3.1. Characterization of Initial Materials
The particle distribution was determined for the ground sample of disintegrated granitic waste and its mixture with the raw clay (
Figure 2,
Table 1).
Particle size analysis revealed that the GW belonged to sandy loam and the CGW to silt loam, both of a grey pattern. The GW showed coarser particle size distribution than the composite sample (CGW). Microscopic identification of both 0.063 mm gray-patterned sieve residues determined the powdery materials are built of quartz, feldspar, and cobwebs of brown biotite and pearl-like muscovite. Aggregate grains of a dark reddish color, most likely of iron-siltstone composition, were also present to a low extent. The samples showed no reaction in contact with 5 vol% HCl. The observed similarities between the residues are one of the factors that confirm the possibility of using both materials in the production of ceramic tiles. Both the samples contained 0.00% of total carbonates, as determined by the volumetric method.
The chemical content (
Table 1) revealed that the GW mainly consisted of SiO
2, with significant quantities of Na
2O and K
2O, and relatively low content of Al
2O
3. The increased content of fluxes makes this material very favorable in terms of obtaining high-strength ceramic tiles. The quantity of Fe
2O
3, TiO
2, and MnO seemed satisfactory to obtain a pale color [
22]. The sample GW exhibited a typical chemical composition of a granite (
Table 1) that is very close to the average chemical analysis of raw granite deposits from which fluxes used by the ceramic tile industry are usually recovered (Na
2O in the range of 0.39–9.12 mass% and K
2O in the range of 0.51–7.63 mass%) [
3]. Based on the chemical composition, GW can be classified as a quartz feldspathic flux of type NKQ1, and is considered a raw granitoid suitable for application in the production of ceramic tiles [
3,
6]. The chemical composition of GW falls within the range of raw granites, but its iron amount is not so low as to be comparable with leucogranites [
3].
The chemical content of some of the trace elements expected to leach from the materials was determined by using ICP (
Table 2). We show that cadmium, chromium, copper, zinc, nickel, and arsenic are contained in a quantity lower than in some natural rocks, which would not cause problematic leaching [
36]. However, the quantities detected in the bulk materials were lower than those leached out from fly ash [
37]. The quantity of lead and barium might be problematic, and also possibly that of chromium. The leaching tests were done according to the procedure proposed in SRPS EN ISO 10545-15 [
38], and none of these trace elements were detected in the distilled water solution.
Mineralogical analysis of the aplitic granite waste showed that the most common minerals were feldspars (albite and orthoclase), quartz, and illite-mica (
Figure 3). Albite was dominant over other minerals, which is usual in hydrothermally altered granitoids [
24,
39]. The kaolinite content was low (about 2%), which indicated a low degree of kaolinization of the granite rock, and consequently a relatively young geologic age of the sediment. Additionally, dolomite, goethite, and vermiculite were detected in minor quantities (
Figure 3,
Table 3). For a more improved ceramic flux, it would be beneficial, although not necessary, to lower the amounts of micas and iron oxide by means such as high-intensity magnetic separation. When 60 mass% of the raw ceramic clay is introduced to 40 mass% of the GW, a dominantly quarzitic sample is obtained, containing 27 mas% of feldspars and 24 mass% of clay minerals (
Figure 3,
Table 3).
The obtained FT-IR bands in the GW and CGW samples (
Figure 4) complemented the composition of the material determined by the XRD and XRF analyses. The sharp bands at about 3616, 3652, and 3688 cm
−1 in the CGW correspond to the -OH groups’ stretching vibrations of illite-mica and some kaolinite [
31]. Additionally, the mild and wide band at 1630 cm
−1 indicates the bending vibration of the physically adsorbed water molecules to illite-mica. A weak band of illite-mica was observed at around 848 cm
−1 [
31]. All the previously mentioned peaks are missing in the GW samples.
The largest bands at 998/1012 cm
−1 and mild shoulders at 1022/1033 cm
−1 in the CGW and GW correspond to the Si-O asymmetrical stretching vibrations of quartz, feldspars, and clay minerals [
40,
41]. The prominent and mild absorption shoulders at about 915/912 cm
−1 represent the Al-OH vibrations of the clay minerals in the CGW and GW, respectively [
31]. The small bands that appeared as the shoulders near the bottom of the most intensive band (CGW:1113 and 1170 cm
−1, GW: 1078 and 1175 cm
−1) may be attributed to the characteristic splitting of feldspar bands [
42].
The distinctive bands at 424/427 and 530/538 cm
−1 (CGW/GW) again present the part of the footprint of feldspars [
40,
42]. Nonetheless, the second band at 530/538 cm
−1 may also correspond to trace amounts of hematite [
31]. Besides, in the sample GW, the band is detected at 590 cm
−1, showing O-Si(Al)-O bending vibrations in feldspars. The same vibrations are noticed at about 652/648 cm
−1 in both samples (CGW/GW), corresponding to orthoclase [
42].
The crystalline form of quartz and symmetrical bending vibrations of the Si-O bond [
31,
40] is detected at 697/701 cm
−1 in CGW/GW, being more prominent in CGW. The amorphous portion of quartz is seen as stretching vibrations in the CGW as a doublet at around 785 and 799 cm
−1. The first band in a triplet, to which the quartz doublet builds, is detected at 760 cm
−1 in CGW, and corresponds to feldspars [
40]. Amorphous quartz is not detected in the GW sample. Several other bands corresponding to Si-O-Si deformation of quartz occur in the 465/467 cm
−1 (CGW/GW) [
31,
40,
41].
The DSC/TGA/DTG analysis of the aplitic granite waste is rarely presented in the literature. The peaks on the DSC diagram of the GW confirmed the significant presence of quartz (
Figure 5a). TG analysis showed that this sample lost a small amount of water. A small mass loss, when heated to 1000 °C (below 1%), confirmed the very low content of clay mineral components. The DSC/TGA result obtained was very similar to the one published in the case of granitic rock [
22]. Thus, this waste material would act as a filler in a mixture with clay by lowering the plasticity due to its high content of quartz, and as a flux at elevated temperatures due to its feldspathic nature [
12,
13,
43,
44]. Quartz is expected to increase the mechanical strength of the products by filling the porosity with melt [
22]. On the other hand, the CGW sample showed a mass loss of 5.1%, which was mainly caused by the clay minerals introduced with raw clay. The removal of free water and interlayer hydroxyl groups appeared at 50 °C in the CGW (
Figure 5b) and contributed to about 0.42 mass% of loss at the corresponding TGA curve. A similar effect is seen in the curve of GW but to a significantly lower extent. The combustion of a small amount of organic matter initiated at about 200 °C, gaining the exothermic maximum at 342 °C in the CGW.
The most intensive endothermic peak seen at about 500 °C corresponds to the dehydroxylation of illite-mica and some kaolinite [
31] when the most intensive mass loss was also observed (
Figure 5b). The ά-β structural conversion of quartz is detected at 573 °C in both the samples, being more prominent in the CGW. The sharp peak in the DTG curve at 706 °C in the GW indicates a small amount of calcite, and less intensive peaks at about 768 °C for both the samples show the presence of a minor MgCO
3 [
31]. A period of almost no thermal changes in both samples was experienced from about 600 to 889 °C, after which mild endothermic reactions occurred, presenting further structural reorganization within the materials and complete crystal water removal [
31]. A low-intensity exothermic peak occurred at 983 °C in the CGW, corresponding to a small amount of the primary mullite formation.
The first known dilatometric analysis of this kind of waste is shown in the following sections. During the dilatometry testing, the GW sample was found to intensively constantly expand by 3.45% when heated to 958 °C, which was followed by fast collection, i.e., sintering (
Figure 6). The effect is taken to reflect a high content of fine-grained feldspars in the material [
44]. On the other side, the CGW has gently expanded by 1.08% up until 949 °C, experiencing low firing shrinkage which is very convenient for the production of ceramic tiles.
Both samples showed a mild spreading of about 0.07%; this is related to the removal of adsorbed water while firing up until about 161 °C. A period of the absence of the dimensional changes lasted up to 243 and 190 °C in the GW and CGW, respectively. A sudden shrinkage of 0.13% was observed in the CGW composite up to 228 °C. This was followed by a period of accelerating expansion to 635 °C in the GW (1.53%), and a constant and intensive expansion of a total of 0.64% in the case of CGW to 614 °C. The expansion is a consequence of the removal of crystalline water from illite-mica and kaolinite, the inversion of quartz, and the decomposition of the organic matter [
31]. The typical acceleration of spreading, recorded between 614 and 635 °C in the composite sample, is associated with the final removal of the OH-groups from clay minerals [
31]. After this period, both samples expanded slightly, i.e., by 0.32% to 836 °C in the case of GW and by 0.17% to 930 °C in the case of CGW. The sample GW experienced a notable expansion of 1.45% in the region between 836 and 896 °C, which was followed by an interval without major changes lasting up to 958 °C. Final shrinkage began at 958 and 949 °C in the GW and CGW, respectively. The later shrinkage was caused by the γ-alumina and mullite formation in the CGW [
3]. The more pronounced shrinkage of 1.03% at the final temperature in the case of the GW sample was a consequence of the sintering of feldspars, which begins at 900 °C [
3]. While cooling, both of the samples shrank by about 0.5% due to the relatively high quantity of quartz [
31].
3.2. The Behavior in Shaping, Drying and Firing
Plasticity according to Pfefferkorn and sensitivity to drying were not possible to perform in the aplitic granite waste because the samples were not stable enough for testing, given that they were low in clay minerals (non-plastic material). The coefficient of plasticity of the CGW was 22.1, which classifies the material as moderately plastic (
Figure 7a). Water needed for plastic forming of the CGW was determined for a deformation ratio of 2.5 and amounted to 20.7%. The sample was weakly sensitive to drying (
Figure 7b), thus experiencing a moist loss during drying in the air of 5.80%, while shrinking to 0.77%.
The important characteristics of the dried and fired samples are shown in
Table 4 and
Table 5. Low water absorption and modulus of rupture of up to 18.6 MPa after firing of the GW at high temperatures are characteristic of granitic materials [
22,
27]. The obtained values of water absorption (1.3%) and modulus of rupture (28.8 MPa) in the CGW were similar to the granite sawing waste in ceramic tile formulation from the literature [
27]. The bulk density of the composite samples was higher than in the GW, meaning the more intensive consolidation of the matrix appeared during the firing process due to the fluxing action on clay minerals.
The temperatures of clinkering and sintering, as obtained from the gresification diagram [
28], are presented in
Figure 8. A very narrow sintering interval of 36 °C in the GW defined by these temperatures range is consistent with the fluxing nature of the test sample. The characteristic temperatures in the case of CGW revealed somewhat lower clinkering and later sintering compared to the GW. The refractoriness of the CGW was obtained much higher (1535 °C) than that of the GW (1273 °C) due to the somewhat higher content of kaolinite and lower amount of quartz. The refractoriness of the sample (GW), presented below, was higher up to about 100 °C than is documented in the literature data for granitic rocks [
22].
The appearance of the samples, along with the L*a*b* color coordinates, are shown in
Figure 9. The samples of GW fired at 1200 °C seemed overfired due to the appearance of a glassy phase on the sample surface. A grey pattern of satisfying color tonality was obtained in the CGW. Both samples showed an increase in lightness and a decrease in red hues with the firing temperature.
Based on the obtained test results concerning the sample of disintegrated granite, the sample presented young sediment, without plasticity and of insufficient quality in terms of ceramic tile production. Its application is possible as one of the components of the raw material mixture for the production of ceramic tiles.
SEM-EDS analysis was done on the GW and CGW fired at 1250 °C (
Figure 10). The outer side of the samples is recorded below.
The results proved the generally expected mineralogical composition similar to the previous studies [
2,
40]. Primary mullite was found in the form of nano- and micro-sized elongated crystals in a mixture with feldspars and quartz, as seen previously [
2,
43]. These elongated crystals are found typically in the case of mullite formed after firing and decomposition of feldspars [
20]. Quartz is seen as partly dissolved in the matrix or the form of irregularly shaped crystals [
2]. Additionally, the CGW is seen to contain some microcracks, which were mainly around quartz grains due to the α → β conversion [
2]. Generally, a usual, porcelain-like, glassy, dense microstructure interrupted by coarser grains of quartz and nano and microcrystals of mullite, feldspars, and quartz is recorded [
40]. In addition, a small amount of rounded open pores is noticed in the case of CGW, since the densification of the matrix during melting and sintering is intensified by the addition of the aplitic granite waste [
2]. The pores of 2.6–5.2 µm in size are seen in
Figure 10, which are significantly smaller than those obtained previously with mineralogically similar waste material [
2]. The GW sample contained somewhat more cracks than the CGW, with highly compact parts due to the more liquid phase that formed in the pure GW. Unusual accumulations in the matrix of the CGW presented minor amounts of rutile and goethite.