DEVELOPMENT OF A PRINTABLE
ALUMINA-BASED COMPOSITION OF
REFRACTORY CASTABLE FOR 3D PRINTING
PRESHAPED PARTS

Sandra Abdelouhab

► To cite this version:

Sandra Abdelouhab. DEVELOPMENT OF A PRINTABLE ALUMINA-BASED COMPOSITION OF REFRACTORY CASTABLE FOR 3D PRINTING PRESHAPED PARTS. UNITECR'23, Sep 2023, Francfort, Germany. 2023. [hal-05119347](https://hal.science/hal-05119347v1)

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# DEVELOPMENT OF A PRINTABLE ALUMINA-BASED COMPOSITION OF REFRACTORY CASTABLE FOR 3D PRINTING PRESHAPED PARTS

Sandra Abdelouhab

BCRC-INISMa, Belgian Ceramic Research Center, Mons, Belgium

## ABSTRACT

Thanks to its high versatility, additive manufacturing is used in many fields such as powder metallurgy, polymers, ceramics and recently in the field of civil engineering.

In the field of refractories, this shaping approach, based mainly on material extrusion, has not yet been adopted whereas it has a significant potential. Indeed, it offers the possibility of producing parts with complex geometries which are difficult to achieve by conventional techniques while avoiding the use of formworks/moulds whose costs from a few hundred to several thousand euros. In addition, a reduction in production costs, environmental and societal impacts can be considered via the reduction of labour and waste, controlled consumption of raw materials via the topological optimisation of parts as well as the reduction of risks on site.

To allow the emergence of this approach in this field, key scientific barriers have to be overcome, especially in the formulation, implementation, and characterization of these materials. This is the purpose of this current study by developing a printable formulation of an alumina based refractory castable. This tailored formulation must have pumpability, extrudability and buildability properties, mandatory for the printability characteristics of such materials. The parts produced by material extrusion will be characterised in terms of thermomechanical performance after printing and firing at high temperature and compared to similar parts obtained by conventional techniques.

## INTRODUCTION

Since the invention of the first 3D printer in 1983, the development of this additive manufacturing technology has been very fast and is currently widely used in several fields, including biomedical applications, jewellery, automotive or aerospace [1].

More recently, the material extrusion technology has been implemented in the building sector as now it is possible to print an entire building with this technique. In the field of refractory materials, this 3D printing technique has not yet been widespread. However, with making a parallel with civil engineering concretes, the use of this technology applied to refractory precast products suggests the same benefits as those in the field of building sector, namely [2]:

- Economic, with a lower time and cost production: usually castable shaping is done with formworks. For refractory pre-shaped parts, formworks can cost from a few hundred to several thousand euros when they are not standardised;

- Environmental, with the reduction of raw material consumption: the 3D-printing process allows a better control of the required material amount for the part construction. This contributes to the waste reduction in addition with the formwork-free construction and less dust generation compared to traditional methods;

- Social, by the reduction of arduous human labour and risk on site through the process automation.

Moreover, this 3D printing technology allows the generation of complex shape designs at a better cost than current casting methods. To print a pre-shaped part, a digital model is firstly created either with a CAD like modelling software or by a 3D scanning of the object. Next, this digital model is sliced by a dedicated software. Finally, a digitally controlled device builds the object slice-by-slice. For the 3D concrete printing (3DCP), the most common used technique is the layered material extrusion [3] even though selective powder binding method have also been reported [4].

For the extrusion method, typically a well-designed cementitious mixture is pumped through a nozzle mounted either on cartesian system or a robotic arm. Based on the 3D digital model, the printing

head moves in order to extrude various filament shapes depending on nozzle shape with a printing speed of 50-500 mm/s. To produce refractory pre-shaped parts with the use of 3D extrusion printer, the refractory mix has to meet specific rheological requirements. Indeed, as for printable civil engineering formulations, printability characteristics are prerequisites:

- Pumpability, which is the ability of the cementitious ink at “fresh” state to be conveyed from the wet material tank to the extrusion nozzle via a pipe under the pump action;

- Extrudability, which means the ability of the cementitious ink to be extruded easily under the form of a continuous filament without clogging or segregation during this phase;

- And finally, buildability, which represents the ability of the printed cementitious ink to support the weight of the upper layers without excessive creep of the lower layers or printed element.

When the formulation combines these 3 properties, the cementitious ink is considered printable. However, these rheological characteristics remain antagonistic. While stiffness and cohesiveness are requested for buildability to support the weight of subsequent layers and limit the deformation of the printed layers, a high flowability is requested for pumpability and extrudability [5]. This highlights the whole issue of the composition and formulation of such printable materials, for which an optimum has to be found between flowability and stiffness/cohesion in order to simultaneously satisfy the properties of pumpability, extrudability and buildability.

In this context, the goal of this study was to develop a refractory cementitious ink formulation gathering the printable characteristics. To reach out this goal, this formulation development has been considered through mainly:

- the characterisation of 3D commercial cementitious inks printable with a large-scale 3D printing device without stiffness additives at the printing nozzle;

- a literature review on refractory gunning mixes.

## MATERIALS AND METHODS

### Materials

*Commercial cementitious inks:* For the development of refractory cementitious ink, two commercial civil engineering cementitious inks have been characterised: 704 CLAVEX + HP mortar (704 CL+HP) and SIKACRETE – 751 3D (751 SI-3D). These commercial inks are printable with a cartesian large-scale 3D printing system (Mini Printer Pro supplied by CONSTRUCTION 3D society). The buildability of 704 CL+HP material is limited as it can quickly present an excessive part deformation or a structural collapse. The second one, 751 SI-3D presents a higher buildability. The table 1 reports the main characteristics of both commercial cementitious inks indicated in the product data sheet.

<table><caption>Tab. 1: Main characteristics of commercial cementitious inks.</caption><thead><tr><th>Sample reference</th><th>704 CL+HP</th><th>751 SI-3D</th></tr></thead><tbody><tr><td>Maximum grain size</td><td>0 - 2 mm</td><td>0 - 1 mm</td></tr><tr><td>Density (g/cm³)</td><td>2.2</td><td>2.14</td></tr><tr><td>Water content for a plastic consistency</td><td>11%</td><td>18%</td></tr><tr><td>Setting times at 20 °C</td><td>6 h – 7 h</td><td>45 min – 60 min</td></tr><tr><td>Practical use life after extrusion (start of the fresh material stiffness)</td><td>60 min</td><td>15 min</td></tr><tr><td>CCS (MPa) at 28 days</td><td>60</td><td>50</td></tr><tr><td>MOR (MPa) at 28 days</td><td>12</td><td>10</td></tr></tbody></table>
Material selection for the refractory cementitious ink (3D-RCI): The following raw materials have been used for the formulation of the refractory cementitious ink: tabular alumina (3 – 1 mm, 1 – 0.5 mm, 0.5 – 0 mm, 0.6 – 0.2 mm, 0.2 – 0 mm - ≤ 0.045 mm; Almatis), reactive alumina (RG4000, Almatis) and calcium aluminate cement (Secar 71, Imerys).

As the 3D printing equipment considered to further print the 3D-RCI material, does not allow the addition of a stiffness agent directly after the extrusion at the printing nozzle, the dry refractory mix has to contain a stiffness agent to ensure an early age stiffness development. It will ensure the refractory ink buildability. However, the stiffness development has to be slow enough to avoid pipeline clogging and short enough to avoid a structural deformation or collapsing. The stiffness agent used in this study was a BASF additive.

A water reducing agent was also used to optimise the start of the stiffness development and the water content in the starting mixture. Indeed, due to its plasticizer effect, it allows to decrease the water content required for a good flowability and in the meantime, it influences the early age of the stiffening development as it has retarding properties. A BASF plasticizer was used in this study as a water reducing agent.

Experimental procedure for the fresh material preparation: The commercial cementitious inks were prepared according to the recommendations of the technical data sheets.

For the developed refractory cementitious ink, a dry mixing of the raw material was performed for 2 minutes then the required water content was added to mixture and the mixing was carried out for a further 3 minutes. An Eirich mixer, R02/E type, with a star rotor was used with a rpm range between 450 for the dry mixing and 750 for the wet mixing.

## Characterisation methods

Bulk Density (BD) and Apparent Porosity (AP): The bulk density and the apparent porosity of the castable samples were measured according to Archimedes' principle in relation with the standard EN ISO 1927-6.

Flow-cone test: The pumpable consistency was characterised by means of a flow-cone test. The fresh wet mix was placed in a cone shape mould with standard dimensions (Hagerman cone). After the flow of the material under its own weight, the largest diameter (D1) and the diameter perpendicular to it (D2) were measured. The flow value (F) is given by the following relationship:

$$F (\%) = \frac{\left(\frac{D_1 + D_2}{2}\right) - D_{inf}}{D_{inf}} \times 100 \qquad \text{Equation 1}$$

Where Dinf is the lower diameter of the cone, which is 100 mm. The upper diameter is 70 mm and height 60 mm.

Gun test: As there are no actual standards for extrudability testing, a manual gun device with a 20 mm circular nozzle diameter was used to assess the extrudability of cementitious inks at laboratory scale. The material was defined as extrudable when it squeezed out of the printing nozzle in a continuous manner while maintaining the required dimensional accuracy and layer quality without causing any clogging or segregation. The manual gun used in this study is shown on figure 1.

<figure><img src="image_1.png" /><figcaption>Fig. 1: Picture of the device used for manual gun tests</figcaption></figure>

Ultrasonic wave transmission measurements: The material buildability can be assessed through the characterisation of the

setting properties determined by ultrasonic wave velocity evolution. Indeed, the increase of the ultrasonic velocity indicates the increase of the stiffness or the mechanical resistance of the material [6]. In this study, the setting properties of all cementitious inks were characterised by using ultrasonic wave transmission measurements performed with a FreshCon system developed at the University of Stuttgart [7]. The FreshCon system consists in a specific mould (for castable or mortar) equipped with two piezoelectric transducers (one transmitter and one receiver). A computer equipped with a data acquisition card is linked to a piezoelectric high voltage pulser to generate a high voltage short rectangular wave (typically 800 V with a pulse width between 2.5 and 5 µs). A wide band ultrasonic (mechanical) wave is generated and picked up by the receiver of which the signal is amplified after traveling through the material. The software automatically computes the travelling time from the transmitter to the receiver and deduces the velocity of the wave. More detailed information on this device can be found in [7].

Mechanical test: Format D samples, according the standard EN ISO 1927-6, have been casted with the 3D-RCI composition, cured during 48 h and dried 24 h at 110°C. Cold modulus of rupture (CMOR) and cold crushing strength (CCS) tests have then been performed with samples thus prepared. For each mechanical trial, three samples have been tested.

## RESULTS

### Formulation of the 3D printing refractory mix

For the formulation development of a refractory cementitious ink, the first targeted, among the three characteristics needed to obtain a printable mix, has been the pumpability. For that, a literature review on shotcreting refractory castables has been carried out, focusing especially on the relationship between the castable ability to be pumped and their particle size distribution. This review revealed that to enhance the pumping ability of the shotcreting castable, the ratio between the coarse grains and fine particles has to be optimised. If the mix presents an excess of aggregates, collisions between particles and friction forces will be enhanced, resulting in an increase of the pumping pressure. The mix will also have low cohesion or a greater tendency to segregate during the transport in the pipeline. At the contrary, if the mix presents an excess of fine particles, the mixture will become less fluid during pumping, which will lead to the clogging of the pumping pipe [8].

To determine the optimum ratio between fine and coarse grains to obtain good pumping properties, authors [9] first formulated three compositions of refractory castables according to the Andreasen's model [10] with q coefficients 0.21, 0.26 and 0.31. The rheological behaviour of these three refractory castables was characterised by measuring the torque as a function of the rotation speed of their mixing tool. Tests have been carried out with and without volume restriction, the latter simulating the flow in a pipe during pumping. The results indicated that the rheological behaviour of castable compositions with q values 0.21 and 0.31 was dilatant for tests without volume restriction then became Newtonian for tests with volume restriction. For the composition, with a q value 0.26, exhibited a pseudo-plastic or a rheo-fluidifying rheological behaviour for tests with or without volume restriction [9].

This pseudo-plastic of rheo-fluidifying behaviour is required when a fluid is subjected to an increase in a shear stress, as it is the case during pumping. Indeed, the resistance of the fluid flow tends to stabilise or even decrease with increasing stress for a rheo-fluidifying rheological behaviour. The pseudo-plastic behaviour for castable compositions with q value 0.26 has been confirmed by authors for the use of different maximum grain size [11].

Based on the literature results, a 3D printing refractory mix was designed according to the Andreasen's model with a q value 0.26. The table 2 reports the cementitious ink composition with the different proportions of each component in weight percentage.
<table><caption>Tab. 2: Refractory ink composition (wt %)</caption><tr><td rowspan="4">Tabular alumina</td><td>T 60 (1-3) mm</td><td>25</td></tr><tr><td>T 60 (0.5-1) mm</td><td>5</td></tr><tr><td>T 60 (0.2-0.6) mm</td><td>10</td></tr><tr><td>T 60 (0-0.5) mm</td><td>15</td></tr><tr><td rowspan="4">Reactive Alumina</td><td>T 60 (0-0.2) mm</td><td>15</td></tr><tr><td>T 60 (0-0.045) mm</td><td>13</td></tr><tr><td>RG4000</td><td>10</td></tr><tr><td>SECAR 71</td><td>5</td></tr></table>

This 3D refractory printing mix designed corresponds to a LCC castable according to the CaO content in the composition. (1.5 wt%). A BASF stiffness agent (0.1 wt%) has been added to the dry mixture.

### Flow-cone test results

After the cementitious ink design, the water content has been determined thanks to the flow-cone test to obtain a pumpable formulation: obtention of a plastic consistency. Firstly, the flow-cone tests have been performed on commercial cementitious inks to establish a flow value range for a plastic consistency. Table 3 reports pictures of commercial materials obtained after adding the mixing water content recommended by the supplier for a plastic consistency and their flow under their own weight after the cone removal. The test results indicate a flow value of 35% and 45% respectively for 751 SI-3D and 704 CL+HP. To obtain approximately the same flow value for the 3D refractory cementitious ink (3D-RCI), a water content close to 8% was needed. This water content led to a flow value of 40% for the 3D-RCI material. A picture after the flow-cone test is also reported on table 3 for the 3D-RCI ink.

<table><caption>Tab. 3: Flow-cone test results</caption><thead><tr><td>Sample</td><td>Water content (%)</td><td>F (%)</td><td>Picture</td></tr></thead><tbody><tr><td>704 CL+HP</td><td>10</td><td>45</td><td></td></tr><tr><td>751 SI-3D</td><td>18</td><td>35</td><td></td></tr><tr><td>3D-RCI</td><td>8</td><td>40</td><td></td></tr></tbody></table>

### Results of ultrasonic wave transmission measurements

To go further on the development of the refractory cementitious ink formulation, characterisation of the setting properties has been performed to determine the elapsed time between the mixing water addition in the dry mix and the time when the system began to stiffen. The knowledge of this elapsed time is extremely important for the 3D printer as there is no addition possibility of a stiffness accelerator at the printing nozzle. If this elapsed time is too short, there is a risk that the material will stiffen in the pipe during transport, causing the clogging pipe. If the time interval is too long, the material may not be suitable for buildability, with the lower layers collapsing under upper layers.

To suitable adjust the stiffness development of the 3D-RCI material, firstly those of commercial materials, has been determined by using ultrasonic wave transmission measurements and then compared to the developed ink.

The Figure 2 reports the ultrasonic wave velocity evolution through the materials 704 CL+HP, 751 SI-3D and 3D-RCI. During experiment, the temperature profile of each material was also recorded as it allows the setting time detection.

<figure><img src="image_2.png" /><figcaption>Fig. 2: Ultrasonic wave velocity and temperature over time</figcaption></figure>

For both commercial materials, the ultrasonic wave signal is detected about 20 or 25 minutes after the water addition in the dry mixture whereas it is about 10 minutes for 3D-RCI material. It corresponds to the beginning of a first increase of the ultrasonic wave velocity. Then a second increase of the wave velocity can be defined corresponding to the material setting. The beginning of this second increase is related to the start of the temperature rise inside the material, and the end of the increase of the wave velocity to the exothermic peak. The first increase of the wave velocity is then defined as the material stiffness. For both commercial inks, the setting material occurs just after the stiffness whereas it is not the case for the developed ink. Its setting occurs few hours later.

Comparing both commercial inks, their velocity curves indicate a higher acceleration of the wave velocity at early age for 751 SI-3D than for 704 CL+HP. As the wave velocity value can be related to the mechanical resistance of the materials [6], at early age it signifies, 751 SI-3D material has a higher mechanical resistance than 704 CL+HP explaining its higher buildability. The 3D-RCI material exhibits the same behaviour at early age as those of 751 SI-3D. However, as the wave velocity signal of 3D-RCI ink appears about 10 minutes after the water addition, this result highlights the fact that its stiffness appears too fast and could induce a clogging of the pipe before its extrusion.

To delay the beginning of 3D-RCI ink stiffness, the content of the stiffness agent has been firstly decreased. It induced only an early age of the setting period but not a delay of the stiffness one. Therefore, a water reducing agent has been added to the dry mixture also acting as stiffness retardant. An optimum ratio between the water reducing agent and the stiffness agent had to be found to obtain an appropriate setting curve corresponding to a ratio of 0.75. Moreover, as the water reducing agent is a plasticizer, its addition in the mixture has decreased the mixing water content down to 7% without a degradation of the flow properties at early age. The flow value of the optimised composition was the same as those of the first formulation: 40%. The ultrasonic wave velocity and temperature over time of the optimised formulation are presented on figure 3.

<figure><img src="image_3.png" /><figcaption>Fig. 3: Ultrasonic wave velocity and temperature over time for 3D-RCI, 3D-RCI optimised, 704 CL+HP and 751 SI-3D</figcaption></figure>

The setting behaviour of the optimised 3D-RCI optimised ink is close to 751 SI-3D ink. Indeed, the stiffness starts approximately at the same time, about 20 minutes after the mixing water addition.
Moreover, the evolution of the wave velocity during the stiffness phase is the same for both materials signifying the same buildability if the consistency is mastered during the 3D printing at large scale.

## Gun tests

The gun test has been used to assess the material extrudability at lab scale. In the meantime, the buildability has been also evaluated by manually printing 4 or 5 stacked layers. Both commercial inks have been tested with also the optimised 3D-RCI material. Pictures of the manually printed parts are shown on table 4.

<table><caption>Tab. 4: Pictures of the manually printing parts</caption><thead><tr><th>Sample reference</th><th>Picture during printing</th><th>Picture of the parts after printing</th></tr></thead><tbody><tr><td>704 CL+HP</td><td></td><td></td></tr><tr><td>751 SI-3D</td><td></td><td></td></tr><tr><td>3D-RCI</td><td></td><td></td></tr></tbody></table>

These manual tests have indicated that the developed refractory ink is extrudable and buildable at lab scale: it was possible to squeeze out of the gun nozzle filaments and to stack several layers without a flow of the lower layers.

## Physical and mechanical characterisation of the developed ink

As the printable refractory composition thus developed seems to be promising for a large-scale printing test, firstly, 4/4/16 samples were produced according to refractory standards to determine the apparent porosity (AP), the bulk density (BD) and the MOR and CCS of 3D-RCI material after a drying at 110°C. The average results of each test are reported on table 5.

<table><caption>Tab. 5: Physical and mechanical characteristics of 3D-RCI materials</caption><thead><tr><th>Sample reference</th><th>BD (g/cm³)</th><th>AP (%)</th><th>MOR (MPa)</th><th>CCS (MPa)</th></tr></thead><tbody><tr><td>3D-RCI</td><td>2.88</td><td>19.7</td><td>8.10</td><td>34.57</td></tr><tr><td>3D-RCI optimised</td><td>2.94</td><td>17.9</td><td>10.36</td><td>40.7</td></tr><tr><td>6S71B [12]</td><td>/</td><td>8</td><td>24</td><td>/</td></tr></tbody></table>

The AP and BD results of 3D-RCI materials show an improvement of the density, porosity, and green mechanical properties for the optimised formulation. It is probably due to the use of the water reducing agent allowing a decrease of the water addition from 8% down 7% without a deterioration of flow properties at early age. The MOR average value is lower and the average AP value higher than those found in the literature for a high alumina LCC castable composed from Andreasen's model with a q value of 0.26 [12], maybe due to no vibration applied on 3D-RCI samples.

## CONCLUSIONS

In this study, a 3D printable refractory cementitious ink has been formulated. To reach out a such goal, the strategy adopted was firstly to design a pumpable mix according to the literature review on shotcreting castables. Then, the contents of the mixing water, the stiffness additive and water reducing agent have been optimised to obtain suitable extrudability and buildability properties according to the 3D concrete equipment which will be further used to print the refractory ink. These last optimisations have been achieved through manual gun tests and ultrasonic wave velocity measurements. The

developed mix corresponds to a classical LCC castable according to the CAC content.

Although such a 3D printable mix has been developed and its printability has been demonstrated at lab scale, large-scale printing tests need to be carried out. In addition, a great deal of works remains to be done. One of them concerns a deeper rheological characterisation of the printing castable with also the mechanical characterisation at the green state and at high temperature of samples collected from printed parts. The influence of the printing parameters also needs to be studied: nozzle speed, pump flow, slice thickness, ...

Upcoming future investigations will be undertaken along these guidelines. An upgrade of the composition by using larger coarse grains (up to 6 mm) or the adding of viscosity modifying agent (VMA) to improve the thixotropic behaviour, may also be of a great interest.

## ACKNOWLEDGEMENTS

BCRC-INISMa acknowledge Walloon Region (SPW-EER) for their financial support of this WIN4COLLECTIVE research project "BEXTRUS". The authors also thank ALMATIS, BELREF and CONSTRUCTION 3D for their support to this study. Jeremy Winand and Maigane Bouillon are also gratefully acknowledged for their lab work contribution for this study.

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