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Article

Microstructural Investigations Regarding Sustainable Recycling of Ceramic Slurry Collected from Industrial Waste Waters

by
Simona Elena Avram
1,
Lucian Barbu Tudoran
2,3,
Stanca Cuc
4,*,
Gheorghe Borodi
3,
Bianca Violeta Birle
1 and
Ioan Petean
5,*
1
Faculty of Materials and Environmental Engineering, Technical University of Cluj-Napoca, 103-105 Muncii Bd., 400641 Cluj-Napoca, Romania
2
Faculty of Biology and Geology, Babes-Bolyai University, 44 Gheorghe Bilaşcu Street, 400015 Cluj-Napoca, Romania
3
National Institute for Research and Development of Isotopic and Molecular Technologies, 65-103 Donath Street, 400293 Cluj-Napoca, Romania
4
Department of Polymer Composites, Raluca Ripan Institute for Research in Chemistry, Babes-Bolyai University, 30 Fantanele Street, 400294 Cluj-Napoca, Romania
5
Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, 11 Arany Janos Street, 400028 Cluj-Napoca, Romania
*
Authors to whom correspondence should be addressed.
Sustainability 2024, 16(3), 1123; https://doi.org/10.3390/su16031123
Submission received: 15 December 2023 / Revised: 11 January 2024 / Accepted: 26 January 2024 / Published: 29 January 2024
(This article belongs to the Section Sustainable Materials)
Browse Figures
Versions Notes

Abstract

:
Ceramic slurry wastes have a significant hazardous potential when dumped. Their recycling as raw material is a sustainable approach for the development of nature-friendly applications. The microstructure and mechanical properties play a key role in the success of this sustainable recycling. Ceramic slurry samples resulting from the wall and floor tiles production facility were analyzed. The mineral composition was investigated by XRD combined with mineralogical microscopy and the microstructure was investigated by SEM microscopy coupled with EDX spectroscopy and elemental mapping. The ceramic slurry contains: quartz, kaolinite, mullite and small amounts of lepidocrocite. Quartz and mullite particles have sizes in the range of 5–100 μm and kaolinite has small particles of around 1 to 30 μm. Iron hydroxide crystallized as lepidocrocite is finely distributed among kaolinite aggregates. It makes the slurry unable to be reused in the technological process because of the glaze staining risk, but it does not affect the material cohesion. Thus, the cylindrical samples were prepared at progressive compactions rates as follows: 1808.55; 1853.46; 1930.79 and 2181.24 kg/m3 and dried. Thereafter, were subjected to a compression test with a lower compression strength of 0.75 MPa for lower density and a higher strength of 1.36 MPa for the higher density. Thus, slurry compaction enhances the kaolinite binding ability. The Young’s Modulus slightly decreases with the compaction increasing due to local microstructure rigidizing. This proves the binding ability of kaolinite, which properly embeds quartz and mullite particles into a coherent and resistant structure. The fractography analysis reveals that fracture starts on the internal pores at low compaction rates and throughout the kaolinite layer in the samples with high compactness. The observed properties indicate that the investigated ceramic slurry is proper as a clay-based binder for sustainable ecological buildings, avoiding the exploitation of new clay quarries. Also, it might be utilized for ecological brick production.

1. Introduction

The actual world environmental conditions require special attention regarding industrial processes and their polluting potential because of their harmful potential. Higher amounts of industrial waste might break the tin equilibrium of the planetary environment by affecting the atmosphere [1,2] and water [3,4] and also might influence climate change [5,6]. The only way to keep the actual environmental equilibrium is by developing sustainable technologies and sustainable materials. This implies industrial waste mitigation by their reuse as sustainable materials [7,8] or for the production of derived sub-products [9,10].
Economical efficiency presumes the maximum valorization of raw materials and the minimization of technological waste. The general trend is to reuse the primary wastes in the same technological process according to circular economy concepts [11,12]. This ensures diminishing amounts of final waste. Their recirculation would be impossible if physicochemical modifications occur within their chemical composition and influence the subsequent microstructure. A circular economy proves to be effective for increasing technological process efficacy by more rigorous management of the raw materials but does not resolve the problem of the final dumped wastes. These industrial wastes are collected in dumps representing a major environmental problem due to their progressive accumulation and their harmful potential [13,14]. The actual challenge is finding effective ways for these dumped industrial wastes to be valorized as sustainable materials.
A sustainable material requires three main environmental characteristics: to be prepared from recycled materials including industrial wastes (in other words, to help diminish the dumps amount), to have a low carbon footprint or to help reduce CO2 emissions and to be utilized for environmentally friendly applications [15,16]. Thus, a sustainable material design implies using a significant amount of industrial wastes (preventing their dumping) and ensures their further processing in an environmentally friendly manner.
The ceramics industry is a very important manufacturing branch spread over the world that implies a high amount of raw materials, high energy consumption and finally, a lot of industrial waste [17,18,19,20]. The literature data refer mainly to broken ceramic bodies or recycled ceramics that are milled and used as raw materials for eco-friendly concrete based on geopolymers [17,18,19,20] and refers less to the ceramic slurries resulting from technological processes. The main references indicate the preparation of special slurries (not waste slurries) based on selected ceramics such as: zirconia [21] and alumina and quartz blended with low traces of kaolinite [22] that are mixed with photo-polymerizable resins. These are used for three-dimensional printing of high-resolution parts with a complicated geometry [23]. The 3D printing process allows precise constructions of the desired details and the subsequent photo-polymerization process (induced by laser or UV light) brings the desired mechanical strength [21,22,23].
The literature data present the sustainable recycling of red mud resulting from Alumina production through the Bayer process, which is used for floor tile manufacturing by adding extra kaolinite [24]. Similar examples were developed by other researchers valorizing such red mud [25,26]. Another sustainable approach for red mud was developed by Liu et.al by the acceleration of the carbonation process at the ambient temperature for the production of imitative ceramic bricks instead of their sintering at high temperatures, reducing the technological process’s carbon footprint [27]. As observed, the literature data show that red mud is intensively investigated from a sustainable point of view, including ceramic floor tile production. Unfortunately, it is a lack of references regarding the recycling of slurries collected from the industrial wastewater generated by the ceramic tile production.
Ceramic tile production technology generates a significant amount of used water during: the raw materials preparation, tile molding (or pressing), firing and glazing. These used waters contain many types of ceramic sediment that are collected and filtered (e.g., pressing filtration). The water resulting after sediment removal is re-circulated into the technological process and the pressed slurry is dumped. It cannot be reused in the technological process because of contamination with iron oxides and hydroxides that would stain the glaze [28,29,30]. Therefore, the resulting slurry is an industrial waste containing a high amount of fine mineral particles. It requires special wet dumping, preventing particle release into the atmosphere and subsequent risk of particulate matter pollution (mainly in the range of PM 2.5 to PM 1), which are very dangerous for living bodies [31,32]. The cumulative effect of the pressed slurry dumping raises a big problem of hundred-of-ton deposits, which might affect the local microenvironment. Such a situation requires a more deeply scientific approach to develop the sustainable recycling of these dangerous wastes. These ceramic slurry dumps should be minimized by the development of a sustainable recycling strategy, including their use for the manufacturing of derived sub-products.
Therefore, the current study aims at a detailed investigation of the chemical composition and microstructure of the ceramic slurry collected from the wastewater resulting from the technological process of wall and floor ceramic tile production in order to establish their sustainability for recycling within ecological applications. Therefore, slurry compaction is used for microstructure conditioning to achieve a comparable compressive strength with similar materials in the literature. Physical processing of the slurry is preferred instead using of chemical enhancers to keep the material sustainable. Thus, the compressive strength is investigated at different compaction rates.

2. Materials and Methods

2.1. Samples Preparation

The wall and floor tile ceramic slurry samples were collected from a production facility in Cluj County, Romania (the facility name is kept anonymous for economic reasons). Two kinds of samples were collected: the wet sediment from the waste water tank and the pressed slurry resulting from the water filtration process. Both samples were investigated regarding their microstructure and chemical and mineralogical composition.
The pressed slurry with a humidity of 30% was molded into cylindrical dies with a diameter of 30 mm at several pressing loads. The macroscopic aspect of the pressed samples is presented in Figure 1a. These samples were prepared in triplicate for statistical measurement.
The samples were extracted from the molds and examined for any structural flaws. All good quality samples were then dried in ambient conditions at a mean temperature of 22 °C for 3 months according to the drying curve in Figure 1b. We aimed for a sustainable drying process avoiding energy consumption and to be as gentle as possible with the binding particles. We assume that slow and progressive drying ensures better consolidation of the sample microstructure. Thus, the humidity decreases progressively from 30% to 22.98% after the first month and is established at 22.96% at the end third month.
The characteristics of dried samples were measured, including diameter, height and mass. The density was calculated for each sample and the mean value was used as the main parameter describing the sample compaction. All obtained values are centralized in Table 1.
The cylindrical samples presented in Figure 1 and described in Table 1 were further subjected to compression strength measurements.

2.2. Investigation Methods

An X-ray diffraction (XRD) investigation was effectuated with a Bruker D8 Advance diffractometer produced by Bruker Co., Karlsruhe, Germany. The patterns were registered at room temperature in the range of 10–80 degrees 2 theta with a speed of 1 degree/min using copper monochromatic radiation CuKα1 (λ = 1.54056 Å). The peak identification was performed with specialized software Match 1.0 including a PDF 2.0 database provided by Crystal Impact Company, Bonn, Berlin. The XRD Powder Diffraction Files used for minerals identification are: PDF 89-8936 for quartz; PDF 01-0527 for kaolinite; PDF 02-0428 for mullite and PDF 76-2301 for lepidocrocite.
Mineralogical optical microscopy (MOM) was effectuated in cross-polarized light with a Laboval 2 mineralogical microscope produced by Carl Zeiss Company, Oberkochen, Germany. We used a digital capture for microphotographs, Samsung 10 MPx produced by Samsung Company, Seoul, Republic of Korea.
The samples’ microstructure and elemental composition were investigated at high resolution using a Scanning Electron Microscope (SEM) Hitachi SU8230 produced by Hitachi Company, Tokyo, Japan, equipped with an elemental analysis device Energy-Dispersive Spectroscopy (EDS) X-Max 1160 EDX produced by Oxford Instruments, Oxford, UK. The investigation was carried out in high vacuum mode at an acceleration voltage of 30 kV. The samples were subjected to a mild metallization with Pt to ensure proper electrical conductivity of the samples but it was thin enough to not interfere with the microstructure. The platinum component was subtracted from the EDS patterns and elemental maps.
The mechanical properties were investigated with a Lloyd LR5k Plus testing machine with a maximum load of 5 kN produced by Ametek Lloyd Instruments, Meerbusch, Germany. The compression testing parameters were: stress loading rate 0.5 N at a speed of 0.2 mm/min. The testing curves were registered with professional Ametek Lloyd software version 1.0 and further processed with Microcal Origin 6.0 software produced by Microcal Company, Northampton, MA, USA. The Young’s modulus was measured automatically by the Ametek Lloyd software version 1.0 from the linear part of the obtained stress–strain curves.
The statistical analysis was effectuated using an Anova Test followed by Tukey’s post hoc test at a significance level p = 0.05 using Microcal Origin Lab 2018b produced by Microcal Company, Northampton, MA, USA.
The fractography images were investigated by SEM microscopy using an Inspect S microscope produced by the FEI Company, Hillsboro, OR, USA. The examination was effectuated without metallization at a low vacuum and at a voltage of 25 kV.

3. Results

The slurry resulting from the wall and floor ceramic tile production technological process requires an advanced investigation of its microstructure and composition in both wet sediment and pressed conditions for sustainability assessment. The mechanical properties of the raw slurry compacts provide important clues for further sustainable recycling possibilities.

3.1. Microstructure and Composition Characterization

The mineral composition of the slurry samples was assessed in a complex manner using XRD spectroscopy coupled with MOM microscopy. The XRD pattern of freshly dried sediment, Figure 2a, reveals a highly crystalline state with intense and narrow diffraction peaks.
The composition is dominated by quartz, followed by kaolinite and mullite. There are also some less intense peaks of lepidocrocite, indicating its presence in low quantities. The pattern baseline is also narrow, indicating the prevalence of well-organized microparticles with fewer ultrafine fractions, which might be gone with the drained water.
The consistency of pressed slurry is more compact than wet sediment but keeps all particles range that was formerly found in the wet state. The obtained XRD pattern, Figure 2b, also reveals very intense and narrow peaks, indicating the highly crystalline state of the sample, but the broadened pattern baseline reveals the significant presence of ultrafine particles. The kaolinite peaks are significantly broadened compared to the sediment sample, proving the presence of a significant amount of ultrafine fractions that require more detailed microscopic investigations. The mineral composition is also dominated by quartz, followed by kaolinite and mullite. We notice that pressed slurry evidenced stronger lepidocrocite peaks, which possibly indicate local concentrations of γ iron hydroxide.
Both sediment and pressed slurry samples were investigated with mineralogical optical microscopy (MOM) in cross-polarized light, Figure 3. Each mineral has its own color range depending on its position relative to the microscope optical axe. Therefore, quartz exhibits green–gray shades, kaolinite has a pale blue–bright white nuance, mullite presents reddish brown to reddish pink shades and lepidocrocite’s nuances vary from the dark brown to intense red [33,34].
Particles within the sediment sample are very well individualized, as seen in Figure 3a, as a consequence of suspension liquid drainage. The observation field reveals a uniform distribution of quartz particles surrounded by kaolinite. Mullite particles are predominantly bigger ones randomly distributed into the quartz–kaolinite mixture. Lepidocrocite also appears as small red dots randomly distributed. This fact indicates that iron hydroxide appears as a distinct component in the sediment sample and it is not bonded to the other minerals. Figure 3a also reveals some boulder-like particles with dark nuance due to their amorphous structure. These are related to glass particles, which are often used as paste stabilizers during tile firing [29,35]. The observed frit particles range from 5 to 70 μm.
The pressed slurry presents a denser aspect, seen in Figure 3b, in which bigger particles of quartz, kaolinite and mullite are embedded into a tight mass of very fine particles containing mainly kaolinite. It is very interesting that the observation field of Figure 3b reveals some bigger kaolinite particles (bright white nuance and tabular shapes) that were not observed in the sediment samples. Amorphous glass particles are also present in the pressed slurry sample. The mineralogical aspect in Figure 3b proves that pressing filtration of the sediments gathered all sediment particles into a compact structure while the filtered water is ready to be reused in the technological flux.
The slurry samples’ minerals amount was determined using the reference intensity ratio method (RIR), which was previously described in the literature [36,37]. The obtained values were correlated with the particle sizes range measured on the MOM images and further centralized in Table 2.
SEM microscopy allows more enhanced microstructural observation of the ceramic slurry samples than mineralogical microscopy. The sediment morphology is dominated by the bigger particles of quartz with about 60 μm diameter and a boulder aspect with sharp edges because of fresh breaks (a fact that is in good agreement with the data in the literature reported for silicate fragmentation [38]) and mullite particles with a size of about 75 μm and a rounded shape. The observed sizes are in good agreement with the data in Table 2. The bigger particles are surrounded by fine formations ranging from 1 to 30 μm evidencing a lamellar–tabular aspect for kaolinite and a granular aspect for the other minerals.
The elemental map corresponding to Figure 4a reveals the atomic species characteristic for each particle. The dominant element is O with 69.8 at.% labeled with cyan, followed by 17.8 at.% Si labeled with light green, 6.9 at.% Al labeled with yellow and 2.3 At.% Ca labeled with blue. There are small traces of 1.6 at.% Na marked with a pink label, 0.9 at.% K marked with a red label and 0.6 at.% Fe marked with an orange label.
Oxygen dominance is ensured by the identified minerals, which have an oxide nature. Thus, the quartz particles appear bright green because of their chemical composition, SiO2, and confirm the microstructural observation of the bigger particles in the SEM image, Figure 4a. The elemental map figures out the presence of the fine quartz particles mixed up with kaolinite. They appear in a light greenish shade due to the aluminum content, Al2Si2O5(OH)4, and can be easily distinguished by their tabular–lamellar aspect. Mullite, All6Si2O13, contains more aluminum than kaolinite, influencing the particle color, which is pale yellow-green with mostly bigger sizes with rounded shapes. The blue particles contain a significant amount of Ca and Na with traces of K, elements related to the glass particles, with the general chemical formula being 6SiO2·CaO·Na2O [39,40].
Iron distribution is very interesting because of orange spot distribution, which is mainly associated with kaolinite particle clusters (the left side of the elemental map in Figure 4a features a large area of iron aggregates). This spatial distribution might be related to the semi-solved rust scales from the mechanical aggregates on the technological flux that are incorporated into the wastewater and further into the wet sediment. These iron clusters make the resulting ceramic slurry unable to be reused as a raw material and consequently, they become a dumped waste.
The pressed slurry, Figure 4b, shows a sharper microstructure than the sediment. The bigger particles of about 50–60 μm belong to quartz with a boulder aspect with freshly broken margins, while mullite particles are rounded. Some finer particles in the range of 10–30 μm are observed in random positions all over the observation field in Figure 4b. This complex granular material is embedded into a dense matrix of fine tabular–lamellar particles of kaolinite. The fine particles matrix has a light greenish shade characteristic due to the kaolinite distinctly observed in the corresponding elemental map. The other particles also appear in a sharp manner, revealing the colors of their specific composition: quartz—bright green, mullite—pale yellow-green, glass—blue. This is in agreement with the amount of the elements: 69.9 at.% O, 17.6 at.% Si, 7.3 at.% Al, 2.0 at.% Ca, 1.7 at.% Na, 0.9% at.% K and 0.5% at.% Fe. Iron distribution is significantly different compared to the sediment sample. Now the orange spots are concentrated in well-defined particles that indicate that the water loss through pressing facilitates crystallization and clustering of the lepidocrocite particles. Their relative sizes range from 2.5 to 25 μm, which is in good agreement with MOM observation.
The microstructural differences between sediment and pressed slurry samples indicate the high binding abilities of kaolinite, which brings together the other particles into a coherent material. We wonder if the mechanical pressure facilitates the pressed slurry toughening due to its microstructural improvements. The answer could be obtained by conducting a mechanical properties test on the raw slurry pressed at different loads.

3.2. Mechanical Testing and Fractography

The samples presented in Figure 1 were subjected to compression testing until they were broken. The solicitation curves are presented in Figure 5. Their allure indicates an interesting behavior situated among plastic and brittle failure, Figure 5a. The lower compaction grade features a relatively plastic behavior with significant strain at a relatively lower load until failure. The testing curves’ allure is modified by the sample compaction grade increasing to an aspect of brittle failure with moderate strain and significant load increase, Figure 5b,c. The most tenacious behavior among tested specimens was obtained at the higher compaction load, the obtained curve, Figure 5d, presents higher strain and stress until failure.
The compressive strength and Young’s Modulus were determined using the testing machine software and their mean values are presented in Figure 6. It results that the compressive strength increases with the specimen density, Figure 6a, while the Young’s modulus slightly decreases with the compaction grade, Figure 6b. Both variations indicate that the microstructural changes induced by the specimen compactions generated a progressive rigidity of the material. Thus, increasing the compressive strength of the sample will decrease its elasticity.
The statistical analysis found two similar populations with a significance level of p > 0.05: the first population has low compressive strength and relatively high Young’s modulus, represented by S1 and S2 specimens. The second population represented by S3 and S4 specimens has increased values of compressive strength and relatively decreased Young’s modulus.
The statistical analysis shows significant differences between those two populations p < 0.05. This means that two fracture mechanisms are involved in the specimens breaking under compressive solicitations. This requires a detailed fractography investigation to be effectuated with SEM microscopy at different magnifications.
The fracture surface of the lower compacted grade sample S1 is investigated with SEM microscopy in Figure 7a. The general view evidences an irregular fracture surface promoted on the internal pores of about 500–700 μm, as observed in the upper left side of Figure 7(a1). The pores’ presence develops collateral cracks propagated in the whole sample volume, as seen in the microscopic detail in Figure 7(a2).
The morphological detail in Figure 7(a3) evidences a lack of particle binding around the pore wall with granular material (such as quartz and mullite) and thus they are less embedded into the kaolinite matrix: the darkened area on the right side of the observation field is the pore, and partly un bound particles are clearly visible. The higher magnification in Figure 7(a4) reveals that the kaolinite particle that ensures the binding process is relatively undisturbed by the specimen failure. The EDS map reveals the kaolinite particles being well attached to the quartz surface. It indicates that the pore structure ensures elastic behavior of the specimen during compression stress but insufficient particle binding on the pore walls facilitates failure at relatively low loads.
S2 compacted at a load of 140 N has a very irregular fracture surface with significant material loss through fallen parts. This behavior is facilitated by the propagation of significant cracks propagated perpendicularly on the main failure front, Figure 7(b1), and the collateral crack is observed on the right side of the observation field. Increasing the magnification, Figure 7(b2) reveals that small pores that occur in the specimen microstructure facilitate collateral cracks and the crack width ranges from 30 to 50 μm. Therefore, kaolinite binding is relatively weak in these areas and allows relative facile disintegration of the specimen cohesion, Figure 7(b3) (the fine debris particles observed in the collateral fracture sustain the material disintegration). The kaolinite binder particles observed at higher magnification present relative stress marks with some excoriation that indicates partial detachment from the quartz particles (a fact sustained by the bright green portions of quartz visible on the EDS maps beside the kaolinite lamellas appearing in yellow-green nuance).
The microstructure is more compact after the compaction load increases at 210 N, S3, evidencing a uniform failure surface with only a few thin collateral fractures of about 5 μm propagated at short distances not longer than 100 μm, Figure 7(c1,c2). Thus, the kaolinite particles embed quartz and mullite particles well, ensuring a coherent material. Higher magnification images in Figure 7(c3,c4) reveal that the failure occurs through the binder layer, causing severe excoriation of the kaolinite particles that were plucked from their normal position. The elemental map in Figure 7(c3) shows the plucked kaolinite particles along with some orange spots that indicate the local presence of lepidocrocite. This might act as a local destabilization agent towards the kaolinite bridges that facilitate the structure failure.
Specimen S4 was compacted at a load of 350 N, ensuring a dense microstructure and a very uniform failure surface, as observed in Figure 7(d1). There are no collateral cracks propagated through the material, proving that kaolinite’s binding abilities during compaction ensure good mechanical behavior of the tested samples. SEM microscopic details in Figure 7(d2,d3) reveal a very good adhesion of kaolinite fine particles to quartz and mullite, ensuring proper compactness. Thus, the compressive effort was dissipated through the granular material, which subjects the kaolinite particles from the binder layer to tensile forces causing their cleavage. This definitely plucks the binder layer as observed in Figure 7(d4). The irregular surface of the cleaved kaolinite confirms the brittle failure mode. Iron traces present over the fractured kaolinite particles are evidenced by the EDS map corresponding to Figure 7(d4). It is an interesting behavior that might influence the sample failure. The lower right side of the EDS map reveals a quartz particle of 10 μm with some fine kaolinite remaining after the failure still attached on its surface.

4. Discussion

The investigated ceramic slurry is a waste product derived from the water generated by the technological process for wall and floor tile production. It collects particulate matter resulting from each step of the working flow processes as well as impurities generated by the working devices. The used water is further processed by decantation and advanced filtration and then re-introduced into the technological flux. The decantation process generates the ceramic slurry as wet sediment. The wet sediment is further processed by a pressing filtration to recover most of the water to be reintroduced in the main flux. It results in a pressed slurry with a humidity of around 30%, which is dumped because of ferrous impurities that might stain the tile glaze [29,30].
Both wet sediment and pressed slurry have the same mineralogical composition based on quartz and kaolinite generated mainly from the ceramic paste preparation and a moderate amount of mullite, about 20 wt.%. Mullite might appear directly from the fired biscuits post-processing, but the rounded shape of the particles as observed by MOM and SEM microscopy indicates that the mullite amount is mostly represented by the rejected tiles with structural faults that were ground and reused as granular raw material. Glass particles were evidenced as amorphous material by MOM microscopy and their SEM–EDX analysis evidenced Ca and Na content. The iron presence was observed by SEM–EDS investigation and XRD revealed its crystalline structure as lepidocrocite, which is γ-FeOOH, the most degraded form of iron hydroxides [41,42]. The elemental composition of both sediment and pressed slurry is similar (only with minor variations), being based on the silicate structure dominated by O, Si and Al [38].
The pressed sample has much more fine particles than observed in the wet sediment. Thus, their microstructure proves to be the main difference: the mineral particles are well individualized with a low agglomeration tendency in the sediment but agglomerated and structured into a compact material in the pressed slurry. The microstructural investigations show that the pressed sample has a composite-like structure with a kaolinite matrix and the filler role is fulfilled by quartz and mullite particles. Such a mineral base is reported in the literature for various sustainable materials such as plasters [43,44] or pozzolan structures for geopolymer production [45,46]. Our microstructural observation evidenced that pressing the slurry with a humidity of around 30% facilitates the kaolinite binder activity, increasing the material cohesion. This is in good agreement with clay binding activity in sand mold preparations for metal casting [47]. Therefore, this hypothesis required detailed investigation.
The obtained results show that slurry compression at low loads in the range of 70–140 N generates a compressive strength of 0.75 to 0.84 MPa. The elasticity modulus decreases from 57.59 to 53.64 MPa as a consequence of the compaction load increasing. The internal pores that ensure an elastic behavior of the specimens under compressive stress causes this behavior, but this easily promotes generalized failure through a lot of collateral cracks oriented perpendicular to the main failure surface. Increasing the compaction load in the range of 210–350 N, the material becomes more structured and kaolinite binding properties are enhanced. Thus, the compressive strength reaches values in the range of 1.11–1.36 MPa, but the Young’s modulus decreases significantly at values around 41.69 MPa. The material toughening through kaolinite platelet binders increases the compressive strength but slightly reduces the specimen’s elasticity. This orientation of the specimen microstructure ensures good behavior of the kaolinite matrix which has a brittle failure under compressive stress involved in the binder layer without propagation of collateral cracks. The high-resolution SEM microscopy reveals the plucked aspect of the failed kaolinite particles. We notice that failed kaolinite particles are often associated with iron hydroxide traces. The compressive strength obtained at high compaction loads is comparable with the data in the literature for clay plasters [48,49]. Horabik and Jozefaciuk show that the kaolinite amount increases the compressive strength of granular material such as quartz, up to 1 MPa at an optimal amount of 32% and without internal pores [50]. The kaolinite amount in our pressed slurry is close to the Horabik optimal value and the obtained compressive strength is very close to that reported in the literature.
It is interesting to observe the compressive strength of other plaster systems as reported in the literature. The lime plasters have a compressive strength of about 20 MPa for curing at a temperature of 100 °C and about 30 MPa at 200 °C [51], while the gypsum plasters feature compressive strengths of about 10 MPa after powder hydration [52,53]. The addition of short fibers [50] or microstructural activators might enhance the gypsum plaster’s compressive strength at 20–30 MPa [54], achieving comparable values to the lime plasters. Both lime and gypsum require enhancers to ensure a proper compressive strength such as short fibers or reticulation agents. Such an approach could be applied to the currently investigated to enhance the binding properties of kaolinite and for microstructure consolidation. The best situation is the usage of another byproduct or waste resulting from a technological process. This will be a future step to increase the sustainability of our ceramic slurry to be used as ecological plaster. For example, the sugarcane filter mud with short fiber content and viscous matter might facilitate spatial reticulation within the currently investigated pressed ceramic slurry. This application fits the current trend for environmental friendly house building, avoiding the exploitation of other clay sources from nature.
The mineralogical and elemental investigation of the pressed slurry proves its high mineral purity except for the small amount of iron hydroxide. It might be used as raw material for less pretentious ceramic applications such as porous garden tiles or bricks. In such cases, its composition and humidity must be properly adjusted according to the technological requirements. This will ensure sustainable recycling of the ceramic slurry, but its structural consolidation requires thermal treatment to ensure the kaolinite’s mullitization process. This technological step requires significant energy consumption, which implies a lot of fuel, which has a negative impact on the carbon footprint. The sustainability of this recycling possibility depends on technological process balancing by the proper adjustment of the slurry composition, humidity, firing temperature and maintaining time. All these aspects will be explored in the following article as future perspectives. The industrial implementation of these sustainable strategies regarding sub-product manufacturing would reduce the slurry deposit in the ceramic tile facility and increase their waste management efficacy.

5. Conclusions

The investigated ceramic slurry resulting from tile production has a composition based on quartz and mullite as granular materials and kaolinite as the binding matrix, forming a composite-like microstructure. The material compaction facilitates kaolinite’s fine particle binding effect, which determines the increase in compressive strength and ensures a fair value of the Young’s modulus. A small amount of iron hydroxide affects the slurry reutilization in the tile fabrication process but it might be used for sustainable applications such as plaster for greenhouses or derived sub-products such as ecological bricks.

Author Contributions

Conceptualization, S.E.A. and I.P.; methodology, S.E.A. and I.P.; software, G.B.; validation, S.E.A. and I.P.; formal analysis, S.C. and B.V.B.; investigation, S.E.A., L.B.T., G.B., S.C., B.V.B. and I.P.; resources, S.E.A. and B.V.B.; data curation, I.P.; writing—original draft preparation, S.E.A. and I.P.; writing—review and editing, I.P.; visualization, L.B.T., S.C. and I.P.; supervision, S.E.A. and I.P.; project administration, S.E.A. and I.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are available upon request from the corresponding author.

Acknowledgments

The authors acknowledge the XRD diffractometer maintenance which was supported by the Ministry of Research, Innovation and Digitization through Program 1-Development of the National Research and Development System, Subprogram 1.2-Institutional Performance-Funding Projects for Excellence in RDI, Contract No. 37PFE/30.12.2021.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Macroscopic aspect of the cylindrical samples (S1, S2, S3 and S4) for compression test (a) and their drying curve (b).
Figure 1. Macroscopic aspect of the cylindrical samples (S1, S2, S3 and S4) for compression test (a) and their drying curve (b).
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Figure 2. XRD patterns for ceramic slurry samples: (a) sediment and (b) pressed.
Figure 2. XRD patterns for ceramic slurry samples: (a) sediment and (b) pressed.
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Figure 3. Mineralogical optical microscopy images for ceramic slurry samples: (a) sediment and (b) pressed.
Figure 3. Mineralogical optical microscopy images for ceramic slurry samples: (a) sediment and (b) pressed.
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Figure 4. SEM images for ceramic slurry samples: (a) sediment and (b) pressed. Corresponding EDS elemental map and spectrum are given on the right side of each SEM image.
Figure 4. SEM images for ceramic slurry samples: (a) sediment and (b) pressed. Corresponding EDS elemental map and spectrum are given on the right side of each SEM image.
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Figure 5. Compression test curves for cylindrical samples: (a) S1, (b) S2, (c) S3 and (d) S4.
Figure 5. Compression test curves for cylindrical samples: (a) S1, (b) S2, (c) S3 and (d) S4.
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Figure 6. Mechanical properties variation with the specimens’ density: (a) Compressive strength and (b) Young’s Modulus. The error bars represent standard deviation and the red line is the linear fit of the values variation trend.
Figure 6. Mechanical properties variation with the specimens’ density: (a) Compressive strength and (b) Young’s Modulus. The error bars represent standard deviation and the red line is the linear fit of the values variation trend.
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Figure 7. Fractography images obtained by SEM microscopy at progressive magnifications: (a) S1, (b) S2, (c) S3 and (d) S4.
Figure 7. Fractography images obtained by SEM microscopy at progressive magnifications: (a) S1, (b) S2, (c) S3 and (d) S4.
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Table 1. Cylindrical sample characteristics.
Table 1. Cylindrical sample characteristics.
Physical
Characteristic		Cylindrical Samples
S1S2S3S4
Pressing load, N70140210350
Diameter, mm30293031
Height, mm15192017
Mass, g19.2723.2927.1327.44
Density, kg/m31812.581850.841913.932132.85
* Mean density, kg/m31808.551853.461930.792181.24
* Mean value was determined for three similar samples.
Table 2. Mineral composition of ceramic slurry samples.
Table 2. Mineral composition of ceramic slurry samples.
Sample Mineral Composition
QuartzKaoliniteMulliteLepidocrocite
SedimentAmount, %4230216
Size, μm10–1001–8010–1203–30
PressedAmount, %3934189
Size, μm2.5–1001–1005–1202.5–25
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Avram, S.E.; Barbu Tudoran, L.; Cuc, S.; Borodi, G.; Birle, B.V.; Petean, I. Microstructural Investigations Regarding Sustainable Recycling of Ceramic Slurry Collected from Industrial Waste Waters. Sustainability 2024, 16, 1123. https://doi.org/10.3390/su16031123

AMA Style

Avram SE, Barbu Tudoran L, Cuc S, Borodi G, Birle BV, Petean I. Microstructural Investigations Regarding Sustainable Recycling of Ceramic Slurry Collected from Industrial Waste Waters. Sustainability. 2024; 16(3):1123. https://doi.org/10.3390/su16031123

Chicago/Turabian Style

Avram, Simona Elena, Lucian Barbu Tudoran, Stanca Cuc, Gheorghe Borodi, Bianca Violeta Birle, and Ioan Petean. 2024. "Microstructural Investigations Regarding Sustainable Recycling of Ceramic Slurry Collected from Industrial Waste Waters" Sustainability 16, no. 3: 1123. https://doi.org/10.3390/su16031123

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