Alumina-mullite Structures Prepared in situ from Calcined Alumina and Colloidal Silica Using Different Processing Methods: Uniaxial Pressing and Direct Casting of Aqueous Suspensions

Abstract

Aqueous dispersions of colloidal silica (CS) are commonly employed in high-alumina refractory castables to prepare in situ mullite, behaving simultaneously as a liquid medium, binder, and SiO₂-source. Despite such technological interest, CS was not explored as a uniaxial pressing additive to replace organic binders and promote earlier strengthening during sintering. This study mixed alumina particles with varying amounts of CS to compare structures composed of the same raw materials, shaped by different processing techniques. Such compositions were pressed as bars or cast into cylinders, and their microstructure and physical properties evolution were assessed during sintering (700-1500ºC). Cast samples developed a homogeneous microstructure comprised of alumina particles surrounded by a gelled CS phase; in contrast, the pressed samples generated SiO₂-rich spherical clusters, originating from the original CS droplets, surrounded by alumina. Such a heterogeneous microstructure persisted during sintering, when the silica nanoparticles crystallized before forming mullite. Directly cast samples developed a granular microstructure of alumina-mullite, with traces of cristobalite. In pressed samples, regions with higher SiO₂ concentration developed acicular mullite crystals, imbibed in a matrix of alumina and cristobalite. Increasing the CS content in both cases enhanced porosity and decreased strength after sintering at temperatures above 1100 °C.

Keywords:
Colloidal silica; alumina; in situ mullite; uniaxial pressing; direct casting

1. Introduction

Mullite (3Al₂O₃.2SiO₂ or Al₆Si₂O₁₃) is the most important aluminosilicate for the ceramic industry due to its high thermal stability, thermal shock, corrosion resistance, and biocompatibility with bone tissues¹-¹². Due to its scarce availability as a naturally occurring raw material, it is produced from the combination of sources of aluminum oxide (Al₂O₃ or alumina, such as calcined alumina, aluminum hydroxide, metallic aluminum, and kaolin) and silicon dioxide (SiO₂ or silica, river sand, micronized quartz, microsilica, silica fume, and clay, for instance). Although fusion or electrofusion (above 2500ºC) produces grains with high density and chemical resistance⁷,¹³-¹⁵, the high energy consumption of such an approach has favored the development of several synthesis methods based on in situ reactions¹⁶-⁵². In these cases, mixtures of compacted fine particles of alumina and silica sources are heated to temperatures in the 1300-1700ºC range. This solid-state reaction is based on the interdiffusion of Al³⁺, Si⁴⁺, and O^2- ions and begins at approximately 1200ºC (Equation 1), when a thin layer of mullite forms amongst the particles, reducing the reaction rate²,¹⁶,¹⁸,⁵³-⁵⁶:

$3{\text{Al}}_2{\text{O}}_3+2{\text{SiO}}_2\to {\text{Al}}_6{\text{Si}}_4{\text{O}}_{13}$ (1)

As heating continues, the reaction rate enters a constant-rate period that lasts until the reactants are completely consumed. Although the composition of mullite remains the same, the excess of alumina or silica leads to the precipitation of other phases. Besides this, impurities can alter its crystalline habit from regular grains (in high-purity systems) to acicular whiskers (in the presence of amorphous silica and alkalis, such as Na₂O)⁵⁷-⁶⁴. Since such a complex process can generate different types of microstructures and a wide range of physical properties, mullite can be employed in several applications, from refractories¹⁴,¹⁵,⁶⁵-⁷⁶, thermal insulators²⁵,²⁷,²⁹,³¹,³⁴-³⁶,⁴¹,⁷⁷-⁸¹, ballistics⁸²,⁸³, parts for water treatment⁸⁴-⁸⁶, solar thermal storage⁸⁷-⁸⁹, and biomaterials⁹⁰-⁹³.

In recent works, monolithic in situ mullite structures have been prepared using the direct casting technique⁷⁷,⁷⁸,⁸¹,⁹⁴-⁹⁶. This technique involves mixing the reactants' particles with water to form a uniform castable suspension. A binding agent then sets the mixture during the curing step, ensuring enough strength for handling before reactive sintering and allowing the production of parts of complex geometry. For instance, porous structures of in situ mullite were prepared by sintering castable suspensions of calcined alumina, aluminum hydroxide (α-Al(OH)₃), and microsilica or micronized quartz, having polymeric resins or hydratable alumina (ρ-Al₂O₃) as binders²⁸-³¹,³⁴,⁵⁰,⁵¹,⁶⁸,⁷⁷-⁸⁰. Other studies that also worked with calcined alumina employed colloidal silica (CS) as the source of SiO₂⁶⁸-⁷¹,⁷⁴,⁸¹,⁹⁷-¹⁰¹. These materials consist of aqueous dispersions of spherical nanometric particles of synthetic amorphous silica stabilized using electrosteric mechanisms¹⁵,¹⁰²-¹⁰⁴. Besides their high purity and tailored particle size distribution, CS dispersions also serve as a liquid medium for mixing, a binder during the curing and drying steps, and a sintering aid, significantly reducing the minimum densification temperature compared to plain alumina¹¹,¹⁵,⁵³,⁶⁵,⁶⁹-⁷¹,⁷⁴. These promising results suggest that CS can also produce straightforward effects on other in situ mullite-forming processing techniques, such as uniaxial pressing¹⁰⁵.

In uniaxial pressing, one of the most used methods for ceramic processing, particles are compacted in a rigid mold using a mechanical load applied in a single vertical direction. The particles are forced to move toward each other by increasing the compacting force until no further movement is possible. As a result, the inter-particle void ratio is significantly reduced, and the final geometry of the part is established with great precision and surface finish. Although a binding agent is not necessary in clay-based plastic systems, for non-plastic compositions, such as zirconia and calcined alumina-based ones, adding paraffin, stearic acid, and dextrin enhances the structure's strength for demolding and handling. Unlike hydraulic binders in castable systems, these organic compounds melt and char during the first heating, so they neither provide strength at high temperatures nor become part of the final composition. Based on these considerations, it seems reasonable that colloidal silica could behave simultaneously as an inorganic binder and a source of SiO₂ for uniaxially pressed compositions, aiming to form in situ mullite¹⁰⁵.

In the present study, in situ mullite was produced using the same sources of calcined alumina and colloidal silica by uniaxial pressing (UP) and direct casting (DC) to evaluate the impact of the processing route on microstructural development and physical properties during sintering. In the first method, a 50 wt% solids concentrated CS dispersion was sprayed over calcined alumina particles before pressing. In the second method, castable suspensions were prepared using the same Al₂O₃-SiO₂ ratios. After consolidation and drying, the samples' microstructure, physical properties (total porosity, rigidity, flexural strength, thermal dimensional variation), and crystalline phase evolution were evaluated after sintering (700-1500ºC).

2. Materials and Methods

The raw materials used in this work were calcined alumina (CA, A1000SG, Almatis, USA), colloidal silica (CS, Levasil CS5028, Nouryon Pulp and Performance, Brazil), dispersant agent (Castment FS20, BASF, Germany), and hydratable alumina (HA, Alphabond 300, Almatis, USA). Their physical properties (Table 1) were previously determined, and their detailed description can be found elsewhere⁶³,⁷²,⁷⁷-⁸⁰.

Table 1
Characteristics of raw materials.

Characteristics	Raw materials
	Calcined Alumina (CA) a	Colloidal Silica (CS) b
Chemical composition (wt%)	Al2O3: 99.8; Na2O: 0.07; Fe2O3: 0.02; MgO: 0.05; SiO2: 0.03; CaO: 0.02.	SiO2: 99.85; Na2O: 0.15
Main crystalline phase (PDF file)	α-Al2O3 (46-1212)	SiO2 (non-crystalline)
Particle size (D50)	0.6 μm	34 nm
Solid density (g.cm-3)b	3.98	2.1
Specific surface area (m2.g-1)b	8.2	83
Solid content (wt%)a	-	50

• ^a
  After thermal treatment at 1000°C.
• ^b
  after drying at 120°C

Two sets of compositions were prepared with the same Al₂O₃-SiO₂ ratio and processed by uniaxial pressing (UP) or direct casting (DC) (Table 2). The castable compositions were prepared by mixing calcined alumina, varying amounts of CS, dispersant, and distilled water in a ball mill (6 mm diameter ZrO₂ spheres, 3:1 spheres-to-alumina mass ratio) for 1 hour at 80 rpm. This condition was previously determined to be necessary to improve particles’ individualization and dispersion, but insufficient to promote a significant change in their size distribution. The CS amounts added to the formulations were selected to match those employed in equivalent pressed systems. The suspensions were transferred to a paddle mixer (RW 20 Digital, IKA, Germany, 600 rpm) and were stirred for 5 min after the addition of 0.1 wt% of fine magnesium oxide particles (MgO sinter, dry-basis, RHI-Magnesita, Brazil) as a gelling agent (Figure 1a). After homogenization, the suspensions were cast in cylindrical non-adherent molds of 70 mm length by 16 mm diameter, for total porosity, flexural elastic modulus, flexural modulus of rupture, and permanent linear thermal variation measurements; and 8 mm length by 6 mm diameter, for dilatometric analysis (Figure 1b). Cast samples remained at 60ºC in closed flasks for 24 h and in a ventilated environment for 24 h for the curing step (Figure 1c). After demolding, they were dried overnight at 120ºC in a ventilated environment.

Table 2
Compositions tested.

a Samples’ identification	Experimental compositions (wt%/vol%)								Theoretical composition after sintering at 1500ºC (wt%)
	During mixing						After drying
	CA	CS		HA	c H2O	Solid load	Al2O3 (CA+HA)	SiO2
		b SiO2	b H2O						α-Al2O3	SiO2	Al6Si4O13
DC_3A_0.0S	73.79/ 45.00	0.00/ 00.00	0.00/ 0.00	5.60/ 5.00	20.60/ 50.00	79.40/ 50.00	100.0/ 100.0	0.00/ 0.00	100.00	0.00	0.00
DC_3A_0.5S	73.82/ 44.14	5.17/ 5.86/	7.75/ 18.45	0.00/ 0.00	13.26/ 31.56	78.99/ 50.00	93.46/ 88.29	6.54/ 11.71	76.80	0.00	23.20
DC_3A_1.0S	67.90/ 38.93	10.19/ 11.07	15.28/ 34.86	0.00/ 0.00	6.63/ 15.14	78.09/ 50.00	86.96/ 77.86	13.04/ 22.14	53.75	0.00	46.25
UP_3A_0.0S	90.91/ 71.53	0.00/ 0.00	0.00/ 0.00	0.00/ 0.00	8.48/ 26.95	91.52/ 73.05	100.0/ 100.0	0.00/ 0.00	100.00	0.00	0.00
UP_3A_0.5S	85.11/ 64.49	5.96/ 8.56	8.94/ 26.95	0.00/ 0.00	0.00/ 0.00	91.06/ 73.05	93.46/ 88.29	6.54/ 11.71	76.80	0.00	23.20
UP_3A_1.0S	72.73/ 45.88	10.91/ 13.04	16.36/ 41.08	0.00/ 0.00	0.00/ 0.00	83.64/ 58.92	86.96/ 77.86	13.04/ 22.14	53.75	0.00	46.25

• ^a
  DC = Directly cast, UP = Uniaxially pressed.
• ^b
  from colloidal silica; c) external source

Figure 1
Steps of preparation of uniaxially pressed and directly cast samples.

For the production of pressed samples, calcined alumina powder was sprayed with different quantities of CS, homogenized with a brush, and passed through a 212 μm opening sieve (Figure 1d). The mixtures were compacted by uniaxial pressing (40 MPa, 60 s, Figure 1e) as prismatic bars (70 mm length by 20 mm width by 6 mm thickness) for total porosity, flexural elastic modulus, flexural modulus of rupture, and permanent linear thermal variation measurements; and as 8 mm length by 6 mm diameter cylinders for dilatometric analysis. After demolding, samples were dried overnight at 120ºC in a ventilated environment (Venticell 50, MMM Group, Germany). The maximum CS content added to the calcined alumina particles was previously determined as the highest possible without liquid segregation during pressing; CS-free samples were also prepared for testing as a reference.

The samples’ mass (M, g) and dimensions (length, L; diameter, D; width, W; thickness, T, cm) were measured in the dried green state (G index). After sintering (S index), samples’ permanent thermal dimensional linear variation (LV, %)⁴¹,⁷⁶,¹⁰⁵ and geometric total porosity (TP, %)⁷⁸, ⁹³ were calculated using Equations 2-4:

$\text{LV}=100\%\times \left({\text{L}}_{\text{S}}-{\text{L}}_{\text{G}}\right)/{\text{L}}_{\text{G}}$ (2)

${\text{TP}}_{\text{Pressed}}=100\%\times \left[1-\left((4\times \text{M})/\left(\pi \times {\text{D}}^2\times \text{L}\times \rho \right)\right)\right]$ (3)

${\text{TP}}_{\text{Cast}}=100\%\times [1-((\text{M})/(\text{L}\times \text{W}\times \text{T}\times \rho ))]$ (4)

where ρ is the solid density (g.cm^-3) of equivalent crushed samples measured by Helium pycnometer technique (Ultrapyc 1200e, Quantachrome Instruments, USA)⁷⁹, ⁹³, ¹⁰⁶ Sintering was conducted in an electric furnace (Lindberg Blue M, Thermo Fisher Scientific, USA), in the range of 700-1500ºC, under a heating rate of 5ºC.min^‑1, 3 h hold, and cooling rate of 10ºC.min^‑1, Figure 1f).

The flexural elastic modulus (E, GPa) was measured using the impulse excitation of vibration technique (Sonelastic, ATCP, Brazil) according to ASTM C1198‑91¹⁰⁷ . The flexural strength (σ, MPa) was measured using a three-point bending test in a universal testing machine (Model WDW-30E, China, 5 kN load cell, 0.5 mm.min^‑1 crosshead speed, ISO 6872) and Equation 5 (for cylinders) and Equation 6 (for prismatic bars):

${\sigma }_{\text{Pressed}}=(3\times \text{P}\times 1\times )/\left(2\times \text{W}\times {\text{T}}^2\right)$ (5)

${\sigma }_{\text{Cast}}=(8\times \text{P}\times 1\times )/\left(\pi \times {\text{D}}^3\right)$ (6)

where P is the rupture load (N), and l is the test span (center-to-center distance between supports, cm).

Dilatometric analyses were conducted in dried green samples (DIL402C, Netzsch, Germany) at 5°C min^‑1 heating rate up to 1500°C under a synthetic air atmosphere.

The phases formed after sintering were identified by X-ray diffraction (XRD, crushed samples, D_Part ≤ 212 μm, Rotaflex RV 200B, Rigaku-Denki Corp.; with CuKα₁ radiation, λ = 0.15406 nm; in the 2θ range from 10º to 100º at a 0.5º.min^‑1 scan rate, against standard JCPDS files; Rietveld method with MATCH! Software, 3.8 version, Germany¹⁰⁸). Scanning electron microscopy coupled to energy-dispersive X-ray microanalysis (SEM/EDX, FEG-SEM, Inspect-F50, FEI, Netherlands) was used to determine the morphology of the fracture cross-sections of the samples after the flexural tests. To reveal the morphology of mullite crystals, pressed samples were immersed in a 0.1 M HF solution for 2 h, washed with distilled water in an ultrasound bath (Q335D, Quimis, Brazil) for 2 h, and dried at 120ºC overnight. Before observation, the samples were glued to a conductive carbon-based tape and coated with platinum using a Q150R sputter (Quorum Technologies, UK).

3. Results and Discussion

Changing the processing method, CS content, and sintering temperature affected the samples' physical properties, as will be discussed in the following sections.

3.1. Cast samples

The CS-free sample showed a microstructure comprised of calcined alumina particles bonded by nanometric pseudo-boehmite (AlOOH) amongst them (Figure 2a), similarly to previous reports⁷⁷,⁷⁸; whereas the CS-containing ones exhibited calcined alumina particles packed with a nanometric colloidal silica uniformly distributed (Figure 3a)⁶³,⁸¹,⁹⁹. As a consequence of such microstructural organization, dried green cast samples presented total porosity levels closely to the volumetric amount of water initially added during mixing (Figure 4a). Such an effect was observed for hydraulic binder-containing systems and CS-containing samples because, during the curing process, the binders hinder the particles’ movement and set their position in the presence of water¹⁵,⁶⁹,⁷⁰,⁷⁷,⁹⁴-⁹⁶,¹⁰⁹. Therefore, the volume originally occupied by water turns into thin pores after drying. The slightly lower porosity measured for CS-bonded samples (53%) compared to HA-bonded ones (50%) was attributed to the more pronounced drying shrinkage experienced by silica-containing compositions⁹⁵,⁹⁶,¹⁰⁵,¹⁰⁹. In CS-free samples, the binding efficiency of hydratable alumina is sufficient to withstand capillary forces among particles generated during water withdrawal, and the original geometry set during curing was mostly preserved⁹⁵,⁹⁶. Gelled colloidal silica, on the other hand, shows its highest binding effect during drying and, therefore, occurs along the shrinkage, leading to lower pore contents (Figure 4a)¹⁵,⁶⁹,⁷⁰. It is also noteworthy that CS-bonded samples presented lower rigidity and flexural strength after drying compared to the HA-bonded ones (Figure 4b-c)⁶³,⁷⁷.

Figure 2
Colloidal silica-free samples processed by direct casting (DC) or uniaxial pressing (UP) after isothermal treatments (120-1500ºC): SEM images of fractured surfaces. The red arrows indicate pseudo-boehmite particles.

Figure 3
Colloidal silica-containing samples (3A_1S) processed by direct casting (DC) or uniaxial pressing (UP) after isothermal treatments (120-1500ºC): SEM images of fractured surfaces. The yellow circles indicate alumina-mullite domains, the red CS indicates a layer of SiO_2.

Figure 4
Effects of colloidal silica addition on total porosity, flexural elastic modulus, and flexural strength of cast (a-c) and pressed (d-e) samples, respectively, after sintering at different temperatures.

During the initial heating, up to 1100°C, the bonding regions in CS-free samples became less defined (Figure 2b) due to the beginning of sintering and grain growth (Figure 5a), while CS-containing samples presented an earlier densification (Figure 5b), with a coral-like microstructure (Figure 3b)⁷⁹,⁸¹ and gain of rigidity and strength above 120ºC and up to 1100ºC (Figure 4b-c)⁷⁹. Two effects explain this behavior. First, in the 120-350ºC temperature range, HA-bonded materials lose strength due to the dehydroxylation of pseudo-boehmite and gibbsite (α-Al(OH)₃) bonding spots (Figure 2b)⁹⁵,⁹⁶,¹¹⁰,¹¹¹. Conversely, gelled CS does not contain any hydroxylated compound in its structure 69,70,81 and, as a result, it becomes stronger after free-water removal. Secondly, at 700ºC, the amorphous SiO₂ that comprises CS reaches its glass transition temperature (Tg, approximately 698ºC)⁸¹,¹⁰²,¹⁰⁵. The particles became soft, deformable, and more prone to bond amongst themselves and with calcined alumina (Figure 3c-d) ⁵²,⁷⁶. Consequently, their densification rate increased. After cooling, these samples exhibited greater sintering shrinkage, rigidity, and strength.

Figure 5
Dynamic thermal dimensional variation and permanent dimensional variation after isothermal treatment for a-b) direct cast and c-d) uniaxially pressed samples.

Above 1100ºC, the higher the CS content, the lower the total porosity, rigidity, and flexural strength. Similar results were observed in previous studies that evaluated in situ mullite systems⁶³,⁸⁰,⁹³. Above 700-800ºC, the softening of amorphous silica favors the coalescence of inter-particle pores, reducing their total fraction and enhancing average diameter (Figure 3c)⁶³,⁸⁰,¹¹² in comparison to the CS-free equivalent sample (Figure 2c). Besides this, the SiO₂ crystallization into cristobalite triggers the interdiffusion of Al³⁺, Si^4+, and O^2- ions at particles’ contact points (Figure 6a-b)³,⁶,¹⁰,¹¹,¹⁶,⁵⁴,¹¹³,¹¹⁴. The first portions of mullite typically form at 1200ºC⁵⁵-⁶⁰, restraining ions' mobility while the crystals formed push particles apart due to their lower density (mullite = 3.2 g.cm^‑3; calcined alumina = 4 g.cm^‑3), hindering densification. After cooling, the microstructure of CS-containing samples sintered at 1300-1500ºC contains a small fraction of large pores (Figure 3d), which are responsible for the lower levels of rigidity and strength (Figures 4b and 4c). The composition of the continuous phase of unreacted crystalline SiO₂ surrounding the mullite and calcined alumina domains (Figs 3c and 3d) was confirmed by XRD (Figure 6). CS-free samples, on the other hand, exhibited a typical dense microstructure comprised of regular alumina grains (Figure 2d)⁶³,⁷²,⁷⁷,⁷⁹.

Figure 6
X-ray diffraction for samples processed by a-c) direct casting and d-f) uniaxial pressing and sintered at 1100ºC, 1300ºC, and 1500ºC. Symbols: A = α-Al₂O₃ (Corindon, JCPDS 1-1243), C = SiO₂ (Cristobalite, JCPDS 1-438), M = Al₆Si₂O₁₃ (Mullite, JCPDS 1-613). The numbers in brackets represent the amount of each phase calculated by Rietveld refinement (in wt%).

3.2. Uniaxially pressed samples (UP)

After drying overnight at 120ºC, all pressed samples showed similar total porosity levels in the 46-48% range (Figure 4d). Previous works that tested similar calcined alumina grades and processing conditions without using any liquid phase reported similar pore content levels⁶³,⁷²,⁷⁹. Therefore, the addition of CS dispersion did not affect the particles’ compacting efficiency. Nevertheless, SEM images of these green-dried samples revealed differences in their microstructure (Figure 7). The one comprised of plain alumina showed a typical compact of irregular particles containing small inter-particle voids amongst them (Figure 2e). For the CS-containing ones, on the other hand, the continuous matrix of calcined alumina particles also exhibited 5-7 μm rounded clusters formed by tightly bonded colloidal silica nanoparticles (Figure 3e). Reviewing the initial processing steps explains such an unusual configuration.

Figure 7
SEM/EDS images of fractured surfaces of a-c) uniaxially pressed samples (UP_3A_1S) and d-f) direct cast samples (DC_3A_1S) after drying at 120ºC.

The ultrasonic nozzle employed to spray CS dispersion over calcined alumina particles produces and ejects droplets of approximately 1-5 μm when working with plain water and 5-10 μm for more viscous liquids. When they fell over the thinner calcined alumina particles (D₅₀ = 0.67 μm), they were instantaneously attracted by strong capillary forces. Consequently, the droplets were lined by a layer of calcined alumina particles, forming stable clusters that resisted the homogenization, compacting, and pressing processes (Figure 7d-f). During drying, water withdrawal gelled the core, which comprised discrete CS particles, setting these heterogeneous domains throughout the continuous matrix of calcined alumina. Similar effects have not been reported in systems containing plain water, isopropanol, or organic binders, as these liquids do not leave any solid residue after drying. In comparison, in the directly cast samples (Figure 7a-c), the SiO₂ particles became uniformly distributed throughout the structure.

Pressed CS-containing samples also presented an early start of densification as they reached the amorphous silica glass transition temperature (698ºC) (Figure 5c-d). After isothermal treatment at 700ºC and 1100ºC, the higher the CS content, the lower the rigidity and strength (Figures 4e-f). It is important to highlight that, whereas the CS-free sample exhibited the first signs of particle necking and sintering (Figure 2e-f), for the CS-containing ones, the clusters of CS particles formed after drying remained stable up to 1100ºC (Figure 3e-f), forming irregular blocks of amorphous SiO₂ (Figure 6d). As heating continues, the reduction in densification rate observed at approximately 1300ºC relates to the expansive in situ mullite formation and crystallization of unreacted SiO₂ (Figure 6e). After sintering at 1300ºC and 1500ºC, such arrangements changed, respectively, to fully sintered granular alumina and to thin mullite crystals imbibed into a continuous matrix of unreacted calcined alumina and in cristobalite particles (Figures 6f, 2g-h, and 3g-h).

3.3. Comparing microstructures and physical properties after sintering

The microstructural evolution was dependent on both the processing route and the thermal treatment. At intermediate temperatures (1100°C-1300°C), solid-state sintering became evident, with the formation of neck areas (Figures 2c and 3c)⁵²,⁵⁴,⁶³,¹¹⁵ and reduced porosity at 1300ºC (Figures 4a and 4d). The CS-containing samples showed a more homogeneous interparticle bonding, with the silica-rich phase coalescing along grain boundaries, resulting in a more uniform pore-size distribution (Figures 3b, 3c, 3f, and 3g)⁵²,⁷⁶,¹¹⁶. At the highest temperature investigated (1500°C), a microstructural difference was more evident. CS-free samples displayed a dense and homogeneous microstructure characterized by well-developed, faceted alumina grains and a low fraction of residual closed porosity (Figures 2d and 2h)⁶³,⁸⁰,¹¹⁷. In CS-containing samples, the intergranular glassy phase became fully developed and facilitated the nucleation and growth of acicular mullite crystals within the matrix, resulting in a composite-like microstructure composed of alumina grains embedded in a mullite–silicon glass phase (Figures 3d and 3h)¹¹⁵,¹¹⁶,¹¹⁸ This transformation was accompanied by significant pore elimination and grain coarsening, particularly in UP specimens, where higher green density intensified densification and microstructural heterogeneity.

Notably, pressed samples presented significantly lower levels of total porosity, permanent sintering shrinkage, rigidity, and flexural strength for CS-containing compositions than those observed for equivalent compositions prepared by direct casting (Figures 4 and 5). Such an unexpected behavior relates to the morphology of the mullite crystals formed during sintering (Figure 8). The in situ formation of mullite is a solid-state reaction based on the interdiffusion of Al³⁺ and Si⁴⁺ ions, and, therefore, the quantity and shape of the resulting crystals can be strongly affected by three main parameters. Firstly, above its glass transition, alkaline impurities, such as Na₂O and K₂O, lower the viscosity of amorphous silica and ease mullite formation due to ions' higher mobility⁵⁷-⁶⁴. Conversely, this hypothesis does not apply when the same raw materials are used in the samples. Secondly, the average sizes of the Al₂O₃ and SiO₂ particles directly impact the mean interdiffusion path length, suggesting that thinner particles would achieve higher mullite yields⁵⁸-⁶⁰,⁸⁰,¹¹⁴. Thirdly, when mullite formation occurs in excess of SiO₂ regarding its stoichiometric ratio, the faster diffusion of Al³⁺ ions compared to the Si⁴⁺ ones favors crystal growth in low-energy planes¹¹¹-¹¹⁴,¹¹⁹-¹²³. Consequently, in such a condition, mullite crystals tend to assume an acicular geometry (Figure 8f); otherwise, less asymmetrical crystalline habits can be observed¹¹²,¹¹⁴. In this study, whereas in directly cast samples the uniform distribution of CS particles produced a thin layer of SiO₂ around calcined alumina particles, in the pressed samples, the clusters formed after drying behaved as large SiO₂ particles surrounded by calcined alumina. Consequently, the high concentration of SiO₂ in those spots produced islands of needle-like mullite crystals that hindered the microstructure’s densification and generated large defects.

Figure 8
Scanning Electron Microscopy images of fractured surfaces of uniaxially pressed samples a-c) after sintering (1500ºC, 3 h) and d-f) after sintering (1500ºC, 3 h) and etching (0.1 M HF solution, 2 h).

4. Conclusions

This work investigated the effects of different processing methods (direct casting, DC, or uniaxial pressing, UP) on the evolution of microstructural and physical properties of a system comprising fine calcined alumina particles bonded with an aqueous dispersion of anionic colloidal silica (CS) during the first heating towards sintering.

Compositions prepared by DC exhibited a more homogeneous microstructure after drying, consisting of thin layers of CS filling the spaces within a continuous, porous matrix of calcined alumina particles. The uniaxially pressed sample, on the other hand, exhibited coarse clusters of CS particles within the calcined alumina matrix. These differences in CS distribution resulted in significant variations in total porosity, rigidity, flexural strength, and densification rate. DC compositions exhibited typical properties evolution for similar systems, where the softening of amorphous silica particles and the in situ formation of mullite halt the densification process, generating microporous Al₂O₃-mullite structures. For the UP ones, on the other hand, the SiO₂ nanoparticle concentration produced rounded domains of acicular mullite crystals, which negatively impacted rigidity and flexural strength. Such results highlight the importance of the early processing stages (mixing, curing, drying) for the microstructure development after sintering.

5. Acknowledgments

The authors acknowledge Brazilian Research Foundations FAPESP (grants 2010-19274-5; 2017/06738-2; 2018/19773-3; 2022/03655-7), CNPq (305877/2017-8; 304081/2020-5; 311010/2023-7), and CAPES (PNPD 88887.648527/2021-00) for supporting this research, and Almatis (Brazil and USA) and Nouryon South America (Brazil) for kindly supplying samples of calcined alumina and colloidal silica, respectively. They are also indebted to the Electron Microscopy Laboratory of the Advanced Materials Research Support Center (SMM/IFSC) for the SEM images. They declare that, to the best of their knowledge, no competing interests (financial or personal) affected the results reported in this paper and that they cited all funding and supporting sources.

• Data Availability
  Data supporting the findings of this study are available from the corresponding author upon reasonable request.

6. References

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Edited by

• Associate Editor:
  Celso Santilli

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

Data supporting the findings of this study are available from the corresponding author upon reasonable request.

Publication Dates

• Publication in this collection
  10 Apr 2026
• Date of issue
  2026

History

• Received
  12 June 2025
• Reviewed
  28 Jan 2026
• Accepted
  02 Mar 2026