Research Progress on Calcium Aluminate Cement as a Binder for Refractory Castables
Calcium aluminate cement (CAC) is an important binder in refractory castables. CAC-bonded castables exhibit good demolding strength after curing at different temperatures for 8–24 hours. Through hydration reactions, calcium aluminate cement forms various hydration products that provide sufficient strength for demolding and rapid drying. As a result, CAC-bonded castables achieve excellent mechanical strength after sintering, along with outstanding high-temperature performance.
Notably, CAC is a key component whose hydration behavior can be adjusted to influence the workability and overall performance of castables. External factors have a considerable impact on the hydration behavior of the cement. It is thus evident that many factors affect the hydration of CAC, such as curing temperature, humidity, particle size, and admixtures.
In view of this, this paper reviews the advantages and disadvantages of various binders, explains the hydration mechanism of CAC, focuses on summarizing the mechanisms influencing CAC hydration, and illustrates the effects of admixtures on the mechanical strength of CAC-bonded castables.
An in-depth investigation of the hydration mechanism of CAC and the factors governing its hydration rate and degree is of great significance for the modification of CAC-bonded castables.
Refractory Castables
1.1 Introduction to Binders
Refractory castables generally consist of two components: the aggregate (i.e., the discontinuous phase) and the bonding matrix (i.e., the continuous phase), which determines the diversity of the castables' properties. The bonding matrix includes fine powders, binders, additives, and other components. The interaction between the aggregate and the matrix governs the final properties of the castables.
Binders serve to tightly bond the aggregate and matrix together, providing strength to castables in the green state and at elevated temperatures by forming high-temperature phases or enhancing sintering kinetics through thermal decomposition. Therefore, the selection of binders and the analysis of their bonding mechanisms have a significant influence on the performance of castables.
Calcium aluminate cement (CAC) is a commonly used binder in refractory castables and is widely applied due to its rapid setting and high demolding strength. However, the use of CAC in systems containing silica fume and MgO has several drawbacks. The CaO present in CAC can react with silica or alumina at high temperatures to form crystalline phases such as tricalcium aluminate, thereby reducing the corrosion resistance and high-temperature mechanical strength of the castables.
Silica sol can react with alumina in refractory castables to form mullite after firing, which helps improve the thermal stability, refractoriness, and physical strength of the refractories. Nevertheless, several problems arise when silica sol is used as a binder for castables: its fluidity and workability are usually poor, resulting in high water demand.
Thus, each type of binder has its own characteristics, with different bonding mechanisms and applicable service environments. A thorough understanding of these bonding mechanisms is of great importance for selecting appropriate binders.
1.2 Classification of Binders
Refractory castables are bonded into an integral body by the action of binders, and the properties of the binders largely determine the physical and mechanical properties of unshaped refractories at ambient, medium, and high temperatures.
According to the different bonding mechanisms of binders used in castables, they can be classified as follows:
Hydration-bonded castables: e.g., calcium aluminate cement-bonded and hydrated alumina (ρ-Al₂O₃)-bonded castables;
Chemically bonded castables: e.g., phosphate-bonded castables, including phosphoric acid-bonded and aluminum dihydrogen phosphate-bonded castables;
Coagulation-bonded castables: e.g., SiO₂, Al₂O₃, and other micro-powder systems, as well as silica sol- or alumina sol-bonded castables.
The advantages and disadvantages of various binders are listed in Table 1. Calcium aluminate cement features a short setting time and high demolding strength, along with good mechanical strength at high temperatures; however, it is not suitable for medium-temperature service environments. In contrast, phosphates and silica sol are more suitable for medium- and low-temperature conditions.
Hydrated alumina, silica sol, and non-silica-based sols have the advantage of low impurity content, which helps improve the purity of castables and thus enhances their physical properties. In addition, calcium aluminate cement, hydrated alumina, and silica sol all require high water addition, resulting in low apparent porosity and poor spalling resistance of the castables.
Calcium Aluminate Cement
The mechanical strength of castables is closely related to the degree of hydration and phase evolution during the hardening process of CAC. Therefore, the hydration behavior of calcium aluminate cement is a key factor affecting the mechanical strength of castables.
2.1 Hydration Mechanism of Calcium Aluminate Cement
The main chemical components of calcium aluminate cement are CaO and Al₂O₃. Hydration reactions produce hydrates that coagulate and harden the coarse and fine particles in castables, thereby improving the strength of the refractories.
The main phases undergoing hydration in calcium aluminate cement are CA (CaO·Al₂O₃), CA₂ (CaO·2Al₂O₃), and C₁₂A₇ (12CaO·7Al₂O₃). CA, accounting for 40%–70%, is the primary hydraulic phase of calcium aluminate cement. With high hydration activity, a fast hydration rate, and rapid hardening, CA provides excellent early strength for castables. CA₂ is the secondary hydraulic phase (<25%), with a long hydration time and slow setting. C₁₂A₇, as a minor phase (<3%), has low refractoriness but a short hydration time and can accelerate the setting of the CA phase.
As shown in Figure 1, different hydration products are formed at different curing temperatures:
When the curing temperature is below 21 °C, the hydration product is acicular-prismatic CaO·Al₂O₃·10H₂O (CAH₁₀);
When the curing temperature is 21–35 °C, the hydration products are platy 2CaO·Al₂O₃·8H₂O (C₂AH₈) and Al₂O₃·3H₂O (AH₃);
When the curing temperature is above 35 °C, the main hydration products are granular 3CaO·Al₂O₃·6H₂O (C₃AH₆) and AH₃.
Table 2 shows the stable states and thermal decomposition temperatures of various hydration products, among which C₃AH₆ is the most stable phase. With increasing curing temperature, the metastable phases CAH₁₀ and C₂AH₈ transform into the stable phases C₃AH₆ and AH₃, and the final hydration products are C₃AH₆ and AH₃.
Since the density of the stable phases is higher than that of the metastable phases, the porosity of the hardened cement increases as hydration proceeds, resulting in volume instability. In particular, C₃AH₆ has the highest density among the hydration products, which causes significant volume reduction of the hydrates and leads to strength loss.
Figure 2 shows the phase evolution of C₃AH₆ and AH₃ during heating. These hydration products eventually form high-melting-point phases such as CA₂ and CA₆ (CaO·6Al₂O₃), which generate ceramic bonding between castable components, thereby enhancing the mechanical properties and thermal stability of the castables.
Since the density of CA₆ is lower than that of alumina and calcium oxide, its formation can compensate for the volume shrinkage of the samples during firing. In addition, flaky and well-packed CA₆ crystals can prevent slag penetration and effectively improve the corrosion resistance of castables.
During the hydration of calcium aluminate cement, the hydration products form an interlocking network structure, which provides demolding strength for castables. However, during heating, these hydration products undergo dehydration and decomposition, forming micropores and disrupting the hydration network.
Furthermore, micro-powders can block the pores of castables, hindering the escape of bound water from the decomposed hydration products. This increases water vapor pressure inside the castables, which can easily lead to internal cracks and even explosive spalling.
As a result, the mechanical strength of castables decreases significantly during firing. At medium temperatures (especially 110–900 °C), castables may experience cracking or even spalling.
The hydration process of calcium aluminate cement can be divided into three stages: the dissolution period, the induction period, and the precipitation period.
First, the surface of cement particles becomes hydroxylated upon contact with water, releasing Al(OH)₄⁻ and Ca²⁺ ions, as shown in Reaction (1).
Some Al(OH)₄⁻ ions can further dissociate in the solution to form Al³⁺ and OH⁻ ions, as shown in Reaction (2), which increases the pH of the solution during calcium aluminate cement hydration.
When the ion concentration in the solution reaches the solubility limit, hydration enters the induction period, and nuclei of hydration products begin to form and grow. When the nuclei reach a critical size, the hydration products start to precipitate, accompanied by heat release, indicating the end of the induction period. Precipitation of hydration products further promotes the dissolution of cement particles. Therefore, the cyclic process of ion dissolution and precipitation continues, consuming most of the cement particles. Due to heterogeneous nucleation on particle surfaces, the precipitated hydration products tend to form strong bonds between adjacent particles, resulting in the setting of the cement.
Calcium aluminate cement containing trace MgAl₂O₄ spinel (CMA) was prepared to investigate the phase distribution and hydration kinetics of the cement. The results show that three mineral phases-CA, CA₂, and MgAl₂O₄ (MA)-are homogeneously formed when the temperature rises to 1300 °C. As shown in Figure 3, the CAC sample exhibits two exothermic peaks, while the CMA sample shows only one exothermic peak. This is because the hydration rate of the CA phase is faster than that of the CA₂ phase, and the MA phase separates CA from CA₂, accelerating the reaction of both CA and CA₂ phases with water. Moreover, the presence of Mg²⁺ ions accelerates the hydration reaction of the cement, causing the exothermic peak of the CA₂ phase to overlap with that of the CA phase. This indicates that the uniform distribution between CA and CA₂ promotes the hydration process.
In addition, it was found that the hydration mechanism of both cements follows the nucleation and crystal growth (NG)–diffusion (D) process. In the early stage of hydration, sufficient water is available and fewer hydration products are formed, so the reaction is dominated by the NG process. As hydration proceeds, more hydration products are generated, hindering ion migration, and the hydration reaction becomes dominated by the D process.
2.2 Influencing Factors on the Hydration of Calcium Aluminate Cement
The hydration behavior of calcium aluminate cement determines its workability and mechanical strength. The hydration process of CAC is affected by various factors, such as curing temperature, humidity, water–cement ratio, nanoparticles, and admixtures.
2.2.1 Effect of Curing Temperature
Curing temperature mainly influences the mass transfer rates of particle dissolution, nucleation, and precipitation on the cement surface, and it has a significant effect on the setting time, the phase composition of hydration products, and the physical strength of CAC-bonded castables. The hydration rate of cement determines its early strength, while the later strength is controlled by the amount and microstructure of hydrates. A low curing temperature slows down the hydration rate, prolongs the hydration process, and reduces the amount of hydrates. Although the type of hydration products does not change at low curing temperatures, their quantity decreases sharply, especially during the early hydration stage, which seriously affects the early strength of the cement.
At low curing temperatures, the hydration rate of CAC is relatively slow, resulting in an increase in the number and size of unreacted CAC particles and an uneven distribution of CA and CA₂. With increasing curing temperature, the size of residual CAC particles decreases, leading to a more uniform distribution of hydration products. In addition, in-situ CA and CA₂ with a more uniform distribution are formed during heating, which improves the medium-temperature strength of castables. Studies have shown that increasing the curing temperature from 5 °C to 50 °C enhances the medium-temperature strength of CAC-bonded castables. A higher curing temperature promotes the hydration degree of CAC, resulting in a more uniform distribution of hydration products in the castable matrix and a more even distribution of CaO. This improved uniformity accelerates the formation and dispersion of CA₆, enhancing the volume stability of the castable.
2.2.2 Effect of Nanoparticles
Nanoparticles, such as nano-TiO₂, nano-Fe₂O₃, nano-CaCO₃, nano-SiO₂, and nano-Al₂O₃, show great potential in accelerating the hydration rate and improving the mechanical strength of cement due to their small particle size, high reactivity, and special functional properties. The effect of nanoparticles on the hydration mechanism of cement is shown in Figure 4.
After cement dissolves in water, nuclei of hydrates form during the induction period, and hydrates precipitate during the acceleration period. As the cement surface is gradually covered by hydrates, further dissolution is inhibited. When cement particles are covered by dense hydrates and water is consumed, the hydration process becomes diffusion-controlled and enters the steady period.
When cement particles are mixed with nanoparticles, the nanoparticles act as nucleation sites for hydration products, promoting the hydration process to enter the acceleration period sooner. As hydration progresses, cement particles are quickly covered by dense hydrates, causing the hydration process to enter the diffusion-controlled stage earlier than in nanoparticle-free systems.
Nano-SiO₂ has a high specific surface area. It exerts a filling effect when distributed around cement particles and within pores, and provides favorable nucleation sites for the precipitation of hydration products, thereby accelerating the hydration reaction of cement. Studies have shown that the addition of colloidal SiO₂ increases the pH of cement paste, promotes the formation of nucleation sites, and consequently accelerates the dissolution of cement particles and the precipitation of hydration products, resulting in a significant increase in the early-stage hydration rate of cement.
However, the formed gel covers the surfaces of unreacted cement particles and inhibits the transport of water and ions between the solution and cement particles, leading to a decrease in the hydration rate at later stages.
The effect of nano-SiO₂ on the hydration process of cement was investigated. As shown in Figure 5, after the samples were immersed in water for one day, the strength loss of CAC gradually decreased with increasing SiO₂ content. This is because nano-silica consumes Ca²⁺ in the paste and forms 2CaO·Al₂O₃·SiO₂·8H₂O (C₂ASH₈), thereby inhibiting the transformation of CAH₁₀ and C₂AH₈ into C₃AH₆.
In addition, C₂ASH₈ fills internal voids and reduces water availability, delaying the hydration of CAC and thus maintaining its long-term strength.
According to the analysis, nanoparticles not only provide additional nucleation sites for hydration products, shorten the induction period, and improve the hydration rate of CAC, but also distribute uniformly within the pores of the cement, reducing porosity and thereby enhancing the mechanical strength of the cement.
2.2.3 Effect of Mineral Admixtures
The addition of mineral admixtures may alter the hydration mechanism and mechanical properties of the CAC–mineral admixture system. Studies have shown that the addition of ground granulated blast-furnace slag (GGBS) to calcium aluminate cement (CAC) increases the heat release rate within the first 0–2 hours and the total heat released within 24 hours. This is because, with increasing GGBS content, the binary system exhibits lower density, providing more available space for hydration.
GGBS generates silicate ions during hydration and releases reactive silica into the solution, which then reacts with hydration products, as shown in Reaction (3) and Reaction (4), promoting the formation of the C₂ASH₈ phase. The transformation of C₂AH₈ into C₂ASH₈ inhibits the formation of C₃AH₆.
The densities of the C₂ASH₈ and C₂AH₈ phases are similar, at 1.937 g/cm³ and 1.950 g/cm³, respectively. Therefore, the compositional changes in the matrix produce a densified pore structure, reducing the overall porosity.
Studies have shown that the addition of treated fly ash (TFA) can reduce the porosity and pore size of CAC paste, thereby improving the mechanical strength of CAC. This is because TFA can form monocarbon aluminate and delay the early formation of hydrates such as C₃AH₆.
From the above analysis, mineral admixtures release reactive silica into the solution during the hydration of CAC, which reacts with hydrates to form the C₂ASH₈ phase and inhibit the formation of C₃AH₆, thereby accelerating the early hydration rate of cement. The fine particles of mineral admixtures also fill the pores of the cement paste, which further helps to improve its mechanical strength.
Modification of Calcium Aluminate Cement-Bonded Castables
Zn(OH)₂ was added to calcium aluminate cement-bonded corundum castables to improve their mechanical strength. The results show that with increasing Zn(OH)₂ content, the medium-temperature strength of CAC-bonded castables is significantly enhanced.
This is because at 140 °C, Zn(OH)₂ decomposes into ZnO, as shown in Reaction (5). At higher temperatures, ZnO reacts with alumina in the binder to form ZnAl₂O₄ (ZA spinel), as shown in Reaction (6). The formation of ZA spinel promotes the development of sintering necks, thereby enhancing the ceramic bonding strength of the castables.
Although the formation of ZA spinel hinders the formation and growth of CA₆, the decomposition of Zn(OH)₂ and the formation of ZA spinel generate micropores, which provide space to accommodate the volume expansion caused by the in-situ formation of CA₂, CA₆, and ZA spinel, thereby reducing the porosity of the castable. Therefore, the introduction of Zn(OH)₂ effectively improves the volume stability of the castable.
Modification of calcium aluminate cement with carbon black was also investigated. It was found that in the carbon black/calcium aluminate cement material prepared by sintering, carbon black is uniformly distributed among the calcium aluminate phases, enhancing water wettability and oxidation resistance, reducing the porosity of the castable, and improving both compressive and flexural strength.
In addition, carbon black is difficult to wet with steel slag, helping to prevent liquid slag penetration and improving the slag resistance of the castable.
Studies have shown that carbon-containing calcium aluminate cement enhances the thermal shock resistance and slag resistance of Al₂O₃-SiC-C castables. During sintering, oxidation of silicon and carbon components in the castable matrix produces SiO and CO gases, which react with carbon powder and silicon powder, respectively, to form silicon carbide whiskers, as shown in Reaction (7) and Reaction (8).
Due to the higher degree of graphitization and better dispersion of carbon nanotubes (CNTs), using CNTs as the carbon source leads to the formation of a continuous network structure of silicon carbide whiskers.
It has been found that under thermal shock, these whiskers can form bridging ligaments ahead of the crack propagation path. Whether the crack propagates along the whisker interface or fractures the whiskers, more energy is required to prevent crack extension. Therefore, the continuous silicon carbide whisker network promotes the formation of stable cracks and effectively strengthens the matrix, significantly improving the thermal shock resistance of the castable.
Furthermore, the continuous network of silicon carbide whiskers can promote the formation of a continuous spinel layer on the slag line, enhancing the corrosion resistance of the castable.
For calcium aluminate cement (CAC)-bonded castables, the use of suitable dispersants can effectively reduce water demand and optimize the workability and mechanical properties of refractory castables.
The effects of three dispersants-naphthalene sulfonate formaldehyde condensate (FDN), sodium tripolyphosphate (STPP), and propionic acid (PA)-on the morphology of cement hydration products and the mechanical properties of refractory castables were investigated.
As shown in Figure 6, the apparent viscosity decreases rapidly and then stabilizes with increasing dispersant content, indicating that all three dispersants exhibit good dispersion effectiveness, among which PA performs the best.
As shown in Figure 7, after the addition of PA, C₃AH₆ transforms into vertical cubic grains with diameters larger than 5 μm, and AH₃ changes from particulate crystals into long columnar aggregates approximately 0.5 μm in length, with their proportion increasing as the PA concentration increases.
As shown in Figure 6, with increasing dispersant content, the electrostatic repulsion between particles in the castable gradually increases. This improves the flow value of the castable, facilitates the release of gas from the interior, reduces porosity, and significantly enhances its mechanical strength. The excellent strength performance may be related to the bridging effect of long-columnar AH₃.
Beyond the optimal content, the increased ion concentration compresses the electric double layer, resulting in a decrease in the flow value of the castable.
As a dispersant, polyphosphate can effectively reduce the water demand of CAC. Under hydrothermal conditions, the acid–base reaction between CAC and polyphosphate can promote the formation of ceramic bonds. Studies have shown that the addition of (NaPO₃)₆ improves the hydration degree of CAC.
At room temperature, (NaPO₃)₆ hinders the transformation of CAH₁₀ and C₂AH₈ into C₃AH₆, whereas after heat treatment at 200 °C, it accelerates the dehydration of CAH₁₀ and C₂AH₈ to form C₃AH₆, which helps reduce the strength loss of the cement. At 200–1000 °C, the paste exhibits more CA and CA₂ phases. This is attributed to the gel-like nature of Ca₁₀(PO₄)₆(OH)₂ formed by the acid–base reaction and its ability to fill voids around hydration products, thereby helping to densify the CAC structure.
In addition, the C-A-P-H and C-P phases generated by the acid–base reaction between (NaPO₃)₆ and calcium aluminate phases contribute to strong bonding of the paste and provide high mechanical strength, ensuring the stability of CAC-bonded castables in high-temperature environments.
In summary, calcium aluminate cement forms denser stable phases during hydration, which increases porosity and decreases strength. The modification of CAC-bonded castables mainly focuses on inhibiting the hydration reaction toward high-density phases, reducing strength loss, and ensuring the long-term strength of the castable.
Furthermore, the hydration products of CAC undergo dehydration and decomposition during heating, which increases porosity and reduces the mechanical strength of CAC-bonded castables. Therefore, reducing the porosity of castables is an important strategy for modifying CAC-bonded castables.
At the same time, increasing the hydration rate can shorten the induction period and raise the heat of hydration, which helps reduce the setting time of cement and enhances the practicality and economy of CAC-bonded castables.
Conclusions and Prospects
Calcium aluminate cement-bonded castables possess many advantages, such as simple operation, short setting time, high demolding strength, high refractoriness, and excellent wear resistance. However, some challenges still exist in the practical application of calcium aluminate cement-bonded castables, including high water demand and low medium-temperature strength.
Although research on calcium aluminate cement has become increasingly in-depth, many issues remain to be solved to further improve the workability and mechanical strength of CAC-bonded castables. Future research on CAC-bonded castables should focus on the following aspects:
In-depth investigation of the hydration mechanism. Although extensive studies have been conducted on the hydration mechanism of calcium aluminate cement, the influences of micro-powders and admixtures on the hydration reaction still need further exploration. Micro-powders and admixtures can accelerate the hydration rate of CAC and reduce the porosity of cement. In addition, mineral admixtures promote the formation of the C₂ASH₈ phase and effectively control the transformation of metastable phases. Future research should emphasize the hydration reaction rate and phase evolution of hydration products, exploring how different admixtures-such as solid waste materials like fly ash and ground granulated blast-furnace slag-can enhance the hydration rate, promote the formation of high-density hydration products, and reduce the porosity of cement paste, thereby improving the workability and mechanical properties of CAC-bonded castables.
Development of suitable dispersants. To overcome the high water demand of calcium aluminate cement, appropriate dispersants should be developed. Certain polymers, such as polyphosphates, are effective in reducing the water demand of CAC and improving the mechanical properties of its bonded castables.
Use of carbon-containing calcium aluminate cement. As a carbon composite powder, carbon-containing calcium aluminate cement shows excellent applicability in castables. Uniformly mixing carbon materials into the castable matrix as a binder promotes the formation of silicon carbide whiskers, which effectively improve the thermal shock resistance and slag resistance of the material. Carbon nanotubes (CNTs) have been shown to form a continuous network of silicon carbide whiskers. Future research should explore various forms of carbon materials to further enhance the thermal and slag resistance of CAC-bonded castables.
Optimization of the drying process. The drying process significantly influences the phase composition and microstructure of castables. In addition to conventional drying methods, microwave drying and freeze drying can effectively control the transformation of metastable phases and adjust the pore structure of the material, thereby affecting the mechanical properties of CAC-bonded castables. Future research should investigate efficient and unconventional drying processes to reduce the formation of stable phases during curing, which helps reduce the porosity of castables.