# ADVANCEMENTS IN REFRACTORY CASTABLES: ENHANCEMENT OF GREEN-STATE AND HIGH-TEMPERATURE PERFORMANCES OF COLLOIDAL SPINEL BONDED CASTABLES

Sandra Abdelouhab, Cathy Delmotte, Isabella Mastroianni, Aurélien Petit, Erwan Brochen, Christian Dannert, Olaf Krause

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Sandra Abdelouhab, Cathy Delmotte, Isabella Mastroianni, Aurélien Petit, Erwan Brochen, et al. ADVANCEMENTS IN REFRACTORY CASTABLES: ENHANCEMENT OF GREEN-STATE AND HIGH-TEMPERATURE PERFORMANCES OF COLLOIDAL SPINEL BONDED CASTABLES. International Colloquium on Refractories 2024, EREF, Sep 2024, Aachen (Aix la Chapelle), Germany. hal-05119384

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# ADVANCEMENTS IN REFRACTORY CASTABLES: ENHANCEMENT OF GREEN-STATE AND HIGH-TEMPERATURE PERFORMANCES OF COLLOIDAL SPINEL BONDED CASTABLES

Sandra Abdelouhab, Cathy Delmotte, Isabella Mastroianni
BCRC-INISMa, Belgian Ceramic Research Center, Mons, Belgium

Aurélien Petit
ENSIL-ENSCI, Limoges, France

Erwan Brochen, Christian Dannert
Forschungsgemeinschaft Feuerfest e. V. at the European Centre for Refractories, Höhr-Grenzhausen, Germany

Olaf Krause
Koblenz University of Applied Sciences, WesterWaldCampus, Germany

## ABSTRACT

Historically, improving refractory castables' high-temperature performance involved reducing calcium aluminate cement (CAC) content. However, even small amounts of CaO caused issues and CAC-bonded castables require careful drying to avoid damage. Cement-free castables with sol-gel binders, like colloidal silica (CS), emerged as alternatives, but CS-bonded castables still face structural stabilities at high temperatures. Given the limitations of CS-based binders, research has shifted to alternative sols like spinel and mullite. Castables bonded with them offer better high-temperature performance but suffer from extended setting times and low green strength due to lower solid content in initial sols (5-10 wt.% vs. 30 wt.% in commercial CS).

In this context, spinel suspensions with 30 - 50 wt.% solid content were developed. Then, the impact of the solid content on setting and green mechanical properties, as well as thermomechanical properties (elastic modulus and HMOR) was analysed. The findings demonstrate that higher solid content enhances green mechanical properties of spinel-bonded castables, though demoulding time is longer than for CS-bonded castables. However, thermomechanical properties are improved notably during initial heating, surpassing those of CS-bonded castables, especially above 1000 °C, where viscous or liquid phases start to form in CS-bonded materials.

## INTRODUCTION

Colloidal silica (CS) bonded castables represent a significant advancement in the field of refractory materials, using a silica sol (a colloidal suspension of nanometric silica particles) as a binder instead of traditional calcium aluminate cement (CAC). The sol transitions into a gel, during drying, leads to a formation of a three-dimensional solid network through silica particle polymerisation. This network surrounds both coarse and fine refractory particles, holding thus, the entire structure together in the green state. The subsequent heating allows the development of the mechanical strength by formation of ceramic bonds through sintering [1].

Using such binder presents some advantages over traditional ones. Firstly, it enhances densification at lower sintering temperatures by employing nano-powders dispersed in water, thereby improving temperature performance [2]. Secondly, it simplifies the drying process as no hydrated phases are generated in the green state, unlike CAC and hydratable alumina (HA) binders. This results in highly permeable structures, reducing overall processing time and safer castable dewatering during the initial heat-up [2]. Additionally, removing the CaO component from CAC, which can negatively impact high-

temperature properties, such as the resistance to fly ash or corrosive gases [3], represents another significant benefit. Despite these advantages, the high-temperature performance of CS-bonded castables is still impaired by the presence of amorphous free silica, which forms viscous or liquid phases at elevated temperatures [1], thus limiting their refractoriness. To address this, alternative colloidal suspensions, such as mullite or spinel sols, are being investigated. Studies on this topic suggest that castables bonded with such alternative colloidal suspensions can offer improved corrosion resistance, thermal stability, and durability compared to those bonded with CAC or CS [4]. However, alternative sol-gel binders face challenges like prolonged setting times and low green mechanical properties due to their lower solid content (5-10 wt.% vs. 30 wt.% for commercial CS). Increasing the solid content in the initial suspension can potentially enhance both green properties and high-temperature performance. Recent research [5] has shown that raising the solid content in a boehmite suspension to 30 wt.% enhances green-state mechanical properties, even surpassing CS-bonded castables while maintaining strong thermomechanical performance. Among the alternative sol-gel binders, colloidal spinel suspensions appear to be the most promising for high-temperature performance.

In this context, the originality of this study lied primarily by developing stable colloidal spinel suspensions with high solid contents. The study further characterized their gelation behaviour via rheological measurements, including a comparison with a commercial CS suspension and with the MgO addition as an accelerating agent of the gelation process. Moreover, a correlation between gelation time and the setting properties of the castables, along with an exploration of green strength properties, was sought to be established. The evolution of thermomechanical properties during the initial heat-up, a topic generally not addressed in the literature or after pre-firing at moderate or high temperatures, was also evaluated versus the solid content in the initial suspension. The thermomechanical performance assessment was conducted through the determination of the elastic modulus over temperature and HMOR (High-Temperature Modulus of Rupture).

## RAW MATERIALS AND METHODS

### Spinel suspensions

The spinel powder used for elaborating the suspensions was the S30CR-grade, supplied by Baikowski. This powder is characterised by a crystalline phase consisting of at least 99% spinel (MgAl₂O₄). The particle size distribution of this product is also carefully controlled, with a D50 ranging from 0.15 to 0.30 µm.

To prepare spinel suspensions with 50 wt.% solid content, 1 kg of nanometric spinel powder was mixed with 1 L of
demineralised water and 2.5 wt.% Dolapix CE64, an electro-steric dispersant. The mixture was stirred for 24 hours with 2 mm alumina grinding media to deagglomerate the powder, followed by 1 hour of attrition milling with 300 µm alumina grinding media to further reduce aggregates. Afterward, 2.5 wt.% dispersant was added again due to observed re-agglomeration, likely from dispersant degradation or increased powder surface area. Fig. 1 illustrates the evolution of the particle size distribution (PSD) of the spinel suspensions, as analysed by laser granulometry, through the process stages. Initially, a bimodal distribution is observed, which transitions to a monomodal distribution with a shoulder after deagglomeration. This shoulder is subsequently reduced due to attrition, resulting in a pure monomodal distribution within the desired range after the final dispersant addition.

<figure><img src="image_1.png" /><figcaption>Fig. 1: PSD of a spinel suspension over the process stages</figcaption></figure>

A diluted spinel suspension with a 30 wt.% solid content was prepared from the initial suspension. The solid content was checked using an infrared balance, and zeta potential and acoustic measurements were conducted to assess stability and particle size of all suspensions (Tab. 1). Results show that all suspensions are highly stable, with zeta potentials exceeding 30 mV in absolute value. Dilution in spinel suspensions increase stability without affecting particle size, while the CS suspension, though less stable, have the finest PSD.

<table><caption>Tab. 1: Zeta potential and particle size analysis</caption><thead><tr><th>Suspension</th><th>Zêta potential (mV)</th><th>D15 (nm)</th><th>D50 (nm)</th><th>D85 (nm)</th></tr></thead><tbody><tr><td>CS (30 wt.%)</td><td>-42.2</td><td>11</td><td>16</td><td>37</td></tr><tr><td>Spinel Sol (30 wt.%)</td><td>-54.5</td><td>47</td><td>74</td><td>116</td></tr><tr><td>Spinel Sol (50 wt.%)</td><td>-45.2</td><td>52</td><td>80</td><td>123</td></tr></tbody></table>

To investigate the impact of a setting agent on accelerating the gelation process, micrometric MgO powder with a D50 of approximately 20 µm (MERCK) was used.

### Colloidal bonded high alumina refractory castables

Refractory castable samples were prepared using tabular alumina (Almatis), reactive alumina (RG400, Almatis), and colloidal suspensions: CS (Ludox, Aldrich) with 30 wt.% solid content, and lab spinel sol with 30 wt.% or 50 wt.%. The designed castable formulations (Tab. 2) followed the Andreasen model with a q coefficient of about 0.26. All castables present the same water content supplied entirely by the suspensions themselves, without any additional water. This approach aimed to evaluate how the suspension's solid content affects castable properties. CS-bonded castables were used as a reference for demoulding time, green strength, and high-temperature performance. After mixing the raw materials in an intensive mixer (type R02E, Eirich) for 3 minutes at 450 rpm, the mixture was casted into standard D moulds according to the EN ISO

1927-6 standard and then vibrated. They were dried in the moulds until they could be removed without deformation, then air-dried for an additional 24 hours, and subsequently cured at 110 °C for 24 hours. The D format samples were either used directly or machined to produce MOR (modulus of rupture), CCS (cold crushing strength) and HMOR (hot modulus of rupture) test pieces. For the determination of the dynamic E modulus over the temperature, samples measuring 15 x 25 x 130 mm were prepared following the same procedure.

<table><caption>Tab. 2: Composition of high alumina castables in wt.%</caption><thead><tr><th>Castable</th><th>CS-Ref</th><th>CSP30</th><th>CSP50</th></tr></thead><tbody><tr><td>Tabular alumina (T60/64)</td><td></td><td></td><td></td></tr><tr><td>2.0-5.0 mm</td><td>15.2</td><td>15.2</td><td>14.7</td></tr><tr><td>1.0-3.0 mm</td><td>15.2</td><td>15/2</td><td>14.7</td></tr><tr><td>0.5-1.0 mm</td><td>10.1</td><td>10.1</td><td>9.8</td></tr><tr><td>0.2-0.6 mm</td><td>10.1</td><td>10.1</td><td>9.8</td></tr><tr><td>0.0-0.5 mm</td><td>10.1</td><td>10.1</td><td>9.8</td></tr><tr><td>0.0-0.2 mm</td><td>15.2</td><td>15.2</td><td>14.7</td></tr><tr><td>0-0.045 mm</td><td>13.2</td><td>13.2</td><td>12.7</td></tr><tr><td>RG 4000</td><td>8.1</td><td>8.1</td><td>7.8</td></tr><tr><td>CS (30 wt.%) – solid / water contents</td><td>2.6 / 6.1</td><td>-</td><td>-</td></tr><tr><td>Spinel Sol (30 wt.%)</td><td>-</td><td>2.6 / 6.1</td><td>-</td></tr><tr><td>Spinel Sol (50 wt.%)</td><td>-</td><td>-</td><td>6.1 / 6.1</td></tr></tbody></table>

### Characterisation methods

#### Rheological characterisations

The oscillatory rheological measurements versus time were conducted using a Mars III rheometer from Haake. These tests were performed in stress control mode with frequencies and stresses predetermined within the linear viscoelastic regime: 1 Hz and 1 Pa for all sols. These measurements allow for differentiation between the elastic and viscous responses of suspensions, represented by the storage modulus (G') and the loss modulus (G''), respectively, enabling a more accurate determination of the gelation process [6].

#### E modulus determination over temperature using IET

The Impulsed Excitation Technique (IET) measures the flexural resonance frequency of a test piece by striking it with a small hammer and recording the resulting vibrations with a transducer. The frequency data, combined with the test piece's dimensions and mass, is used to calculate the elastic modulus (E) as a function of temperature using the Equation 1. The tests were conducted with a GrindoSonic MK7 device coupled with a high-temperature furnace, allowing for E modulus measurement up to 1500 °C.

$$E = 0,9465 \times \left( \frac{m \times f_r^2}{\omega} \right) \times \left( \frac{L^2}{t^3} \right) \times T \quad (\text{Equation 1})$$

where $f_r$ is the resonance frequency (s⁻¹), m the test piece mass (g), L the length (mm), w the width (mm), t the thickness (mm), T a correction factor (approximately 1.1).

#### Determination of MOR and CCS in the green state and HMOR over the temperature

MOR and CCS were measured after curing using three-point bending and compression tests on D format samples, following EN ISO 1927-5 and 1927-6 standards. Six test pieces per composition were tested.

After curing, D format samples were cut down to dimensions 25 x 25 x 150 mm and subsequently dried at 110 °C for 24 hours. HMOR tests was then conducted at temperatures from 250 °C to 1500 °C, every each 250 °C, according to EN 993-7 standard. Three test pieces per composition were tested at each temperature.

### RESULTS AND DISCUSSIONS

#### Gelation process characterisation over the time

##### Without MgO setting agent
The G', G'' curves of each suspension (Fig. 2) can be divided into four stages (see G'/G'' curves for the 30 wt.% spinel suspension inserted in Fig. 2).

<figure><img src="image_2.png" /><figcaption>Fig. 2: Evolution of the G', G'' moduli over time</figcaption></figure>

In the first stage, the moduli G' and G'' remain stable over time, with G' being lower than G'', indicating that the suspension is more in a liquid state than a solid state, as G' represents the solid contribution and G'' the liquid contribution to the suspension's behaviour. In the second stage, G'' slightly increases over time, and during this period, G' shows a very rapid decrease to a minimum value followed immediately by an abrupt increase, with G' still being lower than G''. Next, in the third stage, there is a simultaneous increase in both G' and G'', with G' becoming higher than G'', indicating the transition from a liquid to a solid state and the onset of gelation reactions. Finally, in a fourth stage, where both parameters slightly increase over time, with G' remaining higher than G'', indicating the ongoing cross-linking of the gel.

For spinel suspensions, the less stable the suspension, the faster the onset time of gelation reactions, and vice versa. In comparison, for the CS suspension, the onset time of gelation reactions lies between those of the 30 wt.% and 50 wt.% spinel suspensions, as shown in Tab. 3.

<table><caption>Tab. 3: Onset time of the gelation reactions</caption><thead><tr><th>Suspension</th><th>Zêta potential (mV)</th><th>Onset time of the gelation reactions (hours)</th></tr></thead><tbody><tr><td>CS (30 wt.%)</td><td>-42.2</td><td>60</td></tr><tr><td>Spinel Sol (30 wt.%)</td><td>-54.5</td><td>110</td></tr><tr><td>Spinel Sol (50 wt.%)</td><td>-45.2</td><td>35</td></tr></tbody></table>

### With MgO setting agent

After assessing the gelation process over the time for each suspension, the impact of adding MgO as a setting agent to spinel suspensions was evaluated. MgO was added at 1 wt.%, 5 wt.%, and 10 wt.% relative to the spinel powder content. The results (Fig. 3) indicate, adding MgO as a setting agent shortens the onset time of gelation reactions, with faster gelation observed as the MgO content increases. For the 30 wt.% spinel suspensions with 5 wt.% MgO and the 50 wt.% spinel suspension with either 5 wt.% or 10 wt.% MgO, a nearly instantaneous gelation occurs. In these cases, stages 1, 2, and 3 are eliminated with the G' modulus exceeding the G'' modulus at the start of the oscillatory tests, indicating that cross-linking gelation reactions are already in progress.

Based on these observations, refractory castable samples were prepared using either initial spinel suspensions alone or with the addition of 1 wt.% and 5 wt.% MgO relative to the spinel powder content in the starting 50 wt.% and 30 wt.% spinel suspensions, respectively, corresponding to 0.13 wt.% and 0.04 wt.% MgO addition in the castable compositions, respectively. This approach aimed to first address the effect of the MgO setting agent on demoulding

time, as it reduces the gelation process to under 24 hours. Secondly, the objective was to assess the impact on both green-state properties and high-temperature performance, with the latter being evaluated exclusively for the spinel-bonded castables without MgO.

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 3: Evolution of G' and G'' moduli over time for 30 wt.% (a) and 50 wt.% (b) spinel suspensions with MgO</figcaption></figure>

### Green-state and high temperature properties of Spinel-bonded castables

#### Demoulding times

To determine the demoulding time, both visual and tactile assessments were used, evaluating when the samples could be removed from their moulds without sagging and if the castable felt solid or firm.

<table><caption>Tab. 4: Demoulding time for each castable composition</caption><thead><tr><th>Castable</th><th>Demoulding time (hours)</th><th>Onset time of the gelation reactions (hours)</th></tr></thead><tbody><tr><td>CS-Ref</td><td>48</td><td>60</td></tr><tr><td>CSP30</td><td>72</td><td>110</td></tr><tr><td>CSP50</td><td>72</td><td>35</td></tr><tr><td>CSP30 + 0.13 wt.% MgO</td><td>24</td><td>20</td></tr><tr><td>CSP50 + 0.04 wt.% MgO</td><td>24</td><td>4</td></tr></tbody></table>

The results (Tab. 4) reveal that there is not necessarily a direct correlation between the gelation onset times of the suspensions and the demoulding times of the castables bonded with them. For instance, the demoulding time is the same for the castable samples bonded with the 50 wt.% spinel as those bonded with the 30 wt.% spinel suspension, despite the gelation onset times of the initial suspensions being 35 and 110 hours, respectively. In contrast, the CS-bonded castables demonstrate faster demoulding time compared to those bonded with spinel solids, even though the CS suspension has a longer gelation onset time compared to the 50 wt.% spinel suspension.

Regarding the effect of MgO addition, it significantly accelerates the setting time of the castable. Samples bonded with either the 30 wt.% spinel suspension and 5 wt.% MgO or the 50 wt.% spinel suspension and 1 wt.% MgO both achieve a demoulding time of 24 hours. This observation aligns with the accelerated gelation process observed for these suspensions with the MgO addition.
### Green-state properties

Regarding the comparison of MOR and CCS values between all castable compositions (Fig. 4), an increase in the solid content in the spinel suspension leads to an increase in the green strength of the castables bonded with it. However, their green mechanical performance does not achieve the same level as the CS-bonded castable, for which, the suspension's solid content is, as a reminder, 30 wt.%. The addition of MgO as a setting agent, while reducing the demoulding time, negatively affects the green-state mechanical performance of the refractory castables bonded with the spinel suspensions. Nevertheless, the same trend is observed as without the use of MgO, namely an increase in mechanical performance with the increase in the solid content of the spinel binder.

<figure><img src="image_5.png" /><figcaption>Fig. 4: Green mechanical resistances of castables</figcaption></figure>

### E modulus and HMOR evolution over the temperature

Fig. 5a indicates for the CS-bonded castable, the E modulus remains stable up to 900 °C, where the sintering begins. Then, as temperature nears 1200 °C, it decreases likely due to viscous phase formation from the amorphous silica. During cooling, solidification of this phase sharply increases the E modulus between 1500 °C and 1200 °C. Below 1200 °C, the E modulus stabilises, ending higher than its initial value.

<figure><img src="image_6.png" /><figcaption>Fig. 5: E modulus (a) and HMOR (b) versus temperature</figcaption></figure>

Spinel-bonded castables have a higher initial E modulus than CS-bonded ones, with a slight decrease near 300 °C due to hydroxide decomposition. They show then a continuous E modulus increase during heating, with a more pronounced rise from 900 °C and 1100 °C for CSP30 and CSP50, respectively, due to matrix sintering. During cooling, the E modulus remains significantly higher than that of CS-bonded castables, highlighting the advantages of colloidal spinel binders at high temperatures. However, CSP50's E modulus at 1500 °C and room temperature after cooling does not exceed that of CSP30, indicating that while increased solid content enhances green properties, it does not necessarily improve high temperature strength. The HMOR results (Fig. 5b) closely align with those of E modulus. For the CS-bonded castable, HMOR is higher than those of spinel-bonded castables, showing a temperature-dependent increase up to 1000 °C, then a significant decrease from 1000 °C to 1500 °C, with the HMOR values becoming lower than those of spinel-bonded castables. In contrast, above 1250 °C, HMOR for CSP30 and CSP50 increases substantially and remains stable at 1500 °C. Nonetheless, similar to the E modulus findings, increasing the solid content in the initial binder suspension does not necessarily enhance HMOR.

## CONCLUSION AND OUTLOOKS

The study highlights the advantages of spinel-bonded castables over colloidal silica (CS) bonded castables at high temperature. Increasing the solid content in spinel suspensions improves the green-state mechanical properties but not necessarily the final high-temperature strength. The MgO addition as setting agent accelerates both gelation and demoulding times but negatively affects green-state properties.

To enable the adoption of spinel-bonded refractory castables in industries, it is essential to improve their green-state performance by developing spinel sols with higher solid contents. Additionally, investigating other setting agents, such as CaO or hydratable alumina, could optimise the setting and demoulding processes, along with comparing the drying rates of these formulations with CS-based castables, which is crucial for industrial uses.

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[4] A. K. Singh, R. Sarkar: "High alumina castables: a comparison among various sol-gel bonding systems", J. Aust. Ceram. Soc., 53, 2017, 553-567

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