The effect of marble waste in the production of low-temperature porous material from Alkali-activate

35.08. wt. %, which correlates well with the reported values

[30,31]. When looked at the phase analysis, quartz, haematite and mullite were detected to be the main crystalline phases in the Tunçbilek FA (Figure 1). The origin of quartz (primary quartz) in the Tunçbilek FA was the source coal and the secondary quartz formed during combustion [32]. The mullite was the product of the solid-state reaction of decomposed clays [32] and/or occurred through the crystallization of the aluminosilicate melt [33]. The existence of haematite, which generally exists in bituminous F class FA, was because of the thermal decomposition of clay minerals during combustion [34]. Calcite was the only dominant crystalline phase in the MW with a low amount of quartz phase (Figure 1) [35]. The fact that the MW contained a high amount of CaO (61.65 wt. %) (Table 2) enabled it to be a suitable material in its use as a pore former. It was also used as a cheap pore-forming agent because it was a waste material [31,32,35]. The thermal behaviour of the Tunçbilek FA was studied by DTA/TG analyses (Figure 2a). First, an endothermic peak on the DTA curve at approximately 160°C corresponded to the evaporation of moisture, that is, dehydration of adsorbed water mechanically bonded in the form of H2O molecules on the surface of the ash particles [36,37]. In the interval between 100 and 450°C, the removal of hydrated water was completed [38]. Subsequently, an exothermic event at 678°C was associated with the combustion of unburned coal [39]. At higher temperatures between 800 and 1200°C, the decomposition of impurities from

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Table 2. Chemical analyses of Tunçbilek FA and MW Chemical composition (%)

Figure 1. XRD spectrum of Tunçbilek FA (a) and MW (b). (C: calcite, Q: Quartz, M: Mullite, H: Hematite)

coal, carbonates, or sulphates took place [36,39]. The TG curve accordingly showed a continuous decrease in weight. The melting temperature of the Tunçbilek FA was 1220°C (Figure 2a). In the DTA analysis of the MW (Figure 2b), a gradual mass loss of approximately 1.5 % was observed up to 575°C, which can be attributed to moisture loss [31,36]. A sharp endothermic reaction occurred between 620 and 760°C with a rapid downward move in the TG curve. This temperature indicated to the decomposition of calcite according to equation 2. Looking at the TG curve, the MW lost about 39% of its initial weight between 620 and 760°C [30,31]. It was considered that the high loss of ignition (LOI) (Figure 2b) (Table 2), would contribute to the formation of a porous structure in this study. Following equation 2 [30,31], it was presumed that the calcium carbonate broke down under heat and converted into CaO (solid) and CO2 (gas), and open-pore structure formed because of the CO2 gas released. CaCO3(s)→CaO(s)+CO2(g)

The size distribution of the Tunçbilek FA (D10 6.09 µm, D50 32.5 µm and D90 124 µm) ranged between 0.46 and 310 µm. However, the particle size distribution of the MW was smaller (D10 2.11 µm, D50 9.17 µm and D90 42.14 µm) than that of the Tunçbilek FA in a close range between 0.46-66 µm (Fig. 3). Therefore, it was considered advantageous in that the pore dimensions are in a narrow range. The Tunçbilek FA and the MW were both used as received with no size-reduction. The Tunçbilek FA mostly consisted of spherical particles so-called microspheres with some irregularly shaped angular particles (Figure 3). The MW had a microstructure consisted of irregular but dimensionally similar particles (Figure 3). In the current experimental work, the production of open-pore ceramic from waste materials FA and MW was investigated. The alkali activation method was adopted to reduce the sintering temperature of green specimens to the lowest possible value. As seen from the DTA data in Figure 2b, the decomposition temperature of the MW was between 620 and 760°C [30,31]. The LOI value of the MW was 39 wt. % [30,31]. However, as mentioned earlier,

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Figure 3. Particle size distribution of Tunçbilek FA and MW.

previous studies on sintering of geopolymers showed that densification and formation of new phases in the structure [23], which improved the properties of geopolymers [19,20], occurred at and above 900°C [19,20]. Therefore, in the present work, the alkali-activated specimens composed of the Tunçbilek FA and the MW were sintered at 900°C, to investigate the effect of sintering on forming an open-pore structure and contribution of new phase development to mechanical properties. Physico-Mechanical Properties of Sintered Porous Geopolymers FTIR analysis is principally based on the absorption of different wavelengths of infrared light and studies the composition and structure of material molecules. Besides, FTIR absorption spectroscopy is an effective tool for characterizing alkali-activated materials because of its well-known sensitivity for materials of short-range structural order [40]. Therefore, FTIR tests were conducted on the raw Tunçbilek FA and alkali-activated compositions to investigate the effect of alkali activation on the FA structural development. The results of the FTIR spectra were depicted in Figs.4a and 4b. In Figure 4a, the effect of alkali activation was determined by examining the raw and alkali-activated FA (MW0) specimen. The central band at around 1022 cm-1, as the main feature of the FTIR spectra of raw FA, exhibited overlapped peaks associated with the asymmetric stretching vibrations of tetrahedral Si-O-Si or Si-O-Al bonds [41]. The intensity of this band is proportional to the reactivity of FA [42]. The peak at 1022 cm-1 shifted towards lower wavenumber of 983 cm-1 indicating the formation of geopolymer structure

due to alkali activation [42,43]. The new peak observed at a lower wavenumber of 983 cm-1 on geopolymerization was directly linked with the transformation of Si-O-Si bonds of amorphous silica into Si-O-Al bonds of poly sialate, which meant the substitution of Si by tetrahedral Al and therefore the formation of the geopolymer structure of aluminosilicate network [44]. Another band, which appeared in the geopolymer specimen at around 1440 cm-1, was absent in the Tunçbilek FA. This characteristic band appeared due to the asymmetric vibrations of CO3-2 ions, which pointed to the presence of sodium carbonate because of the carbonation reaction between excess sodium and atmospheric carbon dioxide [42,44]. The band 777 cm-1 was connected with the symmetric stretching vibration of Si-O-Si, which was characteristic of quartz. Quartz was also detected in the XRD analysis of the Tunçbilek FA (Figure 1), which supported the FTIR results. The last band observed at around 443 cm-1 was assigned to Si-O- and Al-O bending vibrations characteristic for silica glass and silicates [42]. Figure 4b shows the FTIR spectra of alkali activated compositions MW0, MW5, MW10, MW20, and MW40. With the addition of the MW, new peaks at frequencies of 1409, 866, 712 cm-1 were detected. These peaks indicated the presence of calcite and their intensities increased with the increase in the MW addition. The FTIR peak at 983 cm-1 (MW0) indicating geopolymerization shifted towards lower wavenumbers 979 (MW5), 969 (MW10), 968 (MW20), and 965 cm-1(MW40) with increasing MW ratios. This result herein could not be interpreted in a similar way to the above discussion for Figure 4a because of the gradual replacement of FA by increasing MW ratio in geopolymer

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Figure 4. FTIR spectra of raw Tunçbilek FA and MW0 geopolymer (a) and geopolymers MW0 (a), MW5 (b), MW10 (c), MW20 (d), and MW40 (e) (b).

compositions. There were several articles on the formation and co-existence of the C-S-H phase within the geopolymer binder in the presence of significant amounts of calcium. A previous study reported that geopolymers with a high amount of Ca generated C-A-S-H phase along with the geopolymeric gel of N-A-S-H. In geopolymerization, Si4+ or Al3+ species react with Ca2+, either in the FA or from external calcium-containing additive, to form calcium silicate hydrate gel (C-S-H), calcium alumino hydrate gel (C-A-H) or calcium aluminosilicate hydrate gel (C-A-S-H) in the presence of water [40,45-48]. Therefore, in the present study, the presumption explained in the following lines was adopted. During geopolymerization reactions, CaO from the MW (CaCO3) was attacked by the alkali activation solution and turned into active species of Ca2+ and O2−. The resultant active Ca2+ ions, along with Si4+ and Al3+ from the dissolution of reactive glassy FA microspheres, reacted with OH− ions of alkali solution. Thus, C-A-S-H and N-A-S-H geopolymer gel formed [48]. It is thought that increasing MW content introduced more Ca2+ ions into the solution and accelerated the continuous development of geopolymer

matrix, while the formation of C-S-H and C-A-S-H phases was promoted, which reflected itself as shifts in FTIR peaks towards lower wavenumbers [41,49]. These structures were expected to contribute to new phases in the sintering process and provide advantages in preserving material integrity. Table 3 shows the change in apparent porosity and water absorption with the addition of increased MW. CO2 gas, released on the decomposition of calcite [30,31,50,51] in the MW (equation 2), exerted pressure against the glassy phase of appropriate viscosity formed during sintering, leading to pore formation. An increase in the MW content increased apparent porosity and water absorption because of increased pore formation. While the apparent porosity and the water absorption of MW0 were respectively 12.7 and 8.92%, they increased to respective ratios of 39.92 and 28.17% in MW40. This result translates itself to more than a three-fold increase in apparent porosity and water absorption values. An increase in apparent porosity and water absorption with increasing content of the MW followed similar trends (Table 3) because water

Table 3. Physical and mechanical properties of sintered porous geopolymers Sample code

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absorption directly relates to the open porosity of the sintered ceramics. When compared with the apparent porosity values of porous ceramics, produced from FA and MW/or calcite wastes by various other methods for different application areas, 39.92% open porosity value of MW40 was satisfactory and even better than those of the previous studies [25]. The water absorption values were in line [41] with or even better than that of the literature [24]. Furthermore, the apparent porosity value of MW40 (39.92%) was also in good agreement with those of openpore ceramics produced from different raw materials by various other methods [52,53]. However, it must be borne in mind that all those studies in the literature were carried out for much longer holding times, and most of them at much higher sintering temperatures. In Table 3, the variation in bulk density and linear firing shrinkage with an increase in the MW ratio was depicted. The bulk density of the sintered alkali-activated specimens slightly decreased, as expected, with increasing apparent porosity (Table 3). MW0 composition of high melting point (1220°C) was successfully sintered at the low temperature of 900°C. High sintering temperatures between 1050-1300°C are needed to sinter F class FA [5457]. Previous research reported that alkali activation was successful in the performance enhancement of ceramics sintered at the low temperatures [21,58]. The reason for lower temperature sintering of MW0 specimen at 900°C was the alkali activation of the Tunçbilek FA before sintering. The material produced from FA, not subjected to alkali activation, could not be sintered at 900°C, and therefore could not maintain its integrity. The bulk density increased from 1.42 to 1.55 g/cm3 at 5 wt. % MW (MW5) and then slightly decreased with increasing ratio of the MW down to 1.42 g/cm3 at MW40 composition. The first increase at 5 wt. % MW ratio may be because of an increasing amount of glassy phase formed by the fluxing action of calcite, which filled the pores in the structure and resulted in densification. In other words, calcite acted as a flux rather than acting as a pore-forming agent at this composition [36]. As the apparent porosity increased with the addition of the MW, the expected decrease in bulk density was not observed [31,32,35,50,51]. It is thought that this was because the MW used as a pore-making agent contained a high proportion of CaO (specific gravity 3.34 gr/cm3). Therefore, although the apparent porosity increased, there was no significant change in the bulk density value. Linear firing shrinkage also increased from almost 0 to 1.44% at MW5 and 1.71% at MW10 compositions, due to improved sintering then slightly decreased similarly with further increase in the MW content down to 0.81% at MW40 composition. The decrease in linear firing shrinkage can be attributed to the expansion that occurred due to increasing MW content acting as a pore former during sintering.

Microstructural Analyses of Sintered Porous Geopolymers Figure 5 shows the SEM images of porous geopolymers produced with different amounts of the MW. The SEM micrograph of MW0 in Figure 5a showed that the structure had open and closed pores with thick pore walls. The structure was dense due to intense sintering at MW0 composition with a low apparent porosity value of 12.7% (Table 3). Due to the breakdown of CaCO3 structure during sintering and the resulting CO2 gas, the porosity increased in microstructures (Figure 5b-e) as the MW ratio increased. Although the number of large pores of MW40 appeared less in Figure 5-e than those of other compositions, it was, indeed, the specimen with the highest apparent porosity (39.92%) (Table 3). Therefore, MW0 and MW40 porous specimens were compared with each other at high magnification in Figure 6 to reveal this difference in porosity more clearly. It was observed that MW0 had large pores and its pore walls were dense (Figure 6a). On the other hand, the SEM analyses determined that MW40 contained many small pores. Consequently, the addition of the MW resulted in a more porous (39.92%) structure (Figure 6b). Figure 7 shows the microstructure of the non-sintered MW40 composition. Compared with the microstructure after sintering (Figure 6b), the structure of the non-sintered specimen was dense, and the number of pores was low. However, after sintering, many new pores formed. Thus, a much more porous structure was observed by the removal of emitted CO2 gas during the sintering process. Phase Evolution of Sintered Porous Geopolymers To study the crystalline phase evolution of fired alkali-activated specimens with increasing CaCO3 (MW) addition was necessary for a better understanding of the modification of CaCO3 on the pore structure. Figure 8 showed the XRD spectra and phase distribution of the specimens with different quantities of CaCO3 sintered at 900 °C for 30 min. The Tunçbilek FA comprised quartz, mullite, and haematite while the MW was composed mainly of calcite and quartz (Figure 1). However, the crystalline phases of quartz, mullite and haematite in the Tunçbilek FA and calcite in the MW disappeared and transformed into the new phases of gehlenite, nepheline, quartz, orthoclase and calcium iron oxide (Figure 8) upon sintering at 900°C. The main crystalline phases in the structure of sintered geopolymers were gehlenite and nepheline. The MW contained a very high earth alkali Ca (61,65 wt. % CaO) ratio and Mg (0.69 wt. % MgO). The SiO2 content of the MW was 2.2 wt. %. On the other hand, the Tunçbilek FA contained earth alkali Ca (1.64 wt. % CaO), Mg (4,48 wt. % MgO) and the reasonably high content of Fe (11.45 wt. % Fe2O3). Besides, Na+ ions were also present in the structure due to alkali activation. All these alkaline earth oxides along with Fe2O3 have fluxing properties and it is thought that they contributed to the sintering of alkali-activated compositions at the low temperature of 900°C. This fluxing effect may reflect

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Figure 5. SEM analyses of sintered porous geopolymers MW0 (a), MW5 (b), MW10 (c), MW20 (d), MW40 (e).

Figure 6. SEM analyses of sintered porous geopolymers; MW0 (a), MW40 (b).

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itself as a lower glass phase viscosity, which helped to overcome the resistance against the CO2 pressure, leading to a porous structure. The phases of mullite, hematite, calcite in the fired geopolymers were totally, and quartz was partially consumed to develop new phases of gehlenite and nepheline (Table 2, Figure 1, Figure 8). Here, the addition of the MW had an effect on the phase structure formed. When the MW ratio was 5, 10 and 20 wt.%, the main phase was nepheline. However, when the MW addition was 40 wt.%, a low ratio of nepheline, orthoclase and calcium iron oxide phases developed in the structure, along

with a large proportion of gehlenite phase. The absence of the free CaO phase was critical for the integrity and strength of the structure. The Na ions in the alkaline activation solution contributed to forming new phases such as nepheline and gehlenite at low temperature (900 °C), resulting in strong porous ceramics. As seen in Figure 8f, a very different phase structure was obtained when the MW40 composition, which was not subjected to alkali activation, was sintered at 900°C. It was determined that there was free CaO at (a) high ratio, quartz, and Ca(OH)2 formed from free CaO in this phase structure. The high amount of CaO is extremely unfavourable in terms of the integrity of the structure. As can be understood from here, it was shown in this study that using a material with high CaCO3 content was possible with alkali activation. In addition, the sintered sample obtained when alkali activation was not applied was easily broken with a small force applied by hand (Figure 8g). Table 3 showed the variation in the compressive and flexural strength of the porous specimens obtained with increasing MW ratio. The addition of the MW caused a decline in mechanical properties. While this decline in strength was rapid up to 20 wt. % MW addition, it diminished at 40 wt. % MW addition. The porous specimen produced without the MW (MW0) had 34.98 MPa compressive and 19.85 MPa flexural strength values, while the 40 wt. % MW added porous specimen (MW40) had 12 MPa compressive and 5.35 MPa flexural strength values. The decrease in strength values was consistent with the increased apparent porosity. However, they were acceptable for open-pore materials [53].

Figure 8. XRD spectra of sintered porous geopolymers; a) MW0, b) MW5, c) MW10, d) MW20, e) MW40, f) un activated MW40, g) un activated MW40 photograph (G: Gehlenite, N: Nepheline, Q: Quartz, O: Orthoclase, C: Calcium Iron Oxide, P: Portlantide (Ca(OH)2)).

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