Low-Temperature Sintering of High-Siliceous Clay under Conditions of Oxygen Deficiency

In the work, the processes of phase reconstruction and properties of ceramics obtained from high-siliceous clay – sand, clay – cullet, and clay – sand – cullet mixtures are considered. It is possible to perform plastic molding of blanks from the given mixtures due to the presence of montmorillonite in clay and sand and sintering at 800 o C for 8 h under conditions of oxygen deficiency. Depending on the composition of the initial mixtures, specimens ranging in color from gray to black can be obtained. It has been established by the XRD, IR spectroscopy, and electron microscopy methods that the synthesized material is glass ceramics consisting of quartz, feldspars, and a glass phase. Depending on the content of the initial components in the mixtures, it is possible to obtain glass ceramics with high strength properties or coarse-pored glass ceramics whose properties are similar to those of foamed ceramics.


Introduction
In [1], a technology that makes it possible to realize the low-temperature synthesis of bricks from high-siliceous clays by the method of plastic molding of blanks was presented, though it is known that ceramics of this type is usually obtained from clays by dry or semidry pressing of blanks with subsequent high-temperature sintering [1][2][3][4][5][6]. It was established that for the initiation of the plastic properties of the material, it is necessary to introduce a small amount of montmorillonite/bentonite and water into high-siliceous clay. In [1], as a plasticizing additive, sand containing a small amount of montmorillonite (Table I) was used. Moreover, to perform low-temperature sintering, a low-melting component should be added to high-siliceous clay. As such, a component, low-melting glass, namely, bottle cullet with a melting point of ~ (750-800) o C, playing the role of flux, was chosen. It turned out that the ceramics obtained from binary and ternary mixtures containing high-siliceous clay, sand, and cullet and sintered at 800 o C for 8 h had high strength properties.
Tab. I Mineralogical composition of the used components [1]. Type  In view of the similarity of the mineralogical composition of the so-called "clay" and "sand" and the dominant content of quartz and plagioclases in them, it is more reasonable to assign these components to quartz-feldspar sands with a somewhat different composition [7][8]. It is known that, on the basis of high-siliceous sand, refractory ceramics is obtained at high firing temperatures [2,3,[9][10][11]. For quartz-feldspar sand, precisely the presence of feldspar containing orthoclase, albite, and anorthite provides the flux-forming effect and sintering at 1050 o С [12][13][14][15]. To decrease further the sintering temperature and obtain high-strength ceramics, different additives, e.g., bone ash, cullet, etc. are introduced into the initial composition or compositions [16][17][18][19][20][21][22][23][24][25][26]. As a rule, ceramics sintered in air is reddish brown.
Since in modern civil engineering, bricks of different color are extensively used, the objective of the present work is three-fold: (1) to develop a plastic molding technology of blanks containing a substantial amount of silica/sand; (2) to develop a technology of lowtemperature synthesis of ceramics whose color ranges from black to gray from high-siliceous clays by analogy with the technology developed in [1]; (3) to evaluate the properties of the synthesized material from the viewpoint of its application as building bricks or a filler in the case of increased contents of sand and cullet, and, hence, an increased amount of the glass phase in the newly formed materials.
To realize objective 1, namely, plastic molding of blanks, it is intended to determine the limits of decrease of the content of the clay component in the mixtures because exactly the total content of montmorillonite in clay and sand determine the plastic properties of the mixtures at a certain content of introduced water. To realize objective 2, black sand containing hematite (Fe 2 O 3 ) was chosen (see Table I) because it is known that the presence of iron oxide in clays under certain sintering conditions (reduction firing of clay) provides a decrease in the sintering temperature and the transformation Fe 2 O 3 → Fe 3 O 4, thus stimulating the appearance of black color in the ceramics [27][28][29]. To realize objective 3, it is necessary to carry out a complex analysis of the properties of the synthesized ceramics, particularly from the viewpoint of the evaluation of the compressive strength, fracture strength, pore formation, and water absorption.

Materials and Experimental Procedures
For study the effect of the composition of the gaseous medium on the lowtemperature sintering process of highly siliceous clays, in this work a double and triple mixtures consisting of yellow clay, black sand, and milled cullet from brown, green, and colorless bottles with low temperatures of melting were used. In the binary clay-sand and clay-glass mixtures, the clay content was varied from 100 wt% to 10 wt%. In the ternary mixtures, the contents of clay, sand, and glass were (wt%): 40:30:30, 29:25:50, and 15:25:60. Blanks were prepared by the plastic molding method from the initial mixtures with 100 g of the dry component per 40 or 30 ml of water. After natural drying in air for 8 days, blanks were carried into a hermetic draught cupboard. Then the chamber was blown by argon. Inside the chamber, blanks were placed in hermetically closable steel capsules. Capsules containing blanks were transferred into a Carbolite CWF 1300 electric furnace. Sintering was carried out at 800 o C for 8 h. The temperature was raised from room temperature up to 800 o C at a rate of 14.1 o C/min. Then heating was turned off, and the furnace was cooled down to room temperature. The sizes of the experimental specimens were 80 mm × 20 mm × 15 mm.
For the research of the initial components, mixtures, and ceramics, we used the following analytical methods: X-ray diffraction (XRD) analysis (a Siemens D-500 diffractometer in Cu K α radiation). An electron microscopy study and electron-probe microanalysis were performed with a SU 5000 Hitachi scanning electron microscope. Infrared spectra were obtained on a Frontier FT-IR 101804 spectrometer. The contents of oxides in specimens were determined with use of an S8 TIGER X-ray fluorescence spectrometer (XRF) (Bruker).
The linear shrinkage of specimens was determined as Σ, % = [(l 0l 1 )/ l 0 ] ·100, where l 0 is the initial length of blanks and l 1 is the length of blanks after sintering (ceramics). The water absorption was determined by the formula: W, % = [(P 1 -P 0 )/ P 0 ] ·100, where P 0 is the initial weight of a specimen and P 1 is the weight of the specimen after water absorption. The mechanical properties of the ceramics were investigated by standard techniques [30] on electromechanical universal tester, Model 312.31, Load Frame 250 kN, Serial 1088 machine. The compression strength was calculated by the formula: C = W/A (kg/cm 2 ), where C is the compression strength, W is the maximum load (kg) indicated by the test machine, and A is the average of the gross areas of the upper and lower bearing surfaces of the specimen (cm²). The flexural strength was calculated by the formula: R flex = 3FL/2bd 2 (kg/cm 2 ), where F is the maximum load indicated by the test machine (kg), L is the length of the support section (cm), b and d are the width and thickness of specimen (cm) [31].
To determine the average value of experimental data for clay and the obtained ceramics, measurements were performed on 10 specimens.

Characterization of the initial materials
According to the XRF data, in all components used in the present work, iron oxide is present (Table II). In temperature treatment of the given materials, with increase in the treatment temperature, under conditions of oxygen deficiency, their color gradually changes (see Fig. 1). Already at T tr. = (500-600) o C, clay turns dark brown, and, at T tr. ≥ 700 o C, becomes dark gray. Sand gradually turns darker and becomes black T tr. ≥ 500 o C. Glass begins to melt at T tr. ˃ 600 o C and gets a gray color. In the range T tr. ~ (800-1000) o C, intensive melting of glass occurs. Due to a decrease in the viscosity, the melt of glass spreads over the substrate surface, and, in cooling, as a result of the generation of microstresses on the "substrate-glass mass layer" boundary, cracking of glass occurs.
Tab. II Сhemical composition of used components.
Note: powder glass consists of cullet of green, brown, and transparent bottle cullet. The change in the color of the used materials indicates the possibility of the partial reduction of hematite, which is present in clay, sand black, and glass (see Tables I and II). The transfer of Fe 2+ ions into plagioclases, silica (SiO 2 ), and glass is not improbable. The formation of new phases of dark ferrosilicate compounds is also possible [32][33][34][35].

Preparation of blanks.
In preparation of blanks by the plastic molding method (see Fig. 2), 40 or 30 ml of water were introduced into mixtures, and wooden molds were filled by plastic mass. For the binary mixtures containing from 10 to 100 wt.% clay, the extrusion method of obtaining blanks can be used. After drying for 8 days, only on the surface of blanks containing 90 wt% sand, "down" appears. It consists of evaporation products of water soluble salts, which are present in clay and sand (see Table II) [36]. A characteristic feature of these blanks is the bad cohesion of sand particles due to the small content of montmorillonite in the composition.

Appearance of ceramic specimens
Note that ceramic specimens sintered at 800 o C differ in color, which, depending on the composition of the initial mixtures, varies from light gray, to gray and black (Fig. 3). This means that, by changing the composition of the initial mixtures in the same sintering regime (temperature, time, composition of the gas atmosphere in the firing furnace), it is possible to obtain bricks with dark color of different intensity in contrast to reddish brown bricks obtained by firing in air. Specimens obtained with a large sand content (≥60 wt%) swell, in the volume of the ceramics, porosity increases. The cause of pore formation in sintering is dehydration processes, dehydroxilation of phyllosilicates, and release of gas/vapor bubbles through melt [2,3,[37][38][39][40]. The surface of such specimens has signs of "vitrification". In the ceramics obtained for the clay-glass mixtures, vitrification signs occur more intensively. The preparation of specimens from the clay-glass mixtures is possible only at a glass content no greater than 50 wt%.

Shrinkage of blanks at sintering
In sintering, the sizes of blanks change. Their changes depend on the composition of the initial mixtures (Fig. 4). For specimens obtained from the clay-sand mixtures, at a sand content ≥50 wt%, the shrinkage gradually decreases due to the increase in the silica (quartz) content in the mixtures. However, at С sand ˃ 50 wt%, as a result of the formation of pores in the ceramics (see also Fig. 3), the process of deformation/"swelling" of ceramic specimens occurs. The linear sizes of the specimens increase. In Fig. 4 a, these changes are as negative values of the shrinkage. As noted, the "swelling" of specimens depends on the content of phyllosilicates in the mixtures and the content of eutectic melt, in the formation of which plagioclases play a significant role. According to Table I, with increase in the sand content in the mixtures, the content of plagioclases rises, and the content of phyllosilicates decreases. This must lead to a decrease in pore formation and partial "healing" of pores by melt.
For the ceramics obtained from the clay-glass mixtures, the shrinkage gradually decreases with increase the glass content up to С glass =50 wt% (Fig. 4 b). However, at С glass ˃ 50 wt%, due to the large content of eutectic melt, blanks lose their shape, take the form of glass mass, and, in cooling, split into fragments/debris.
The shrinkage of the ceramic specimens obtained from the ternary mixtures ( Fig. 4 c), as in the previous cases, depends on the silica content in billets, the amount of formed melt, and evidently depends on the porosity of specimens. This character of the shrinkage of ceramic specimens is radically different from the character of the shrinkage of similar blanks in firing in air. According to [1], with increase in the content of sand and/or glass in mixtures, the shrinkage of ceramic specimens increases, whereas in this case, it decreases. To understand this difference, it is necessary to analyze the phase composition and porosity of specimens.

X-ray data
The results of the investigation of the phase composition of specimens by the XRD method show (Fig. 5) that quartz and feldspars are the main crystalline phases [41][42][43]. With increase in the contents of sand glass in the initial compositions, the contribution of the amorphous phase to the ceramics, which is recorded as a diffuse halo in XRD patterns, increases. The data of the semi quantitative analysis of the content of these phases indicate that, depending on the composition of the initial mixtures, in the ceramics, the quartz content changes, new feldspars appear, and the content of the amorphous phase changes (Fig. 6). It can be concluded that, in sintering, the interaction processes of the components with the participation of quartz actively proceed in the formed eutectic melt. In cooling, a material consisting of quartz, feldspars, and the amorphous phase forms.

IR data
IR spectra of ceramic specimens are a superposition (Fig. 7) of the spectra of quartz, feldspars, and glass [41,42,[44][45][46]. Depending on the individual contribution of one or another phase, the observed shift of the intensive absorption band ranges from  ~ 1090 cm -1 to  ~ 850 cm -1 , which indicates that the content of the glass phase in specimens rises with increase in the contents of sand и glass in the initial mixtures. At the same time, in the frequency range  ~ (800-600) cm -1 , the shape of the absorption band changes substantially. For the ceramics obtained from the initial mixtures consisting of clay and sand, the shape of the absorption band corresponds to a greater extent to that of sodium feldspar, whereas, as the content of the cullet introduced into the initial mixtures increases, it corresponds to that of potassium feldspar (see the upper right part in Fig. 7). In the case where large amounts of glass are introduced into binary and ternary initial mixtures, the IR spectra of specimens are most similar to that of the glass phase.
Thus, the IR spectroscopy data agree with the XRD data, indicating that, in sintering of blanks, interaction processes of components, which lead to both the formation of both new feldspars and the glass phase, occur.

SEM and EDS results
The SEM data indicate that, after introduction of sand and cullet into ceramic specimens, the sizes of areas joined by melt increase. This is most clearly recorded in specimens obtained from the clay-glass mixtures (Fig. 8). The X-ray microanalysis data demonstrate ( Fig. 9-11) that, on fractures of all specimens, predominantly Si, Al, and O are present. The amount of Ca, Mn, Mg, K, Na, and Fe is much smaller, but their distribution is relatively homogeneous, which is explained by the spread of formed melt over the whole volume of the specimen. It follows from the data of Table III that the sand and glass additives influence the change in the contents of the elements in the ceramics. Though the evaluation of the content of the elements was performed in small areas of specimens (20 ×10 m) and is not exact for the whole specimen, we, nevertheless, can arrive at the conclusion that the Fe content depends to a substantial extent on the amount of the introduced sand, which contains the largest amount of iron oxide (see Table II). At the same time, with increase in the content of glass introduced into the initial mixtures, the iron content in the ceramics decreases because, in the used glass powder, the Fe 2 O 3 content is small (see Table II).    The study of the porous structure of specimens indicates (Fig. 12 a) that, at C sand ≥ 50 wt%, for the binary clay-sand mixture, an increase in the porosity of the ceramics is observed. Taking into account that, in sintering, new feldspars and the glass phase are formed (as evidenced by the XRD data, IR spectroscopy data, and the vitrified appearance of specimens), we can conclude that precisely the interaction processes of the components are the source of pore formation. Exactly these processes lead to an increase in the volume of specimens (see Fig. 4 a). The ceramics sintered from the clay-sand mixtures with C sand ≥ 60 wt% can be classified with foamed ceramics. Similar results on the low-temperature synthesis (850 o C for 2 h) of mesoporous ceramics were obtained in [47]. In the ceramics obtained from the binary clay-glass mixtures with C glass ˃ 50 wt%, (see Fig. 12 b), a decrease in the porosity due to a substantial increase in the content of eutectic melt and filling of a large part of open and interconnecting pores by it was noted. This ceramics is similar in properties to glass, has a low toughness, loses its shape, and readily fractures in a fast cooling regime (see also Fig. 3). Thus, it is possible to add at most 50 wt% of low-melting glass to this high-silica clay.
The character of changes in the porous structure of the ceramics obtained from the ternary clay-sand-glass mixtures is also caused by the interaction processes of the components, the evolution of vaporous and gaseous products, the increase in the content of the glass phase, and the filling of a part of formed pores by melt (see Figs. 12 c, 4 c). Undoubtedly, it is possible to extend substantially the composite composition of the initial mixtures that provide the preservation of the shape of specimens, but, in sintering, the glass phase with a low toughness must not dominate.
The appearance of the dark color of different tints of the ceramic specimens in the stationary sintering regime of different mixtures arouses particular interest (Fig. 3). As it follows from Table II, in all used initial components, the content of iron oxide is not so high.
Only in sand, the Fe 2 O 3 content is ~4.3 wt%. However, the XRF method shows that the total content of iron oxide in the mixtures can attain required values. Black ceramics is usually obtained in the reduction-firing regime of clay at iron oxide content in it above 5 wt% [27]. The basis of the appearance of the dark color is a number of processes, namely, the incomplete reduction Fе 2 Оз to FеО and formation of magnetite Fе 2+ Fе 2 3+ (Fе 3 O 4 ), fayalite Fе 2 SiO 4 , and hematite α-Fе 2 Оз in bricks, and the presence of [Fe 3+ O 4 ] 4and [Fe 3+ O 6 ] 9anions in the composition of the glass phase and in metakaolin. According to the EDS analysis data, iron is present in all ceramic specimens (see Table III). It can be noted that the larger the total content of Fe/Fe 2 O 3 in the initial mixtures, the darker the color that of the sintered brick products. This concerns the clay-sand mixtures. Correspondingly, the smaller the content of Fe/Fe 2 O 3 in the initial mixtures, the lighter the color of the ceramics. This concerns the clayglass and clay-sand-glass mixtures (see Fig. 3) because, in glass, the content of iron oxide is smallest.
Tab. III Contents of elements in ceramic specimens on an area 20 ×10 m according to EDS data. The question arises as to what dominant process provides dark color of specimens. In view of the fact that, at T sint. = 800 o C, a large amount of the liquid phase appears, the active reconstruction of feldspars with the participation of silica occurs, and phyllosilicates are absent, it can be concluded that the main sources of the appearance of dark color are the formation of [Fe 3+ O 4 ] 4and [Fe 3+ O 6 ] 9anions in the composition of the glass phase and feldspars, and the formation of Fе 2 SiO 4 . Since the fayalite phase was not detected in the X-ray diffraction patterns, the source of the appearance of the dark color is predominantly the glass phase and feldspars alloyed with iron ions. This is confirmed by the X-ray diffraction patterns of specimens synthesized from the mixtures with large contents of sand and/or glass (see Fig.  5).

Water absorption in ceramic specimens
Depending on the field of application of building ceramics (materials for external and internal walls, facing and roofing materials, etc.), different requirements on water absorption are imposed upon brick products. In the present work, it was established that, for the ceramics obtained from the binary clay-sand mixtures, the water absorption (W) is about 23-25 % (Fig.  13 a) at a sand content of at most 50 wt%, which differs from the standard value for wall ceramics, for which W ~ (6-16) %. For the ceramics obtained from the binary clay-glass mixtures, with increase in the glass content up to 50 wt%, W decreases from 8 down to 5 % (Fig. 13 b). For the ceramics obtained from the ternary mixtures, as the total content of sand and glass decreases down to 50 wt%, W decreases from 15 down to 3 % (Fig. 13 с). It is possible to assume that by selection of the sand-to-glass ratio, it is possible to reduce the value of the water absorption. The water absorption of this ceramics must approach that for stone products, which will provide its application for the brickwork of the lower stories of buildings. According to the XRD and IR spectoscopy data (see Figs. [5][6][7]12), these changes in W depend on the content of the amorphous (vitreous) phase in specimens. The larger the content of the amorphous phase, the smaller the water absorption because open pores are filled by liquid melt.

Mechanical properties of ceramics
Since the compressive strength and fracture strength of building bricks depend on the content, composition, and morphology of the binder, the function of which is fulfilled by lowmelting eutectic and/or the glass phase [1][2][3]48], the observed changes in the compressive strength of the ceramics obtained from the clay-sand mixtures at C clay ˂ 80 wt% (Fig. 14 a) can be associated with the increasing deficiency of the binder (namely, the glass phase), though the fracture strength begins to decrease only at C clay ˂ 60 wt% (Fig. 14 a'), when quartz begins to precipitate from the eutectic melt (see Fig. 6 a), and the porosity of the ceramics increases (see Fig. 13 a). At C clay ˂ 50 wt%, when porous ceramics are formed (see Fig. 12 a), the compressive strength and fracture are reduced. However, such samples can be used as heat-insulating aggregates (analogue of expanded clay) [2][3].
For the clay-glass mixture, with increase in the glass content in the initial mixtures, the compressive strength increases (see Fig. 14 b) as a result of the increase in the content of melt (see Fig. 6 b) and filling the pore space by it (Fig. 13 b). However, the fracture strength decreases at C clay ˂ 80 wt% due to the brittleness of the glass phase (Fig. 14 b'). The ceramics obtained from the ternary mixtures is characterized by the highest strength properties (see Fig.  14 c, c'). In it, a smaller amount of quartz is contained, different feldspars form, and a sufficient amount of the glass phase, which joins quartz and feldspars and partially fills the pore space formed in sintering, is preserved (see Figs. 5 c, 6 c, 13 c). Note that the strength properties of bricks sintered under the conditions of oxygen deficiency at T sint. = 800 o C for t sint. = 8 h do not have a standard. In [50], strength data for bricks obtained after firing in air by the traditional technology (T sint. = 1000 o C for t sint. = 24 h) are presented. The compressive strength and flexural strength are 110 kg/cm 2 and 57 kg/cm 2 , respectively. These values are higher than those for black bricks obtained from the binary mixtures in the present work. However, as seen in Figs. 14 c, c', by varying the composition of the ternary mixtures (clay, sand, and glass), black ceramics with high strength properties is easily obtained in the energysaving sintering regime.

Conclusion
The performed investigations have shown that by using high-siliceous clay and sand similar in mineralogical composition with additives of low-melting glass, it is possible to realize plastic molding of blanks.
The sintering of mixture with a total content of iron oxide of about 5 wt% under reducing conditions at T sint. = 800 o C for 8 h leads to the formation of glass ceramics consisting of quartz, feldspars, and the glass phase. The basis of preparation of ceramics of different color spectrum (from gray to black) is diffusion of iron ions into eutectic melt.
By changing the contents of clay, sand, and glass in sintering, it is possible to obtain two types of ceramic materials: (a) in the form of building bricks; (b) in the form of porous fillers (expanded clay). To obtain products of type a, it is necessary to use the binary mixtures x wt% clay + y wt% sand, where x ≥ 80 wt%; x wt% clay + z wt% glass, where x ≤ 50 wt%; and ternary mixtures x wt% clay + y wt% sand + z wt% glass, where y + z ≥ 60 wt%. To obtain products of type b, it is necessary to use binary mixtures x wt% clay + y wt% sand, where x ≤ 40 wt%.