The study of thermal behavior of montmorillonite and hydromica brick clays in predicting tunnel kiln firing curve

Construction and Building Materials 150 (2017) 872–879

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The study of thermal behavior of montmorillonite and hydromica brick clays in predicting tunnel kiln firing curve M.V. Vasic´ a,⇑, L. Pezo b, J.D. Zdravkovic´ c, Z. Bacˇkalic´ d, Z. Radojevic´ a a

Institute for Testing of Materials IMS, Bulevar vojvode Mišic´a 43, 11000 Belgrade, Serbia University of Belgrade, Institute of General and Physical Chemistry, Studentski trg 12, 11000 Belgrade, Serbia c Innovation Centre – Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia d Brick factory ‘‘Toza Markovic´”, Bašaidski drum 62, 23300 Kikinda, Serbia b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Brick clays of montmorillonite and

hydromica type were tested.  Thermal and dilatometry analysis

along with technological parameters, were determined.  All the results were used in construction of firing curves in a tunnel kiln.

a r t i c l e

i n f o

Article history: Received 9 April 2017 Received in revised form 11 June 2017 Accepted 12 June 2017

Keywords: Brick clays Thermal analyses Firing curves Process optimization

a b s t r a c t The aim of this study was to test montmorillonite and hydromica type of brick clays by using simultaneous thermal analysis and dilatometry in an assessment of the suitability of brick clays to produce building elements. The plasticity coefficient and drying susceptibility were determined to discover the behavior of brick clays. Fired products’ characteristics were studied by performing water absorption and compressive strength tests. All the methods were employed in the construction of the firing curves in a tunnel kiln. The results could increase the degree of certainty to lead the production process towards obtaining the desired features of brick elements. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction A technological process in the brick industry consists of primary processing and shaping, and later drying and firing. The good knowledge of material behavior during heating is important for the proper design of technological operations and the whole production process. Complementary methods for physical and ⇑ Corresponding author. E-mail address: [email protected] (M.V. Vasic´). http://dx.doi.org/10.1016/j.conbuildmat.2017.06.068 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

mineralogical brick clays study, differential thermal analysis (DTA), thermo-gravimetrical analysis (TGA) and dilatometry, are, besides technological characteristics and optimal drying and firing curves, the basic indicator which give information about the applicability of a certain brick clay. DTA is used for mineralogical characterization of brick clays, and also gives valuable data on a moment of formation, the time needed, nature and the intensity of certain thermo-effects, which occur in the tested material during heating. TGA gives information on dehydration of brick clays, records weight loss during drying and firing, and provides insight

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into the behavior of raw materials while heated. TGA curves, in a starting period until 120 °C, show speed and quantity (in%) of water loss from a material, and, based on that, give valuable data on drying of the testing material. Further, during heating from 120 °C to 1000 °C, these curves give a picture of changes that occur while firing, and so indicate the necessary precautions in the choice of firing process speed in the critical temperature range, i.e. the intervals in which a phase transformation, the combustion of organic matter, decomposition reaction of carbonates or hydroxides etc., occur [1–6]. The importance of dilatometric studies lies in the possibility to track relative and total shrinkage and expansion of brick clays during the firing process, as well as a determination of materials‘ sensitivity during cooling. The dilatometric curve in cooling stage, concerning recrystallization of quartz from to modification, gives a possibility to nearly estimate the sensitivity of brick clay during cooling stage [7]. This study summarizes the thermal behavior of brick clays having a different mineralogical composition. Natural occurring montmorillonite and hydromica type brick clays were investigated. All important characteristics that define behavior during shaping and drying, as well as the fired samples quality, were determined. The obtained results were used in the design of a firing process in a tunnel kiln. 2. Materials and methods In this research, 6 brick clays of different mineralogical composition were tested on thermal behavior, in order to determine their applicability in the brick industry. Natural occurring montmorillonite (brick clays A, B, and C) and hydromica (brick clays D, E, and F) type brick clays from several locations in Serbia were investigated.

873

The shrinkage of samples is monitored continuously for 24 h during drying in the air as a function of weight loss The obtained Bigot’s curve determines graphically the critical point which separates the two drying phases. The first phase corresponds to the removal of colloidal water, while it comes to the linear shrinkage as a function of weight loss while drying. After the inflection point, the external dimensions of the specimen remain slightly constant, while the removal of interstitial water takes place [14,15]. Drying shrinkage was determined as the change in length and its‘ initial value [9]. The test was done on 5 samples of tiles (120  50  14 mm3) using vernier caliper having precision 0.01 mm.

2.2.2. Testing the physical–mechanical properties of fired samples The extruder-shaped laboratory products were first dried in the air by turning upside-down every few hours until they obtained light gray homogeneous color. Afterward, the samples were carefully dried in the oven for about 24 h, to ensure complete removal of the moisture. The firing was done in the oxygen atmosphere electricity powered kiln, with the average heating speed of 1.4 °C/min until 610 °C, and later with the rate of 2.5 °C/min until the final given temperature of 960 °C was reached, at which the samples were treated for 2 h. Firing regime applied in this study was slightly slower than that specified in the literature, in order to enable all the reactions complete [11]. Water absorption was evaluated by immersion of samples in distilled water for 24 h, according to standard SRPS EN 771-21 [16]. Compressive strength was determined using laboratory hydraulic press (Alfred Amsler, CHD) with a measuring range of 100/200/500/1000 kN and resolution of 0.1/0.2/0.5/1 kN. The samples were flattened before testing to ensure that the surfaces were parallel. Compressive strength was then tested on samples with the bottom area of 0.002 m2 for blocks, and a loading rate of 0.6 kN/s. The strength results reported were the average of three specimens with a variation of no more than 10% [17]. For comparison, these tests were performed on samples fired at 960 °C.

3. Results and discussion 3.1. Behavior during drying

2.1. Determination of mineralogical and thermal properties of the brick clays The qualitative mineralogical analysis was carried out by XRD (powder diffractometer Philips PW-1050, kCu-Ka radiation, scanning speed 0.05°/s). The samples were tested in powder, bulk form, and also as oriented aggregates [8]. The DTA and TGA curves were obtained using an SDT Q600 (TA Instruments) device with platinum/platinum-rhodium thermocouple. The temperature was raised from room temperature to 1000 °C, under the atmosphere of air (flow rate: 100 cm3 min 1), with the heating rate of 20 °C min 1. A low amount of samples was tested (maximal 20 mg), in order to gain the higher resolution of peaks and faster heating. The dilatometric analysis was carried out using a Linseis dilatometer with a rhodium oven and thermo-pair platinum – rhodium (L76). The temperature increment was 10 °C min 1, and the sample (20  50  14 mm3) was held up at the final temperature of 1000 °C for 1 h. The extruded sample was dried overnight at 105 °C and then heated in the dilatometer. Length changes were recorded every minute during the heating stage. 2.2. Technology features of brick clays 2.2.1. Behavior during shaping and drying The as-received brick clays were quartered and dried in the oven overnight at 105 °C. Afterwards, the samples were ground in a 3 mm sieve mill and then mixed with sufficient water to obtain plastic mass (moist content about 20 wt.%). The prepared moist brick clays were kept in sealed nylon bags for 24 h to ensure homogeneity of moisture. The samples in the shape of hollow blocks with vertical voids (55.3  36  36 mm3) were prepared in an Händle extruder reaching 90% vacuum [9]. Plasticity is often determined by a method according to Pfefferkorn, wherein, in a physical sense, it represents the content of water in the material which causes the deformation of the test sample. At least 5 test cylinders containing different moisture content were manually shaped in the mold having a diameter of 33 mm and a height of 40 mm. The weight integrated into the apparatus gravitationally descends on the mold-shaped moist sample to test, which leads to the reduction of its original height, depending on the moisture content. The ratio of the initial and the residual height of the sample depending upon the moist content of the samples is displayed graphically, and the diagram at the point H = 12.1 mm reads the plasticity of the material, the plasticity coefficient – PC [10,11]. The moist samples, shaped under vacuum, were used to obtain the drying capacity of the clays [12,13]. For every test, the two samples (approx. dimensions 20  14  14 mm3) were made by hand in a mold. They were weighed and measured using vernier caliper, and carefully put in the A.D.A.M.E.L. baralettograph.

Tested brick clays plasticity results are shown in Table 1. The montmorillonite clays showed very high plasticity, according to Pfefferkorn method. These clays were easy to shape into different forms of brick products, as a result of suitable mineralogical composition, which means that the content of montmorillonite was not too high. The presence of adsorbed water between layers in clay minerals in high quantity allows these layers to very easy shear under the influence of an external force, due to which these clays can be easily shaped. Plasticity of the hydromica clays showed to be of middle values – moderate (clay E) and good (D and F). Carbonates, in particular, fine-grained calcite, reduce the plasticity and drying shrinkage, as well as the strength of the dry products [11]. One of the important steps in a testing of behavior during drying is to analyze Bigot‘s curves. Results of critical point (DSk and DGk) coordinates are presented in Table 1, and the Bigot‘s curves are shown in Fig. 1. Since the physical properties depend on the mineralogical composition (if all other characteristics are the same – moisture content, particle size, etc.), the inflection point at these curves differs for a various mineralogical content of brick clays [18,19]. The montmorillonite clays showed greater susceptibility to drying (higher water loss in critical point) and also shrinkage, compared to the hydromica clays, which was expected concerning the plasticity. All montmorillonite clays have shown a loss of mass in critical points greater than 10%, which was placing them in highly susceptible to drying. Higher content of quartz induced lower susceptibility to drying in clays B and E. The lowest drying susceptibility and plasticity were found in the hydromica type clay sample which had the highest contents of quartz and carbonates (sample E). Linear shrinkage results showed high values in the case of montmorillonite clays, and remarkably lower in the hydromica clays, except the clay F.

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Table 1 Technological properties of tested brick clays during drying. Characteristics

Montmorillonite type clays

Plasticity Critical point coordinates – Bigot‘s curve Linear shrinkage (%)

DSk (%) DGk (%)

Hydromica type clays

A

B

C

D

E

F

Very high plasticity 7.7 11.2 8.5

Very high plasticity 7.5 11.7 9.0

Very high plasticity 7.2 12.0 9.5

Good plasticity 5.6 8.1 5.6

Moderate plasticity 3.6 6.0 3.5

Good plasticity 6.5 9.4 8.0

DSk – shrinkage in critical point, DGk – weight loss in critical point.

Fig. 1. Bigot’s curves of (a) montmorillonite and (b) hydromica type clays (DS – shrinkage, DGk – weight loss).

3.2. Mineralogical analysis The qualitative presence of minerals in tested brick clays, as well as sampling locations and brick clay types, are presented in Table 2. Characterization of XRPD patterns indicated the presence of quartz in all the samples; chlorite was found in samples D, E, and F; calcite was detected in all the samples except C, and dolomite was detected only in E and F. 3.3. Thermal analysis Six clay brick samples were subjected to the simultaneous thermal analysis including differential thermal analysis (DTA) and thermogravimetric analysis (TG). An amount of approximately 18 ± 0.5 mg of each powder sample was analyzed. The thermograms of DTA and TG are shown in Figs. 2 and 3. Results of TG analysis are summarized in Table 3. Dehydration, dehydroxylation, organic matter decomposition, and carbonates loss were detected in almost all the samples, with different intensities in corresponding DTA peaks. In montmorillonite clay bricks, the weight loss occurring from room temperature to 200 °C is related to the removal of adsorbed and the interlayer water [20,21]. For sample A, dehydration is

finished in one step, while for B and C it was a two step process. For the hydromica type clays in the same temperature range, the removal of adsorbed and the interlayer water is finished in one step. Thermogravimetric results showed that the maximum weight loss up to 200 °C was observed for sample C (Table 3). This fact is attributed to the highest content of adsorbed interlayer water, which is in direct relation to the quantities of montmorillonite minerals. In the case of the hydromica type clays, the adsorbed water content decreased in line F, E, D, while being similar in F and E (Fig. 2). Dehydration process manifested for all brick clays as an endothermic effect in the temperature interval 100–300 °C (Fig. 3), characteristic for adsorbed and the interlayer water removal (water held between the basal planes of the lattice structure, i.e. swelling water) [20,22,23]. In contrast to the A-C samples, DTA peaks for D-F samples were smaller and up to 300 °C having more pronounced double endothermic peak: D at 157 °C and 203 °C; E at 163 °C and 237 °C and F at 177 °C and 286 °C. These differences in heat effects were attributed to the evolution of the interlayer water and the water associated with the hydration of the interlayer exchangeable cations [22]. In addition, dual character of the low-temperature endothermic system (Fig. 3) could lead to the conclusion that

Table 2 Contents of minerals in brick clays. Q Brick Brick Brick Brick Brick Brick

Clay Clay Clay Clay Clay Clay

A B C D E F

Montmorillonite Type Hydromica Type

+ +l +h + +h +

H

+ +h +h

Ch

+l +l +

M

C

+ + +h

+ +l + +h +

D

Sampling location

+h +

Negotin Leskovac Šabac Novi Pazar Banatski Karlovac Banatski Karlovac

Q – quartz; layered silicates (H – hydromica, Ch – chlorite, M – montmorillonite), carbonates (C – calcite, D – dolomite), +h – present in high rate, + – present, +l – present in low rate, – not present.

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Fig. 2. TGA curves of: a) montmorillonite and b) hydromica type clays.

Fig. 3. DTA curves of: a) montmorillonite brick type clays b) hydromica brick type clays.

Table 3 Thermogravimetric data for montmorillonite and hydromica type clays. Sample

Stage

Temperature range (°C)

Weight loss (Wt.%)

A

1 2 1 2 1 2 1 2 1 2 1 2

25–200 200–1000 25–200 200–1000 25–200 200–1000 25–200 200–1000 25–200 200–1000 25–200 200–1000

4.3 7.2 4.7 8.8 5.8 4.2 1.2 9.2 2.3 9.8 2.6 7.6

B C D E F

another reaction took place at 270 °C in both brick clay types, and that was an exothermic crystallization of goethite to hematite, or generally dehydroxylation of hydrated iron oxides [1]. From 300 to 500 °C the presence of low-intensity exothermic effect is identified in all samples due to the combustion of a small quantity of organic matter [24,25]. The hydromica type clays generally contained the lower amount of organic matter in comparison to that of the montmorillonite type samples. The highest amount of organic matter was observed for sample B, which had the largest mass loss by heating up to 500 °C (Fig. 2, a). In montmorillonite clay types the dehydroxylation process commences before the loss of the sorbed water was complete. This was clearly evident from TG curves in which a gradual, continual mass loss between the two inflections was exhibited (Fig. 2, a) [26]. The dehydroxylation process for the montmorillonite type samples was manifested as an endothermic effect on corresponding DTA curves with peak minima at about 587 °C in A, 536 °C in

B and 550 °C in C. Among the montmorillonite type clays, sample C showed the largest peak area on DTA curve, ascribed to the dehydroxylation (Fig. 3, a), probably due to the highest content of montmorillonite. The dehydroxylation process for the hydromica clay types in corresponding DTA curves is shown also as an endothermic effect with peak minima observed at about 550 °C in D, 580 °C in E and 550 °C in F. The second endothermic peak for hydromicas samples was more intensive than the first one in D and E (Fig. 3, b), but it was conversely in the case of the clay brick F because chlorite shows the same, but more pronounced, thermal effects compared to hydromica. According to the XRD analysis, quartz was present in all the samples to some extent, but the recrystallization of quartz from to modification was not visible in DTA records. For montmorillonite clay types destruction of the crystal lattice of montmorillonite and allocation much of constitutional (lattice) water coincides with the endothermic effect which follows the recrystallization of quartz from to modification, while for hydromicas samples this phenomenon is related to the high degree of overlapping with the loss of OH--ions (crystal water) and the destruction of the crystal lattice of hydromica and chlorite (Fig. 3). The dehydroxylation process is practically finished up to 600 °C for both clay types. Based on the peak intensity and position [1,27], the carbonates quantity in brick clays was generally low. The carbonates decomposition generally occurs between 600 and 800 °C. For montmorillonite clay samples the highest content of carbonates was detected for sample A, where TG curve shows partial overlapping of dehydroxylation and calcite decomposition that ends up to 800 °C [21]. For sample A, calcite decomposition was also visible in the DTA curve as a third endothermic effect with

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the minimum at 800 °C. The substantially lower content of calcite in sample B in comparison to sample A is evident from the fact that weight loss from 600 to 800 °C is less than 1%. (Fig. 2, a). In this temperature range, the weight loss of sample C is negligible, which supports previous findings related to the carbonate absence (Table 2). The third endothermic effect in sample C at 807 °C is related to the destruction the crystal lattice of smectite. Concerning the position of the decalcification peak, in samples A and less, in B, calcite was present in the coarser particle sizes and mainly in the monocrystalline form. This effect is often followed by an exothermic reaction, weaker or stronger, which is caused by the crystallization of amorphous decay products of montmorillonite (montmorillonite recrystallization), when probably it comes to the formation of a spinel [1,20,22,28]. In the case of the hydromica type clays (Fig. 2, b) thermal decomposition from 600 to 800 °C exhibit higher weight loss in comparison to montmorillonite clay types (Fig. 2). The endothermic effects in this temperature interval were related to dehydroxylation (crystal water) and the destruction of a crystal lattice of hydromica and chlorite, along with carbonate degradation, respectively [14,21]. The highest carbonate content was observed in sample E, and in some lower extent in D, F, and A samples, while in B it was found in traces. In addition, the intensive mass loss between 700 °C and 850 °C, followed by endothermic effects in that range in corresponding DTA curves, was particularly emphasized in brick clays E and F due to the presence of dolomite. From 850 °C to 950 °C, the third endothermic reaction in the hydromicas clay samples, was being associated with the final destruction of the hydromica mineral structure. Due to the presence of dolomite and quartz in brick clays E and F (Fig. 3, b), reaction around 900 °C resulted in the formation of diopside, CaMgSi2O6 [29]. Finally, an exothermal reaction between 900 °C and 1000 °C, was an effect of the creation of amorphous glassy phase and transformation of metaillite into spinel [22,28,30,31]. This third endothermic peak, followed by exothermal reactions, is characteristic of three-layer clay minerals since it is not seen in the case of kaolinite and the halloysites [22,30]. With the temperatures above 800 °C, the mass change was very slow since the dehydroxylation is completed, and is followed by the high-temperature reactions in illite (sintering and vitrification) [13]. 3.4. Dilatometry analyses The dilatometric tests allow measurement of volume change patterns of a sample when heated. Since the volume change is the consequence of physical and chemical reactions that occur in the test material, it is obvious that the various clay mineral composition will show the different dilatometric curves flow. The results

of the dilatometric tests for the studied brick clays are shown in Fig. 4. The dilatometric curves of the montmorillonite brick clays (Fig. 4, a) showed gentle expansion of clays, and afterward greater shrinkage at about 150–180 °C due to the loss of adsorbed water [32]. These changes were more pronounced in sample C, due to the high quantity of adsorbed water and the montmorillonite type clay. Above 200 °C, the thermal dilatation process was predominant, with the emergence of the uniform expansion up to about 500 °C. An expansion increase in the interval 500–600 °C is mainly due to the – quartz inversion, which was the most pronounced in sample C [33,34]. In the temperature range from 500 °C to 700 °C, montmorillonite clays lose crystal water, which is followed by a larger or smaller shrinkage depending on the quantity of that water. Above around 800 °C, intense contraction took place as a result of the complete separation of crystalline water, degradation of the crystal lattice of montmorillonite and dissociation of carbonates [35], and actually the formation of vitreous phase [32]. From the flow of the dilatometric curves of illite (hydromica) clays (Fig. 4, b), intensive spreading up to 600 °C can be noticed; at temperatures from about 800 °C, it comes to decomposition of the crystal lattice of illite and carbonates, which causes sudden shrinkage in this temperature region. The shrinkage of the sample caused by sintering and changes in the metaillite lattice reaches 1% [30,34]. The sample F showed ‘‘U” shape part of the dilatometric curve, with a maximum peak at about 860 °C. This phenomenon was not viewed as having much significance in the literature, according to the findings of the present study. The effect is explained by the expansive formation of anorthite in samples rich in carbonates [36,37]. 3.5. Physical and mechanical properties of the fired samples Compressive strength and water absorption of samples fired at 960 °C are shown in Table 4. It was found that the water absorption of fired samples depended directly on the mineralogical composition of the used raw materials. It was obvious that, during the same firing regime and conditions, the montmorillonite clays gave the products with lower porosity (lower water absorption values) and higher density, when compared to the hydromica clays. The mineralogical content of montmorillonite clays caused significantly higher drying shrinkage, and thus the higher density of fired samples. Carbonates content in the hydromica clays was relatively high, and it induced higher porosities and lower compressive strength. The highest compressive strength and the lowest water absorption in the two groups showed samples B and F, because of the highest organic

Fig. 4. Dilatometric curves of (a) montmorillonite and (b) hydromica type clays.

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Table 4 Technological properties of fired brick samples. Properties

Water Absorption (%) Compressive strength (MPa)

Montmorillonite clays

Hydromica clays

A

B

C

D

E

F

14.0 22.56

11.2 44.13

12.0 26.48

17.2 28.44

21.0 10.79

15.0 29.42

matter content and the increased clay minerals content within the two groups, respectively.

3.6. Designing the firing curves To determine the firing curves, clay must be tested by DTA, TGA, dilatometry, and also shrinkage during drying and properties after firing should be tested [38]. To design the optimum firing curve, one must first determine its ideal shape. This shape was then modified by applying corrections based on events in material recorded by thermal analysis. The curve essentially uses a dilatometric curve to design the shape and then can be modified by the DTA information. Firstly, J. Funk [39] described this procedure, but the authors of this research found it more convenient to apply as presented in the somewhat newer literature [40]. The both procedures give the same results. Basically, the dilatometric curve should change its’ axes, so the Y-axes stands for temperature. Every point of a curve which turns back in a negative direction of X-axes should be flipped as in a mirror to grow towards the right. X-axes show time of firing and must be predicted generally by experience, depending on the brick clay used, type of kiln, the position, and type of burners, etc. A firing process in a tunnel kiln can be divided into 3 phases: preheating (until about 750 °C), firing and cooling zone. Preheating phase duration was determined mainly by the composition of brick clay, because of the large differences in temperatures between upper and lower rows of goods, which can come to even 250 °C in conventional tunnel kilns (kilns without recirculation and side pulse burners). In this phase, the most susceptible are the montmorillonite type clays because of the residual hygroscopic and the interlayer water separates at the beginning of the process. Products based on smectite absorb moisture from the air after drying and before entering the furnace. In the case of the fine structure when the surface of a product is dry, water can remain trapped inside and the rapidly rising temperature often causes cracks and even swelling. The hydromica clays also lose water up to about 200 °C (according to the DTA peaks), but to a lesser extent and showing a much lower shrinkage. The length of the preheating stage is approximate 10–28 h. The firing zone starts from 750 °C, where the burner groups are placed. The duration of the top firing temperature is different in relation to the position of products (top or bottom rows) and is usually about 12 h. It should be appreciated that the maximum firing temperature for brick products varies from 870 to 1100 °C. The cooling phase is conditioned by the furnace thermal technical measures and is of approximate duration 10–20 h. Until about 620 °C, the cooling can be performed at the speed 100–200 °C/h. The phase transformation of quartz (620–520 °C) must be kept slow because the products are sensitive due to abrupt changes in size. This was evident from the cooling stage in the dilatometric curves (not presented in the figures). During this period, the rate of cooling of the roofing tiles is below 20 °C/hour, and in the case of hollow blocks, it is between 20 and 30 °C per hour. After 500 °C products are not sensitive anymore and cooling rates are dependable by the removal of heat in the dryer.

A previous experience in the design of the furnaces, and later also the recording of tunnel kiln regimes for the last 10 years have suggested the next firing times through the stages: – the montmorillonite type clay (sensitive in both the heating stage and the cooling stage: about 28 h in preheating stage, the firing phase for 12 h, 20 h in the cooling stage; a total of 60 h; – the illite-montmorillonite clays: 18 h in preheating stage, the firing phase for 12 h, 15 h in the cooling phase; a total of 45 h; – the illite-chlorite clays: 10 h in preheating stage, the firing phase for 12 h, 10 h in the cooling stage; a total of 32 h. Some of these experience-based conclusions are found also in the literature, where the authors stated that ‘‘Assuming a traditional setting height of 14 bricks high (1300 mm) firing cycle time can vary from 30 to 60 h depending on the clay being used.” [38]. Reconstruction of the tunnel furnace can reduce the firing time as follows: intense recirculation in the stage of preheating, installation of side impulse burners in the heating stage, which would regulate gradual and uniform heating of products at the height of the packing etc. Based on all previously explained, the firing curves were constructed and presented in Fig. 5. Besides, firing regimes in a tunnel kiln, proposed in this research, are shown in Table 5. First of all, a significantly higher quantity of adsorbed water in montmorillonite clays, compared to hydromica ones, indicated that brick clays A, B and C will be more susceptible to drying until 100 °C, which can also be seen in Table 1. Besides, knowing that montmorillonite clays lose adsorbed water up to about 200 °C (peaks in DTA curves in Fig. 2, a), the products must be additionally dried in a tunnel kiln. TGA curves (Fig. 3, a) showed that the quantity of adsorbed water decreased in line C, B, A; and that was also a trend in which firing velocity in the preheating zone fell. A similar quantity of adsorbed water in the hydromica clays caused choosing just a bit slower firing at the beginning of the firing process in the case of brick clay D, in comparison to E and F (Fig. 5, b). Generally, that was one of the reasons the whole firing process of the hydromica clays can be shorter compared to the montmorillonite clays. Very useful data for technological process of production of brick products of the tested clays can be obtained from the analyzed data. Namely, montmorillonite clays, due to their extreme plasticity (Table 3), may lead to difficulties during drying and firing, which can be partially remedied by adding the matter that can decrease the plasticity (i.e. quartz) [11]. In the process of drying the product made of montmorillonite clays, without a doubt, the greatest difficulty was the large linear shrinkage, which requires very long drying process in order to avoid the appearance of cracks in the dried products, and thus avoiding the appearance of an increased percentage of waste. This certainly affects the cost of production of montmorillonite clay bricks, compared to the hydromica clays. Further, a very important information about the technological production process, was that due to the significantly higher quantities of adsorbed and the constitutional water, the process of firing was considerably longer, as compared to the process of firing of the hydromica clay products, because of the afore-

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Fig. 5. Firing curves in a tunnel kiln: a) montmorillonite type clays, b) hydromica type clays.

Table 5 Firing regimes in a tunnel kiln.

Brick Clay A

Montmorillonite Type

Brick Clay B Brick Clay C Brick Clay D Brick Clay E Brick Clay F

Hydromica Type

Duration (h) Average velocity Duration (h) Average velocity Duration (h) Average velocity Duration (h) Average velocity Duration (h) Average velocity Duration (h) Average velocity

(°C/h) (°C/h) (°C/h) (°C/h) (°C/h) (°C/h)

Preheating phase duration (h)

Firing phase duration (h)

Cooling phase duration (h)

The whole process duration (h)

Final firing temperature (°C)

24 37.9 26 36.2 32 28.0 8 116.9 10 97.5 11 88.6

12

20 45.5 20 47.0 20 44.8 10 93.5 10 97.5 10 97.5

56

935

58

965

64

920

30

960

32

1000

33

1000

mentioned reasons, it was necessary to reduce the velocity of heating gradient (Fig. 5). While studying the firing curve of montmorillonite clays, it was evident that, given the intense endothermic reactions, small gradients of heating rate were predicted, underlying the relatively long time for passage of products through the tunnel kiln. In order to achieve the final firing temperatures for products made of brick clays marked with A, B, and C, it was necessary that the products were pushed through the kiln for clay A in a period of 24 h, for clay B in 26 h and for clay C in 32 h. In addition, for quite a long process of firing, the presence of quartz was responsible, which underwent the phase transition from to -quartz during cooling at 575 °C followed by significant dimensional changes, which requires, in the temperature range of about 620–520 °C, the reduced velocity of cooling gradient. The maximum temperature for firing clays D, E, and F can be achieved in 8, 10, and 11 h. The mineralogical nature of the hydromica clays enables faster warming, and thus directly reduces the total transit time of products through the furnace. When comparing the total time during the process of firing of the hydromica clays, it can be seen that, on average, the time required for firing process of the montmorillonite clays was for about 30 h shorter. The final firing temperatures were found at the end of shrinkage in the dilatometric curves and were as follows: sample A (935 °C), B (965 °C), C (920 °C); sample D (960 °C), and samples E and F (1000 °C). Substantially higher sensitivity to drying of the montmorillonite clays, in relation to the hydromica type, was very clearly illustrated also in Bigot’s curves (Table 1). While, for example, clay B on drying shrank 7.3% and lost 12% of adsorbed water (critical point of the curve), these values for hydromica clay E were twice as lower. In light of these data and TGA, the results obtained on the dilatometer can be explained (Fig. 4) and the behavior of these

12 12 12 12 12

clays in the production process predicted. It can be seen in the hydromica clays, that during the wide temperature interval to about 500 °C it comes to the continuous expansion of samples with nearly constant speed. During the same temperature interval in montmorillonite clay (clay B), in particular, up to 200 °C, it comes to a partial shrinkage as a result of the loss of large amounts of water, and the already mentioned, significant shrinkage during drying. The further course of montmorillonite clays dilatometer curves, to about 500 °C, was almost the same as in the hydromica type, only the relatively expansion was lower equally to the size of the resulting expansion in the period to 200 °C. In the temperature range of about 500–700 °C, in both clays (B and E), there were very small dimensional changes, to above 750–800 °C, when there was a pronounced linear shrinkage in both kinds of clay, in particular as a consequence of the decomposition of carbonates present in them. Integrated effect of dimensional changes, in the montmorillonite type clays, was such that final product did not practically change dimensions compared to sample before firing. The hydromica type clays, though, showed relative linear expansion at about 1%. The results obtained by DTA helped to complement and confirm thermogravimetric analysis data (Fig. 3). It can be seen that the total weight loss for both montmorillonite and hydromica clays while annealing up to 1000 °C was similar, but the distribution of losses was different. The weight loss up to about 200 °C, related to dehydrating of adsorbed water, was much higher in the montmorillonite type clays. On the other hand, weight loss in the montmorillonite clays was up to about 600 °C almost constant, while in the hydromica type clays, these losses between 100 °C and 500 °C were lower, but more intense at temperatures of 600–800 °C. These data were used in the design of the industrial firing curves in a tunnel kiln (Fig. 5). For these reasons, the process of firing of the montmorillonite type clays was up to 900 °C, but especially up to 600 °C, nearly two times longer than in the case of hydromica

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type clays. In the temperature ranges in which there was a low thermal expansion or contraction, the products can be heated quickly. Contrary, when expansion or contraction rates are high, kiln temperatures should be raised slowly to minimize the internal stresses. 4. Conclusions Brick clays of the montmorillonite type had lower contents of carbonates than the hydromica clays. Heavy clay E showed large quantities of dolomite and calcite, according to endothermic peaks in DTA curve, and this result indicates that it was unsuitable for producing certain brick products such as, for example, facing bricks, given the potential for efflorescence and destruction of finished products’ surface. This research showed that it is possible to design a firing regime in a tunnel kiln, mainly on the basis of the dilatometric curves. Additional data were also needed and they comprise XRD, DTA, TGA, plasticity and drying susceptibility tests, as well as drying shrinkage. It was found that firing for the montmorillonite type clays must be twice as the longer compared to the same process required in the case of hydromica clays. Products made of the montmorillonite type clays showed higher compressive strength, but also an increased cost of production. Acknowledgements Support for this work by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Grants No. III45008, TR31055 and III45007) is gratefully acknowledged. References [1] M. Arsenovic´, L. Pezo, L. Mancˇic´, Z. Radojevic´, Thermal and mineralogical characterization of loess heavy clays for potential use in brick industry, Thermochim. Acta 580 (2014) 38–45. [2] F.H. Clews, Heavy Clay Technology, The British Ceramic Research Association, Stoke-on-Trent/London, 1955. [3] V.S. Ramachandran, R.M. Paroli, J.J. Beaudoin, A.H. Delgado, Handbook of thermal analysis of construction materials, Noyes Publications, Norwich, NewYork/USA, 2002. [4] H.A.A. Gibbs, L.W. O’Garro, A.M. Newton, Differential thermal analysis: a means of identifying soil samples from Barbados, Trinidad, Tobago, St. Vincent, Jamaica and Guyana, Thermochim. Acta 363 (2000) 71–79. [5] M. Pansu, J. Gauthezrou, Handbook of Soil Analysis: Mineralogical, Organic and Inorganic Methods, Springer, Berlin/Heidelberg/New York, 2003. [6] M.E. Brown, Introduction to Thermal Analysis, Kluwer Academic Publishers, New York/Boston/Dordrecht/London/Moscow, 2001. [7] H. Mekkia, M. Anderson, M. Benzina, E. Ammara, Valorization of olive mill wastewater by its incorporation in building bricks, J. Hazard. Mater. 158 (2008) 308–315. [8] M. Arsenovic´, L. Pezo, Z. Radojevic´, Response surface method as a tool for heavy clay firing process optimization: Roofing tiles, Process. Appl. Ceram. 6 (2012) 209–214. [9] M. Arsenovic´, Z. Radojevic´, S. Stankovic´, Removal of toxic metals from industrial sludge by fixing in brick structure, Constr. Build. Mater. 37 (2012) 7–14. [10] F.A. de Andrade, H.A. Al-Qureshi, D. Hotza, Measuring and modeling the plasticity of clays, Mater. Res. 13(3) (2010) 395–399. Available online: http:// www.scielo.br/pdf/mr/v13n3/19.pdf (accessed 31.03.17.) [11] M. Arsenovic´, Optimization and prediction of the quality of materials, process and final properties of heavy clay products by mathematical modeling of the characteristic parameters, (Ph.D. dissertation). University of Belgrade, 2013. [12] M.V. Arsenovic´, L.L. Pezo, Z.M. Radojevic´, S.M. Stankovic´, Serbian heavy clays behavior: application in rouch ceramics, Hem. Ind. 67 (5) (2013) 811–822. [13] M. Arsenovic´, Z. Radojevic´, Zˇ. Jakšic´, L. Pezo, Mathematical approach to application of industrial wastes in clay brick production—Part II: optimization, Ceram. Int. 41 (2015) 4899–4905.

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