Correlation Study on the Effect of Sintering Mechanism with the Properties of Geopolymer-Based Ceramic

Nepheline geopolymer-based ceramics are emerging as a promising alternative to traditional ceramics due to their eco-friendly production and sustainable nature. Therefore, this study aims to comprehensively investigate the relationship between mechanical behaviour and sintering mechanisms in the production of kaolin geopolymer-based nepheline ceramics. Sodium hydroxide and sodium silicate were mixed to act as the alkaline activator to facilitate the geopolymerization process. The experimental analysis involved varying the sintering temperature within the range of 200°C to 1200°C. The findings from the correlation study highlight that the flexural strength and densification process is in linear relation with R2 of 0.9369, whilst the water absorption and volumetric shrinkage exhibited an inversely linear relationship with the R2 value of 0.8733. The maximum flexural strength of 78.92 MPa and density of 2.56 g/cm3 were achieved when sintered at 1200°C. Meanwhile, the water absorption decreases with the increase of volumetric shrinkage, which might relate to the densification process of the geopolymer-based nepheline ceramic. The outcome of this research contributes a deeper understanding of the interplay between mechanical behaviour and sintering mechanism, enabling the design of superior sintered materials.


Introduction
Ceramics have been an integral part of human civilization for thousands of years, serving as essential materials in various industries, including construction, electronics, automotive, and healthcare [1][2][3][4][5].Even though ceramics have been used independently for a wide range of applications due to their excellent properties such as significant compressive strength, low thermal conductivity, remarkable hardness, and chemical inertness, however, they still possess some disadvantages.Conventionally, ceramics have been produced using methods such as hot pressing, pressure-less sintering and hot isostatic pressing, with high-temperature sintering which might go up to 1800°C [6].This method is not desirable as it contributes to high production costs, and also could lead to abnormal grain growth.While these conventional techniques have been successful in producing high-quality ceramics, they often involve energy-intensive processes and may have limitations in terms of material properties and sustainability.Investigating alternate methods of ceramic manufacture that overcome the drawbacks of traditional ones has garnered more attention in recent years.A novel method of using geopolymer as the precursor in ceramics production had been introduced due to its robustness and versatility which results in speedy strength development [7].
Geopolymers, a class of inorganic polymers, have emerged as a promising solution in this regard.Due to their two primary benefits; low energy usage and absence of CO2 emissions during preparation, they have attracted scientific research recently [8].Davidovits proposed in 1978 that the by-product of alkaline solution's reaction with aluminium (Al) and silicon (Si) may be used to produce geopolymers [9].Geopolymers are formed by the reaction of aluminosilicate materials activated in an alkaline medium consisting of the mixture of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3), resulting in a three-dimensional network structure of polymeric Si-O-Al that is extremely firm.The polycondensation process of the geopolymer occurs at a temperature below 100°C in ambient pressure.Due to its distinctive characteristics, which contribute to excellent chemical and fire resistance as well as relatively high mechanical strength, kaolin has been widely employed as an aluminosilicate source [10,11], where the main composition is silica (SiO2) and alumina (Al2O3).The unique chemical composition and structure of geopolymers offer several advantages over traditional ceramics, including lower sintering temperature requirements, enhanced mechanical properties, superior chemical resistance, and lower environmental impact.
Geopolymer-based ceramics are a relatively novel method that creates high-quality ceramics with greater mechanical strength as well as lower porosity, which is advantageous for some applications.Geopolymer-based ceramics possess low sintering temperature that gives advantages from both environmental and technological points of view [12].The geopolymer-based ceramics are more sustainable compared to conventional ceramics as they can utilize waste as the source materials, reduce carbon dioxide emissions, and has better resistance to chemical and fire [13].
One critical aspect of ceramic production is the sintering process, which involves heating the compacted ceramic material to a high temperature to achieve densification and bonding between particles.The sintering mechanism employed during this process greatly influences the final microstructure, density, porosity, and mechanical properties of the ceramic material.Understanding the correlation between the sintering mechanism and the properties of ceramics is crucial for optimizing the manufacturing process and tailoring the material properties to specific applications.Geopolymer-based ceramics exhibit unique sintering characteristics due to their chemical composition and reaction mechanism.Therefore, a comprehensive study is needed to explore the relationship between the sintering mechanism and the properties of geopolymer-based ceramics.
This paper aims to present a correlation study that focuses on the effect of different sintering mechanisms on the properties of geopolymer-based ceramics.The resulting data will be analysed statistically to determine the significance of the sintering mechanism on the geopolymer-based ceramic properties.The findings of this correlation study will contribute to a better understanding of the sintering process and provide valuable insights for optimizing the manufacturing process.This knowledge will enable the development of tailored geopolymer-based ceramic materials with enhanced performance and application potential in various industries, including high-temperature coatings, structural components, and advanced functional ceramics.Ultimately, the utilization of geopolymer-based ceramics can lead to more sustainable and efficient ceramic production processes with improved material properties and reduced environmental impact.

Materials
The experiment encompassed the utilization of kaolin, denoted as Al2Si2O5(OH)4, acquired from Associated Kaolin Industries Malaysia, as the primary raw material.The chemical composition of the kaolin was meticulously determined by X-Ray Fluorescence (XRF) analysis as per shown in Table 1 below.Based on the result, the dominant compounds found within the composition of the raw kaolin material are SiO2 and Al2O3.To propel the geopolymerization process with utmost efficacy, the kaolin was mixed with a 12M alkaline activator, comprising of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3), meticulously blended with a fixed ratio of 0.24.These solutions were sourced from Formosoda-P, Taiwan and South Pacific Chemicals Industries Sdn.Bhd.(SPCI), Malaysia, respectively.

Experimental procedures
In formulating the geopolymer systems, the activation of kaolin was accomplished by employing an alkaline activator solution comprising NaOH and Na2SiO3.The solid-to-liquid and Na2SiO3/NaOH ratios were 1.0 and 0.24, respectively [14], thereby yielding a cohesive geopolymer paste with a slurry-like consistency.Following the preparation stage, the kaolin-geopolymer composite underwent a thorough drying and overnight curing process, after which it was milled and sieved to obtain a finely powdered form.The resulting geopolymer green body was then subjected to a compaction process at a pressure of 4.5 tonnes for a duration of 2 minutes, effectively shaping it into a bar-like structure.The compacted specimens were subsequently subjected to sintering at various temperatures within the range of 200°C to 1200°C with a fixed heating rate of 5°C/min and holding time of 180 mins to study the effects of sintering mechanisms on the properties of the produced geopolymer-based nepheline ceramics.

Mechanical testing
The flexural strength test for the sintered and unsintered geopolymer samples was carried out according to ASTM C-1163b, where the support span was 30 mm long, and the crosshead speed is 0.3 mm/min [15].The density was calculated, and the water absorption tests were performed following the guidelines outlined in ASTM C642-13 [16].The weight of the samples before and after immersion was documented, and the water absorption percentages were determined using the equation presented in Eq. 1 as follows.
Water absorption (%) Where ∆W is the difference in weight and W0 is the initial weight before immersion.The volumetric shrinkage was manually measured by calculating the changes in the volume upon the sintering process to examine the effects of heat exposure on the performance of geopolymer-based ceramic.
the influence of temperature on the properties of geopolymer ceramics.

Correlation between sintering temperature with density and flexural strength of geopolymerbased ceramic
The properties of nepheline ceramics-based geopolymer are influenced by the sintering temperature.
Figure 1 shows the trend for the density and flexural strength of the samples as the sintering temperature increase.The findings demonstrate that the maximum flexural strength of 78.9 MPa was attained when the specimens were subjected to sintering at 1200°C.A smooth matrix was formed as a result of the development of a broad sintered area, which was aided by the effective diffusion of particles within the samples.The unsintered sample and sample sintered at 200°C had the lowest density of 1.452 g/cm 3 and 1.614 g/cm 3 , respectively.The density then increases along the increment of the sintering temperature, whereas geopolymer-based nepheline ceramics sintered at 1200°C recorded the highest density at 2.564 g/cm 3 .The correlation of the flexural strength and the density of the geopolymer-based nepheline ceramics as shown in Figure 2 indicates a strong relationship of R 2 is 0.9369.The correlation between these properties can be regarded satisfy with each other.An excellent strength of ceramics is provided by the densified structure which is obtained at a high sintering temperature of 1200°C.As reported by Le et al. [17], the increase in flexural strength is attributed to the melting and densification effects of the sintering mechanism.The melting of the geopolymer matrix caused densification and facilitate an increase in the strength of the geopolymer-based ceramics.The elevated sintering temperature facilitated densification within the geopolymer matrix, which allowed it to fill the voids and cracks, thus leading consequently leading to improved strength.This process generated a more condensed structure, thereby enhancing the overall strength performance [18].

Correlation between sintering temperature with volumetric shrinkage and water absorption of geopolymer-based ceramic
Correlation between volumetric shrinkage and the water absorption of the nepheline ceramics in terms of the sintering mechanism is plotted in Figure 3.An inversely linear relation is obtained from the water absorption and volumetric shrinkage of the geopolymer-based ceramics with an R 2 value of 0.8733.Meanwhile, Figure 4 shows the trend for the volumetric shrinkage and water absorption influenced by sintering temperature.As the sintering temperature increase, the water absorption decreases from 28.64% to 0.24%, while the volumetric shrinkage increases to the maximum value of 29.91% when sintered at 1200°C.A reduction in the amount of water absorbed is observed as the volumetric shrinkage increase which might be due to the decrease of the permeable voids and density of microstructural in the geopolymer matrix.As the geopolymerbased ceramics undergo shrinkage, it leads to a reduction in the overall porosity, thus, resulting in lower water absorption [19].

Conclusions
The sintering mechanism study was conducted to investigate the effect of geopolymer-based nepheline ceramics exposed to various temperatures of 200°C, 400°C, 600°C, 800°C, 1000°C, and 1200°C on its mechanical properties.A compact and smooth surface with the highest strength and density of 78.92 MPa and 2.56 g/cm 3 , respectively, was achieved at a sintering temperature of 1200°C.The correlation from the effect of sintering temperature on the kaolin geopolymer-based ceramic indicates a strong relationship.The correlation between the flexural strength the density shows a high value of R 2 (0.9369), which is expected as sintering leads to particle rearrangement and grain growth in the geopolymer matrices, thus promoting in high flexural strength of the ceramics.Meanwhile, the correlation between water absorption and volumetric shrinkage presented an inversely linear relation with an R 2 value of 0.8733.Moreover, the study has provided insights into the mechanisms involved during the sintering process of geopolymer-based ceramics.This correlation study has deepened the understanding of the effect of different sintering mechanisms on the properties of geopolymer-based ceramics.The insights gained from this research will serve as a foundation for further advancements in the fabrication and design of geopolymer-based ceramic materials.By harnessing the knowledge obtained, we can continue to develop high-performance ceramics with tailored properties, leading to advancements in various industries and a more sustainable future for ceramic production.

Figure 1 .
Figure 1.Density and flexural strength of the geopolymer-based nepheline ceramics a function of sintering temperature

Figure 2 .
Figure 2. Relationship between density and flexural strength.

Figure 3 .
Figure 3. Relation between shrinkage and water absorption.

Figure 4 .
Figure 4. Volumetric shrinkage and water absorption of the geopolymer-based nepheline ceramics as a function of sintering temperature.

Table 1 .
Chemical composition of raw kaolin.