Enhanced formation of magnesium silica hydrates (M-S-H) using sodium metasilicate and caustic magnesia in magnesia castables

Magnesium-silica-hydrates (M-S-H) is a promising binder for magnesia castables due to its bonding strength and progressive dehydration behavior over a wide temperature range during the heating-up stage. Sodium metasilicate and caustic magnesia were used to form M-S-H in magnesia castables. The results showed that M-S-H was remarkably produced in the caustic magnesia-microsilica slurries containing sodium metasilicate with increasing pH value, which activated the hydrolysis of microsilica into silicic ions and enhanced the M-S-H formation. When 0.3 wt% caustic magnesia and 0.05 wt% sodium metasilicate as additives were incorporated into magnesia castables, the cold crushing strength and cold modulus of rupture of castables after drying at 110 °C reached the maximum value of 68.3 MPa, which corresponded to ~ 40% improvement in comparison with those of caustic magnesia and sodium metasilicate-free magnesia castables. Besides, the enhanced formation of M-S-H bonding system contributed to a better explosion resistance of magnesia castables.     

Microsilica or MgO grain size: Which one mostly affects the in situ spinel refractory castable expansion?

Microsilica is commonly added to alumina–magnesia castables to counterbalance the in situ spinel expansion. This effect is attained by the generation of a low-melting temperature phase, which also affects the expansive reaction kinetics. Additionally, the MgAl₂O₄ formation depends on the grain size of the reactants. The use of coarse magnesia grains results in lower Mg²⁺ dissolution and could lead, at 1500 °C, to forsterite development (Mg₂SiO₄). For finer MgO, silica was detected at the edge of the spinel grains. Considering these aspects, this work evaluated the effect of microsilica content for different magnesia grain sizes (<45 or <100 μm). Due to a faster spinel formation for the fine MgO source, microsilica counterbalanced the MgAl₂O₄ expansion. Conversely, for the coarser MgO, silica increased the Mg²⁺ dissolution, speeding up the spinel formation and expansion. Therefore, microsilica presented opposite roles, pointing out that it does not always counterbalance the spinel expansion. This work also indicated the need for a systemic approach for the expanding design of alumina–magnesia refractory castables.     

Formation of magnesium silicate hydrate in the Mg(OH)2-SiO2 suspensions and its influence on the properties of magnesia castables

Brucite is an alternative magnesium precursor for magnesia castables. The hydration process of brucite-microsilica suspensions at 50?°C was firstly studied to identify the magnesium-silicate-hydrate (M-S-H) formation. The existence of M-S-H was dependent on the characteristics of brucite. The brucite with smaller grain size exhibited higher reactivity and favored the formation of M-S-H. The early strength of magnesia castables with addition of the 1?wt% reactive brucite powder was increased, which was related to the modified microstructure via filling effect of brucite fine particles and the formation of M-S-H. Explosion resistance of castables was improved as well attributed to the enhanced early strength and the M-S-H phase in the presence of reactive brucite. Besides, the facilitated formation of forsterite bonding phase in the brucite containing castables during firing process and the thermal-mechanical strength such as hot modulus of rupture and refractoriness under load were significantly increased.     

Effect of Microsilica Addition on Properties of CMA72 Bonded Al_2O_3-MgO Castables

CMA72 bonded Al_2O_3-MgO castable is promising for application of steel ladle wall,because of unique combination of thermo-mechanical properties,slag corrosion resistance and cost benefit. In these castables,microsilica can be introduced to counterbalance the expansion generated by spinel formation. In this paper,the effect of microsilica dosage on properties of castables was evaluated. Expansion,expressed by the permanent linear change( PLC),is highly dependent on the dosage of microsilica. Unexpected expansion occurs when the dosage of microsilica is too low due to dominant effect of spinel and CA_6 formation. Too high dosage results in sintering shrinkage,which is related to amount of liquid phase generated by microsilica addition. In addition,HMOR declines dramatically with increasing microsilica dosage. Considering the balance between expansion control and hot property retention,1. 0 mass% of microsilica is recommended for the castable containing 4 mass%of magnesia.     

The corrosion resistance of microsilica-containing Al2O3–MgO and Al2O3–spinel castables

Microsilica addition in Al₂O₃–MgO and Al₂O₃–spinel castables helps to improve their flowability and partially accommodate their residual expansion after firing. Nevertheless, there is a lack of conclusive statements in the literature regarding the effects of microsilica on one of the main requisites for steel ladle refractories: corrosion resistance. In the present work, the performance of alumina–magnesia and alumina–spinel with or without microsilica when in contact with a steel ladle slag was evaluated based on three aspects: the material's physical properties, its chemical composition and the microstructural features before the slag attack. According to the attained results, microsilica induced liquid formation and pore growth during sintering, favoring the physical slag infiltration. Moreover, due to this liquid, CA₆ was formed in the matrix, mainly for the Al₂O₃–spinel composition, which also favored the castable dissolution into the molten slag.     

Microstructural evolution of magnesia-based castables containing microsilica

The higher performance of refractory materials applied in steelmaking vessels is mainly associated with the development of high-magnesia bricks. However, the same success has not yet been attained for the production of high-quality magnesia-based castables, due to the well-known magnesia hydration trend. In order to overcome this drawback, microsilica addition was tested as an anti-hydration additive in the present work. As it also leads to liquid formation at high temperatures in high-alumina CAC-containing castable compositions, the microstructural development of microsilica-containing magnesia-based castables was also analyzed by scanning electron microscopy, thermodynamics simulations and sintering assisted tests. According to the results, microsilica hindered the magnesia hydration and provided an additional bonding mechanism due to the reaction with MgO and water. Moreover, it helped to control the material's volumetric change by reducing the expansion associated with the spinel formation and also the shrinkage level afterwards.     

基于水氯镁石合成水合硅酸镁及其在镁质浇注料的应用

水氯镁石是一种非常具有应用前景的镁盐资源,其储量丰富,成本低廉。以青海盐湖水氯镁石和水玻璃合成不同MgO/SiO₂摩尔比(0.5:1,1:1,1.5:1)的水合硅酸镁(M-S-H)凝胶,采用XRD、SEM、红外和核磁共振等测试手段研究M-S-H的合成机理和结构特征,进而将合成的M-S-H与硅微粉复合制备镁质浇注料,探究M-S-H结构对浇注料结合特性的影响规律。结果表明:不同MgO/SiO₂摩尔比的M-S-H呈层状堆叠结构,MgO/SiO₂摩尔比为1:1时M-S-H的层间自由水少,结晶度最高;M-S-H替代部分硅微粉制备镁质浇注料能显著提高1 550 ℃热处理后浇注料的力学性能,其中MgO/SiO₂摩尔比为1:1的M-S-H复合硅微粉制备的镁质浇注料综合性能最佳,与添加6%(质量分数)硅微粉制备的镁质浇注料相比,其常温抗折强度和高温抗折强度分别提高75%和8%。     

Kinetics and mechanistic analysis of caustic magnesia hydration

The kinetics of magnesia hydration to produce magnesium hydroxide is crucial for process design and control, and for the production of an Mg(OH)₂ powder with desirable particle morphology. In this study, highly pure magnesia has been hydrated in a batch reactor. The effects of the following variables were evaluated experimentally: temperature (308–363 K), reaction time (0.5–5 h), initial slurry density (1–25%wt) and particle size in the ranges ?212 + 75 µm and ?45 + 38 µm. Experimental data indicate increasing magnesia hydration rates with increasing temperature, as expected. In addition, it has been observed that the hydration of magnesia increases significantly up to about 4–5%wt initial slurry density, stabilising afterwards. On the other hand, the reaction was almost unaffected when magnesia with different particle sizes were hydrated because of similar specific surface areas involved. A reaction mechanism to explain the oxide dissolution and the hydroxide precipitation has been proposed, assuming no significant change in the initial solids size and dissolution rate as the controlling step. The calculated activation energy value of 62.3 kJ mol^?1 corroborates the mechanism proposed in this study and compares well with values previously reported in the literature. Copyright © 2004 Society of Chemical Industry     

Systemic analysis of MgO hydration effects on alumina–magnesia refractory castables

The use of magnesia sources with high specific surface area and small particle size in the Al₂O₃–MgO system can induce faster in situ spinel (MgAl₂O₄) formation in castable compositions, improving the slag corrosion resistance. However, the higher reactivity of these raw materials lead to an intensive brucite formation (followed by volumetric expansion), spoiling the castable's properties during the curing and drying steps. Considering these aspects, a systemic analysis of three magnesia sources (dead-burnt and caustic ones) was carried out in order to evaluate: (1) their hydration impact on the refractory castables properties, and (2) their bonding ability in cement-free compositions. Mechanical strength, thermogravimetric and Young's modulus tests were conducted during the castables’ curing and drying steps. According to the results, the elastic modulus measurement is an efficient tool to evaluate the magnesia hydration. The addition of proper amounts of calcium aluminate cement and/or silica fume to the castables can inhibit the crack formation and provide suitable mechanical properties. The results also show that under certain conditions, MgO can be used as a binder, replacing calcium aluminate cement and leading to a significant reduction in the castables costs with no drawbacks to their refractoriness.     

钢包渣线用镁锆铬质无炭浇注料的研制

精炼钢包渣线使用含炭耐火材料存在着使钢水增碳的问题 ,需要研制抗渣侵蚀和渗透能力较强的无炭钢包渣线材料。以轻烧氧化镁和锆英石为原料 ,采用湿法共磨工艺 ,合成了镁锆质熟料 ,并以此合成料为主要原料 ,加入工业Cr2 O3细粉、锆英石细粉及硅微粉等研制了无炭的镁锆铬质钢包浇注料。实验证明 ,制得的镁锆铬质浇注料高温强度较高 ,高温收缩率较小 ,抗渣侵蚀和渗透能力较强。     

MgO fumes as a potential binder for in situ spinel containing refractory castables

MgO is pointed out as an alternative binder for refractory materials, mainly for systems where the presence of CaO might not be desired. Selecting the most suitable magnesia source is an important step as its purity and reactivity should influence the hydration reaction, leading to binding effect or cracks. This work investigated the design of vibratable high-alumina compositions bonded with MgO fumes [which is a very fine powdered oxide (d < 3?µm) resulting from the production process of electrofused magnesia] and/or dead-burnt magnesia (d < 212?µm). Acetic and formic acids were added to the castables during their processing steps in order to adjust the density of active sites for Mg(OH)₂ formation and control the crystal growth of this phase. The green mechanical strength and thermomechanical performance (cold and hot mechanical strength, thermal shock, refractoriness under load, corrosion, etc.) of designed MgO-bonded compositions were analyzed. Improved green mechanical strength and crack-free samples were obtained when adding up to 6?wt% of MgO fumes to the refractories and processing them with aqueous solutions with 3?wt% of formic acid. The compositions with 6?wt% of magnesia fumes resulted in samples with flexural strength in the range of 12.0?MPa after curing at 50?°C/24?h and similar green mechanical strength (12.9?MPa) as the ones bonded with 4.0?wt% of calcium aluminate cement after drying at 110?°C for 24?h, which highlights the great potential of this MgO source. Despite the enhanced green mechanical strength, alumina-based castables containing 6?wt% of MgO (fumes, dead-burnt or their blend) showed low mechanical strength at intermediate temperatures and high linear expansion, as a consequence of the in situ spinel phase formation above 1200?°C. Thus, better densification, improved HMOR, thermal shock resistance and corrosion behavior were obtained for the castables prepared with less MgO fume contents.     

Role of sodium hexametaphosphate in MgO/SiO2 cement pastes

The extent of reaction between magnesium oxide (MgO) and silica fume (SiO₂) is normally limited and mixes require high water contents to give suitable rheology. The use of considerably lower water contents and the formation of magnesium silicate hydrate (M-S-H) gel as a binding phase is made possible by adding sodium hexametaphosphate (Na-HMP) to the mix water prior to the addition of MgO and SiO₂. This results in the formation of extensive reaction products and cured samples with high compressive strength and low porosity. In this work, the effect of Na-HMP on the hydration of MgO/SiO₂ mixes is investigated using high water to solids ratio samples to allow monitoring of pH and the solution chemistry during hydration. It is shown that a relatively small amount of Na-HMP inhibits the formation of Mg(OH)₂ when MgO is hydrolyzed. It is proposed that this is due to adsorption of phosphate species on the MgO which inhibits the nucleation of the Mg(OH)₂. This gives rise to high Mg^2 + species in solution and elevated pH (> 12) conditions relative to when Mg(OH)₂ forms. In contrast, the phosphate does not suppress formation of M-S-H gel. In combination with the enhanced dissolution rate of SiO₂ at high pH, M-S-H gel can form quickly without competition for Mg^2 + ions by Mg(OH)₂ precipitation. Incorporating the optimum concentration of Na-HMP into the mix water therefore transforms the properties of cement paste and mortar samples formed by reacting MgO and SiO₂.     

Synthesis and evaluation of Mg-Al hydrotalcite formation and its influence on the microstructural evolution of spinel-forming refractory castables under intermediate temperatures

The mechanical properties of in-situ spinel (MgAl₂O₄)-forming alumina-based castables under intermediate temperatures are of critical importance before the refractory lining system reaches normal operating conditions. The objective of this study is to elucidate the role of the hydrotalcite formed within a fine-grained castables matrix, in which no strength loss of the MgO-bonded alumina-based castables without silica fume was observed. Numerous fundamental studies were conducted to examine the factors influencing hydrotalcite formation within the blended pastes composed of MgO and Al₂O₃ nanopowders; dead burned or fused magnesia and Al₂O₃ nanopowder; dead burned magnesia and water-dispersed sol of fumed alumina by using: XRD and DSC-TG-EGA(MS). The XRD, FTIR and ²⁷Al MAS NMR analysis of the hydrotalcite calcination products revealed that the spinel begins to form at temperatures as low as 700 °C. Finally, the physical properties, phase composition and microstructure of the refractory castables bonded with the hydrotalcite decomposition-routed nanostructured spinel were evaluated.     

Fundamentals and applications on in situ spinel formation mechanisms in Al2O3–MgO refractory castables

Although the physical expansion associated with the in situ formation of magnesium–aluminate spinel (MgAl₂O₄) is well-reported, some questions related to this behavior, such as the different volume change values experimentally attained when compared to theoretical one and the pore generation after the reaction, remain open. Thus, the main objective of this work is to shed some light on these questions by evaluating a cement-bonded alumina–magnesia castables, designed using dead-burnt magnesia of different particle size ranges. Microstructural observations suggested that the faster Mg²⁺ migration during the spinel formation led to vacancy accumulation and, consequently, to pore generation, as a direct result of the Kirkendall effect. Additionally, the overall expansion of alumina–magnesia castables seemed to be ruled by two main factors: its sintering efficiency and the different possibilities of the Al₂O₃ and MgO interactions in the mixture. Those consequences, however, do not usually affect the castable corrosion behavior in industrial applications, due to the benefits imposed by the structural constraint.     

Effect of microsilica addition on the properties of colloidal silica bonded bauxite-andalusite based castables

Colloidal silica bonded bauxite-andalusite based castables were prepared using homogenized bauxite and andalusite as aggregates, andalusite fines, corundum fines, ultrafine Al₂O₃ as matrixes and colloidal silica as binders. Effects of microsilica addition on the green strength, physical properties, hot strength and thermal shock resistance of castables were investigated. Moreover, phase composition and morphological evolution of specimens were characterized by XRD and SEM analysis. Green strength after demoulding, cold strength and hot strength as well as thermal shock resistance of the castables are enhanced with microsilica addition, which attribute to generating more chemical bond (–Si–O–Si–) after demoulding and heating at intermediate temperature (up to 1100 °C), and creating a stronger mullite bonding at higher temperature (1400 °C) compare to the specimens without microsilica.     

Improvement of Durability of Purging Plugs Using MgO Micropowder for Refining Ladles

To modulate the matrix of purging plugs, MgO micropowder was introduced as a replacement to magnesia powder in alumina–magnesia castables, and the effect of MgO micropowder on the properties of alumina–magnesia castables and the possibility of developing chrome‐free castables were investigated. Experimental results showed that the introduction of MgO micropowder resulted in an improvement in the volume stability, strength, and thermal shock resistance of alumina–magnesia castables due to its high surface energy and small particle size. However, excessive amounts of MgO micropowder led to a lower densification, and there was a slight degradation in the performance of the alumina–magnesia castables. The slag resistance of the prepared alumina–magnesia castables was significantly better than that of the alumina–chrome castables. Microstructure and energy spectrum analysis showed that the formation of a solidified reaction layer, mainly consisting of spinel and CaAl₁₂O₁₉, was the major cause of the observed difference in slag resistance. In addition, the alumina–magnesia castables had a lower linear thermal expansion coefficient than that of the alumina–chrome castables at each experimental temperature, which effectively decreased the thermal stress during its service period, thus exhibiting good thermal shock resistance.     

Optimized microstructure with nano-alumina and its effects on properties of corundum spinel castables

The mechanical properties, thermal shock resistance, and microstructure evolution of corundum spinel castables with different nano-alumina contents were investigated. The results show that the addition of nano-alumina has advantages on the microstructure evolution and properties of castables. An optimum nano-alumina amount is 2 wt%. When nano-alumina addition changes from 0 to 2 wt%, the cold modulus of rupture (CMOR) and cold compressive strength (CCS) improved by 69.7% and 78.1% after firing at 1100°C, respectively. The CMOR and CCS increased by 42.5% and 35.2% after firing at 1500°C, respectively. Meanwhile, the hot modulus of rupture (HMOR) was enhanced by 21.5% to 13.4 MPa. The retained Young's modulus values of castables were improved from 49.6% to 59.6% after eight thermal shock cycles. Furthermore, the HMOR and the retained Young's modulus values of castables slightly reduced when nano-alumina content up to 3 wt%. XRD, DSC, SEM, and EDS analyses revealed that the addition of nano-alumina leads to the formation of calcium dialuminate (CaO·2Al₂O₃) at 1100°C and it is beneficial to the formation of more platelet hibonite (CaO·6Al₂O₃) at 1500°C. As a result of using nano-alumina with large surface area, the solid phase sintering of the nanoscale particles can occur at lower temperatures. Moreover, the mechanical properties and thermal shock resistance of the castables were improved remarkably.     

Spinel-containing alumina-based refractory castables

Due to their high corrosion resistance to basic slags, either pre-formed or in situ spinel (MgAl₂O₄) containing refractory castables are nowadays widely used as steel ladle linings. Nevertheless, whereas the pre-formed spinel castables present high volumetric stability and a well-known processing technology, the in situ spinel castables still require further understanding due to the challenges related to magnesia hydration and their expansive behaviour at high temperatures. Therefore, the objective of this paper is to review the knowledge already available for high-alumina spinel-containing castables (preformed and in situ) in order to provide a support for novel technological developments in the area. The main variables considered are the spinel content and grain size, the effect of calcium aluminate cement and hydratable alumina on the general castables’ properties, the influence of different alumina and magnesia sources and the silica fume content. Nowadays research subjects, including the use of mineralising compounds, the addition of nano-scaled particles and the evaluation of the effect of expansion under constraint will also be addressed, pointing out alternatives for the design of high-performance alumina-magnesia refractory castables.     

Reconstruction and hydration of hydrotalcite response to thermal activation temperature: Enhancement of properties for magnesia castables

Optimization binding system for refractory castables is significant to enhance the service performance. Hydrotalcite has been considered a promoter for high-temperature performance of basic castables, however, its binding property remains to be improved before practical application. In this work, the thermal activated Mg–Al hydrotalcites were incorporated in magnesia castables, and the mutual influence of pre-calcination temperature on the hydration, microstructure, and strength of castables was investigated. The obtained results indicated that the reconstruction of calcined hydrotalcite took place in the hydration process and effectively motivated the hydrolysis. Hydrate was thus promoted and a relatively dense microstructure of magnesia castables was confirmed by X-ray computed tomography analysis. Hydrotalcite pre-calcinated at 300 °C contributed to the highest early strength for castable, and the high-temperature properties also performed better than that of other pre-calcinated hydrotalcite-adding. The enhancement mechanisms of calcined hydrotalcite were attributed to the two following reasons: (ⅰ) the modified microstructure of magnesia castables from the early stage by hydration process, (ⅱ) the further enhanced sinterability inspired by the appropriate thermal activation effect.     

硅微粉对超低水泥浇注料流动性的影响

采用不同硅微粉,研究了其对高铝浇注料基质泥浆粘度的影响,结果表明,基质泥浆粘度随硅微粉加入量增加而增大.这是因为硅微粉与水反应形成水化产物,水化产物发生聚合,分子的体积增大,使浆体层流阻力增大,导致泥浆粘度上升.而随硅微粉含量增加,浇注料的振动流动性增加,这与硅微粉的填充减水机理密不可分.