Development of an Energy-and Water-Saving Manufacturing Technology of Brick Products

In the work, the carrying of realizing three problems in the manufacture of brick products, namely, energy saving, water saving, and processing of large-scale waste (ecological problem), are considered. Four types of bricks have been obtained and investigated: red clay, red clay–milled cullet mixture, red clay– milled basalt (tezontle) mixture, and red clay–milled glass–milled tezontle mixture. To form the semi-finished products, water or wet waste of activated sludge were added to the dry mixtures. It is established that the presence of low-melting glass into double and triple compositions reduces the sintering temperature of ceramic products down to 900-1000 °C and sintering time to 8-12 h while maintaining good strength properties of ceramics. This means that the energy-saving technology is provided. The use of waste activated sludge (biowaste) with high water content allows us to exclude the introduction of water into dry mixtures at the stage of molding. This means that the water-saving technology is achieved. The introduction of a different content of burnt out bio waste contributes to the formation of brick products of different porosity. Variations on mixtures compositions and sintering modes make possible to synthesize ceramics of different applications: as a stone products, bricks for external and internal walls, porous bricks, and a filtering ceramics.


Introduction
Mankind is now confronted with the problems of improving ecology, energy saving, and water saving, which call for their urgent solution.The greatest part of these problems arose as a result of the technogenic activity of people.For instance, the manufacture of glass for different applications and glassware is accompanied by the accumulation of considerable quantities of cullet [1,2].The use of natural stones such as basalts in ceramic production is usually connected with their grinding and accumulation of milled waste [3].The purification of household and industrial sewage at water treatment plants with the use of activated sludge leads to the necessity for developing methods of processing the biowaste which are dangerous for the environment [4][5][6].All aforementioned wastes are large-scale and require processing.
It is desirable that the technology be simple and realizable for large-scale production of a new type of materials.The manufacture of brick products that requires the use of inexpensive raw materials (low-melting clays) is such a large-scale production [6][7][8][9][10][11].
The wide range of brick products, including the preparation of porous ceramics with water absorption above 5 mass % (e.g., ceramic bricks) or dense ceramics with water absorption under 5 % (e.g., clinker), opens a wide of possibilities of using additives for the modification of the sintering process as demonstrated [12][13][14][15][16].
It is known that the preparation of bricks is performed in the narrow temperature range T sint.= 1000-1050 °С for a long time.As a rule, the temperature treatment time is about 26-56 h, and the burning time is about 10-12 h.The synthesis of high-density high-strength ceramics from mixtures of clay, feldspar, and quartz is carried out at T sint.= 1200-1280 °С.
The aim of this investigation is to obtain different types of building ceramics (from porous bricks to stone products) with reduced both sintering time of semi products due to a decrease in the treatment temperature and time with retention of standard physical and mechanical properties of this type of products.
In 1972 it was shown [33] and then confirmed in [34] that the introduction of waste glass into clay decreases the sintering temperature of bricks by ~200 °C.Therefore, in the manufacture of bricks, it is reasonable to use bottle cullet, the softening point and melting point of which range from 700 to 800 °C.
As the second (refractory) component, tezontle was chosen.Tezontle belongs to basalts, which are rocks of volcanic origin.In the mineral composition of basalts, volcanic glass, microlites of plagioclase, titan magnetite, magnetite, and other silicate minerals are present.Basalts that are most commonly used as building materials are environmentally friendly materials [2,16,17,19,35,36].The melting point of basalts ranges from 1200 to 1500 °C.It is assumed that the fusible glass-tezontle composition can be regarded as an eutectic system with a melting point below 1000 °C.In the stage of burning (sintering), the destruction products of clay (alumina-silicates) have to take part in the formation of new multicomponent crystalline and amorphous phases and low-temperature eutectics.
In the intermediate stage of preparation of semi-products, water is added to clay to impart plasticity to the material (for the molding of specimens).However, after removal from the cleansing tank, waste activated sludge contains a big amount of water.Therefore, it can be used not only as a burnable additive in the sintering stage, but also for the molding of billets.
The synthesized products were investigated by X-ray diffraction (XRD) in Cu K α radiation (a DRON-3M diffractometer).SEM and EDS measurements were carried out with a LEO 1450 VP scanning electron microscope.Water absorption was determined by the formula: W, % = 100 -[(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.Compression and fracture tests were performed by standard techniques.

Sintering of composite billets prepared with the use of water 3.1.1. Initial materials
Red clay consists of montmorillonite, quarts, cristobalite, and feldspar.Tezontle contains chained and skeletonized silicates, silica (cristobalite and quartz), and iron oxides (wustite and magnetite).The compositions of low-melting glass and clay are presented in Table I.All crystallograms of these materials are shown in Fig. 1a.After treatment at 1000 °C during 1 h, the XRD pattern of only red clay changes substantially because, in montmorillonite, dehydration and dehydroxylation processes occur [37][38][39] (Fig. 1b).The composition of bio waste is presented in Table II.The temperature treatment of bio waste in air is accompanied by the burning-out of the organic component and a change in the color of the residue (Fig. 2).

Sintering of composite billets prepared with the use of water
The phase composition of sintered specimens prepared from different mixtures (Figs. 3, 4) is determined by the content of components in the initial mixtures and depends on the burning temperature and (T sint. ) and time (t sint. ) of the specimens.The characteristic feature of binary and ternary mixtures containing tezontle is a substantial change in the content of silicate phases, quartz, and cristobalite, which enables us to assume that, in the glass-tezontle and glass-tezontle-sillimanite systems, eutectic melts form in the temperature range 800-1000 °C and that, in cooling, silicates similar in composition to basalts crystallize from these multicomponent melts [35,36].SEM investigations showed that, independently of the composition of the initial mixtures, all ceramic specimens are porous.However, at introduction of glass or/and tezontle are added to the clay, the contribution of large pores is significantly reduced (Fig. 5 a-c).Ceramics is transformed into fine-pored material.Accordingly, the size of the glassy inclusions decreases (see Fig. 5 a'-c ').The structure of the material becomes more homogeneous.These transformations indicate the active development of the processes of interaction of the components of mixtures and formation of eutectics with a lower melting point [40][41][42].It should be noted that the destruction of specimens occurs most often at the boundary of the ceramics and large glassy inclusions (see Fig. 5a).The water-absorption tests of specimens showed that, for all specimens, W depends on the composition of the initial mixtures, sintering temperature, and time (Fig. 6).In view of the fact that, during the sintering of the mixtures, open and closed pores of different size are formed, the reduction in the water absorption can be explained by the decrease in the content of open pores as a result of the increase in the content of the liquid glass phase, which forms not only due to the appearance of melt during decomposition of the clay mineral [43,44] and melting of additives of low-melting glass, but also as result of the formation of low-melting eutectics.So, for a mixture of clay-glass (see Fig. 6a), a clear decrease in water absorption due to an increase in the content of liquid glass phase and a decrease in its viscosity with increasing sintering temperature.For the clay-tezontle mixture (see Fig. 6b), effective filling of open pores with melt is carried out at 1000 o C and depends on the sintering time.For ternary mixtures (see Fig. 6 c,d), in view of the total increase in the melt content, a low-porous ceramic can be obtained already with short-term sintering (~ 4 h).However, with an increase in the sintering time to 6 h, the water absorption changes insignificantly even with increasing T sint .By comparing the change in water absorption (W) with X-ray data (Fig. 3 c, d) and SEM (Fig. 5 a'-c '), we can conclude that in ternary mixtures at T sint.= (900-1000) o C between components mixtures, the processes of interaction and phase formation continue.They was accompanied by gas evolution and pore formation, which are immediately filled with on eutectic melt.Thus, the obtained results show that by varying the main technological parameters (c, T sint., T sint.,) it is possible to change not only the content of the liquid phase, but also its viscosity and, therefore, the porosity of the synthesized ceramics.A simplified scheme of formation of low-porosity ceramics is shown in Fig. 7.
As can be seen from Table III, the compressive strength and breaking strength also depend on the main parameters of preparation of the ceramics, namely, the composition of the mixture, sintering temperature, and sintering time.The dependence of the mechanical properties of the ceramics on the composition of the initial mixture, and sintering regimes will make possible to choose treatment regimes that will provide the preparation of ceramics in the form of brick products for different applications: for exterior and interior walls of buildings, in the form of ceramic tiles (clinker bricks), and for filtering of aqueous suspensions.
The obtained data enable us to conclude that, during temperature treatment of red clay (without additives) and the compositions used in this work (clay-tezontle-glass), different sintering mechanisms are achieved, particularly in the low-temperature region of treatment.For instance, in the first case, the formation of the liquid glass phase, containing quartz, cristobalite, feldspar, which are present in the initial clay, and crystalline phases formed as a result of dehydration of montmorillonite and decomposition of a number of impurity compounds is registered at T sint.≥ 900 °С.However, after addition of glass and/or tezontle to clay, the glass phase forms at lower temperatures (T~ 700 °С) due to the low-melting point of the used glass or as a result of the formation of the eutectic melt of glass and tezontle.This is why the sintering process can be performed at lower temperatures for a shorter heating time.Thus, an energy-saving manufacturing technology of brick products can be produced.

Sintering of composite billets prepared with the use of biowaste
Since biowaste contain substantial quantities of free water, its introduction into red clay must provide plastic molding of billets [40,41,[45][46][47].
It follows from Table II that a number of inorganic compounds, namely, clay, sand, and feldspar, enter into their composition.This is why, during treatment in air, the burning-out of a part of biowaste occurs, and the residue contains inorganic compounds (Fig. 2).
The phase composition of sintered specimens prepared from different mixtures is actually a set of phases characteristic of specimens obtained with the use of water (see Figs. 3,  4).This is explained by the fact that biowaste burn out (at ~600 °C for 30 min) well before the beginning of structural-phase treansformations in clay and the mixtures.The contents of phases in the sintered specimens depend on the content of components in the initial mixtures (c), T sint., and t sint.The characteristic feature of binary and ternary mixtures, as in the case of using water in the prepartion of billets, there is a substantial change in the content of the silicate phases, quartz, and cristobalite, which enables us to assume the formation of eutectic melts in the glass-tezontle and glass-tezontle-sillimanite systems in the temperature range 800-1000 °C and subsequent crystallization of silicates that are similar in composition to basalts from these multicomponent melts.
SEM results showed that, regardless the composition of the initial mixtures, ceramic specimens are highly porous (Fig. 8 a-c) with an inhomogeneous distribution of elements, caused by the ingomogeneity of the mixtures (Fig. 8 d, e).The high porosity of the ceramics implies the presence of closed and open pores, which are important for the evaluation of the brick products properties.The water-absorption testing of the ceramics showed that, in all specimens, by increasing the sintering temperature and time, water absorption decreases (Figs. 9 a,b -12 a,b), which is explained by an increase in the content of the liquid phase in the sintering process (due to the appearance of melt during decomposition of the clay mineral [43,44], melting of glass additives, and formation of lowmelting eutectics).As the biowaste content in the clay and initial mixtures increases, the porosity of specimens increases even at T sint.= 1000 °C (see Figs. 9с, 10c, 11c, and 12с).This is due to the more intensive vapor and gas release during destruction/burning-out of the organic component of biowaste and formation of a looser semiproduct.As is seen from Table IV and Figs.9d -12d, the compressive strength and breaking strength of specimens also depend on the main parameters of preparation of the ceramics, namely, the composition of the mixtures, sintering temperature, sintering time, and quantity of biowaste.Therefore, it is possible to obtain ceramics differing in strength and porosity with the use of biowaste and find appropriate applications depending on their properties.
The investigations carried out showed that biowaste with high water content can be used for the plastic molding of billets from clay, the clay-glass, clay-tezontle, and clay-glasstezontle compositions.Thus, the introduction of additives of milled solid waste (glass-tezontle) and wet biowaste into red clay make it possible to realize an energy-and water-saving manufacturing technology of ceramics of not only different strength, but also different porosity.A simplified scheme of pore formation in the ceramics prepared by plastic molding with the use of water and biowaste is shown in Fig. 13.

Conclusions
The use of compositions of red clay and sawing/milling waste of basalt (tezontle) and cullet makes it possible to reduce both the sintering temperature and time of ceramic products for different applications within the framework of the traditional technology, i.e., to realize an energy-saving technology.
The base of the energy-saving technology is the principle of formation of lowtemperature eutectics that form as a result of the melting of glass-basalt-aluminum silicates.
The use of biowaste from water treatment plants enables one to perform plastic molding of products without using free water and, therefore, to reach a water-saving technology.
The introduction of milled large-scale low-melting glass waste, basalt waste, and biowaste into red clay makes it possible to solve ecological problems of environment preservation.It is possible to obtain ceramics for different applications by varying the contents of components in mixtures and sintering regime (temperature and time).

Fig. 1 .
Fig. 1.Fragment of crystallograms of used clay, tezontle, and glass in the initial state (a) and after treatment at 1000 °C during 1 h (b).

Fig. 2 .
Fig. 2. Weight loss of waste sludge vs. time of treatment at T = 600 °C in air (1), and change in the color of specimens (2, the upper part of the figure).

Fig. 4 .
Fig. 4. X-ray diffraction patterns of specimens prepared from different mixtures with the use of biowaste after treatment at 1000 °C during different sintering times.p -peridot, p Lplagioclase, f -feldspar, s -sillimanite, c -cristobalite, q -quartz, p y -pyroxene.

Fig. 7 .
Fig. 7. Simplified scheme of molding of a ceramic material.

Fig. 9 .
Fig. 9. Absorption of water by the ceramics prepared from red clay vs. sintering temperature (a), sintering time (b), and biowaste content in the initial clay (c).In (c): dependence of the compressive strength on the sintering temperature.

Fig. 10 .
Fig. 10.Dependence of the absorption of water by the ceramics obtained from the 70 wt.%red clay -30 wt.% tezontle mixture on the sintering temperature (a), sintering time (b), and content of biowaste in the initial clay (c).In (d): dependence of the compressive strength on the sintering temperature.

Fig. 11 .
Fig. 11.Absorption of water by the ceramics prepared from the 40 wt.% red clay-60 wt.% tezontle mixture vs. the sintering temperature (a), sintering time (b), and biowaste content in initial clay (c).In (d): dependence of the compressive strength on the sintering temperature.
Al 2 O 3 Fe 2 O 3 Na 2 O CaO MgO TiO 2 K 2 O Σ rest Glass 72.03 1.989 not 13.964 7.006 4.005 not 1.001 0.005 Tab.I Chemical composition of the used glass and clay.Composition, wt.% Component SiO 2