Effect of calcined halloysite addition on mechanical strength and deformation behavior of ceramic tableware

Abstract

This study investigates the effect of adding 0.1, 0.3, 0.6, 1.0, 2.0 wt.% halloysite clay, which is calcined at 600 °C, to the standard ceramic tableware slurry. The test specimens were fabricated by pressing of granulated compositions and subsequently sintering at 1180 °C for 2 h. Detailed deformation, mechanical and microstructure characterization studies were done on sintered samples. All sintered samples with halloysite exhibited higher bending strength than the standard sample. The sample with 1.0 wt.% calcined halloysite showed the highest bending strength (12.80 MPa) and the lowest pyroplastic deformation (1.15 × 10⁻⁵ cm⁻¹) among all the samples. Phase analysis and microstructural investigations revealed a significant increase in needle-like mullite phases within the microstructure at this optimal halloysite concentration. These findings suggest that the augmented interlocking of mullite grains contributes to improved mechanical strength and reduced deformation. This study demonstrates halloysite addition might enhance the technological properties of conventional tableware to achieve a reduced thickness in line with current market trends and environmental considerations.

Resumen

Este estudio investiga el efecto de la adición de 0.1, 0.3, 0.6, 1.0, 2.0% en peso de arcilla halloysita, que se calcina a 600 °C, a la pasta de vajilla cerámica estándar. Las probetas se fabricaron por prensado de composiciones granuladas y posterior sinterización a 1180 °C durante 2 horas. Las muestras sinterizadas se sometieron a estudios detallados de deformación, mecánica y caracterización microestructural. Todas las muestras sinterizadas con halloysita mostraron una mayor resistencia a la flexión que la muestra estándar. La muestra con 1.0% en peso de halloysita calcinada mostró la mayor resistencia a la flexión (12.80 MPa) y la menor deformación piroplástica (1.15 × 10^–5 cm^–1) entre todas las muestras. El análisis de fases y las investigaciones microestructurales revelaron un aumento significativo de las fases de mullita en forma de aguja dentro de la microestructura a esta concentración óptima de halloysita. Estos resultados sugieren que el aumento del entrelazamiento de los granos de mullita contribuye a mejorar la resistencia mecánica y a reducir la deformación. Este estudio demuestra que la adición de halloysita podría mejorar las propiedades tecnológicas de las vajillas convencionales para conseguir un espesor reducido en línea con las tendencias actuales del mercado y las consideraciones medioambientales.

Introduction

Ceramic tableware remains a ubiquitous product in daily life, driven by its esthetic appeal, durability, and food safety attributes. Recent market trends emphasize the demand for thinner, lighter tableware designs that not only enhance user experience but also contribute to environmental sustainability by reducing raw material consumption and CO₂ emissions during production and transport [1,2].

However, the manufacturing of thin ceramic tableware is challenged by pyroplastic deformation during firing, a phenomenon that compromises dimensional stability and mechanical integrity [3–5]. Firing at elevated temperatures induces physical and chemical transformations within the microstructure, such as liquid phase formation, particle dissolution, and crystallization [6]. Since pyroplastic deformation results from complex microstructural changes during firing, controlling these transformations is essential to maintain product quality. Minimizing deformation while enhancing bending strength is therefore critical for producing durable, reliable thin ceramics.

Previous studies have highlighted the influence of crystal shape and size on ceramics’ bending strength and pyroplastic deformation behavior [7–9]. Martín-Márquez et al. [10] conducted a study to investigate the effect of microstructural characteristics, e.g., crystalline and amorphous phases, on the mechanical properties of a standard porcelain stoneware composition (50% kaolinitic clay, 40% feldspar, and 10% quartz) fired at 1200–1300 °C. The results showed that the increase in the percentage of mullite phase and the needle shape morphology of secondary mullite in the microstructure were the main factors for the improved mechanical properties of the sintered samples. Capoglu [11] studied a translucent whiteware composition made from coarse and fine-milled preferred components with minimal clay addition. The sintered microstructure consisted of anorthite, mullite, and a glassy phase. Mullite crystals formed a three-dimensional network that mitigated pyroplastic deformation during glaze firing.

The properties of ceramics can be improved by modifying the proportions of raw materials or incorporating minerals into the chemical composition [12]. The minerals for the production of mullite ceramics typically include natural clay minerals [13] or mixtures of silica (SiO₂) and alumina (Al₂O₃) [14]. Among various additives, halloysite clay stands out due to its high alumina content and unique tubular morphology, which promotes mullite formation upon calcination [15–18]. Its tubular morphology is the main distinguishing property between halloysite and kaolinite [15,16]. Harabi et al. [17] fabricated porous membrane supports based on mullite and anorthite using halloysite clay raw material and calcite (CaCO₃). The halloysite addition facilitated the formation of interlocked elongated microcrystals of mullite. This was a significant factor for the high mechanical strength (87 MPa) of low-temperature sintered porous mullite membrane supports. Selli et al. [18] investigated the effect of adding calcined halloysite at 600 °C to standard wall tiles in varying proportions (from 0.1% to 1.0% by weight). Electron microscopy revealed needle-like mullite phases within the microstructure of the halloysite samples. The sample with 0.6% calcined halloysite showed a significantly higher bending strength value (28.75 MPa) than standard compositions (20 MPa). The study reported that even small amounts of halloysite can substantially improve wet and dry strength, making it a promising additive in ceramic production.

While halloysite's potential to enhance mechanical properties is reported, its simultaneous effect on reducing pyroplastic deformation in commercial ceramic slurries remains underexplored. This study aims to fill this gap by systematically investigating the influence of calcined halloysite addition on the microstructure, mechanical performance, and pyroplastic deformation behavior of conventional ceramic tableware formulations. For that, standard tableware slurry has been reformulated by introducing various percentages of calcined halloysite (at 600 °C calcination temperature). The findings showed that the addition of calcined halloysite into commercial slurry significantly reduced deformation (∼18% recovery) while enhancing the mechanical strength (∼38% increase) of the samples. We demonstrate a promising approach to produce thinner, stronger, and more dimensionally stable ceramic tableware, aligning with both market demands and environmental considerations.

Experimental

Preparation of the compositions

In this study, the compositions were prepared from standard tableware slurry and calcined halloysite. In each composition, the amount of slurry was kept constant and calcined halloysite was added and mixed at 0.1, 0.3, 0.6, 1.0, and 2.0 wt.% regarding to slurry. Previous research on ceramic wall tile bodies has reported that low-level additions of halloysite or related clay minerals (typically below 3 wt.%) can enhance sintering, microstructural densification and mechanical performance, whereas higher additions may negatively affect processing due to increased viscosity and particle interactions [18,19]. Guided by these findings, a similar low-addition interval of 0.1–2.0 wt.% was determined appropriate for tableware compositions.

Regarding their amount of calcined halloysite, the prepared compositions were denoted as HT-0.1, HT-0.3, HT-0.6, HT-1.0, and HT-2.0, respectively. The standard slurry without halloysite was labeled as STD. The main steps of the preparation of samples are shown in Fig. 1.

Fig. 1.

The schematic diagram of the experimental process.

The standard tableware slurry was purchased from REFSAN Industrial Company. The halloysite raw material was kindly provided by Esan Industrial Raw Materials Company. The chemical compositions of the halloysite clay and standard tableware body composition were analyzed using an XRF Instrument (Philips Model PW 2400 XRF). Their chemical analysis results are presented in Table 1.

Table 1.

Chemical analysis of the halloysite and standard slurry (wt.%, XRF).

Oxides	Halloysite clay	Standard slurry
SiO2	45.20	50.40
Al2O3	38.80	20.60
Fe2O3	0.58	1.00
TiO2	0.27	0.70
CaO	0.10	10.70
MgO	0.26	0.40
Na2O	0.10	0.50
K2O	0.26	1.10
L.O.I*	14.43	14.6

*

L.O.I: Loss on Ignition.

Halloysite clay was calcined to eliminate water, volatiles, and other impurities. With the calcination process, the raw halloysite is converted into calcined halloysite, which has several features that make it appropriate for a wide range of industrial and scientific uses [20,21]. 600 °C was selected for calcination in this study since our previous study showed that this temperature yields optimal performance [18]. To prepare the calcined halloysite, the following procedures were conducted. First, the halloysite clay was wet milled by using a laboratory scale ball mill for 30 min in distilled water and alumina grinding media. The slurry was sieved with a 45 μm aperture, and withdrawn a small quantity from this mixture to measure particle size distribution. The particle size distribution was determined by laser diffraction analysis (Malvern Master Sizer 2000), which revealed an unimodal particle size distribution with a maximum peak of 11 μm (Fig. 2). In our previous study [18], it was demonstrated that fine-grained halloysite enhances the green strength of ceramic bodies compared to its coarse-grained counterpart, primarily due to improved packing efficiency and higher reactivity. Based on these findings, the same particle size range was adopted in the present study to ensure consistency and to further investigate its influence on the final properties of the ceramic formulations. It is worth noting that a finer calcined halloysite may enhance some properties by promoting early-stage sintering and more uniform mullite nucleation during firing. However, excessive fineness can also increase slurry viscosity and reduce workability, therefore an optimal particle size distribution should be targeted for industrial applications [16].

Fig. 2.

Particle size distribution of the milled halloysite powder.

The remaining slurry was dried in an oven overnight at 100 °C. Then the halloysite powder was granulated by moistening and sieving. The details of the granulation process were explained elsewhere [22]. Subsequently, the granules were placed into an alumina crucible and subjected to calcination in an electric furnace. The calcination was performed at 600 °C for 2 h (10 °C/min) in air environment.

The compositions with the addition of calcined halloysite were prepared separately by mixing the calcined granules with the standard slurry at 0.1, 0.3, 0.6, 1.0, and 2.0 wt.%. After homogeneous mixing of the granules with the slurry, these compositions were oven-dried at 100 °C for 24 h. Following, they were sprayed with water to wet them and then granulated. The granules of each composition were placed in the cavity of a steel die and shaped into test specimens by applying a pressure of 30 MPa with the use of a hydraulic press. The rectangular shaped bar samples of 75 mm × 7 mm × 5 mm in size were prepared for the mechanical study. The rectangular shaped bar samples were also pressed with the dimensions of 120 mm × 20 mm × 5 mm in order to determine pyroplastic deformation behavior. The test samples were sintered at 1180 °C for 2 h at a rate of 3 °C/min in a Nabertherm electric furnace. The sintered specimens were furnace-cooled at 3 °C/min. After sintering, the samples were characterized for elastic modulus, bending strength test, and pyroplastic index measurement. In addition, mineralogical and microstructural analyses were done.

Characterization of the samples

The modulus of elasticity of ten rectangular sintered samples was determined in accordance with the ASTM C1259-94 standard. To find the frequency of the samples at room temperature, a Grindo-Sonic instrument was employed. Following, the bending strength of these samples was determined using an electronic universal tester (Model 5569, Instron Ltd.) by a three-point bending test with a lower span of 50 mm and a crosshead speed of 1 mm/min, in accordance with the ASTM standard C1161-90. The magnitude of pyroplastic deformation was determined by calculating pyroplastic index (PI). For this purpose, the curvature of a specimen supported by two refractory supports was measured after the firing was completed. The detailed procedure determining the pyroplasticity index is explained elsewhere [23]. The pyroplasticity index is calculated according to Eq. (1).

with s: Maximum deformation (cm), b: Bar thickness (cm), l: Distance between supports (cm).

The phase analysis of the halloysite both in its as received form and after the calcination process was performed by X-ray diffraction (XRD) method. The sintered test samples were also subjected to XRD analysis. For this, a PANalytical Empyrean diffractometer working at 40 kV and 40 mA was used. They were scanned at 2°/min speed in the range of 5–70° with CuK_α radiation (λ = 0.154 nm). JADE software with the JCPDS database was used for phase identification. The estimated content of glassy and crystalline phases was calculated from the XRD patterns. For quantitative analysis, a baseline was drawn across each diffractogram to integrate the peak areas. The area of crystalline phase was calculated using the number of counts multiplied by the angle 2θ[24]. The amount of amorphous (glassy) phase was calculated by the difference of the crystalline phase area from the total area of all peaks. The percentage of each of the identified crystalline phases was determined using the X’Pert HighScore Plus Software. Scanning electron microscopy (SEM) and X-ray spectroscopy (EDS) analysis were performed to examine the microstructure and phases formed in the samples after sintering. The analysis was conducted by a Phillips XL30 SFEG scanning electron microscope. SEM images were captured in secondary electron (SE) mode. The SEM specimen's surface was ground and polished, then chemically etched in HF solution to remove the glassy phase and reveal the crystalline phases, and coated with gold/carbon.

Results and discussion

Phase analyses

The phase diffraction pattern of initial halloysite clay prior to calcination is given in Fig. 3. Halloysite chemical formula is represented by Al₂Si₂O₅(OH)₄nH₂O. It is a hydrated kaolinite polymorph, comprising 1:1 aluminosilicate layers with a layer of water molecules between them. When n = 2, the mineral is formed halloysite-10Å, reflecting its layer periodicity of 10 Å. The pattern in Fig. 3 confirmed that the halloysite powder exhibits a distinctive (001) peak at 2θ:∼8.96° which correlates with a basal spacing of 0.998 nm. This indicates that the material is in the form of halloysite-10Å (JCPDS # 029-1489). The diffraction pattern showed characteristic peaks for hydrated halloysite at 20.218°, 26.742°, 35.058 (2θ). The structure exhibited also the presence of quartz peaks (JCPDS # 046-1045) [18].

Fig. 3.

XRD pattern of halloysite powder as received (h: halloysite, q: quartz).

The phase diffraction pattern of halloysite clay after calcination at 600 °C is given in Fig. 4. After heating at 600 °C, halloysite-10Å underwent an irreversible transformation to halloysite-7Å due to the loss of the interlayer water, exhibiting the formula Al₂Si₂O₅(OH)₄, n = 0. As evident, the spectrum of halloysite powder treated at 600 °C no longer exhibits a peak at 8.96° (2θ). It only displayed a broad reflection at approximately 21.1°(2θ), resulting from the dehydroxylation of halloysite and revealing the formation of an amorphous phase [15,25,26]. Similar findings were reported by Peng Yuan et al. [25], who utilized XRD to characterize the thermal transformation of halloysite, showing key phase changes. They reported that a significant loss of crystallinity was observed due to dehydroxylation between 500 °C and 900 °C, resulting in the formation of an amorphous phase named metahalloysite. During this stage, silica and alumina originally locate in tetrahedral and octahedral sheets gradually separates, causing loss of long-range order. The halloysite's rough tubular morphology and mesoporosity remains mostly stable below 900 °C. Calcination at 1000 °C causes distortion of the tubular nanoparticles, while higher temperatures led to increased distortion and eventual destruction of the tubular structure. Given that our previous study [18] and the other study [25] confirm the calcination temperature does not degrade the halloysite's rod-like structure, such degradation is not anticipated in this study.

Fig. 4.

XRD pattern of halloysite powder after calcination at 600 °C.

Complex formation with partially or fully dehydrated halloysite is influenced by factors such as particle size, crystallinity, Fe content, and the presence and types of impurities. Highly crystalline halloysite exhibits stronger interlayer hydrogen bonding and more ordered octahedral sheet structures, which delay dehydroxylation during heating. Conversely, lower crystallinity halloysites contain more defects and stacking disorder, facilitating transformation into a reactive amorphous phase at lower temperatures [25–28]. This behavior aligns with the mechanism observed in kaolinite–metakaolin systems, where structural disorder enhances reactivity and promotes mullite nucleation during subsequent firing [16,27]. In the present study, halloysite exhibiting moderate crystallinity was preferred, which is considered advantageous for facilitating efficient amorphization and enhancing sintering performance.

Fig. 5 shows the XRD pattern of the STD, HT-0.1, HT-0.3, HT-0.6, HT-1.0, and HT-2.0 samples sintered at 1180 °C. The graph demonstrates that all samples contain anorthite (CaO·Al₂O₃·2SiO₂) (JCPDS # 073-0265), quartz (JCPDS # 083-0539), and mullite crystals (3Al₂O₃·2SiO₂) (JCPDS # 079-1275). Moreover, a quantitative analysis was performed to determine the quantity of crystalline and amorphous phases present. The results are presented in Fig. 6. It can be observed that the samples incorporating varying percentages of calcined halloysite exhibited a higher amount of the mullite crystalline phase in comparison to the standard composition. The observed increase in mullite content was attributed to the presence of halloysite, which is in accordance with the findings of previous studies in the literature [15,17,29]. In consideration of the comparable anorthite content, a meaningful comparison could be made between HT-1.0 and the STD sample. The HT-1.0 displayed a reduction in quartz and an increase in mullite content in comparison to the STD sample, thereby demonstrating the influence of halloysite on the formation of the mullite phase. In addition, the amorphous phase increases with halloysite addition (except for HT-1.0), suggesting the development of a larger glassy matrix. During firing, the decomposition of halloysite produces amorphous aluminosilicate species that partially melt and contribute to the formation of this extended glassy phase.

Fig. 5.

XRD patterns of the samples after sintering at 1180 °C (m: mullite, h: halloysite, q: quartz).

Fig. 6.

Phases and their % amount in the samples sintered at 1180 °C

Microstructural analyses

The microstructure images of the STD, HT-0.6, HT-1.0 and HT-2.0 samples after sintering at 1180 °C are given in Fig. 7a–e. The SEM examination provides direct evidence of the morphological evolution caused by halloysite addition. The elemental compositions of the formed phases were determined using EDX analysis (Fig. 7e). The EDS result of region A presents the dominance of calcium (Ca), aluminum (Al), and silicon (Si) peaks, thereby revealing that these interconnected clusters are anorthite crystals. The configuration of the anorthite crystals indicates that anorthite occurred in the primary (spheroidal) formation [30]. The EDS analysis taken from angular grains marked as region Q, displays primarily a silicon (Si) peak, and it is attributed to quartz. Here, weak Ca and Al peaks are also detected. The needle-like grains, observed in region M, are identified as secondary mullite grains, indicated by the most prominent peaks which are silicon and aluminum. Here, a weak Ca peak is also detected. Those weak peaks are observed as an interpenetrating effect of crystals in adjacent regions. As a result, all samples exhibit a multiphase microstructure containing quartz, anorthite, and mullite crystals embedded in an amorphous glassy matrix, consistent with the XRD results.

Fig. 7.

Microstructure analysis of (a) STD, (b) HT-0.6, (c) HT-1.0, (d) HT-2.0 and (e) EDX analysis of the A, M, and Q regions.

In the images, each specimen exhibits porosities within the microstructure, as some are pointed by the red arrow. In the STD sample (Fig. 7a), needle-like mullite grains are observed within some pores while other pores remain unoccupied. The pores appear relatively larger in size and more abundant compared to the other samples. HT-0.6 sample shows partial pore filling with newly formed mullite grains; however, the distribution of these crystals remains heterogeneous in overall microstructure (Fig. 7b). In contrast, the HT-1.0 sample demonstrates fewer and smaller pores with all of them filled with mullite crystals (Fig. 7c). When the amount of halloysite increased to 1.0 wt.%, there was an increase in the degree of mullite in the microstructure with a more homogeneous distribution. The elongated morphology of these needles is attributed to their enrichment in Al₂O₃ content with added halloysite [31]. This composition facilitates the rapid growth of mullite crystals. As the halloysite content is increased to higher levels (HT-2.0), the tendency for particle agglomeration becomes more pronounced. This agglomeration disrupts the uniform distribution and continuity of the mullite network, leading to the formation of a less cohesive microstructure. The SEM image revealed the presence of irregularly distributed and unfilled pores throughout the structure (Fig. 7d).

Mechanical properties

Fig. 8 shows the three-point bending strength and elastic modulus graph of the samples sintered at 1180 °C. In Fig. 8, the bending strength of the bodies exhibited a composition-dependent trend with increasing calcined halloysite content from 0 to 2 wt.%. As the halloysite concentration increases from 0 to 0.1 wt.%, the bending strength increases from 9.23 MPa to 12.75 MPa. The addition of calcined halloysite, even in small amounts, positively influenced the sample resistance to bending forces. An almost stable trend was observed in the further increase of the halloysite value from 0.1 to 0.3 and 0.6 wt.%. Notably, the strength value peaked at 1.0 wt.% with a value of 12.80 MPa, and subsequently decreased with a higher concentration of calcined halloysite, reaching 11.26 MPa for HT-2.0. Although HT-2.0's bending strength was lower than that of other samples in the halloysite added series, this value was still above that of the standard body. This mechanical behavior closely correlates with the evolution of crystalline and amorphous phases within the microstructure of the samples [32]. The type and quantity of these phases are presented in Fig. 6. At low halloysite additions (≤0.6 wt.%), the amorphous phase remained predominant accompanied by an increase in crystalline phases, primarily attributed to the formation of anorthite and mullite. Accordingly, the noticeable improvement in bending strength at 0.1 wt.% can be associated with the rearrangement of the microstructure and the contribution of the halloysite-derived amorphous phase to early-stage densification during firing. The strengthening effect exhibited only minimal variation up to the HT-1.0 composition because the additive content remained relatively low, and the enhanced mullite formation although beneficial, may not have significantly altered the overall microstructural continuity of the ceramic matrix. Furthermore, the amorphous matrix lacked a well-interlocked crystalline framework capable of efficiently transferring stress. These interpretations are supported by the microstructural observation presented in Fig. 7b. At 1.0 wt.% halloysite addition, the balance between amorphous and crystalline phases reached an optimum, resulting in the highest bending strength (12.8 MPa). Fig. 6 shows that the amorphous fraction decreased, while the amount of mullite remained higher compared to the STD sample in HT-1.0 sample. In addition, Fig. 7c reveals the formation of a well-distributed, interlocked crystalline network within the glassy matrix. As reported by Harabi et al. [17], the initial halloysite morphology promoted the formation of interlocked elongated mullite and anorthite microcrystals, resulting in a bridged microstructure that enhanced mechanical strength. This microstructural configuration effectively bridges residual pores and enhances stress transfer across grain boundaries, thereby improving the overall mechanical integrity of the ceramic body [17].

Fig. 8.

The bending strength and elastic modulus values of sintered samples depending on the compositions.

At 2.0 wt.%, however, the excess halloysite tends to particle agglomerate and does not disperse effectively within the body. This resulted in localized porosity and microstructural heterogeneity, which can act as preferential sites for stress concentration and crack initiation (Fig. 7d). Furthermore, the bending strength results are in good agreement with the quartz-mullite relationship reported by Martín-Márquez et al. [10] who demonstrated that a decrease in quartz content combined with enhanced mullite formation during firing positively contributes to the bending strength.

These findings indicate that the incorporation of small amounts of halloysite is beneficial for adequate densification and refining the microstructure, whereas excessive additions tend to compromise microstructural integrity and adversely affect mechanical performance. The elastic modulus graph, an index of material rigidity [11], is also given in Fig. 8. Regarding elastic modulus, the standard sample exhibited a value of 17.97 GPa. The elastic modulus generally decreased with halloysite addition, as evidenced by dropping to the lowest value (12.35 GPa) with a 2.0 wt.% halloysite addition. The second lowest elastic modulus is observed in HT-0.6 sample with a 12.39 GPa value. On the other hand, the 1.0 wt.% addition yielded the second-highest value (14.91 GPa) recorded after that of the standard body. This variation in elastic modulus with calcined halloysite incorporation indicates a complex interaction between the halloysite-derived phases and the surrounding ceramic matrix during sintering. Notably, the HT-1.0 sample exhibited both the highest bending strength and a relatively high elastic modulus, suggesting that a balanced phase composition and the formation of a well-connected microstructure contribute to enhanced mechanical performance.

These findings are in agreement with other studies in the literature and demonstrate that the addition of calcined halloysite is an effective method to improve mechanical properties [17,18].

Pyroplastic deformation measurements

The graph of pyroplastic index values change of sintered samples depending on the calcined halloysite addition into the standard slurry is given in Fig. 9. Almost all samples with added calcined halloysite generally showed a degree of deformation comparable to that of the STD sample (1.41 × 10⁻⁵ cm⁻¹). At low halloysite contents (0.1 and 0.3 wt.%), the pyroplastic deformation index slightly increases to 1.63 × 10⁻⁵ and 1.57 × 10⁻⁵ cm⁻¹, respectively. Interestingly, HT-0.6 had the highest value in deformation, reaching 2.25 × 10⁻⁵ cm⁻¹, indicating that this concentration may lead to instability during firing. The influence of calcined halloysite on pyroplastic deformation is closely related to the microstructural changes occurring during firing [4,33]. The decomposition of calcined halloysite yields amorphous aluminosilicate species that melt at relatively lower temperatures, thereby increasing the amount of amorphous phase [16], as shown in Fig. 7. This enhanced glassy phase facilitates viscous flow during firing. The formation of mullite contributes to the development of a rigid, interconnected crystalline network that limits viscous flow and thus enhances resistance to shape distortion at high temperatures, reducing pyroplastic deformation [11]. However, the increased liquid-phase volume (Fig. 6) and the nonhomogeneous phase distribution observed in the microstructure (Fig. 7b) weakens the structural stability of the ceramic body. It is also noteworthy that the HT-0.6 specimen exhibited the lowest quartz content (3.03%) and the highest anorthite content (21.92%) (Fig. 6). This sample showed the highest value in deformation with a considerably high anorthite amount in phase ratios compared to other samples with halloysite. It can be concluded that the generated anorthite crystals in the microstructure were not able to withstand the pyroplastic deformation during firing. This finding is in good agreement with the results given by Capoglu [11]. Significantly, the addition of 1.0 wt.% calcined halloysite significantly improved the resistance to pyroplastic deformation. HT-1.0 sample exhibited the lowest pyroplastic index value (1.15 × 10⁻⁵ cm⁻¹) among all samples tested, demonstrating that an optimal halloysite addition can enhance resistance to pyroplastic deformation by maintaining a proper balance between crystalline reinforcement and amorphous-phase fluidity. It contained the lowest amorphous phase fraction. As shown in Fig. 7c, the HT-1.0 sample exhibited a homogeneous distribution of crystalline phases, with pores effectively filled by mullite within the microstructure, indicating a well developed phase equilibrium during firing. At the highest halloysite content tested (2.0 wt.%), the deformation index rose again to 1.68 × 10⁻⁵ cm⁻¹, suggesting a reduction in dimensional stability relative to the 1.0 wt.% sample, The excess halloysite content, accompanied by particle agglomeration, leads to heterogeneous mullite formation as shown in Fig. 7d, which weakens the structural integrity under load. At the same time, the amorphous phase fraction increases again, leading to higher liquid-phase mobility that, in the absence of a homogeneous crystalline framework, enhances viscous deformation and results in a higher pyroplastic index at elevated halloysite levels. The balance between crystalline reinforcement and amorphous viscosity appears to govern the overall pyroplastic behavior of the system.

Fig. 9.

Pyroplastic index values of sintered samples depending on the compositions.

Conclusions

This study highlights the significant impact of calcined halloysite on the deformation, mechanical and microstructural properties of tableware bodies sintered at 1180 °C. The findings demonstrate that incorporation of calcined halloysite can improve bending strength with only a minor effect on the elastic modulus. At the low halloysite additions (≤0.6 wt.%), the amorphous phase remained predominant, with an increase in crystalline phases (anorthite and mullite). The initial improvement in bending strength at 0.1 wt.% is attributed to microstructural rearrangement and the contribution of halloysite-derived amorphous aluminosilicate species to early densification. HT-0.6 sample—with the lowest quartz (3.03%) and highest anorthite (21.9%) contents—displayed the highest pyroplastic deformation due to a shift in crystalline balance and reduced structural stability while the HT-1.0 sample exhibited the lowest deformation (1.15 × 10⁻⁵ cm⁻¹), showing needle-like mullite crystals in the microstructure with a more homogeneous distribution that effectively filled pores within the matrix. The interlocking nature of mullite crystals contributed to improved strength and reduced deformation. At higher halloysite addition (2.0 wt.%), microstructural and phase-related limitations became evident. Excess halloysite promoted the particle agglomeration and heterogeneous mullite formation, interrupting crystalline continuity and increasing residual porosity which serve as stress concentrators and limit mechanical improvement.

In summary, the 1.0 wt.% halloysite addition represents an optimum composition that provides (i) an interlocked and continuous mullite framework, (ii) efficient pore filling, and (iii) a favorable mullite/anorthite/quartz/amorphous phase balance. These factors collectively result in the best combination of bending strength and pyroplastic deformation resistance. The findings suggest that calcined halloysite additive may pave the way for further exploration and development of thinner ceramics for dinnerware applications.

Conflict of interest

The authors declare that they have no known competing financial interest or personal relationship that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by Canakkale Onsekiz Mart University The Scientific Research Coordination Unit, Project number: FBA-2022-3826.