Showing papers on "Calcium aluminates published in 2017"

Characteristics and Modification of Non-metallic Inclusions in Titanium-Stabilized AISI 409 Ferritic Stainless Steel

Abstract: This study describes an investigation into the improvement of castability, final surface quality and formability of titanium-stabilized AISI 409 ferritic stainless steel on an industrial scale. Non-metallic inclusions found in this industrially produced stainless steel were first characterized using SEM-EDS analyses through the INCA-Steel software platform. Inclusions were found to consist of a MgO·Al2O3 spinel core, which acted as heterogeneous nucleation site for titanium solubility products. Plant-scale experiments were conducted to either prevent the formation of spinel, or to modify it by calcium treatment. Modification to spherical dual-phase spinel-liquid matrix inclusions was achieved with calcium addition, which eliminated submerged entry nozzle clogging for this grade. Complete modification to homogeneous liquid calcium aluminates was achieved at high levels of dissolved aluminum. A mechanism was suggested to explain the extent of modification achieved.

Investigation of hydration products of fly ash–slag pastes

Abstract: The paper presents the results of the research identifying the hydration products present in hardened fly ash–slag pastes considered as alternative non-clinker binder. Thermal analysis (TG/DTG), infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used as complementary methods. The pastes contained the binder made of two components: fly ash from circular fluidized bed combustion of brown coal and ground granulated blast furnace slag. The components were mixed accordingly to five formulas differing in proportions, but with constant water/binder ratio (w/b = 0.5). No chemical activators were used to initiate the binding reaction of blast furnace slag. The results of thermal analysis, IR spectroscopy and scanning electron microscopy as well as the increase of compressive and flexural strength in time revealed the progression of hydration process in time and formation of products similar to ones present in hardened cement pastes. SEM observations of pastes after 90 days of curing showed fibrous (type I) and similar to honeycomb form of C–S–H phase, ettringite and calcium aluminates.

Calcium Aluminate Reactive Coatings on Carbon‐Bonded Alumina Filters for Clean Steel Approaches

Abstract: New carbon-bonded filters with coatings based on calcium aluminates are developed. The samples are tested for 10 s in contact with a steel melt containing endogenous inclusions at 1650 °C, in order to explore the behavior and impact of these materials on the filtration process. Investigation of the filters after the test is carried out by optical and scanning electron microscopy. In addition, steel samples are analyzed with a special automatic SEM. The population of the detected inclusions is classified in terms of size and chemistry. Both compositions deliver excellent filtration results.

A comparative study of hydration kinetics of different cements by thermogravimetry on calcined mass basis

Abstract: The capture of CO2 and SO2 from industrial gas effluents has been done usually by lime-containing products. For this purpose, cement pastes also can be used, due mainly to their calcium hydroxide content formed during hydration. To select the best cement for this purpose, TG and DTG curves of different Portland cement pastes (types I, II, III and G), prepared with a water-to-cement ratio (W/C) equal to 0.5, were analyzed at different ages, at same operating conditions. The curves were transformed into respective cement calcined and initial mass basis, to have a common and same composition reference basis, for a correct quantitative hydration data comparison. This procedure also shows that there is an unavoidable partial drying effect of the pastes before starting their analysis, which randomly decreases the W/C ratio at which were prepared, which indicates that, when results are compared on respective paste initial mass basis, assuming that the ratio W/C has not changed, possible calculation errors may be done. Type I, II and G analyzed cements have shown similar hydration characteristics as a function of time, while the analyzed type III cement has shown a different hydration behavior, mainly due to its highest Al2O3 and lowest SO3 contents, promoting the formation of hydrated calcium aluminates, by the pozzolanic action of the excess of alumina, consuming Ca(OH)2, which final content at 28 days was the lowest one, among the hydrated cements.

Synthesis, formation mechanism and hydraulic activity of novel composite cements belonging to the system CaO–Al 2 O 3 –ZrO 2

Abstract: This paper describes a new approach for synthesizing novel composite cements belonging to the system CaO–Al2O3–ZrO2 via the high-temperature reactions within the Ca7ZrAl6O18–Al2O3 system and characterizes products using X-ray diffraction (XRD), Rietveld XRD quantification, infrared spectroscopy, scanning electron microscopy combined with energy-dispersive spectrometry and heat flow calorimetry techniques. Quantitative comparison of the phase transformation in the samples 25 mass% Al2O3–75 mass% Ca7ZrAl6O18, 50 mass% Al2O3–50 mass% Ca7ZrAl6O18 and 75 mass% Al2O3–25 mass% Ca7ZrAl6O18 sintered in the temperature range 1300–1500 °C was presented. Detailed investigations of the reaction mechanism in the synthesis process in these samples show that calcium zirconium aluminate is unstable when it is heated with alumina, and it undergoes chemical reactions forming calcium aluminates with different CaO/Al2O3 molar ratios and Zr-bearing compounds (CaZrO3 or ZrO2). Probability of liquid-phase sintering of ceramics prepared from alumina and Ca7ZrAl6O18 that provides particle rearrangement increases with increasing the CaO/Al2O3 mass ratio of composites. Specifically, this paper discusses composition design and microstructural characterization for achieving modern calcia–alumina–zriconia composite cements. A model that explains the chemical and microstructural changes within the Ca7ZrAl6O18–Al2O3 system due to high temperature was proposed. This study extensively investigates the effects of the cement phase composition on the hydraulic activity in the Ca-aluminates and Zr-bearing compounds containing blended binder pastes.

Simultaneous Modification of Alumina and MgO·Al 2 O 3 Inclusions by Calcium Treatment During Electroslag Remelting of Stainless Tool Steel

Abstract: Calcium modification of both alumina and MgO·Al2O3 inclusions during protective gas electroslag remelting (P-ESR) of 8Cr17MoV stainless steel and its effect on nitrides and primary carbides were studied by analyzing the transient evolution of oxide and sulfide inclusions in the P-ESR process. The oxide inclusions that were not removed during P-ESR without calcium treatment were found to retain their original state until in as-cast ingot. Calcium treatment modified all MgO·Al2O3 and alumina inclusions that had not been removed in the P-ESR process to liquid/partially liquid CaO-Al2O3-(MgO) with uniformly distributed elements, in addition to a small proportion of partially modified inclusions of a CaO-MgO-Al2O3 core surrounded by a liquid CaO-Al2O3. The modification of low-MgO-containing MgO·Al2O3 inclusions involves the preferential reduction of MgO from the MgO·Al2O3 inclusion by calcium and the reaction of calcium with Al2O3 in the inclusion. It is the incomplete/complete reduction of MgO from the spinel by calcium that contributes to the modification of spinels. Alumina inclusions were liquefied by direct reaction with calcium. Calcium treatment during P-ESR refining also provided an effective approach to prevent the formation of nitrides and primary carbides in stainless steel through modifying their preferred nucleation sites (alumina and MgO·Al2O3 inclusions) to calcium aluminates, which made no contribution to improving the steel cleanliness.

Formation and Evolution of Non-Metallic Inclusions during Deoxidation by Al Addition in BOF Crude Steel

Abstract: In order to know the types of inclusions in BOF crude steel and their evolution during Al deoxidation, steel samples are taken from industry and deoxidation with aluminum experiments are carried out in laboratory. The results show that the BOF crude steel is an important source of inclusions. A lot of (Fe, Mn)O inclusions and some BOF slag droplets (mainly calcium silicates) were found in BOF crude steel. During Al deoxidation, alumina clusters could be formed because of homogeneous nucleation, while some singular alumina particles can also be generated on (Fe, Mn)O inclusions due to heterogeneous nucleation. Besides, (Fe, Mn)O would be reduced by dissolved Al into alumina completely. The unstable oxides in calcium silicates will be reduced by dissolved Al in liquid steel to form calcium aluminates. The study results are very helpful to understand the origin and evolution of different types of inclusions in industry.

Dissolution kinetics of hydrated calcium aluminates (AFm-Cl) as a function of pH and at room temperature

Abstract: The determination of reliable weathering/dissolution rates for cement phases is of fundamental importance for the modelling of the temporal evolution of both radioactive waste repositories and CO 2 geological storage sites (e.g. waste matrix, plug in boreholes). Here, the dissolution kinetics of AFm-Cl (hydrated calcium aluminates containing interlayer Cl) has been studied using flow-through experiments conducted at pH values ranging from 9.2 to 13. Mineralogical (XRD) and chemical (EPMA, TEM) analyses have been performed to determine the evolution of the phases during the dissolution experiments. For pH values between 10 and 13, the dissolution of AFm-Cl is congruent (i.e. Ca/Al ratios close to 2 both for solids and outlet concentrations). In contrast, the precipitation of amorphous Al-phases and possibly amorphous mixed Al/Ca phases is observed at pH 9.2, leading to Ca/Al ratios in the outlet solutions higher than those of the initial solid. Therefore, at pH 9.2, even if Cl – /OH – exchange occurs, estimation of dissolution rate from released Cl appears to be the best proxy. Dissolution rates were normalized to the final specific surface areas (ranging from 6.1 to 35.4 m 2 g −1). Dissolution rate appears to be pH-independent and therefore the far-from-equilibrium dissolution rate at room temperature is expressed as: logR(mol m –2 s –1) = –9.23 ± 0.18

Tracing the Origin of Non-ferrous Oxides in Lamination Defects on Hot-Rolled Coils: Mold Slag Entrainment vs Submerged Entry Nozzle Reaction Products

Abstract: In this work, lamination defects (slivers) on hot-rolled coils of Ca-treated steel were investigated for microstructure and composition using optical and scanning electron microscopy combined with microanalysis (SEM/EDS). The goal was to identify possible origins for the observed defects which contain a complex assemblage of phases, such as different types of calcium aluminates (CA, CA2, CA6), melilite (C2AS), spinel (MA), and a newly identified phase, CNA2. Mold slag similar to that employed during the cast was absent. Analysis of the bulk composition of some of the defects indicated these to be too rich in alumina to be derived from mold slag through steel-slag redox exchange. In contrast, microstructural observation of the inner side of the submerged entry nozzles (SEN) used during casting showed deposits with compositions comparable to those of the defect material. Based on an estimation of the chemical evolution of mold slag interacting with steel, it is found that the defects are not likely to be entrained mold slag but remobilized SEN deposits, as supported by several microstructural and trace phase criteria. However, it should be noted that extensive reduction of mold slag by steel can lead to compositions rich in sodic-calcic aluminates (CNA2). Therefore, differentiation between specific locations of the defect materials within a casting system requires detailed analysis from the potential sources of origin as well as from the materials found in the defects.

Strength Properties and Micro-structure of Steel Slag Based Hardened Cementitious Composite with Graphene Oxide

Abstract: For the rapid hardening of concrete, various types of calcium aluminate composite have been used for special purpose as shotcrete or rapid repairing in construction field. However, high cost of calcium aluminates is demerit in the point of expanding usage. In recent years, a new binder for rapid hardening has been introduced as the pulverized ladle furnace slag (RC-LFS) with main components of C 12 A 7 and β-C 2 S. Like other traditional cementitious materials, it’s use will preserve natural resources and has high environmental advantage with economic efficiency, by transferring the industry byproducts into high value materials. Even though flake state, graphene oxide has high tensile strength and electric conductivity. Then, it can be used to increase strength and thermal properties of concrete and its efficiency depends on the dispersion in the cement matrix. In this paper, we used graphene oxide for increasing the engineering properties in RC-LFS concrete with GGBFS. Test results showed that the 0.05w% of graphene oxide increase the flexural strength to 26% and compressive strength to 16%.

Interaction between magnesia-spinel refractory grains and cement clinker at high temperature

Abstract: Interaction between cement clinker and refractory grains plays a key role in determining lifetime of refractory bricks. In the current study, magnesia-spinel refractory grains were corroded in cement slag at various temperatures. During the corrosion test, the cement clinker melted at above 1300 °C, and molten slag interacted strongly with the refractory particles. Appearance of calcium aluminates around the spinel indicates the spinel have chemically reacted with molten slag. However, there is no variation of the compositions of slag around the periclase, which suggests no reaction has occurred between the periclase and cement slag. In addition, the reaction products are concentrated in local regions around reacting particles, and the aluminum content in bulk slag does not increase as the concentration of refractory particles increases. Therefore, diffusion of reaction products is much slower than the chemical reaction, and diffusion rate determines overall rate of corrosion.