Fired ceramics 100% from lignite fly ash and waste glass cullet mixtures

Author’s Accepted Manuscript Fired ceramics 100% from lignite fly ash and waste glass cullet mixtures V. Karayannis, A. Moutsatsou, A. Domopoulou, E. Katsika, C. Drossou, A. Baklavaridis www.elsevier.com/locate/jobe

PII: DOI: Reference:

S2352-7102(16)30258-3 http://dx.doi.org/10.1016/j.jobe.2017.09.006 JOBE326

To appear in: Journal of Building Engineering Received date: 24 October 2016 Revised date: 4 September 2017 Accepted date: 12 September 2017 Cite this article as: V. Karayannis, A. Moutsatsou, A. Domopoulou, E. Katsika, C. Drossou and A. Baklavaridis, Fired ceramics 100% from lignite fly ash and waste glass cullet mixtures, Journal of Building Engineering, http://dx.doi.org/10.1016/j.jobe.2017.09.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fired ceramics 100% from lignite fly ash and waste glass cullet mixtures V. Karayannis1*, A. Moutsatsou2, A. Domopoulou1, E. Katsika2, C. Drossou2, A. Baklavaridis1 1

Department of Environmental Engineering, Western Macedonia University of Applied Sciences, Kila, 50100, Kozani, Greece 2 School of Chemical Engineering, National Technical University of Athens, Zografou Campus, 15773, Athens, Greece

*Corresponding author: E-mail: [email protected]; [email protected]; Tel +30 24610 68022; +30 6976447511

Abstract In the present study, the development of new building ceramics is investigated, using 100% lignite fly ash (FA) and waste glass cullet (WGC) mixtures as secondary industrial raw materials towards circular economy. Thus, compacted and sintered (at 700 and 900oC) ceramic bodies based on binary WGC/FA mixtures, with WGC loadings up to 15%, were fabricated. The utilization of WGC (amorphous) aimed at lowering the sintering temperature of the mixture, for energy reduction purposes, via a better heat flux regulation in the material. The successful consolidation/densification of the ceramic microstructures, mainly composed of different silica phases, was achieved upon synergistic sintering at 900oC for 2h. Moreover, the successful consolidation/densification was confirmed by the SEM micrograph observation and the porosity evaluation from the SEM micrographs. The addition of WGC yielded to a drastic decrease in the porosity values (down to 12%) for the samples sintered at 900oC for 2h. This porosity decrease favored, in turn, the substantial microhardness increase (up to 3833 HV) due to the pore sealing by the glassy phase of WGC. Moreover, an exponential relationship between microhardness and porosity was revealed. Finally, further investigation of the processing conditions is currently underway towards the optimization of the attained ceramic microstructures in order to meet the requirements of specific applications. Keywords: building ceramics; lignite fly ash; glass cullet; firing; characterization.

1. INTRODUCTION A recent trend in building engineering is the valorization of various waste materials into addedvalue building products. Specifically, novel ceramic materials, for building engineering applications, can be developed by properly formulating and processing mixtures of solid industrial byproducts. In principal, these byproducts contain valuable oxides such as silica (SiO2) and alumina (Al2O3) that improve the mechanical and other properties of the ceramics. The main advantages of the valorization of industrial byproducts is the production cost reduction, resources conservation and environmental protection. In that sense, the usage of industrial byproducts is strongly encouraged according to the current environmental policies and circular economy [1-4]. Up to now, considerable research efforts have been reported regarding the investigation of the production process for the conversion of various silicate wastes into useful materials destined to building, construction and other technical applications [5-7]. Particularly, siliceous fly ash (FA), produced in massive quantities from power station coal-fed combustors, is considered an attractive starting material for the development of cost-effective ceramic and glass-ceramic products. These

products include traditional ceramics, thermal insulating bricks, wall tiles, porcelain stoneware tiles etc. In that sense, the utilization of fly ash in the aforementioned applications is expected to increase in the next few years [8-11]. Nevertheless, chemical composition and the particle size distribution of FA play a significant role on the attained microstructure and properties of the final product. On the other hand, waste glass cullet (WGC) is considered a prime secondary material, since it is 100% recyclable. Compositionally, WGC can be characterized as a rich-in-silica material. It possesses interesting physical and mechanical properties, while it may be easily separated from the overall solid waste. Nevertheless, several thousand tons of WGC are collected annually. Therefore, it is nowadays imperative to explore new potential application for glass utilization, which extend beyond traditional glass production. Very recently, the valorization of waste glass in construction materials was found to be rather promising. Specifically, important applications of WGC include its use in the manufacturing of cement or clay bricks, pressed tiles, glass wool, smalt and decorative cementitious products, facing materials for architectural applications, heat- and sound-insulating materials and glass-ceramics [12-18]. Glass-ceramics are polycrystalline materials with fine microstructure, usually produced upon controlled glass crystallization-devitrification. In fired ceramics’ manufacturing, WGC can be used as a binder and heat-flux regulator, since it exhibits lower softening temperature than the other constituents of the ceramic material. This, in turn, leads to reductions in firing temperature, time and energy consumption during thermal processing. Furthermore, the possible synergistic effect of different constituents in the raw material mixture, may yield to materials with improved physico-mechanical properties and chemical durability. The aforementioned properties may also be tailored accordingly for specific applications of potential commercial interest. Moreover, the environmentally dangerous components, included in waste, can be beneficially immobilized in the ceramic phase. Moreover, the substitution of clayey raw materials with WGC in the ceramic industry, may contribute to CO2 emissions reduction, since considerable CO2 emissions are produced due to carbonates’ decomposition during the thermal processing of clay [19]. Consequently, the investigation of the handling, processing and performance of WGC containing ceramics is considered to be rather useful for the industrial exploitation of WGC. Moreover, the acquired knowledge and expertise from such investigations may be useful for establishing specifications and market criteria, which are crucial aspects for decision making related to production scale-up [20, 21]. In the last few years, various industrial solid residue mixtures, including combustion ashes, (e.g. FA) and WGC originating from different sources, have been considered for ceramic and glassceramic materials applications. Specifically, R.C. Da Silva et al. reported the preparation and characterization of ceramic glazes made from combinations of different industrial wastes [22]. Furthermore, E. Furlani et al. investigated the effect of the sintering process on various properties of ceramics containing paper sludge, glass cullet and different types of clayey materials [23]. Additionally, A. Karamberi et al. synthesized glass-ceramics using glass cullet and vitrified industrial by-products and studied the structure and the morphology of the materials produced [24]. Moreover, S.C. Kou et al. studied the effect of recycled glass powder and reject fly ash on the mechanical properties of fibre-reinforced ultrahigh performance concrete [25]. Furthermore, P. Pisciella et al. examined the chemical durability of glasses obtained by vitrification of industrial wastes. Last but not least, A. R. Boccaccini et al. produced glass matrix composites WGC and FA reinforced with alumina platelets and studied the effect of alumina platelets on the Young's modulus, modulus of rupture, hardness and fracture toughness [26]. In the present research, the development of fired compacted ceramics, containing only lignite FA and WGC is investigated. Specifically, the sintering behavior of the mixtures is explored as a function of the % relative mixture composition and the firing/sintering conditions. The mineralogical composition of the ceramics was also evaluated and the effect of the glassy phase on the final materials properties is discussed. Moreover, the effect of the morphological characteristics and the porosity on the physical performance of the produced materials was investigated.

2. EXPERIMENTAL 2.1. Secondary raw materials FA used in this study derived from the electrostatic precipitators of Megalopolis lignite-fed power plant. This type of FA is strongly siliceous (51.26 % SiO2), while lower amounts of Ca-bearing species are also present (11.82 wt.% CaO, including only a 0.95 wt.% of free CaO), and therefore this fly ash is barely a Class-C ash (CaO barely over 10 wt.%). The Si/Al ratio for this type of FA is 2.64. Moreover, it has a specific gravity of 2.50 g/cm3, a specific area of 3.87 m2/g, a mean pore diameter of 165.1 (Å) and a pore volume of 0.016 cm3/g. WGC was obtained from pulverized (for 10 min) waste glass (Fritsch, Pulverisette 2). SEM micrographs of FA and WGC are provided in Figures 1a and 1b respectively. The chemical composition (Spectro X-Lab 2000 XRF) of WGC, used as admixture, is given in Table 1. The WGC particle size distribution (Malvern Mastersizer-S) is shown in Figure 2.

Figure 1. SEM micrographs of siliceous FA (a) and WGC (b) used in this study.

Table 1. Chemical composition of WGC. Glass type (Si-Na-Ca) flint

SiO2 70.6

Al2O3 1.75

K2O 0.55

Chemical composition (%) MgO CaO Na2O SO3 2.45 10.70 13.25 0.45

Fe2O3 0.45

Cr2O3 -

Co2O3 -

Figure 2. Particle size distribution of WGC. 2.2. Development of ceramics from FA-WGC mixtures 2.2.1. Sample preparation - Compaction Binary mixtures of FA and WGC in different loadings (0, 5, 10, 15 wt.% WGC) were uniaxially cold pressed in a stainless steel die using a hydraulic press (Specac, 15011), in ambient temperature (25oC). In this way, disc-shaped specimens, with diameter 13 mm, were produced. All samples produced in this study are summarized in Table 2. Table 2: The composition and the processing conditions of samples produced in this study Sample Name Samples’ composition Processing conditions WGC (% wt.) FA (% wt.) non-sintered (only FA 0 100 compacted at 25oC) WGC-FA-0595 5 95 -//WGC-FA-1090 10 90 -//WGC-FA-1585 15 85 -//o FA-700 0 100 700 C for 2h -//WGC-FA-0595-700 5 95 -//WGC-FA-1090-700 10 90 -//WGC-FA-1585-700 15 85 o FA-900 0 100 900 C for 2h -//WGC-FA-0595-900 5 95 -//WGC-FA-1090-900 10 90 -//WGC-FA-1585-900 15 85 2.2.2. Firing – Sintering The pressed compacts were heated in a programmable laboratory furnace chamber (Thermoconcept, ΚL06/13). The samples were initially preheated from room temperature (25oC) up to 550oC and held at that temperature for 1h in order to burn-out any residual carbon in FA and any organic substances possibly present in WGC waste glass as well as to activate WGC [12]. Actually, it has been reported that potential premature densification may inhibit the complete carbon removal,

which, in turn, is being influenced by both internal and external mass transfer [7]. Furthermore, the samples were heated up to a certain firing/sintering temperature (700 or 900oC), with 10oC/min heating rate. It is worth noting that, a relatively moderate heating rate (i.e. 10oC/min) was used, in order to avoid significant temperature gradients between the surface and the interior of the sample. These temperature gradients could lead to the development of process-induced stresses which endanger the sintered products’ integrity. This can be important especially for the production of large-sized products consisting of low thermal conductivity raw materials such as FA, which mainly comprises of hollow sphere-shaped particles (see Figure 1a). Finally, all specimens were held at constant firing/sintering temperature (700 or 900oC) for 2h and then gradually cooled to ambient temperature. In order to ensure safe handling and consolidation of the samples subjected to the firing/sintering procedure, their density and strength were evaluated so as to optimize the compaction pressure. It is worth noting that, thermal treatment at 700oC for 2h yielded to samples with reduced densification and integrity. Nevertheless, thermal treatment at 900oC for 2h successfully yielded to consolidated ceramic materials (yellowish-brown colored) for all mixture compositions examined. In this way, the sintering temperature needed for the consolidation of the ceramic microstructures was reduced from 1050oC, which is needed for the successful consolidation of pure siliceous FA ceramics, down to 900oC using WGC as admixture [8]. 2.2.3. Characterization Phase characterization of as-received lignite fly ash samples and fired/sintered ceramic samples was realized by X-Ray Diffraction (XRD Siemens Diffractometer D-5000). The produced ceramic microstructures were studied using Scanning Electron Microscopy (SEM - Jeol, JSM-6400 and JSM-6610LV) coupled with Energy Dispersive X-Ray (EDX) Spectroscopy. The porosity of the ceramic samples produced, was evaluated by image analysis software (ImageJ 1.50), using a wellestablished SEM-based methodology [27]. It is worth noting that only lower magnification images were used for the evaluation of the porosity in order to take into account the widest possible area. At least five images were evaluated from each sample. Microhardness measurements were conducted by using a Vickers indenter (Shimadzu, HMV-2T) by applying 200 g load on the samples’ surface. The mean microhardness values were determined over five valid indentations per sample. Apparent density was determined according to Archimedes principle, by using a Shimadzu SMK401ΑUW220V apparatus. 3. RESULTS AND DISCUSSION XRD patterns of the produced materials were recorded in order shed light into their crystalline phase composition. In Figure 3, XRD patterns of the green and sintered (900oC, 2h) samples, with WGC loadings 0-15%, are shown. All specimens were principally composed of different crystalline phases of silica (SiO2). This is highly expected taking into consideration the strong siliceous character of the FA used. The addition of WGC does not seem to lead to any mineralogical phase change, since WGC is amorphous. Hence, crystalline phases of silica (SiO2) are also observed in the sintered FA-WGC mixtures. Furthermore, it should be noted that the peaks in the samples with WGC content up to 10% (i.e. samples WGC-FA-0595-900 and WGC-FA-1090-900) exhibit considerably lower intensity than the sintered FA sample. This implies a glassy phase increase in these certain WGC compositions. However, the presence of silica resulting from glass content crystallization at the sintering conditions applied can not be excluded. Interestingly, as shown in Figure 3, the intensity of the peaks, which are attributed to the crystalline silica phases, is much higher in the sintered mixtures with high WGC content (i.e. 15%) than the one recorded for the mixtures with lower WGC content (5 and 10%). It should be mentioned here that sintering with simultaneous crystallization of powdered glass represents an interesting processing route particularly for glass-ceramics and especially for those originating from wastes [15]. The role of silica is also underlined in the synthesis of glass-ceramics using WGC and vitrified industrial byproducts through devitrification/crystallization process [21]. Besides, iron oxide is also detected

both in the pristine FA and in sintered FA-WGC mixtures. It originates from a noticeable iron content occurring in Megalopolis lignite. Moreover, no gehlenite was identified in the XRD patterns (Figure 3), even in the specimens sintered at 900oC, which is considered beneficial to the mechanical resilience of the produced materials.

Figure 3. Typical XRD spectra of as-received FA and the FA-WGC compacted mixtures (0, 5, 10, 15 wt.% WGC) sintered at 900oC for 2h. SEM micrographs for all specimens are provided in Figure 4. The specimens fired at 700oC (half left of Figure 4), exhibit poor densification resulting in highly porous microstructures, whereas samples at 900oC (half right of figure 4), a continuous network of characteristic solid-state sintering necks are observed (e.g. Figure 4l). These necks, which should be composed of glassy (amorphous) phases, may contribute to the effective formation of solidified microstructures. Moreover, the hard and resistant quartz and other silicon oxide crystalline phases, that are the predominant phases, should also contribute to the final consolidation and quality of the obtained products. In Figure 5, a representative EDX spectra of the WGC-FA-1585-900 sample is shown. This EDX spectra denotes that Si is the major element in the sample, with Al, Ca (mainly derive from FA) and Fe elements also present, but in significant lower quantities. Some other minor elements (Na, Mg, S, K, Ba and Fe) were also detected in the EDX spectra. It is worth noting that the presence of Na, K and Ca facilitates the softening of WGC because these elements were found to assist network structure collapse [28]. WGC exhibits softening temperature approximately 650°C due to its glassy nature [26]. Therefore, the WGC utilization as admixture into siliceous FA, in temperatures higher than 700 oC, probably increases the heat flux between the constituents. Therefore, the addition of WGC yields to higher glassy phase transformation in the ceramic body, especially when it is sintered up to 900°C. In that sense, WGC should act as “welding connection” between granules, which enhances the ceramic matrix consolidation. These observations are in accordance with the literature findings. Specifically it has been reported that clay bricks incorporating WGC, the amorphous nature of WGC particles enhanced the sintering process, leading to highly consolidated microstructures with improved strength [11,16].

Figure 4. SEM micrographs of FA-WGC mixtures with different WGC loadings: 0% (a,b,c,d), 5% (e,f,g,h), 10% (i,j,k,l) and 15% (m,n,o,p) sintered for 2h at 700 oC (half left of the image) and 900oC (half right of the image).

Figure 5. EDX analysis spectra taken from the sample WGC-FA-1585-900.

In order to quantitatively evaluate the consolidation/densification of the samples, their porosity was calculated from the SEM images. In Figure 6, the percentage porosity as a function of WGC loading (%wt) is presented. The error bars in the graph correspond to the standard deviation from the average porosity value. A sharp porosity decrease is observed with the addition of only 5% WGC,

in both firing temperatures (700 and 900oC). A further porosity decrease is achieved in higher WGC loadings (10%) for the samples fired at 900oC. Specifically, porosity values down to 12% can be reached in this case. In order to further promote densification, an increase in the firing temperature and/or a decrease in the particle size distribution of WGC should be considered [22]. Moreover, rapid sintering processes that reduce the sintering activation energy of WGC and FA could be considered. In this way, higher bulk density and smaller pore size can be achieved for the sintered products [29]. Nevertheless, a certain degree of porosity may be desirable in specific applications, such as weight reduction, insulation and other. 36

700oC 900oC

32

Porosity (%)

28 24 20 16 12 8 4 0

5

10

15

WGC loading (% wt)

Figure 6. Porosity as a function of WGC loading. The line, which connects the experimental points, is used to indicate the trend of the measurements. In Figure 7 microhardness Vickers (HV) as a function of WGC loading (% wt) is presented. For the pristine FA, relatively low microhardness values were obtained. Specifically, the microhardness was 581 HV and 765 HV at 700 and 900oC respectively. However much higher microharness values (almost double compared to the initial ones) were recorded by the addition of WGC up to 10%. Further increase of the WGC content did not affect substantially the attained microhardness values for the samples processed at 700 oC. Nevertheless, for the samples processed at higher temperature (i.e. 900 oC), notably higher microhardness values were recorded (3833 HV). This may be ascribed to the enhanced mechanical resilience due to the higher degree of densification/consolidation (porosity reduction), as mentioned earlier. The indentation cavity, generated during microhardness testing, is created by elastic-plastic deformation in dense materials. However, in porous materials, an additional contribution to the indentation mechanism exists: the filling of the porosity which is produced by the considerable indentation stresses beneath the indenter [30]. In that sense, the hardness increase in decreasingly porous materials is dominated by this porosity filling [30]. Moreover, J. Luo et. al reported that the porosity should be related to microhardness by following an exponential relationship [31]. In order to scrutinize the hardnessporosity relationship in our case a microhardness vs porosity graph was produced (Figure 8) for the samples fired at 900oC. An exponential equation (Eq. 1) was used to fit the experimental data: (1) where P the porosity (%) and mH is the microhardness (HV), with a,b and c being the fitting coefficients. In our case a, b and c obtained the values 12.388, -89.918 and 0.998 respectively. As observed in Figure 8, the predicted values from equation 1 (red line) fit rather well to the experimental data.

4500

700 oC 900 oC

4000

Microhardness (HV)

3500 3000 2500 2000 1500 1000 500 0 0

5

10

15

WGC loading (% wt)

Figure 7. Microhardness Vickers (HV) as a function of WGC loading (% wt). The line, which connects the experimental points, is used to indicate the trend of the measurements.

36 32

Model

Asymptotic1

Equation

y = a-b*c^x

Reduced Chi-Sqr

0.46686 0.98865

Adj. R-Square

Value

Porosity (%)

28

Porosity (%)

Standard Error

a

12.38756

b

-89.91831

31.63589

c

0.99754

4.7393E-4

0.55678

24 20 16 12 8 4 0

500

1000

1500

2000

2500

3000

3500

4000

Microhardness (HV)

Figure 8. Porosity (%) vs Microhardness Vickers (HV) for the sample processed at 900 oC. The black line, which connects the experimental points, is only used to indicate the trend of the measurements. The red line corresponds to equation (1) that was used to fit the experimental data.

4. CONCLUSIONS Ceramics based on binary WGC/FA mixtures were successfully fabricated and tested in this study. The successful consolidation/densification of the ceramic microstructures, principally composed of different phases of silica, was achieved upon synergistic sintering of the WGC/FA mixtures at 900oC for 2h. The utilization of WGC (highly amorphous) contributes to lowering the sintering temperature of the mixture, via the better heat flux regulation inside the material. The successful consolidation was confirmed from SEM micrograph observation and the porosity measurement from the SEM micrographs. The addition of WGC yielded to a drastic decrease in porosity (down to 12%). This porosity decrease favored, in turn, the substantial microhardness increase due to the pore sealing ability of WGC. Moreover, microhardness and porosity were found to be connected via an exponential relationship. Finally, further investigation of the processing conditions is currently underway so as to better tailor the ceramic microstructures to meet the requirements for specific building applications.

ACKNOWLEDGEMENT This research has been co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: ARCHIMEDES III. Investing in knowledge society through the European Social Fund (Scientific Coordinator: Prof. V.G. Karayannis).

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Highlights  Development of fired ceramics 100% from lignite fly ash and glass cullet mixtures  Beneficial synergistic effect of different constituents in the raw material mixture  Production of successfully densified ceramic microstructures  Lowering of ceramic firing temperature towards energy savings  Sustainable use of industrial secondary resources