Alumina-rich magnesia–alumina spinel (MgAl2O4) is increasingly used as a high-performance ceramic in extreme-service environments, yet the temperature dependence of its exsolution equilibrium has not been quantified; and this information is essential for optimizing in-situ toughening of refractory castables. This study systematically elucidates the isothermal exsolution kinetics and equilibrium limits of an 85-grade, alumina-rich spinel between 1100 and 1600 °C. Using a 325-mesh electrofused feedstock and dwell times ranging from 0.5 h to 7 d, we quantified phase abundances and lattice stoichiometries by Rietveld refinement of X-ray diffraction data coupled with energy-dispersive X-ray spectroscopy. Below 1300 °C the lattice remains supersaturated for up to 7 d without detectable α-Al2O3 precipitation, establishing 1300 °C as the minimum kinetic threshold for measurable exsolution. The maximum extent of exsolution (lowest residual Al2O3, 78.3 wt.

90-grade alumina-rich spinel fines were ball-milled for various durations to obtain three distinct size fractions (D-50 similar to 5.5, 3.8 and 1.5 mu m). After heat treatment at 1600 degrees C for 3 h, the exsolution extent was quantified by Rietveld refinement of X-ray diffraction data. Each fraction was incorporated into a calcium aluminate cement (CAC) bonded corundum castable at a fixed 10 wt% dosage; phase evolution, microtexture and mechanical properties were systematically compared. The findings demonstrate that decreasing the alumina-rich spinel particle size markedly increases the amount of corundum exsolution. Shorter diffusion paths and higher surface-area/strain energy in finer particles accelerate Al3+ rejection and amplify the thermodynamic driving force for corundum exsolution. Finer spinel disperses more uniformly throughout the castable, producing a denser and more evenly distributed CA(6) interpenetration network via reaction between exsolved corundum and the cement phase, which significantly improves bulk density and mechanical properties. This insight clarifies the intrinsic mechanism by which reduced spinel particle size enhances mechanical performance through intensified exsolution and optimization of the CA(6) interpenetrating network.

The alumina could dissolve into the magnesia–alumina spinel when the Al2O3 content of spinel does not reach the theoretical maximum content at a certain temperature. The effect of spinel solid solution behavior on the bonding between CaO·6Al2O3 and spinel, and consequently on the castable strength following sintering at various temperatures, has been investigated. The results indicate that 72-spinel (containing 72 wt.

This study introduces an oxalic acid-activated calcium silicate cement (OACS), where the oxalic acid precursor can be synthesized from CO2 via electrochemical methods, thereby establishing it as a typical low-carbon cement. Experimental results indicate that the combination of oxalic acid and gamma-dicalcium silicate exhibits rapid hardening during reaction. The setting time can be effectively adjusted through (1) lowering the mixing water temperature and (2) partial substitution of oxalic acid with sodium oxalate. Phase characterization reveals crystalline calcium oxalate dihydrate precipitates and extensively polymerized silica gel as the principal reaction products of OACS. Microstructural analysis shows a spatial configuration with silica gel encapsulating unreacted calcium silicate particles, while calcium oxalate deposits occupy interparticle spaces. Progressive pore-filling effects during curing reduce paste porosity over curing time, resulting in OACS pastes achieving a remarkable 3-day compressive strength of approximately 50 MPa, demonstrating excellent early-age performance.

Despite extensive research on the properties and applications of magnesia alumina spinel (MgAl2O4), the exsolution behavior of spinels with different alumina contents under varying heat-treatment temperatures remains poorly understood. This study investigates the exsolution behavior and crystal structure changes of spinel with different alumina contents under various heat treatment temperatures. We found that onset temperature for exsolution decreases with increasing alumina content. Specifically, 78-spinel and 85-spinel begin to exsolve at 1300 degrees C, while 90-spinel starts at 1200 degrees C. At 1300 degrees C, the alumina content in spinel reaches a minimum, with 78-spinel having the lowest alumina content (76.7 %), followed by 85-spinel (78 %) and 90-spinel (80 %). Experimental exsolution equilibrium alumina contents were higher than theoretical values due to differences in chemical driving forces and ion migration behaviors. During exsolution, Al3 + ions occupying tetrahedral sites are preferentially expelled, leading to a measurable expansion of the spinel lattice.

The alumina foamed ceramics were prepared using alpha-alumina powder and calcium aluminate cement as raw materials by the direct-foaming method. The objective was to accelerate the hydration process of the calcium aluminate cement and promote the sintering process of the ceramics by introducing silica fume. The effect of the silica fume introduction on phase composition and microstructure as well as the correlation between the microstructure evolution and ceramic properties were investigated by XRD, SEM and EDS. The results revealed that the introduction of silica fume facilitated the nucleation reaction of the hydration progress of calcium aluminate cement and formed more hydrate nucleus, which accelerated the setting rate of ceramic suspension and decreased the pore size of ceramics. Additionally, the addition of silica fume was beneficial to the formation of transitional liquid and promoted the sintering, leading to an enhancement of the compressive strength. However, the excess liquid led to abnormal growth of alumina grains and the formation of lower-strength phases, giving rise to a reduction of strength. The optimal specimen is the ceramic with 2 wt% silica fume, which exhibits a high porosity of 80.2 %, a high compressive strength of 25.33 MPa, and a low thermal conductivity of 0.227 W/ (m center dot K).

Magnesia-alumina spinel undergoes a solid solution reaction during firing, which potentially impact the microstructure as well as the mechanical properties of the CAC-bonded corundum castables. Moreover, spinels possessing the same chemical composition yet produced via different methods may exhibit diverse solid solution behaviors. Consequently, this study firstly compares the solid solution behaviors of sintered and fused spinel powders, both of which have an identical alumina content of 72 wt%, and then investigates the effect of solid solution on the microstructure and mechanical properties of CAC-bonded corundum castables. The results indicate that both sintered and fused spinel powders experience solid solution reactions during the firing process, leading to the formation of spinel-calcium hexaluminate (CA6) connection structures. This transformation alters the fracture behavior from intergranular to transgranular mode, thereby enhancing the mechanical properties of castables. When the spinel content is 10 wt%, the cold crushing strength of the castables containing sintered spinel reaches 235.9 MPa, which is 28.6% higher than that of the castables without spinel. The spinel-CA6 connection structures increase with the elevation of the spinel content. Additionally, due to its smaller grain size, sintered spinel undergoes higher degree of solid solution reaction, forming tighter connection structures, which contributes to the improvement of the overall mechanical properties.

The alumina-based foamed ceramics were prepared using alpha-alumina powder as raw materials by direct-foaming method. Calcium aluminate cement (CAC) was chosen as a binder to accelerate the solidification of foamed suspensions, and the effect of curing temperature on the rheological properties of suspensions, phase composition, microstructure, pore characteristics, compressive strength and thermal conductivity of foamed ceramics was investigated. What's more important, the thermal conductivity was analyzed using the Grey relational degree model to explore the effect of the pore characteristics on the thermal conductivity of foamed ceramics. The findings revealed that the higher curing temperature led to the increase in viscosity and stability of the foamed suspensions, as well as a faster hydration rate of CAC, which resulted in a reduction in the pore size of the fired ceramics from 312 mu m to 160 mu m and the thermal conductivity (1000 degrees C) from 0.309 W/(m center dot K) to 0.248 W/(m center dot K). It is worth noting that as the curing temperature rose, the porosity of the ceramics increased while the compressive strength improved from 0.83 MPa to 1.26 MPa, due to the smaller pore size, the more uniform pore size distribution and more regular pore roundness. According to the Grey relational degree model, pores in the range of 250-300 mu m had the most significant impact on the thermal conductivity of the foamed ceramics.

The strength development of CAC-bonded castables at low ambient temperatures is of low speed, leading to extended curing time and reduced production efficiency for the castable factories at the winter time. Traditional accelerating agents primarily work by enhancing cement dissolution to speed up hydration. Recent studies suggest that certain nanoparticles can also facilitate hydration by accelerating the nucleation and growth of hydration products. This study proposes to prepare nanoscale hydration products by prehydration of CAC suspension at 10 degrees C, which is then added to CAC-bonded castables acting as nucleation seeds to accelerate the hydration of CAC, thereby speeding up the development of mechanical properties of castables. The results show that nanoscale hydrates CAH10 and AH3 form on the surface of prehydrated CAC particles in the suspensions. The suspension prehydrated for 4 h most significantly enhances the early strength of castables, as the quantity and size of the hydrates reach an optimal state. Additionally, the incorporation of prehydrated CAC suspensions has a slight negative impact on the mechanical properties after heat treatment at 800 degrees C and 1100 degrees C, but has little effect on the mechanical properties of castables after calcination at 1450 degrees C. This method could effectively improve the production efficiency of castables without using special agents with the added benefit of increased resource utilization rate.

This study investigates the effect of the exsolution behavior of alumina-rich spinel on the formation and distribution of CA6 (CaAl12O19) in corundum castables bonded with calcium aluminate cement. In this study, alumina-rich spinel is substituted for tabular corundum in the same proportions and grain size. The matrices after curing were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The phase composition and microstructure of the matrices containing alumina-rich spinel were analyzed after firing at 1600 °C. These results showed that the addition of alumina-rich spinel significantly improved the mechanical strength of the castables. This improvement was attributed to the alumina produced by spinel exsolution during firing at 1600 °C, which reacted in situ with CA2 (CaAl4O7) to form CA6. CA6 connects the different particles and forms an interspersed interlocking structure within the spinel. The CA6-MA interspersed interlocking structure replaces part of the CA6-Al2O3 structure and significantly improves the mechanical strength of the castables.

Accelerated carbonation endows the ordinary Portland cement with high mechanical property and large amount of carbon dioxide sequestration. However, the moisture and carbon dioxide in the air lead to the pre-hydration and pre-carbonation of ordinary Portland cement during production, transportation and storage, which is known as ageing. It is still unknown whether the carbonation reactivity of the aged cement is reduced compared to the normal cement. This study investigates the carbonation behavior variation of the aged and normal cements, and compares it with their hydration property difference. The results indicate that the pre-hydration and precarbonation products cover the surface of clinkers of the aged cement. The compressive strength of aged cement pastes after hydration for 3 d is much lower than that of the normal cement pastes, but increases fast and becomes comparable to that of normal cement after hydration for 7 d and longer. The compressive strength of the aged cement compacted pastes during accelerated carbonation is much lower than that of the carbonated normal cement compacted pastes, no matter at the early age or the later age. It is deduced that the slightly detached ageing products act as seedings which lead to the concentrated hydration heat release and is conducive to the further dissolution and hydration of aged cement. However, as the water film is thin in cement compacted pastes for carbonation, the detached and un-detached products still cover the aged cement particles and hinder the entire carbonation reaction.

Carbonation curing imparts high early strength and substantial CO2 sequestration to carbonatable calcium silicate cement. However, the insufficient carbonation in the interior of the carbonated cement paste negatively impacts its mechanical properties. This study presents a novel method of internal carbonation curing for gamma-dicalcium silicate (C2S(gamma)) using porous lightweight zeolite aggregate saturated with ammonium bicarbonate (NH4HCO3) solution. The findings show that NH4HCO3 rapidly reacts with C2S(gamma) to produce calcium carbonates, confirming the viability of NH4HCO3 as an internal curing agent. Mechanical property tests demonstrated that specimens containing zeolite pre-soaked in 1.5 mol/L NH4HCO3 solution achieved the highest compressive strength after carbonation and extended curing, with a 33.36 MPa increase over the control. Microstructural analysis revealed that internal curing with NH4HCO3 solution enhanced carbonation extent around zeolite. The secondary calcium carbonate (CaCO3 (II)) formed by internal curing strengthens the framework established by primary calcium carbonate (CaCO3 (I)) from external carbonation, thus bolstering the overall strength.

The addition of CaCO3 powder in calcium aluminate cement (CAC) is reported to accelerate the hydration of CAC and generate mono-carbonate C(4)A (C) over barH(11), which is conducive to shortening the curing time of CAC-bonded castables at ambient temperatures. Although the hydration of CAC is relatively fast, there is still residual un-hydrated CAC in castables after curing. The residual CAC would continue to hydrate to produce C(3)AH(6) and AH(3) during the subsequent drying procedures in the absence of CaCO3. But it is worth pondering which type of hydrate would be generated in the presence of CaCO3 during the drying at around 110 degrees C. In this study, the effect of micro-sized CaCO3 addition on the hydrate formation of CAC-bonded castables during drying at 110 degrees C was investigated. The results indicate that the C(4)A (C) over barH(11) formed at the curing temperature of 30 degrees C is not decomposed or converted during drying. The CaCO3 remained after curing does not participate in the hydration of residual CAC during drying at 110 degrees C, and thus the residual CAC continues to hydrate to generate C(3)AH(6) rather than C(4)A (C) over barH(11).

Employing calcium silicate minerals with low calcium-to-silicate ratios, such as CS, C3S2, and C2S(gamma), in the preparation of carbonatable calcium silicate cement (CCSC) presents a significant advantage in reducing carbon dioxide emissions. However, the current approach of forming CCSC mainly through mold pressing methods limits its application. The preparation of castable CCSC encounters two main challenges: low hydration activity hampers the attainment of adequate demolding strength, and a high water-to-cement ratio blocks CO2 transmission pathways, resulting in a low carbonation degree. To address these issues, this study proposes incorporating a small amount of calcium aluminate cement (CAC) into the C2S(gamma) mortar to rapidly provide demolding strength. Subsequently, a pre-drying treatment is applied to remove a portion of free water and form pore channels, facilitating subsequent accelerated carbonation. The results showed that the addition of 15 wt% CAC enabled the C2S(gamma) mortar to achieve a compressive strength greater than 2 MPa after 24 h of curing. A pre- drying time of 1 h was found to be conducive to enhancing strength during the subsequent carbonation curing process. During carbonation curing, the compressive strength of the CAC-C2S(gamma) mortar increased rapidly, reaching 48.95 MPa and 50.87 MPa after 12 and 24 h of carbonation curing, respectively. The hydration products of CAC did not participate in the carbonation reaction but interconnected with the carbonation products, jointly contributing to the strength of the CAC-C2S(gamma) mortar.

The relatively low hydration rate of calcium aluminate cement (CAC) at 15-35 degrees C leads to retarded setting time and reduced production efficiency of CAC-bonded castables. To accelerate the hydration rate of CAC within this temperature range, the present work proposes the fabrication of nano-hydrates acting as seeds by pre-hydration of CAC in suspensions. CAC powders are dispersed in water with water-to-solid ratio of 4.5 and stirred for varying durations at 25 degrees C to achieve different pre-hydration degrees. Then the pre-hydrated CAC suspensions replace a small part of cement in the pastes. The influence of the CAC suspensions addition on the hydration heat release and strength development of fresh CAC pastes at 25 degrees C is investigated. The results indicate that nano-scale hydrates C2AH8 and AH3 form on the surface of pre-hydrated CAC particles in the suspensions and grow rapidly with the extension of stirring duration. The presence of nano-scale hydrates in the pre-hydrated CAC suspensions is found to shorten the induction period of CAC hydration and thus be beneficial to the accelerated hardening of CAC pastes. The pre-hydrated CAC suspension stirred for 3.5 h has the most significant effect on enhancing early strength of the CAC pastes. At this stage, the pre-hydrated CAC within the suspension is in the accelerated period, and has just begun to precipitate large amounts of nano-scale C2AH8 and AH3 acting as seeds to catalyze the hydration and refine the pore structure of CAC pastes.

According to the magnesia-alumina phase diagram, the alumina-rich spinel may undergo exsolution of corundum during firing when incorporated into the castable. This research aimed to explore the phenomenon of alumina- rich spinel exsolution and its subsequent impact on the distribution of CaO.6Al2O3 inside the spinel and the strength of the castables when subjected to a firing temperature of 1600 degrees C. The results indicated that the amount of exsolved corundum escalated as the alumina content within the spinel rose after heat treatment, and this exsolved corundum was predominantly found within the original spinel particle. The CaO.2Al2O3, which formed when calcium aluminate cement was heated above 1200 degrees C, reacted preferentially with the exsolved corundum rather than with pre-added alpha-Al2O3 leading to the formation of CaO.6Al2O3 inside the spinel after firing at temperatures exceeding 1400 degrees C. This reaction formed an interpenetrated "spinel-CaO.6Al2O3" structure during firing, which enhanced the strength of the castables.

Free water available in calcium aluminate cement (CAC)-bonded castables is crucial for the hydration of CAC and the conversion of hydration products in the curing and drying processes, as both the hydration and conversion reactions are dissolution–precipitation reactions. To elucidate the effect of different levels of free water loss upon the hydration of CAC, the conversion of hydration products and the mechanical strength of the CAC-bonded castables, the CAC-bonded castables were subjected to sealed and unsealed curing conditions at 50 °C and drying at 110 °C. The results demonstrate that the fast removal of free water during unsealed curing would hinder the conversion from 2CaO·Al 2 O 3 ·8H 2 O to 3CaO·Al 2 O 3 ·6H 2 O and consequently prevent the deterioration of strength. As a comparison, although sealed-cured samples have less water loss and high degree of hydration of CAC, they still show lower strength than the unsealed samples after curing. The following drying process further accelerates the hydration of residual calcium aluminate clinkers for both the sealed and unsealed samples, but still does not favor the conversion from 2CaO·Al 2 O 3 ·8H 2 O to 3CaO·Al 2 O 3 ·6H 2 O in the unsealed-cured samples.

为弄清可水合氧化铝(HA)结合浇注料中温强度低的原因,以板状刚玉、活性氧化铝粉、HA为原料制备刚玉浇注料.研究了HA添加量(w,1％、3％、6％、9％)和热处理温度(110℃保温24 h,400、600、800、1000、1100和1250℃保温5 h)对浇注料强度和显微结构的影响.结果表明:1)养护和干燥过程中,HA通过水化反应生成蜂窝状产物勃姆石和拜耳石,起到结构骨架的作用,为浇注料提供强度.2)HA结合浇注料在1000℃热处理后强度最低;在低于1000℃时,热处理温度的升高会逐渐破坏水化产物的结构,降低浇注料的强度;高于1000℃时,浇注料基质中发生了局部烧结,使得强度有所回升.3)提高HA的加入量无法提升浇注料的中温强度,说明中温热处理使得HA的水化产物结构完全坍塌,无法起到结构支撑的作用.

Calcium aluminate cement (CAC) was used in this work as a novel binder of the foam slurries to prepare Al2O3 foamed ceramics. The different foam stabilizers employed in the slurries would not only impact the foam stability, but also affect the setting behavior of CAC-containing foam slurries, consequently influencing the pore size and the heat-insulating property of the Al2O3 foamed ceramics. In this paper, modified polyethoxylated silicone (MPS), sodium alginate (SA) and carboxymethyl cellulose (CMC) were selected respectively as the foam stabilizer. The effects of the different foam stabilizers on the setting behavior of foam slurries, and the dependence of the pore size and heat-insulating property of CAC-bonded foamed ceramics on the setting behavior of foam slurries were investigated. It is found that SA promotes the hydration of CAC in the foam slurries, while MPS and CMC postpone the hydration of CAC in the foam slurries; and the foamed ceramics with SA have better structural integrity, higher porosity, smaller average pore size and lower thermal conductivity than those with MPS or CMC.