Structure and properties of lightweight magnesia refractory castables with porous matrix

To improve the energy-saving capacity of magnesia refractory castables for working lining of high-temperature kilns, this study presents the researches on microstructure and properties of lightweight magnesia refractory castables with porous matrix fabricated by direct foaming method. The results show that formation of closed-pores in the matrix significantly enhanced high-temperature thermal insulation performance of castables with minor changes of slag corrosion resistance. The thermal conductivity of the lightweight magnesia castables at 1000 °C was below 1.2 W/m·K, which is 47.8% lower than that of the referenced magnesia castable. The increasing content of SDS (foaming agent, over 0.02 wt%) led to increments of size and number of large-sized pores, resulting in the significantly decreased density and mechanical performances. The slag resistance mechanism reveals that, in addition to intergranular penetration, the accumulation of slag and penetration between adjacent pores were the major ways of slag mass transfer in lightweight magnesia castables. In conclusion, controlling the size (below 53.2 μm), number and distribution of closed-pores in the matrix is effective to realize the coupling of high thermal insulation, mechanical properties and slag resistance for lightweight magnesia castables used in the metallurgical field.     

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.     

Controlling hydration of hydratable alumina via co-grinding with Mg–Al hydrotalcite

The on-site industrial application of hydratable alumina (HA)-bonded castables is inhibited by the high hydration rate of HA. In this study, the hydration behavior of HA co-ground with sheetlike Mg–Al hydrotalcite (MAT) is investigated. The properties of castables bonded with MAT-bearing HA are systematically assessed. The hydration rate of HA co-ground MAT decreases as this allows MAT sheets to be effectively inserted into the microcracks of HA particles during grinding, thus decreasing the direct contact area between HA and water. The strength of MAT-bearing castables (0.5 and 1 wt%) fired at 800°C improved slightly owning to the generation of magnesia–alumina spinel. The mechanical strength of castables fired at 1100 and 1550°C decreased as the MAT content increased owing to an increase in porosity. Based on an analysis of the hydration behavior of HA and the properties of HA-bonded castables, the optimal MAT/HA weight ratio is approximately 1/10.     

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.     

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.     

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.     

Magnesia grain size effect on in situ spinel refractory castables

Alumina–magnesia castables present an expansive behavior due to in situ spinel formation and the reaction is affected by the magnesia source and its grain size. In this study, castables were prepared with different magnesia grain sizes (<45 or <100 μm) and the samples were evaluated by an assisted sintering technique, mechanical strength, creep resistance and microstructural evaluation. The expansion, the resulting phases and properties scaled with the magnesia grain size. The use of coarse MgO grain (<100 μm) led to phases which were predicted by local equilibrium phase diagrams, such as forsterite and monticellite. Due to a completely different microstructure developed for the castable containing large grains, a large number of cracks were generated, worsening the mechanical strength and creep resistance. Magnesia grain size selection is thus a key issue for alumina–magnesia castable design, as it affects the material's performance during service. As the change of a single parameter affected the final castable microstructure and properties, this study is a typical example concerning the complexity of refractory ceramic systems.     

M- S-H formation in MgO-SiO2slurries via wet milling for magnesia based castables

It is believed that the formation of hydration phase, MgO-SiO₂-H₂O (M-S-H), contributes to good workability and reliable comprehensive properties for magnesia based castables. In order to stimulate the formation of M-S-H in magnesia based castables and understand the minimum introduction of microslica amount, wet milling process was used to promote the dissolution of MgO and SiO₂ in this work. The slurry containing different content of microsilica with wet milling technology and the castables with/without wet milling slurry were prepared. The effects of microsilica content on the formation of hydration phases were analyzed by XRD, FT-IR and TG/DSC and the properties of magnesia based castables were evaluated by explosion resistance, CMOR, HMOR and so on. The results showed that the formation of M-S-H was accelerated because of the dissolution of Mg²⁺ and HSiO₃^? in wet milling process. Higher amount of M-S-H led to a tight bonding in the early stage, and a denser structure after firing at high temperature due to the limited formation of brucite and in-situ formation of evenly distributed forsterite phase. In addition, much higher HMOR were obtained when less microsilica was added, attributing to the suppressed formation of low-melting-point liquid. Therefore, 2–3 wt% microsilica addition was recommended in this process.     

The role of hydraulic binders on magnesia containing refractory castables: Calcium aluminate cement and hydratable alumina

Previous work by the authors has shown that the effects of calcium aluminate cement (CAC) and hydratable alumina (HA) can modify the magnesia hydration behavior in aqueous suspensions. As a consequence of these studies, the present paper highlights how varying the content of these binders can affect magnesia hydration in refractory castables using pH, apparent volumetric expansion, mechanical strength and porosity measurements and hydration–dehydration tests. Furthermore, as mechanical strength, porosity and refractoriness also play an important role in these materials, binder-free, magnesia-free and magnesia-and-binder-free samples were also tested as references. It was found that the deleterious effects of magnesia hydration can be greatly minimized by the binder and its selection content.     

MgO—Al2O3系浇注料的设计

以相关知识和实践经验为依据，探讨了ＭｇＯ－Ａｌ２Ｏ３系中铝镁质（含刚玉－尖晶石质和矾土熟料－尖晶石质）浇注料的基质料的设计和结合剂的合理选择问题。     

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.     

Alumina-rich refractory concretes with added spinel,periclase and dolomite: A comparative study of their microstructural evolution with temperature

Three processing routes for obtaining refractory castables within the alumina-rich zone of the Al₂O₃–MgO–CaO ternary phase equilibrium diagram were studied starting from mixtures of: (a) calcined alumina, synthetic spinel and calcium aluminate cements, (b) calcined alumina, magnesia and calcium aluminate cement, and (c) calcined alumina, dolomite and calcium aluminate cement. The evolution of the phases and microstructure was studied as a function of temperature and the processing route for both refractory concretes and their corresponding matrices (<125 μm).     

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.     

Research on Composite Slag Dam Cast with Magnesia Castables and Alumina Magnesia Castables

To enhance the service life of magnesia based slag dam,composite slag dam was designed to be cast with alumina magnesia castables in slag line and magnesia castables in molten steel zone.Workability of the magnesia castables for the slag dam was improved and a suitable vibration shaping method was adopted to combine it with alumina magnesia castables.The result shows:(1)workability and setting performance of magnesia castables can be improved to match with alumina magnesia castables by adjusting setting ret...     

不同养护制度下大掺量石灰石煅烧黏土UHPC早期水化及力学性能发展

采用大掺量(60%,质量分数)石灰石煅烧黏土替代水泥熟料设计制备了超高性能混凝土(UHPC),通过抗压强度测试、X射线衍射(XRD)分析、等温量热分析以及综合热分析研究了其在标准养护和蒸汽养护下的早期水化行为和力学性能发展。研究发现,蒸汽养护显著改善了由高水泥替代量造成的1 d及3 d抗压强度损失,标准养护则使7 d抗压强度更为优异,煅烧黏土和石灰石质量比为2∶1时各个龄期的强度发展最佳。在蒸汽养护条件下水化进程得到大幅提前,出现明显的铝酸盐相反应放热峰。蒸汽养护加剧了煅烧黏土对氢氧化钙的消耗,煅烧黏土和石灰石质量比为2∶1时才检测到单碳型碳铝酸盐的形成,表明在低水胶比环境下,早期煅烧黏土与石灰石的协同作用主要取决于活性组分煅烧黏土的含量。     

Citric acid as anti-hydration additive for magnesia containing refractory castables

Magnesia hydration is a key concern in refractory castable processing. The volumetric expansion that follows this reaction can result in cracks or even explosion during the first heating-up. Citric acid (CA) and other chelants can significantly reduce MgO hydration rate in aqueous suspensions by forming an insoluble magnesium citrate protective coating on the magnesia particles’ surface. In the present work, the performance of CA as an anti-hydration additive in refractory castables was evaluated by hydration tests, mechanical strength and apparent volumetric expansion (AVE) measurements and thermogravimetry. The results attained have shown that CA effectiveness depends strongly on the amount added and by the interaction with other raw materials in the composition, in particular calcium aluminate cement.     

Effect of hydromagnesite on the hydration of hydratable alumina and properties of corundum-based castables

Hydratable alumina (HA) was premixed with hydromagnesite (BMC) to investigate the BMC impact on the hydration behavior of HA and the thermo-mechanical properties of HA bonded (HAB) castables. The phase composition, microstructure and mass changes of dried HA samples, were characterized by XRD, SEM, and TG, respectively. Flow ability, microstructure, and thermo-mechanical properties of HAB castables were studied. Results indicated that BMC effectively lowered HA hydration rate due to the decreased specific surface area. The hot modulus of rupture strength of castables was improved because the sintering of Al₂O₃ was enhanced by the MgO from BMC decomposition.     

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.     

Effect of grinding on the hydration of hydratable alumina and properties of hydratable alumina-bonded castables

Hydratable alumina (HA) is a superior Ca-free refractory binder, but the quick hydration rate restricts the working time of castables bonded with HA. In this work, HA was grounded for 1 h and 6 h by a rotational ball mill to study the effect of grinding on the hydration of HA and properties of HA-bonded castables. HA samples with and without grinding were cured at 30 °C and then terminated by freeze-vacuum drying. The phase composition and microstructure of the dried HA samples were then examined. Moreover, flow ability and mechanical strength of castables containing ungrounded and grounded HA were also investigated. The results indicate that the specific area of HA particles were decreased by grinding as the micro-pores and micro-cracks on the surface of HA particles were blocked by smaller HA particles, thereby decreasing the hydration rate of HA and increasing the flow ability of castables.     

Effect of the calcined andalusite aggregates on the micro-crack formation and thermal shock resistance of mullite castables

The effect of calcined andalusite aggregates on the micro-crack formation and thermal shock resistance of mullite castables was investigated and analyzed. The mullite castables were prepared from andalusite aggregates calcined at high temperatures. The results show that the micro-cracks from the transformation of andalusite to mullite can effectively relieve thermal stress, improving the thermal shock resistance of mullite castables. The micro-cracks generated decreased with increasing calcine temperatures of the andalusite aggregates. When the calcine temperature was increased from 1300 °C to 1500 °C, the thermal shock resistance of mullite castables was found to continuously increase. However, the thermal shock resistance of mullite castables with the andalusite aggregates calcined at 1600 °C is lower than those with the andalusite aggregates calcined at 1500 °C.