Heating rate effect during sintering on the technological properties of Brazilian red ceramics

Unbalanced energy consumption in the production of ceramic artifacts is responsible for considerable undesirable impacts, such as increased emissions of polluting gases, excessive consumption of fuel materials, land degradation, and unpredictable financial costs. By contrast, the practice of optimizing the ceramic sintering, which in associated with firing of high temperature, can result in increased productivity and reduced production costs preserving an environmentally friendly production system. Moreover, it allows further improvements in the quality of the final product. This work compares the effect of different sintering cycles, with heating rates of 2, 15, and 30 °C/min, on the technological properties of a Brazilian industrial clay ceramic body. Initially the clay ceramic specimens were characterized in terms of mineralogical, chemical, and physical properties. Specimens were prepared by extrusion and fired at temperatures of 800, 900, and 1000 °C. The evaluated properties by standard tests were water absorption, post-firing linear shrinkage, and flexural strength. It was found that owing to sintering carried out at higher heating rates, red ceramic products with superior technological properties were obtained. This is an unprecedented conclusion for common clay ceramics produced in Brazil. Based on the promising obtained results, in a clear and detailed way, the benefits of rapid sintering cycle application for conventional brick production are evidenced.


Introduction
The Brazilian ceramic sector has a significant economic relevance for the country, mainly in the generation of jobs and regional income as well as a support to building construction. This sector is not only one of the world's largest but also diversified in products comprising several segments [1][2][3]. The most important products are abrasives, bioceramics, sanitary ware, refractory materials, ceramic coatings, and clay ceramic with reddish color, commonly known as red ceramics [1,2]. The segment of red ceramics includes materials used in building construction, such as bricks, blocks, tiles, pipes, and hollow elements [1,3]. All of them are obtained by sintering, which usually confers the typical reddish color owing to the high iron content. Red ceramics are categorized in the non-metallic minerals sector of the Brazilian mineral processing industry as associated with the civil engineering production chain [3][4][5][6][7]. It involves approximately 7,000 companies, most of them micro or small family-owned enterprises that are responsible for almost 300 thousand direct jobs, with annual revenues around 5 billion US dollars [3,4]. In the turning of the century, along 1990 and 2011, the per capita consumption of the products of the red ceramic segment in Brazil increased by 350% [4]. Currently, the country's construction industry, as the main driver of the national red ceramic segment, is facing unfavorable economic condition. Thus, reducing production costs through technological innovations in processing is the current strategy for the segment to stay competitive in the market.
Studies have shown that optimizing the sintering step of ceramic parts considerably reduces the financial costs of the process, at least by about 20%, which is a highly favorable feature in a scenario of economic crisis [8,9]. Another relevant aspect of the production of ceramic artifacts refers to environmental issues. Indeed, the traditional red clay ceramic production processes involving raw material mixing, processing, shaping, and firing generate a huge environmental liability. For example, the extraction of raw materials such as clay might cause permanent soil degradation while firing impairs air quality with emission of toxic gases. Firing in particular impairs air quality, with emissions of toxic gases while the extraction of raw materials such as clay might cause permanent soil degradation [10,11]. Firing in particular corresponds to the main stage to consolidate the red ceramic structure and develop properties by mechanisms in solid state sintering [12].
The sintering is a fundamental processing stage in the ceramic production and significantly related to the quality of the final product [9,[12][13][14]. In addition, this stage is the one with the largest economical participation in the production chain. A rapid sintering cycle contributes not only to reduce cost but also to quickly generate thermal energy and efficiently transmit it from the surface to the interior of the ceramic piece. This accelerates the physical and chemical reaction responsible for consolidating the ceramic structure [9,15,16].
Worldwide, in most modern large-scale advanced ceramic tile industries, the production is already carried out through the rapid sintering cycle process in roller kilns [17]. Smallscale common clay ceramic industries, however, still use dry pressing conformation process, typical of their traditional practice for bricks production [17,18]. The implementation of the rapid sintering was consolidated since the 1980s in the sector of ceramic tiles and sanitary wares [19]. Previously, this process was carried out slowly, in the whole segment of ceramics production. Today, this technology is comparable to that of reference countries, such as Italy and Spain [18]. Regarding the Brazilian industrial sector of common red ceramics, no significant technological improvements have been made since its implementation, from the second half of the twentieth century. It consists mostly of small and medium-sized companies with limited mechanization. Indeed, only relatively few Brazilian companies employ the rapid sintering cycle in the specific production of tiles and sanitary wares [4].
Today Brazil produces a huge number of red ceramic pieces, which are generally applied in civil construction, such as structural blocks and bricks. One of the major problems related to the clay ceramic products is their low added value and the simplicity of the industries in the sector [20]. Among the largest production centers of red ceramics in Brazil stands the one located in Campos dos Goytacazes (Campos for short), state of Rio de Janeiro, Brazil, with large clay deposits, which motivated the development of the local red ceramic industry. Campos also benefits from available natural gas, as firing fuel, and the proximity to the metropolis of Rio de Janeiro, the second largest consumer market in the country [21]. The Campos ceramic industries are mainly focused on the manufacture of red brick, a low-added value product that corresponds to 90% of their production. In addition, there is also a small manufacture of roofing tiles, extruded floors, and cladding plates, all from local clays and structural block [22].
The Campos clays present, as their main characteristics, the predominance of kaolinite mineral with high plasticity [23]. Local industrial clay ceramic body masses are generally empirically elaborated by mixing of "strong" and "weak," i.e., more and less plastic clays, respectively. Eventually, some industries use quartz, sand in the clay ceramic, and mass mixture. The composition of this mixture is quite variable and depends on intended product to be manufactured and the characteristics of the clays [1,2,24,25]. As any typical Brazilian red ceramic industries, those in Campos use slow sintering rates comprising more than 1 day of firing.
In this scenario, the aim of the present work was a first time evaluation of the influence, in a laboratory scale, of the effect of the sintering cycle conducted with different heating and cooling rates, on the technological properties of red ceramic specimens made by extrusion, which is the typical red bricks manufacturing process by the Campos local industries.

Materials and methods
The experimental procedure was divided into four stages. The first included collecting the different clays and sand for the mixture as well as processing and characterization of the red ceramic specimens. The second comprised the preparation of the clay body (CB) mass formulations. The third consisted of specimens conformation and firing while in the fourth stage, the sintered red ceramics physical and mechanical properties were evaluated by means of standard technological tests. Figure 1 shows a schematic diagram illustrating the different stages of this experimental program.
The typical CB employed in this work was collected at a brick industry in Campos. It was already prepared, mixed, laminated, and stored at the ceramic industry in order to obtain samples suitable for the physical, chemical, and mineralogical characterization. The precursor CB underwent a series of steps that involved drying, comminution, homogenization, and sieving operations with appropriate granulometry to perform each following technological test. Conventional tests were carried out to characterize the CB: granulometry, plasticity, chemical analysis, mineralogical analysis, and dilatometry, all according to Brazilian standards [26][27][28]. The chemical analysis was performed using X-ray fluorescence (XRF) in a Unicam Solar 969 model. The minerals identification was determined by X-ray diffraction (XRD) in a Shimadzu XRD7000 equipment, operating with Cu-Kα radiation and 2θ scan ranging from 3° to 60°, with step of 0.02° and time of 5 s. The dilatometric test was performed on Netzsch DIL 402 PC equipment, with heating rates of 2, 15, and 30 °C/min and final temperature of 1050 °C.
Prismatic specimens were formed by extrusion in a Verdés brand laboratory extruder in which the die shape has nominal dimensions of 30 × 20 cm (width and height). Its length of 80 cm was defined by the distance between the cutting lines. To achieve the appropriate plastic consistency for conformation, 30% moisture was added to the CB.
After shaping, the samples were dried in a laboratory oven at a temperature of 60 °C for 24 h and then at a temperature of 110 °C for final drying until reaching a constant weight. The apparent dry bulk density determination was performed as per ASTM C373 [29]. Three sintering cycles were used to evaluate the influence of the heating/cooling rate on the ceramic properties. In order to verify if there is interaction between heating/cooling rates and sintering temperature, the samples were fired at 800, 900, and 1000 °C using three different heating rates of 2, 15, and 30 °C/min for each temperature. After reaching the desired temperature, cooling was carried out at the same rate of heating. The sintering was carried out in a MAITEC FSCQ-1300/3 laboratory furnace, suitable for fast sintering. Figure 2 shows some images of the specimens after the firing stage.
The sintered red ceramic specimens were subjected to the following tests: three-point bending flexural, water absorption, apparent porosity, and sintering linear shrinkage. The flexural test was performed by three-point bending in an Instron 5582 universal test machine with maximum capacity of 100 kN, operating with a constant speed of 0.5 mm/min until the specimen was ruptured. According to the rupture load found, the calculation of the flexural rupture stress is performed. The evaluation of flexural three-point bending is an important normative parameter for the application of red ceramic materials.
Water absorption and apparent porosity were performed as per ASTM C373 standard [29]. To perform the water absorption test, the pieces produced were weighed after remaining 24 h in an oven at 100 °C; then, the pieces are placed in a container with water and kept at room temperature, which takes an average of 24 h. Soon after, the specimens are removed and weighed to record their mass. Absorption is determined based on the difference between wet mass and dry mass divided by dry mass. Apparent porosity was determined based on data determined by water absorption. The linear shrinkage of the fired pieces was determined with the support of a digital caliper (resolution ± 0.01 mm), comparing the dimensional values before and after the burning of the specimens.
For statistical analysis, considering the uniformity of the experimental environment, a randomized design with 13 replicates was used in a 3 × 3 factorial scheme totaling 117 experimental units. The data were subjected to the normality Lilliefors variance and to the homogeneity of Crochran&Barttlet test. Since data transformation was not necessary, results were then examined in terms of the analysis of variance (ANOVA) and the Tukey test (P ≤ 0.05).

Characterization of the clay body (CB)
The results of clay body (CB) mineralogical analysis by XRD are shown in Fig. 3. The diffractogram reveals that CB is composed of quartz (Q), mica (M), gibbsite (Gi) and less intense peaks related to some potassic feldspar (Fk). Table 1 presents the CB chemical analysis. One should notice the predominance of SiO 2 , Al 2 O 3 , and Fe 2 O 3 , which reveals a typical composition of clay-based raw material for red ceramics [30,31]. However, the lower percentage of Fe 2 O 3 was responsible to provide a lighter red color after sintering. In addition, it is possible to observe a relatively high percentage of Al 2 O 3 , which tends to increase the CB refractoriness, when compared to other clays used to produce roofing tiles with recognized technical quality [32]. The silica present in kaolinite clays is typical of the study region and is present in the free form, as quartz, and in the combined form, as the alumina (Al 2 O 3 ) observed, to form the aluminosilicates of this ceramic mass [21][22][23][24]. The loss on fire (LoF) comes mainly from the dehydroxylation of the clay minerals and the gibbsite [33]. Figure 4 shows the granulometric distribution results indicated in a Winkler diagram [34]. Corroborating the results of the chemical composition, the CB has higher amount of clay fraction, characterized by particles with equivalent spherical diameter (d eq ) less than 2 μm. It also has a higher sand fraction content, characterized by particles with d eq > 20 μm. Thus, according to the diagram in Fig. 4, the CB position is very close to that appropriate for roofing tiles production. In relation to the Atterberg limits shown in Fig. 5, the CB is in the acceptable extrusion ranges for red bricks processing [35,36].

Apparent dry density (DD), water absorption (WA), and apparent porosity (AP)
The apparent dry density (DD) of the clay ceramic specimens under different treatments is shown in Table 2. Statistically different variations (P ≤ 0.05) in DD are verified when evaluating the general averages for the heating rate (HR) and the sintering temperature (ST), as well as by comparing the interaction between these two factors. In the shaping by extrusion, which is the most used method for red clay ceramic bricks production in Brazil, it is not possible to directly control the specimens' weight. The same thing happens in the shaping by pressing. Thus, variations in density may occur. The water absorption (WA) values under the different treatments are shown in Table 3. As expected, an increase in ST resulted in WA reduction. However, promising results are observed when comparing the averages between the different heating rates. The increase in HR from 2 to 15 °C/min was able to reduce the WA, proportionally, by about 6.4%. The HR of 30 °C/min was also able to reduce the WA when compared to the conventional HR (2 °C/min). However, it revealed a higher value than the intermediate HR (15 °C/ min).
The apparent porosity (AP), which is a parameter associated with pores interconnected with the exterior of the structure, is markedly related to the WA. In fact, an apparent inverse relationship is observed between WA and AP when comparing the results in Tables 3 and 4. It is also possible to note the existence of this inverse relationship between the DD and WA, justifying the differences associated with distinct HRs. It can be seen that the average values of WA for each HR follow the contrary behavior of the DD. Indeed, HR at 15 °C/min is related to the highest DD, followed by 30 °C/min and 2 °C/min. In the inverse way, the lowest WA was found at HR of 15 °C/min, followed by 30 °C/min and 2 °C/min. However, when assessing the interaction between HR and ST, this trend is not observed by setting ST equal to 1000 °C, which shows homogeneity in the DD, while the WA has undergone significant changes with the HR increasing. To make this information visually more clear, the data are shown in the graphs of Fig. 6, which contains the results of DDs and WAs for the different HRs and STs. It is important to mention that the density results shown in Fig. 6a are relative to the apparent dry density and the specimens have not yet been sintered. However, this representation was found necessary in order to clarify the understanding in how the differences in DD may lead to changes in other properties, especially the WA (Fig. 6b).
Thus, considering only the ST at 1000 °C, for the same DD of 1.82 ± 0.02 g/cm, the WA was significantly higher (P ≤ 0.05). When the HR of 2 °C/min was used (20.52 ± 0.49%), it compares to the rates of 15 °C/ min and 30 °C/min, respectively, of 19.25 ± 0.33% and 19.19 ± 0.39%, that did not differ among themselves. At the 1000 °C, by increasing the HR from 2 °C/min to 15 °C/ min, an approximately 6% reduction in WA was achieved. However, no statistical variation was noted with increasing the HR from 15 °C/min to 30 °C/min.  In order to better understand the relationship between these properties, the Pearson (R) correlation was calculated for all the variables studied. The lower variance by the Tukey test (P < 0.05) and correlation superior to 60% were stipulated, so that only the interactions between WA and AP, as well as WA and DD, were chosen, and are represented in Table 4. For WA and AP correlation, they are already well described in the literature [22,30,31], showing that pores interconnected with the surface are those responsible for absorbing water from the environment.
The direct correlation between DD with the other properties, shown in Table 5, such as WA, AP, and mechanical strength, makes DD a property of great interest, especially for the ceramic industry [22,[37][38][39][40]. Indeed, the higher the DD, the smaller the AP becomes and, as a consequence, the smaller the WA the greater the strength. It should be emphasized that packaging degree and DD are different concepts. However, as there is no variation in the grains density, it is correct to extend the effect of DD correlation to the other properties, as long as one takes care to use the same clay specimens to produce the specimens.
This discussion is pertinent for the correct results evaluation, since the differences found between HR from 2 °C/ min to 15 °C/min and 30 °C/min, for ST of 800 and 900 °C,

Linear shrinkage (LS) and bending rupture stress (BRS)
Another interesting parameter is the linear shrinkage (LS) with results presented in Table 6, which also indicates that the sintering occurred more efficiently with increasing HR. This property is correlated with the degree of sintering. In a simplified explanation, sintering is a process of mass transport in which material is displaced from its original positions to another, resulting in closing of empty space (porosity reduction) and consequent structural shrinkage [12]. In this way, (LS) is a parameter that helps the sintering degree evaluation, being related to the volumetric reduction of total porosity, i.e., both open and closed pores.
In this context, Table 6 shows that there is an increase in LS with an increase in HR from 2 °C/min to 15 °C/min and 30 °C/min. The same trend is observed when analyzing, separately, the interaction between the HR versus ST variables. In all cases, i.e., for all the STs evaluated, there is an increase in LS when comparing the conventional sintering cycle (2 °C/min) with the fast-sintering cycle (15 °C/min and 30 °C/min).
With respect to the bending rupture stress results, shown in Table 7, it is revealed that the highest strength of 7.32 MPa was obtained by the sintered specimens with HR of 15 °C/min, followed by 6.03 MPa for TA at 2 °C/ min and 5.07 for TA at 30 °C/min. At 30 °C/min, this might be detrimental, due to the fact that the different crystalline phases, as well as the amorphous matrix, have distinct thermal expansion coefficients. Thus, an excessive increase in HR generates tensions that are responsible for surpassing the material mechanical resistance causing cracks and reducing its mechanical strength [9]. In addition, impairing in strength is also attributed to the volumetric expansion caused by the quartz allotropic transformation, in the sand, which at higher HRs is capable of generating stresses above those the products may support.
The aforementioned results clearly demonstrate that sintered samples at higher HRs exhibited greater densification rates, assessed indirectly through the presented properties, than samples sintered at lower HRs. This shows that the rate of material transport depends not only on the ST but also on the HR. Moreover, the HR has such a significant influence on the rate of material transport that it exceeds the effect of reducing the sintering time.

Dilatometric characterization of specimens
Dilatometry experiments were performed with the clay body using three sintering cycles with different HRs of 2 °C/min, 15 °C/min, and 30 °C/min. Corresponding dilatograms are shown in Fig. 7. Regarding this figure, it is important to mention that the dilatometry tests were repeated and the same behavior was observed.
The curves in Fig. 5 describe similar qualitative behavior. In fact, they correspond to the same events, but with different HR and ST intensities, so that some events occur at higher temperatures for higher heating rates. The shifting in temperature observed in Fig. 5 was expected, since the temperature sensor of the equipment indicates the environment temperature in which the sample is located. Due to the thermal gradient experienced by any material subjected to heating, the environment temperature does not necessarily match the temperature of the sample. Thus, HRs should correspond to higher temperature gradients.
After initial shrinkage, caused by elimination of free and adsorbed water, the sintering starts producing kaolinite Table 3 Water absorption (WA, %) of specimens as a function of sintering temperature (ST) and heating rates (HRs) *Means followed by the same uppercase letters in the column and lower case letters in the row, do not differ according to the ANOVA and Tukey test (P ≤ 0.05) dehydroxylation. At this moment, there is an increase of particles' specific surface area [24], resulting consolidation of the sintered structure and an increase and crystallinity loss, which may favor mass transport. Simultaneously, the quartz allotropic transformation, indicated by the structure expansion, occurs at different temperatures as a function of HR, being 575, 598, and 618 °C at 2 °C/min, 15 °C/min, and 30 °C/min respectively. After the complete kaolinite dehydroxylation, the liquid phase begins to form through eutectics reaction involving the silica contained in the metacaulinite and the alkali oxides contained in the potassic feldspars [20]. At this stage there is the greatest intensity discrepancy of LS values displayed in Table 6, when comparing the HR of 2 °C/min with 15 °C/min and 30 °C/min. The end of this stage can be identified by the change in the curve slope, initiating the spinel and/or primary mullite formation [41][42][43][44].

HR (°C/min) ST (°C) Mean
The dependence that the sintering rate has on the HR has already been reported in metals and ceramics. da Silva et al. [45] found that the shrinkage rate of hard metal structures (WC-Co) shows a strong dependence on HR. Saleiro and Holanda [46] identified that a red clay body sintering at HRs of 1, 10, and 20 °C/min culminated in similar properties, favoring energy savings. Santos et al. [47] sintered hard metal alloys via pulsed plasma with HRs above 50 °C/min and found that the structure densification rate has a HR dependence. In fact, it is well known that pulsed plasma sintering can densify structures in short time intervals and one of the characteristics of this technique is the high HR employed.
Jones and Miles [48] and Harmer and Brook [49], cited by Johnson et al. [50], sintered alumina materials at high   HRs, between 40 °C/min and 200 °C/min, and reported high densification rates under these conditions. In addition, they observed that grain sizes were significantly lower when compared to sintering at slower HRs. These authors attributed the high densification rates and the finer grain sizes to the suppression of surface diffusion occurring by the rapid passage through the temperature regime where surface diffusion rate is intense, avoiding the deleterious effects of this process [51,52].
In the conventional sintering process, much of the time is spent in temperature ranges where the effects of grain growth, through surface diffusion, outweigh the densification produced by diffusion in grain boundaries and in the crystalline lattice itself. Other studies [53,54] reaffirm that grain growth reduces the driving force for sintering. This is explained by the increase in the average distance required for diffusion, where mass transfer from the grain contours to the neck formation occurs, which is the mechanism responsible for porosity reduction [55,56].

Summary and conclusions
A comparison between different sintering cycles at 800, 900, and 1000 °C with distinct heating rates from relatively slow 2 °C/min to a fast 30 °C/min, revealed significant differences in technological properties of a typical Brazilian ceramic clay body used in red bricks, blocks, roofing, and floor tiles.
• It can be concluded that the differences found between heating rates from 2 °C/min, 15 °C/min, and 30 °C/ min, for sintering temperature of 800 and 900 °C, can be explained by differences in apparent dry density, which presented R ≥|60%|. However, this fact does not explain the difference in sintering temperature at 1000 °C (R = − 22%), so it can be inferred that this difference was due to the increase of the heating rates. • There is still an increase in sintering linear shrinkage with an increase in heating rates from 2 °C/min to 15 °C/ min and 30 °C/min. Going from an average value of 0.52% (800 °C) to 2.04% (1000 °C), a variation of almost 400%, which will certainly harm ceramic pieces. • Regarding the bending rupture stress, it was observed that higher sintering rates promote a reduction in the strength value, which is also influenced by the final sintering temperature; for parts sintered at 800 °C, the loss of strength between rates of 2 and 30 °C/min was about 2.5%, while at temperatures of 1000 °C the loss averaged 26%. • Higher heating rates during the sintering of red clay ceramics make it possible to obtain products with superior technological properties. This is an unprecedented conclusion for common red ceramics, based on promising results that evidenced, in a clear and detailed way, the benefits of rapid sintering cycle application for traditional bricks production. • The practice of reducing the sintering cycle time by raising the heating rate may become a major technological advance, enabling environmental gains for the entire Brazilian red ceramic industrial sector involved, as well as economic benefits, through energy consumption reduction in the sintering stage associated with productivity increase.

Declarations
Ethics approval Not applicable.