect of mechanical activation on the hydraulic properties of stainless steel slags

This work aims to assess the possibility of using ladle metallurgy and argon oxygen decarburization stainless steel slag as a hydraulic binder after mechanical activation. Prolonged milling in ethanol suspension resulted in 10-fold increase of the surface area and increase of the amorphous phase. Calorimetric analysis of slags mixed with water indicated the occurrence of exothermic reactions. XRD results revealed that periclase, merwinite, γ-C₂S and bredigite, decreased with hydration time. Thermogravimetric analyses indicated that the main hydration products are most probably C–S–H, CH and MH. The hydrated products in both slags were similar to C–S–H gel. WDS analysis demonstrated Ca and Si to be widespread in the structure. Formation of M–S–H gel or incorporation of Mg in the C–S–H gel remains uncertain. The 90 days compressive strength of mortars prepared from slags reached approximately 20% for LM and 10% for AOD of the compressive strength of mortars prepared from OPC.     

Reactivity and phase composition of Ca2SiO4 binders made by annealing of alpha-dicalcium silicate hydrate

This study investigates the production of highly reactive dicalcium silicate Ca₂SiO₄ (C₂S).To that end, binders were synthesised by annealing of alpha-dicalcium silicate hydrate (α-C₂SH) between 400 and 800 °C. Two different heating sequences were tested. The phase compositions were determined by means of XRD. Depending on the annealing temperature and the heating conditions the cementitious materials consist of an X-ray-amorphous content as well as x-Ca₂SiO₄ (x-C₂S) and γ-Ca₂SiO₄ (γ-C₂S). The hydration kinetics of some selected binders were investigated by means of isothermal calorimetry. The specific reactivity of the phases produced by means of annealing was determined during the first 40 h of hydration by use of XRD and TGA.The resulting binders show the highest reactivity when low annealing temperatures (< 500 °C) were used. After 72 h, degrees of hydration of about 89% are achieved. The most reactive component is the X-ray-amorphous content, followed by x-C₂S.     

Thin fly ash/ ladle furnace slag geopolymer: Effect of elevated temperature exposure on flexural properties and morphological characteristics

The flexural properties and thermal performance of 10 mm-thin geopolymers made from fly ash and ladle furnace slag were evaluated before and after exposure to elevated temperatures (300 °C, 600 °C, 900 °C, 1100 °C and 1150 °C). Class F fly ash was mixed with liquid sodium silicate (Na₂SiO₃) and 12 M sodium hydroxide (NaOH) solution using aluminosilicate/activator ratio of 1:2.5 and Na₂SiO₃/NaOH ratio of 1:4 to synthesise thin fly ash (FA) geopolymers. 40 wt% of ladle furnace slag was partially replacing fly ash to produce fly ash/slag-based (FAS) geopolymers. Thermal treatment enhanced the flexural strength of thin geopolymers. In comparison to the unexposed specimen, the flexural strength of FA geopolymers at 1150 °C and FAS geopolymers 1100 °C was increased by 161.3% to 16.2 MPa and 208.9% to 24.1 MPa, respectively. A more uniform heating was achieved in thin geopolymers which favoured the phase transformation at high temperatures and contributed to the substantial increase in flexural strength. The joint effect of elevated temperature exposure and the incorporation of ladle furnace slag further improved the flexural strength of thin geopolymers. The calcium-rich slag refined the pore structure and increased the crystallinity of thin geopolymers which aided in high strength development.     

Preparation and hydration of β-C2S without stabilizer

The preparation method of β-C ₂S without stabilizer, and their hydration characteristics were studied. β-C₂S is formed when γ-C₂S is heated at about 1000°C or about 1500°C. The hydration kinetics of β-C₂S produced from α′-C₂S is markedly different from that from α-phase. Ca(OH)₂ is hardly produced in the hydration of β-C₂S without stabilizer. Formed C-S-H has a composition of $\text{C/S}\text{? 2}$.     

重构钢渣的胶凝性分析

在炼钢转炉排渣的同时,将一定比例的电炉还原渣和煤渣加入到渣盘中,利用熔融钢渣的余热对钢渣的组成和结构进行在线重构。结果表明,重构处理明显降低了钢渣中的fCaO含量,改善了钢渣的易磨性和压蒸安定性,钢渣粉的28d活性指数提高了10%～20%。     

Preparation and performance of slag-based binders for the cementation of fine tailings

To achieve effective cementation of fine tailings, slag-based binders were prepared using Portland cement clinker stimulation, early strength activator (ESA, mixture of anhydrite and triethanolamine at 97:3 (w/w)) activation and slag pulverization methods. The compressive strength, hydration products, slag reaction degree and non-evaporable water content of the consolidated samples under different curing times were analyzed to clarify the application performance and early strength action mechanisms of this slag-based binder. The results showed that clinker alone was able to effectively stimulate the slag’s cementitious property, but the cementation strength was relatively low. The addition of ESA in the clinker activated slag promoted the conversion of C₄AH₁₃ into ettringite (AFt) and accelerated the consumption of Ca(OH)₂, all of which significantly improved the early cementation strength of fine tailings. Slag pulverization promoted the slag reaction degree and increased the yield of hydrated products, which led to a further increase in the early strength of the slag-based binder. Eventually, a more efficient and higher early strength slag-based binder was prepared with the composition of 27% clinker, 10% ESA and 63% pulverized slag, and the cementation strength at 3 curing days for the fine tailings sample was 231% more than that of P.O 42.5 Portland cement.     

Formation of liquid-phase isolation layer on the corroded interface of MgO/Al2O3-SiC-C refractory and molten steel: Role of SiC

Formation of a dense layer on corroded interface to suppress corrosion is always desired, but it is controlled by numerous environmental conditions. In this work, corroded microstructures of MgO/Al₂O₃-SiC-C refractories in metal bath area of ladle furnace were investigated after industrial trails. A liquid-phase isolation layer in which MgO islands and liquid phases was established on the corroded interface of refractories with 6 wt% coarse/fine SiC-additive. The formed isolation layer against steel/slag attacks led to an approximate 30% improvement in corrosion resistance than that of refractory with 3 wt% fine SiC-additive. More importantly, the liquid-phase isolation layer blocked the direct mass transfer between molten steel and refractories while it decreased exogenous pollution from refractories. SiC-additive affected the formation process of isolation layer by controlling the generation/migration of Mg(g) on refractory' surface. A further formation mechanism of liquid-phase isolation layer was discussed in detail and role of SiC was elucidated.     

Post-mortem analysis of MgO–Al2O3–C bricks containing bauxite used in steel ladle walls

A serious problem with integrated masonry linings ladle was that the ladle wall bricks were difficult to be partially replaced. Thus, the service life of the ladle was determined by the properties of ladle wall bricks. In this study, the new generation of MgO–Al₂O₃–C bricks with bauxite aggregates was tested in a ladle lining of an integrated steel ladle for 103 cycles, and the corroded microstructure of the used brick was investigated. A multilayered structure of bauxite aggregates could be observed in the used brick, which inhibited the slag penetration along the boundary of the magnesia aggregates and reduced the degree of the aggregates fall off from the used bricks. Besides, during the process of slag penetration, bauxite aggregates could melt into the slag, which increased the viscosity of the slag and weakened the penetration ability of the slag. The MgO–Al₂O₃–C bricks with bauxite addition could improve the service life of the ladle to a certain extent.     

Slag corrosion mechanism of lightweight Al2O3–MgO castable in different atmospheric conditions

The slag resistance of ladle lining refractory in different atmospheric conditions is of utmost importance due to the strong variation of atmospheric environment in ladle during the process cycle. In this study, by adopting dynamic induction furnace corrosion test, the corrosion mechanism of lightweight Al₂O₃–MgO castable with different environmental oxygen partial pressure was investigated through macro and microanalysis, XRD, and thermodynamic simulation. The atmospheric condition was set to P (O₂) = 0.21 atm or P(Ar) = 1.0 atm. The attained results showed that a reduced slag corrosion but intensified slag penetration happened at low environmental oxygen partial pressure condition. With P(O₂) = 0.21 atm, Mn and Fe in slag were present in the form of divalent and/or trivalent cations and were incorporated into spinel to form MnFe₂O₄ solutions and MgAl₂O₄ solutions during the corrosion process. Since Fe, Mn ions were largely consumed, liquid with high viscosity formed and the continuing infiltration was suppressed. Under P(Ar) = 1.0 atm, Fe ions in the slag were totally reduced into elemental iron while Mn existed mainly in the state of MnO, and the amount of liquid slag was therefore reduced, leading to significantly weakened corrosion on castable aggregates. Corrosion reaction products under this condition were mainly MgAl₂O₄ and CA₆.     

Alkali-activated blends of calcium aluminate cement and slag/diatomite

The present study deals with the formulation of new cementitious materials via the alkaline activation of an industrial by-product (blast furnace slag) or a natural rock (diatomite) in the presence of reactive aluminium sourced from calcium aluminate cement (CAC). Two blends, one containing 20% CAC and 80% slag and the other 20% CAC and 80% diatomite, were prepared and activated with sodium sulphate or a sodium hydroxide solution. The hardened materials were characterised with X-ray diffraction (XRD) as well as ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) and tested for their 2-day mechanical strength. The main reaction product was a cementitious gel that precipitated with crystalline phases such as ettringite, U phase and katoite. While the slag blend reacted to generate a C–(A)–S–H-like gel under moderately alkaline conditions, diatomite reactivity proved to be very low under such conditions. The greater reactivity of both slag and diatomite at high pH (high alkalinity) favoured their interaction with CAC.     

水淬渣的胶凝活性及其形成机理

采用重新熔融水淬的方法制备不同水淬矿渣样品,综合进行净浆强度实验和碱溶出实验研究,借助XRD,DTA, TEM, SEM和EDX等分析手段,研究了矿渣形成过程、形成结构和胶凝性能之间的关系,并在此基础上探讨了矿渣潜在胶凝活性的形成机理. 研究表明,在合适的排渣温度和冷却速度条件下,水淬渣能够形成分相结构,对水淬渣胶凝活性产生重要影响. 分相矿渣具有富钙相和富硅相,富钙相的迅速溶解和富硅相的缓慢溶解是水淬渣水化过程中产生足够早期强度和发挥稳定后期强度的主要原因. 由于玻璃分相的影响,排渣过程存在最佳工艺条件,即最佳的排渣过程能使水淬渣形成玻璃分相结构,从而具有最好的活性.     

Formation of an intermediate C-S-H phase during the hydrothermal synthesis of gyrolite

An intermediate C-S-H phase, similar to the Assarsson's “Phase Z”, with basal spacing of 16Å formed when β-C₂S or γ-C₂S reacted with silicon dioxide (molar ratio CaO/SiO₂ = 0.5–0.28) for 8 hours at 190°C under hydrothermal conditions. This intermediate C-S-H phase transformed to gyrolite upon prolonged hydrothermal treatment.     

钢渣尾泥-矿渣-脱硫石膏三元体系水化硬化特性

为促进大宗化利用钢渣尾泥,以河北迁安的钢渣尾泥为研究对象,借助X射线衍射(XRD)、场发射扫描电镜(FE-SEM)、傅里叶红外光谱(FT-IR)、热重-差热分析(TG-DTA)测试方法,研究了钢渣尾泥在矿渣-脱硫石膏体系中的水化硬化特性。研究表明,经机械粉磨后的钢渣尾泥仍表现出较好的水硬胶凝特性,与普通钢渣-矿渣-脱硫石膏体系相比具有早期强度高的优势,其水化产物主要为钙矾石(AFt)和水化硅酸钙(C-S-H)凝胶。在水化反应过程中:钢渣尾泥为体系提供碱性环境,促使矿渣中玻璃体解离;矿渣水化不断消耗羟基,进一步促进了钢渣尾泥的水化;脱硫石膏为体系提供大量的Ca²⁺和SO^2-₄,这些离子与体系中的凝胶反应生成AFt。三者相互渗透协同反应推动了水化反应持续进行。     

Corrosion of refractory alumina plates used in the sliding gate system of steelmaking ladle: Chemical experiment

The aim of this study is to investigate the corrosion process of the refractory Al₂O₃-ZrO_2-C slide gate plate caused by chemical interactions with secondary refining slag. The plate is part of the steelmaking ladle slide gate system, therefore its lifespan extension and integrity are of concern. Post mortem plates with slag adhered on their channel surface were analysed and static cup corrosion test experiments were performed in a controlled atmosphere furnace for 1 h at 1600 °C. The corrosion product phases Gehlenite (Al₂Ca₂O₇Si) and Spinel (MgAl₂O₄) identified by DRX were formed after the experiment, showing that the slag is able to chemically corrode the Al₂O₃-ZrO₂-C plates. In addition, a comparison of the interface region between the slag and the post mortem plate and the interface region between the slag and the static cup corrosion test specimens showed that the corrosion products are swept away by the fluid flow. Even though chemical corrosion by slag has been identified as one of the causes of the plate channel degradation, the study suggests that it acts synergistically with other mechanisms of degradation.     

Preparation and ladle slag resistance mechanism of MgAlON bonded Al2O3 -MgAlON-Zr2Al3C4-(Al2CO)1-x(AlN)x refractories

MgAlON bonded Al₂O₃-MgAlON-Zr₂Al₃C₄-(Al₂CO)_1-x(AlN)_x refractories were prepared at high temperatures from 1300?℃ to 1600?℃ in N₂-flowing based on the design of Al-AlN core-shell. The refractory prepared at 1500?℃ was chosen to conduct the ladle slag resistance test at 1600?℃ in air. All the refractories are composed of MgAlON, Zr₂Al₃C₄, (Al₂CO)_1-x(AlN)_x and Al₂O₃. The ladle slag resistance test for the chosen refractory presents a good result and there exist two different reaction layers —— the reacted slag layer and the spinel solid solution layer. The reacted slag layer consists of spinel solid solution, Ca-ZrO₂ and gehlenite/gehlenite solution, where the formation of those phases has changed the chemical composition and phase composition of the original ladle slag. The compact spinel solid solution layer forms by the decomposition of MgAlON into rich-Al₂O₃ spinel and the incorporation of Mn²⁺ and Fe^2+/3+ into rich-Al₂O₃ spinel, which plays an important role in slag penetration resistance.     

Corrosion modeling of magnesia aggregates in contact with CaO–MgO–SiO2 slags

Ladle refining is an efficient process for improvement of quality of steel on secondary metallurgy under harsh conditions. Magnesia refractories with high purity are important raw materials for ladle lining in high-quality steel production. However, the penetration by CaO–MgO–SiO₂ slags damages magnesia refractories, which considerably limits their service life. Abundant grain boundaries in magnesia create channels for slag penetration and lead to the destruction of the structure. The effect of the microstructure on the slag corrosion behavior of magnesia aggregates requires further systematic investigation. In this study, a corrosion model was established to describe the slag penetration process of magnesia aggregates. The effects of the grain-boundary size and slag CaO/SiO₂ mass ratio (C/S ratio) on slag penetration were investigated, and the possibility of the microstructure optimization of magnesia aggregates was discussed. The results indicated that magnesia aggregates exhibited excellent slag resistance for slag with a C/S ratio above 1.5 or even 2.0. When the slag C/S ratio was lower than 1.5, the dissolution rate of magnesia decreased more rapidly with the increase in the slag C/S ratio. In addition, the much smaller grain-boundary size increased the slag penetration resistance by promoting the formation of a dense isolation layer and inhibiting further penetration processes. The calculation results agreed well with the experimental results, suggesting that the corrosion model is promising for predicting slag corrosion.