A Ceramic Foam Structure Design with the Valorization of Fly Ash Cenospheres: A Promising Avenue for Sustainable Bioscaffolds ^†

Abstract

Nowadays, there is wide advocacy for a transition to circular economic models. Fly Ash (FA) in particular is a major by-product of coal combustion and its annual waste has reached one million tonnes. Cenospheres (CSs) are considered as possibly the most valuable element within FA. Thus, in this research, polymeric foam replication was employed to fabricate ceramic foams based on a CS matrix, for potential biomedical applications. For the fabrication of foams, four types of natural marine sponges were used as templates along with a binder agent. The specimens were sintered at 1200 °C for 1 h. The results were encouraging as the specimens obtained retained the given shape and geometry. Further research will enhance the potential of such materials for future use in biomedical engineering.

1. Introduction

The reckless generation of wastes and their accumulation has raised concerns regarding environmental sustainability. The present economic system depends on a linear economic model, in which the materials follow a standard path, starting from resource extraction, then moving on to final product formation and disposal. This process burdens the environment, causing its degradation [1]. As an outcome, resources start to deplete, while wastes still recklessly accumulate [2,3]. In this light, there is a wide advocacy for altering the economic model, from the linear to a circular one. This model is called the circular economic (CE) model and seems to be sustainable.

Major contributors of global pollution are fossil fuels. Because of its abundance, coal is one of the most common fossil fuels used for energy production [4]. Coal combustion leads to ash production, which is classified into two categories—bottom and fly ash (FA)—and both of them act as pollutants. FA is divided into Class C and Class F FA, depending on the calcium content [5].

The management of the produced FA remains a challenge. Specifically, the annual production of FA is very high and it reaches 750 million tones [6,7]. Current management practices include deposits in land or agricultural landfills, thus burdening the ecological system and human health [6,8].

FA consists of Cenospheres (CSs), metal oxides, solid fractions, and unburned carbon [6,7]. Among them, CS is considered as the element of the highest value [5,7,9]. CSs are hollow spheres in microns, composed mainly of aluminosilicates. Their main properties are hardness, light weight and chemical inertness [5,9]. The fact that CS is lightweight makes it suitable to form composite lightweight materials, with polymers and metals [6]. Specifically, CS is applied in electromagnetic shielding, heat shielding, to enhance acoustic properties and in construction, like building materials and concrete [6,10,11]. Moreover, the introduction of CS in agricultural materials, is also, studied, as the research focuses on the incorporation of CS in hydrogels to achieve controlled insecticide release [12]. Furthermore, CS could also, be applied in the biomedical sector, as it can form lightweight composite materials and foams for different biomedical applications, such as drug carriers and scaffolds for tissue engineering and wound healing [9].

This research focuses on elaborating a ceramic foam for potential biomedical application as sustainable scaffold by valorizing fly ash cenospheres (CSs). The ceramic foam was created employing the polymeric foam replication technique, using natural sponges as templates. The specimens were sintered at 1200 °C for 1 h and then they characterized.

2. Materials and Methods

CSs with a high content of silicon oxide (SiO₂) and aluminum oxide (AlO₂), 58.67% and 34.55%, respectively, were acquired from Plomp Mineral Services (Sleeuwijk, Netherlands). Four types of natural sponges were acquired from the Greek island of Kalymnos in the Mediterranean Sea. Those four types were as follows: Grass, Silk Fina, Hoenycomb and Hard. With the exception of the bleached sponge, all of the sponges had their original color. PVA was serves as binder agent, with a 30,000–70,000 g/mol molecular weight (Sigma-Aldrich, Darmstadt, Germany).

The fabrication process included the mix of distilled water with PVA and CS to create a ceramic slurry. Three ceramic slurries were created and denoted depending on the ratio of CS/PVA: 1, 1/2, 1, and 2. Before mixing, PVA was magnetically stirred to 90 °C for 1 h. Then, the solution was let to drop to room temperature while stirring and the addition of CS was followed. This stirring process lasted another 1 h. All of the sponges, with the exception of Honeycomb, whose structure required compression for the immersion, were cubed and submerged in the slurry. The pressing of Honeycomb was uniaxial, applying 5 tn. The specimens were dried for 24 h and then immersed again into the slurry. Each immersion lasted 10 min under magnetic stirring. Then, the specimens were squeezed to remove all the excess slurry and dried for 4 h at 100 °C. Sintering of 1 h at 1200 °C, with a heating ratio of 2 °C/min and a soaking step at 400 °C for 1 h, was the final step of the fabrication process.

SEM and EDS analysis exhibited information regarding the microstructure and elemental composition.

3. Results and Discussion

The foam obtained from Silk Fina exhibited minor shrinkage, which was anticipated. Their structure was maintained, but the materials were brittle. Ceramic foam from Hard sponges exhibited less hardness than those of Silk Fina, while their geometry and shape remained unchanged. Honeycomb sponge led to amorphous ceramic foams, which is attributed to the loss of pastille shape of the sponge after the immersion into the ceramic slurry. Ceramic foams of this template also demonstrated brittleness. Grass sponges underperformed, as particles failed to aggregate, resulting in powdery substance after sintering. Figure 1 demonstrates ceramic foams of the three successful templates.

SEM analysis revealed the presence of pores and their interconnectivity. Despite this, some spherical structures were destroyed. Thus, it is believed that the binder agent did not perform successfully, forming slurries with not the appropriate thixotropy and viscosity. Subsequently, this had the result of forming brittle materials. EDS analysis indicated the dominance of oxygen, due to the oxide presence. The next two elements were silicon and aluminum. Moreover, carbon was present. Its presence can be possibly justified by the partial burning of residual carbon in the original CS powder. Figure 2 illustrates SEM and EDS analysis of Silk Fina-derived foam.

4. Conclusions

Towards enhancing circularity, a ceramic foam was produced with the valorization of CS, a waste derived from the energy sector as part of FA. The ceramic foams were elaborated by a polymeric replication technique and sintered at 1200 °C for 1 h, accompanied by SEM and EDS assessment to analyze the microstructure and elemental features. Silk Fina and Hard sponges led to foams with great replication in shape and geometry, while Honeycomb led to amorphous foams. All of the materials were characterized by brittleness, probably attributed to the underperformance of PVA as a binder agent.

The fabrication of bioscaffolds from CS using natural sponges as templates seems promising. The results from the fabricated foams are encouraging. Nonetheless, to substantiate the potential use of such a material in biomedical engineering, a first step of enhancing the manufacturing process is essential, and this must involve the optimized selection of a more appropriate and sustainable binding agent.