Innovative Manufacturing Processes of Silicate Materials

1. Introduction

Silicate ceramic materials are likely the oldest manufactured materials in human history. For centuries, they have played a crucial role in domestic life, construction, and artisanal crafts. In the modern era, their relevance remains strong, largely due to their outstanding refractory and electrical insulating properties [1,2,3]. Consequently, silicate ceramics continue to represent the most widely produced class of ceramics by volume today.

Since the mid-twentieth century, however, the rise of technical ceramics has introduced growing competition for silicate ceramics in advanced technological applications. Nonetheless, silicate ceramics retain significant advantages in numerous domains, operating at ambient to moderate temperatures (up to 1200 °C). These materials are sourced from abundant natural resources, require relatively low to moderate sintering temperatures, and offer greater potential for recyclability.

Recent advancements in fabrication techniques, coupled with the emergence of innovative materials such as geopolymers [4], have sparked renewed interest in silicate ceramics. They are increasingly being explored in strategic sectors such as energy [5], biomedicine [6], and electronics [7]. This resurgence is particularly relevant in the context of current global priorities centered on sustainability and circular economy principles [8].

Unlocking the full potential of silicate ceramics across diverse industries requires a deep understanding and precise control of their properties and microstructures—an objective that hinges on the development and application of innovative manufacturing processes. In this light, the Special Issue of the journal Ceramics, entitled “Innovative Manufacturing Processes of Silicate Materials”, aims to highlight recent progress in this evolving field. This issue comprises ten contributions that offer new insights into key research areas, including additive manufacturing, geopolymer processing, refractory technologies, and functional applications.

2. Overview of the Contributions

Since the 1990s, additive manufacturing has emerged as a revolutionary approach to producing objects. Initially confined to research laboratories, it has gradually transitioned to industrial-scale production and is now being applied across all classes of materials.

The additive manufacturing of silicate ceramics can be implemented through various techniques, depending on the desired characteristics of the final part and the type of feedstock used [9,10,11]. Among these, robocasting stands out as one of the most mature and widely studied approaches [12]. However, several challenges remain, notably the formation of defects caused by non-uniform shrinkage during the drying stage [13,14,15]. In their contribution, N. Lauro et al. [16] present innovative methods for measuring shrinkage using non-destructive optical vision techniques combined with computer-controlled data acquisition. Applied to the robocasting of a porcelain-based paste, these methods revealed anisotropic shrinkage, which was attributed to the preferred grain orientation induced by the extrusion process. This work is particularly valuable as it offers promising tools for the real-time monitoring and control of the drying process, potentially improving the dimensional accuracy and reliability of robocasted components.

Robocasting also shows great promise as a shaping technique for geopolymers. The geopolymerization process enables the formation of inorganic silicate materials that exhibit excellent mechanical strength, chemical resistance, and long-term durability [4,17,18]. As a result, there is a growing body of research dedicated to developing geopolymers for applications traditionally dominated by conventional ceramics [19,20]. Particularly, porous geopolymer materials are gaining attention for their suitability in diverse applications, including radioactive waste encapsulation, the adsorption of hazardous molecules and synthetic dyes, and heterogeneous catalysis [21,22,23,24,25,26]. In this context, C. Zoude et al. [27] explored the influence of curing parameters, specifically humidity and temperature, on the mechanical properties of metakaolin-based geopolymer filaments fabricated via 3D printing. Their study also compared the performance of samples produced through robocasting with those obtained via conventional molding. The results provide valuable insights into the optimization of the processing conditions for geopolymer-based additive manufacturing.

Beyond the curing conditions, the print quality of robocasted parts—regardless of the material used—depends critically on the rheological behavior of the extruded paste. In this regard, A. Gasmi et al. [28] conducted a comprehensive study on the fabrication of geopolymer composites, with particular emphasis on rheological characterization. Their research focused on how the amount and morphology of mineral fillers affect the paste’s flow properties, aiming to ensure structural integrity, dimensional precision, and minimal sagging during printing.

Geopolymer composites represent a promising pathway for producing ceramic-like materials without the need for high-temperature sintering. In these systems, the geopolymer matrix provides a “cold” consolidation mechanism, binding the filler particles through chemical reactions that can also enhance the thermal and mechanical stability of the final material [29,30]. Expanding on this approach, F. Casarrubios et al. [31] investigated the development of cordierite–geopolymer composites for potential applications in filtration and catalysis at temperatures of up to 1000 °C. Their study placed particular emphasis on the recycling of cordierite powders from industrial waste and examined the effects of both the K/Al ratio and cordierite content on key performance parameters, including dimensional and porosity stability during heating, Young’s modulus, and the coefficient of thermal expansion. These insights highlight the potential of geopolymer-based systems for sustainable, high-performance applications in harsh environments.

In addition to extrusion-based techniques such as robocasting, vat photopolymerization methods like stereolithography (SLA) and digital light processing (DLP) have gained increasing attention for the additive manufacturing of ceramics, particularly when complex geometries and high resolution are required. In this context, Yurii Milovanov et al. [32] employed DLP to fabricate mullite-based porous substrates intended for use in humidity sensors. Their work demonstrates that mullite is a promising functional material for the development of resistive sensors capable of operating under extreme environmental conditions.

More broadly, porous ceramics are of considerable interest across a wide array of fields, including filtration, catalysis, biomedical applications, and environmental technologies, due to their thermal stability, mechanical robustness, ease of cleaning, and extended service life [33]. As a result, the processing of porous ceramics has been extensively explored over the past few decades. Numerous fabrication strategies have been developed, such as partial sintering, freeze-casting, sacrificial template replication, and direct foaming. Notably, producing porous ceramics from natural silicate materials presents several advantages, particularly for water filtration applications, where cost, sustainability, and material availability are critical factors. In their review article, I-Q Maury Njoya et al. [34] provide a comprehensive overview of the techniques and raw material systems used for fabricating porous clay-based ceramics, focusing on key performance parameters such as porosity, mechanical strength, permeability, and filtration efficiency.

In the field of refractories, the increasing deployment of biomass combustion boilers has created a demand for materials with enhanced resistance to alkali corrosion. In particular, the aggressive conditions encountered during the combustion of certain biomass species—where temperatures can reach up to 1200 °C—render traditional castables containing aluminosilicate aggregates unsuitable in the long term. In their study, J. Malaiškienė et al. [35] demonstrate that the key properties of conventional and medium-cement fireclay refractory castables can be significantly improved through impregnation with liquid sodium silicate glass. This treatment results in microstructural densification and reduced porosity, while also promoting crack healing and facilitating the formation of anorthite. However, it is noted that the thermal shock resistance of the treated materials decreases due to an increase in Young’s modulus, highlighting a trade-off between chemical durability and mechanical resilience.

Finally, silicate-based materials exhibit strong potential for functional applications. For example, in the biomedical field, Y. Sugiura et al. [36] investigated the synthesis and characterization of silica-substituted octacalcium phosphate (OCP, Ca₈(PO₄)₄(HPO₄)₂·5H₂O) blocks incorporating dicarboxylic acids, specifically thiomalic acid (SH-malate). Their study demonstrates the successful fabrication of monophasic OCP blocks simultaneously substituted with silica and dicarboxylic acid molecules. Notably, the resulting composites exhibit enhanced mechanical strength compared to their non-substituted counterparts. These findings suggest promising avenues for the development of advanced bone replacement materials, offering improved mechanical performance and potential for more efficient bone regeneration and healing.

Beyond biomedical applications, silicate-based materials also hold significant promise in catalysis due to their tunable surface chemistry and porosity. In this context, A. Guerrero-Torres et al. [37] present a novel approach for dispersing metal particles on sepiolite supports via a microwave-assisted acid treatment. This process partially leaches the octahedral layer of the sepiolite structure, leading to an increased specific surface area and pore volume, while simultaneously altering the surface chemistry through the removal of Mg²⁺ species. The resulting modified sepiolite exhibits characteristics favorable for catalytic applications, particularly as a support for the dispersion of active phases. The use of microwave irradiation not only accelerates the treatment process, but also enables the use of significantly more diluted acid solutions, enhancing both efficiency and sustainability. The study further evaluates the impact of these structural modifications on catalytic performance, specifically in the gas-phase hydrogenation of furfural.

Within the diverse landscape of silicate materials, glasses and glass–ceramics represent a particularly attractive class for optical and electro-optical applications, due to their unique ability to incorporate and control functional nanostructures. In this context, G. Shakhgildyan et al. [38] investigate the precipitation of gold nanoparticles (NPs) in a ZnO–Al₂O₃–SiO₂ glass matrix as a means of generating tunable localized surface plasmon resonance (LSPR). Their work reveals that the spectral position and bandwidth of the LSPR can be precisely modulated through thermal treatment parameters. This controlled nanoparticle formation, leading to ultra-broad LSPR bands, opens up new avenues for enhancing the luminescence of rare-earth-doped systems and contributes to the development of advanced photonic components and devices.

3. Conclusions

The contributions compiled in this Special Issue illustrate the remarkable versatility and continued evolution of silicate-based materials across a broad spectrum of applications. From structural ceramics and refractories engineered for extreme environments, to additive manufacturing innovations, environmental remediation strategies, and advanced photonic systems, the articles reflect the dynamic interplay between composition, processing, and functionality. These works underscore the relevance of both traditional and emerging approaches—ranging from geopolymer technology to nanoengineering, expanding the performance boundaries of silicate materials. Collectively, they offer new insights and technological perspectives that will undoubtedly guide future research and industrial development in the field of ceramic science.