DEVELOPMENT OF A BOEHMITE COLLOIDAL
SUSPENSION AS NEW BINDER FOR
REFRACTORY CASTABLES

Sandra Abdelouhab, Cathy Delmotte, Charlotte Lang, Véronique Vandeneede, Michael Thiesen, Luis Ibarra, Olaf Krause, Erwan Brochen, Christian Dannert

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Sandra Abdelouhab, Cathy Delmotte, Charlotte Lang, Véronique Vandeneede, Michael Thiesen, et al. DEVELOPMENT OF A BOEHMITE COLLOIDAL SUSPENSION AS NEW BINDER FOR REFRACTORY CASTABLES. International Colloquium on Refractories 2022, ECREF, Sep 2022, Aachen (Aix la Chapelle), Germany. hal-05119326

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# DEVELOPMENT OF A BOEHMITE COLLOIDAL SUSPENSION AS NEW BINDER FOR REFRACTORY CASTABLES

Sandra Abdelouhab, Cathy Delmotte, Charlotte Lang and Véronique Vandeneede
BCRC-INISMa, Belgian Ceramic Research Centre, Mons, Belgium

Michael Thiesen, Luis Ibarra and Olaf Krause
Koblenz University of Applied Sciences, Höhr-Grenzhausen, Germany

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

## ABSTRACT

Conventional monolithic refractories bonded with CAC (calcium aluminate cement) have a limitation when they are used at high temperature in aggressive environments. The calcium in the CAC binder reacts with corrosive components (dairy, fly ash, gas) to form liquid/viscous compounds in the refractories which prematurely deteriorate the material.

Refractory monolithics bonded with HA (hydratable alumina) perform better in these environments. However, the drying step of such materials needs considerable time and therefore energy to avoid cracking/explosion of the lining during the evacuation of the mixing water during drying. To compensate the disadvantages of CAC and HA bonded monolithics, CS (colloidal silica) bonded refractory castables have been developed, with the benefits of rapid drying and chemical inertness at high temperature. However, the presence of amorphous silica in the composition of these refractory concretes leads to the appearance of liquid/viscous phases at high temperature and therefore induces a reduction in their thermomechanical resistance.

A new colloidal bonding system which could improve the thermomechanical and thermochemical performances of monolithic refractories bonded with colloidal suspensions was thus developed. In this context, the colloidal binder considered was a boehmite colloidal suspension.

## INTRODUCTION

To improve the properties of refractory castables at high temperature, the decrease of calcium aluminate cement (CAC) content in the composition was done by moving from castables with medium or regular cement content (MCC or RCC) towards low cement content (LCC) or even ultra-low cement content (ULCC). However, the presence of CaO remaining in the mix is detrimental to the high temperature properties especially in corrosive environment [1, 2]. In addition, CAC bonded castables must be dried carefully up to 600 °C to avoid premature cracks or even an explosion of a part of the lining or preshaped products [3, 4].

Therefore, to overcome these issues, cement-free castables were developed by using alternative and innovative binders such as hydratable alumina (HA), colloidal silica (CS) and colloidal alumina (CA). The green mechanical strength of castables bonded with HA is provided thanks to the interlocking of the bayerite crystals and the pseudo-boehmite gel formed during HA hydration [5]. It fills pores and interfacial defects but also forms honeycomb structure at the aggregates surface allowing the joining of the adjacent grains to each other and to the surrounding matrix. The use of this binder also allows the obtention of castables with higher refractoriness compared to CAC ones [1, 2]. HA bonded castables also have a volume stability at high temperature due to the absence of in-situ generated CA₂ and CA₆ phases [6]. However, because HA is amorphous and has high specific surface area, large pore volume content, HA bonded castables have a longer mixing time and a high water demand, but also short working times due to great chemical reactivity of HA [7, 8]. In addition, after hardening, these castables are less permeable than CAC bonded ones, resulting in lower explosion resistance during the dewatering process than CAC bonded castables [9, 10].

The use of colloidal suspensions as binders for refractory castables was then considered. The most used and commercially available are the colloidal silica (CS) and alumina suspensions (CA). A lot of

works has been done with these binders and can be found in the literature [11]. These colloidal binders allow a quick dry-out stages and maintaining or even increasing the mechanical strength at a temperature range between 600 °C-1000 °C where this one decreases for CAC or HA bonded castables. Furthermore, CS or CA bonded castables display high thermomechanical properties (wear resistance, HMOR and thermal shock resistance) for a large temperature range. However, for the CS bonded castables, the presence of silica can still be a drawback for some steel applications [12]. It will also be an issue for the new steel process using direct reduction iron ore by hydrogen.

In this context, the CA as binder is considered with more reliable. However, most of works found in the literature, led by Prof. Pandolfelli and co-workers [11], are dedicated to castable formulations with a use of commercially available suspensions with high solid contents. Other studies also report the use of “home-made” alumina suspensions obtained by sol-gel routes but with a low solid content [13]. Also, most of the time, these CA binders contain well crystallised alumina hydrates (gibbsite or bayerite), and these works deal mainly with the mechanical performances at different temperatures and the corrosion or wear resistances at high temperature. Few of them deals with the use of other alumina hydrates such as boehmite as colloidal binder source which could potentially gather the advantages of HA and CA binders: castables with better mechanical strength after drying at 110 °C for those bonded with HA compared to those bonded with CS or CA [14], quick drying for castable bonded with CS or CA binders [11], a maintaining or even an increase of mechanical strength at firing temperature ranging from 600 °C-1000 °C instead of a decrease for HA bonded due to higher sinterability of colloidal binders as the temperature increases. Moreover, works dedicated to the consolidation (setting) reactions of CA slurries deal mainly with this CA consolidation through coagulation mechanisms and few deal with consolidation through a gelling mechanism [15].

Therefore, the goal of the present study was firstly to produce a pseudo-boehmite colloidal suspension, then to study the consolidation of this suspension through a gelling mechanism by adding different MgO contents as setting agent. A comparison was made with a commercially available alumina suspension.

## MATERIALS AND METHODS

### Materials

The raw material used for making a boehmite sol is a very high pure pseudo-boehmite powder (PS-B), supplied by DEQUACHIM SA. The typical properties of this powder are given in Tab. 1.

<table><caption>Tab. 1: Typical characteristics of PS-B powder</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Chemical<br>composition (XRF)<br>analysis</th><th colspan="3">Particle size (µm) reached at a<br>cumulative percentage</th></tr><tr><th>10%(D10)</th><th>50%(D50)</th><th>90%(D90)</th></tr></thead><tbody><tr><td>PS-B</td><td>99.9%</td><td>14</td><td>55</td><td>106</td></tr></tbody></table>

The commercial colloidal alumina sol used in this study is supplied by Evonik Degussa GmbH/DE: AERODISP W 640 ZX (CA40wt%). The typical properties of this colloidal alumina suspension, containing fumed aluminium oxide, are shown in Tab. 2.

<table><caption>Tab. 2: Data sheet information of CA40wt% sol</caption><thead><tr><th>Sample</th><th>Al₂O₃ (%)</th><th>pH</th><th>Particle stabilisation</th><th>Particle size</th></tr></thead><tbody><tr><td>CA40wt%</td><td>40</td><td>6-9</td><td>Negative surface charge</td><td>80 nm</td></tr></tbody></table>
The MgO used to study the setting time of the PS-B and CA40wt% sols are either micro (45 µm) or nanometric (40 nm-60 nm) and supplied by respectively Merck and Fisher.

## Characterization methods

### Rheological tests

The viscosity and oscillatory measurements as a function of time or temperature were performed on a rheometer device Mars III from Haake. They were done at stress control mode with frequencies and stresses preliminary determined within the linear viscoelastic regime: 1 Hz and 1 Pa for PS-B and CA40wt% sols.

### Zeta potential measurements

The stability of the different sols (commercially and home-made) was analysed by a zetameter device (AcoustoSizer, Colloidal Dynamics, USA). It uses an electroacoustic technique by applying an alternating electric field to force the particles in the colloid to move backward and forward in a high frequency oscillation. This particle movement results in sound waves of the same frequency as the electric field, which are picked up by a transducer. By measuring the amplitude of these sound waves (Electrokinetic Sonic Amplitude, ESA) over a range of frequencies, it makes possible to calculate the particle mobility and, hence, the zeta potential. Measurements were made over a range of frequencies from 1 to 20 MHz with the AcoustoSizer device.

### Particle size analysis

The AcoustoSizer device is complemented with Ultrasonic Attenuation Measurements. In this technique, the amount by which a sound wave is attenuated as it crosses the colloid is determined. The particle size distribution is then obtained from the measurement of this quantity as a function of the frequency.

The particle size distribution of the different sols was also analysed by laser diffraction analysis (Mastersizer 2000 device, Malvern Panalytical, UK, equipped with the Hydro units allowing analysis with wet dispersion). The analysis consists in measuring the angular variation of intensity of light scattered as a laser beam passes through a dispersed particulate sample. Large particles scatter light at small angles relative to the laser beam and small particles scatter light at large angles. The angular scattering intensity data is then analysed to calculate the size of the particles responsible for creating the scattering pattern, using the Mie theory of light scattering. The particle size is reported as a volume equivalent sphere diameter. The particle size range able to be analysed is between 20 nm and 2 mm. For particle lower than 100 µm, the analysis is made by wet route.

## RESULTS AND DISCUSSION

### Preparation of the pseudo-boehmite sol (PS-B sol)

To decrease the particle size of the starting powder, the protocol established was firstly to perform a wet milling under vibrating energy with a low amplitude (Sweco mill, M18). 500 g of PS-B powder was mixed with 1000 mL of distilled water. Alumina milling medias (50 vol%, 2 mm) were then added to the suspension and the milling was performed between 36 h and 40 h.

After this first step, the suspension was filtered to withdraw the milling medias and the pH was adjusted to 3 with concentrated nitric acid. Indeed, the boehmite powder in solution is stable at pH close to 3 [16]. The obtained suspension was then homogenised by simple agitation during 24 h. The solid content of the PS-B suspension was determined with an infrared moisture analyser and was close to 30 wt%. The particle size distribution obtained is presented in Tab. 3. The determination of particle size distribution by laser diffraction analysis under wet route for PS-B after vibrating milling was done with water adjusted at pH = 3 to avoid a particle agglomeration.

<table><caption>Tab. 3: Particle sizes for a cumulative percentage reaching 10%, 50% and 90% for PS-B powder before and after vibrating milling</caption><thead><tr><th rowspan="2">Sample</th><th colspan="3">Particle size (µm)</th></tr><tr><th>D10</th><th>D50</th><th>D90</th></tr></thead><tbody><tr><td>PS-B before vibrating milling</td><td>14</td><td>55</td><td>106</td></tr><tr><td>PS-B after vibrating milling</td><td>1.4</td><td>3.8</td><td>7.8</td></tr></tbody></table>

The results indicate a decrease in the PS-B particle size after a vibrating milling treatment. The corresponding particle size, reached at a cumulative percentage 90%, passed from 106 µm to 7.8 µm. However, particles size was not at nanometric scale. Therefore, it needed a second step to reduce again the particle sizes. It was done by attrition milling. In this case, the slurry is pumped continuously through the grinding volume containing alumina milling medias (500 µm) retained in the grinding chamber by a sieve. The medias are agitated by blades or arms. This agitation induces the apparition of compression and shear stresses due to direct collisions between particles, and stresses friction due the collision between particles and milling medias. High collision frequencies lead to fast grinding kinetics.

During the attrition milling, the particle size was checked every 15 min - 30 min to determine the end of the milling, i.e., when the particles size distribution (PSD) is stable over time. A milling of 3 h was necessary. The pH must also be adjusted to 3 during the milling and after to stabilise the suspension. Indeed, as previously mentioned, the boehmite powder is stable in suspension at pH = 3. The dispersion effect is achieved by two factors. The first is the adsorption of H⁺ protons from the acid on the surface. In addition, the peptization treatment with acid leads to a partial dissolution of the boehmite particles. As a result, large amounts of polymeric Al₁³ cations are adsorbed on the surface of the particles offering a higher repulsive force [16]. However, during the milling, the specific surface of particle increases due to the size decrease. Therefore, more protons are needed to cover entirely the surface of each particle and thus the adding of acid during and after the milling. The solid content was again checked after the second milling treatment and this one was kept close to 30 wt%.

### Characterisation of the PS-B and commercial CA40wt% sols

#### Characterisation and comparison of particle size distribution of PS-B and CA40wt% sols

The PSD for both sols was determined by laser diffraction (Fig. 1 and Tab. 4). For comparison, the particle size distribution of PS-B sol before attrition milling is also reported.

<figure><img src="image_1.png" /><figcaption>Fig. 1: Particle size distribution of PS-B and CA40wt% sols</figcaption></figure>

<table><caption>Tab. 4: Particle sizes for a cumulative percentage reaching 10%, 50% and 90% for PS-B powder before and after attrition milling</caption><thead><tr><th rowspan="2">Sample</th><th colspan="3">Particle sizes (µm)</th></tr><tr><th>D10</th><th>D50</th><th>D90</th></tr></thead><tbody><tr><td>PS-B before attrition milling</td><td>1.40</td><td>3.80</td><td>7.80</td></tr><tr><td>PS-B after attrition milling</td><td>0.14</td><td>0.20</td><td>0.34</td></tr><tr><td>CA40wt%</td><td>0.13</td><td>0.17</td><td>0.25</td></tr></tbody></table>

The attrition milling treatment allowed the reduction of the PS-B particles size at the nanometric scale. The corresponding particles size for a cumulative percentage reached at 90% passed from 7.8 µm after vibrating milling to 0.34 µm after attrition one. The PSD curve is very similar to those obtained for the CA40wt% sol.

#### Zeta potential and attenuation measurements of PS-B and CA40wt% The sol stability was determined by measuring the zeta potential of suspensions and simultaneously attenuation measurements were performed. Hereafter, in Tab. 5, are presented the results obtained for the PS-B and CA40wt% sols.
More the MgO content added to the sol, more the setting reactions were sped up for MgO powder having micrometric particle sizes. For MgO powder having nanometric particle size this behaviour was not so obvious. Moreover, the use of micrometric MgO seemed to react stronger with the CA40wt% than the nanometric MgO. Indeed, for the same MgO content added to the sol, the setting reaction took place faster with micrometric MgO than with nanometric MgO. It is surprising because usually the smaller the particle sizes, the more reactive they are. It is not well understood, but one reason could be an agglomeration of the nanometric MgO particles in the sol leading to a decrease of the setting agent reactivity.

<figure><img src="image_2.png" /><figcaption>Fig. 4: Viscosity as a function of time for different MgO contents and particle sizes (a), comparison without MgO (b)</figcaption></figure>

Setting reaction study of PS-B sol

As for CA40wt% sol, hereafter is presented the viscosity profiles as a function of time for different MgO contents added in PS-B sol.

<figure><img src="image_3.png" /><figcaption>Fig. 5: Viscosity profiles as a function of time for different nanometric MgO contents</figcaption></figure>

Here, the higher the nanometric MgO content added to the sol, the faster the setting reactions took place. The viscosity profile for the tests performed with 1 wt% micrometric MgO particles show a consolidation mechanism still faster than those with 1 wt% nanometric MgO: within 0h-5 h for 1 wt% µMgO in place of 0h-10h for 1 wt% nanoMgO.

The tables hereafter summarise the different time ranges at which the setting reactions occurred for the different MgO contents and MgO particle sizes used for each sol.

<table><caption>Tab. 6: Time range of setting reactions for CA40wt% sol</caption><thead><tr><th rowspan="2"></th><th>Without MgO</th><th>Setting time range</th></tr><tr><th>0</th><th>60 h-65 h</th></tr></thead><tbody><tr><th rowspan="5">CA40wt% sol</th><th>µMgO content (wt%)</th><th>Setting time range</th></tr><tr><td>1.3</td><td>12 h-14 h</td></tr><tr><td>2.6</td><td>9 h-11 h</td></tr><tr><td>5.2</td><td>8h30-10 h</td></tr><tr><td>20</td><td>1 h-5 h</td></tr><tr><th rowspan="2">nanoMgO content (w%)</th><th>Setting time range</th></tr><tr><td>1.3</td><td>15 h-16 h</td></tr><tr><td>2.6</td><td>15h30-16 h</td></tr></tbody></table>

<table><caption>Tab. 7: Time range of setting reactions for PS-B sol</caption><thead><tr><th rowspan="5">PB-S sol</th><th>Without MgO</th><th>Setting time range</th></tr><tr><th>0</th><th>23h-33h</th></tr><tr><th>µMgO content (wt%)</th><th>Setting time range</th></tr><tr><td>1</td><td>0h-5h</td></tr><tr><td>5</td><td>Within 1min</td></tr><tr><th rowspan="4">nanoMgO content (w%)</th><th>Setting time range</th></tr><tr><td>0.25</td><td>12h-22h</td></tr><tr><td>0.5</td><td>2h-10h</td></tr><tr><td>1</td><td>0h-10h</td></tr><tr><td>5</td><td>Within 1min</td></tr></thead></table>

These results confirm the fact that the setting reactions were delayed by adding nanometric MgO powder in both sol in comparison with the adding of micrometric MgO powder. When the reaction setting time is compared between both sols, it is obvious that the PS-B sol is more reactive with or without MgO setting agent than CA40wt% whereas it contains less solid content in the suspension. Indeed, without MgO, the setting reactions took place within around 30 hours for PS-B sol instead of 60 hours for CA40wt% sol. The high reactivity of the PS-B sol compared to CA40wt% one with MgO setting agents was probably due to the pseudo-boehmite nanoparticles which are smaller than those of CA40wt% according to the attenuation measurements and SEM pictures, inducing a higher reactivity.

The last results indicate the high potential of the use of pseudo-boehmite sol as binder in refractory castables. Indeed, it can probably induce higher green strengths compared to commercial alumina sol and silica sol due to a higher reactivity with setting agent but also better performances at high temperature due to a higher sinterability and the absence of SiO₂ helping to retard the building of melting phases in the refractory matrix. However, this high reactivity towards setting agents can be damaging during the mixing of monolithic products bonded with it. Indeed, some authors have reported for refractory castables bonded with CS that silica particle from CS suspension can be immediately gelled around MgO particles and thus hindering a suitable flowability and workability [17, 18].

Conclusion

The present study has revealed firstly that it is possible to produce, at lab scale by milling a starting powder, a suitable colloidal suspension (with a high solid content) which could then be used as binder for refractory castables. The pseudo-boehmite sol thus produced with 30 wt% in solid content presents similar characteristics to a commercial colloidal alumina suspension containing 40 wt%: approximately the same particle size and the same rheological behaviour versus temperature.

However, the study of the setting mechanism trough a gelling process by adding different MgO contents with different particle sizes in the sols, has indicated the pseudo-boehmite sol is more reactive with MgO particles than the commercial alumina sol. It is probably due to the pseudo-boehmite particle sizes slightly finer than the alumina one inducing a higher reactivity. It can potentially help to achieve higher green mechanical strength and better performance at high temperature. Nevertheless, caution must be undertaken to ensure suitable flowability and workability properties of castables bonded with this pseudo-boehmite sol. The effect of the use of this colloidal binder source in a refractory castable composition will be investigated in upcoming future works in terms of green and high temperature mechanical strengths.
Acknowledgement

The BCRC-INISMa, the HSK and FGF acknowledge Walloon
Region (SPW-EER) and the German Federal Ministry for Economic
Affairs and Climate Action for their financial support of this
CORNET research project "COLCAST".

Natalie Derveau and Maigane Bouillon are also gratefully
acknowledged for their lab work contribution for this study.

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