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www.elsevier.es/bsecv

1 Original

2 Exploring the diversity of clays: Impacts

3 of temperature on physicochemical changes,
4 mechanical characteristics, and permeability,
5 and their relevance to membrane applications

6 Q1 Khadija Elataoui a , Mohamed Amine Harech a,∗ , Hajar Qobay a , Noureddine Elbinna a ,
7 Hakima Aouad a , Mohamed Waqif b , Latifa Saadi b
8
a Laboratory of Materials Science and Processes Optimization, Department of Chemistry, Faculty of Sciences Semlalia Marrakech,
9 Cadi Ayyad University, 40000 Marrakech, Morocco
10
b Laboratory Innovative Materials, Energy and Sustainable Development Laboratory, Department of Chemistry, 40000 Marrakech,

11 Morocco
12

13 a r t i c l e i n f o a b s t r a c t
14

15 Article history: This study investigates the characterization of two clays obtained from the Safi and Fez
16 Received 19 January 2024 regions, focusing on their analysis for filtration membrane applications. Various analytical
17 Accepted 16 July 2024 techniques were employed, including chemical composition analysis, elemental analysis,
18 Available online xxx mineralogical characterization, carbonate content determination, color assessment, plas-
19 ticity evaluation, thermal treatment analysis (DTA-TG), mineralogical transformation study,
20 Keywords: fusion tests, membrane tests, and scanning electron microscopy (SEM).
21 Membranes The results reveal significant differences between the two clays regarding their chem-
22 Characterization ical composition. The red clay exhibits a mineralogical composition comprising quartz,
23 Thermal treatment calcite, dolomite, hematite, illite, and kaolinite, whereas the gray clay contains quartz, cal-
24 Red clay cite, dolomite, illite, talc, and montmorillonite. Furthermore, upon thermal treatment, both
25 Gray clay clays exhibit changes in their physical properties.
Despite the decrease in porosity and water absorption, as well as the increase in compres-
sion strength for both clays, the permeability of the grey clay increases, unlike the red clay,
which exhibits a constant permeability beyond 1000 ◦ C.
These findings highlight the diversity and industrial significance of clays from the Safi and
Fez regions for filtration membrane applications. The contrasting properties of red and gray
clays provide insights into their potential utilization in different industries. Exploring these
clays’ behavior can lead to better filtration membranes and new industrial applications.
© 2024 The Authors. Published by Elsevier España, S.L.U. on behalf of SECV. This is an
open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/
by-nc-nd/4.0/).

∗
Corresponding author.
E-mail address: mharech@gmail.com (M.A. Harech).
https://doi.org/10.1016/j.bsecv.2024.07.001
0366-3175/© 2024 The Authors. Published by Elsevier España, S.L.U. on behalf of SECV. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: K. Elataoui, et al., Exploring the diversity of clays: Impacts of temperature on physicochemical
changes, mechanical characteristics, and permeability, and their relevance to membrane applications, Bol. Soc. Esp. Cerám.BSECV
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(2024),
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https://doi.org/10.1016/j.bsecv.2024.07.001
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Explorando la diversidad de las arcillas: impacto de la temperatura en

cambios fisicoquímicos, características mecánicas y permeabilidad, y su
relevancia en aplicaciones de membranas

26 r e s u m e n
27

28 Palabras clave: Este estudio investiga la caracterización de dos arcillas obtenidas de las regiones de Safi y
29 Membranas Fez, centrándose en su análisis para aplicaciones de membranas de filtración. Se emplearon
30 Caracterización diversas técnicas analíticas, incluyendo análisis de composición química, análisis elemen-
31 Tratamiento térmico tal, caracterización mineralógica, determinación del contenido de carbonato, evaluación del
32 Arcilla roja color, evaluación de plasticidad, análisis térmico (DTA-TG), estudio de transformación min-
33 Arcilla gris eralógica, pruebas de fusión, pruebas de membrana y microscopía electrónica de barrido
34 (SEM).
35 Los resultados revelan diferencias significativas entre las dos arcillas en cuanto a su com-
36 posición química. La arcilla roja presenta una composición mineralógica que incluye cuarzo,
37 calcita, dolomita, hematita, illita y caolinita, mientras que la arcilla gris contiene cuarzo, cal-
38 cita, dolomita, illita, talco y montmorillonita. Además, tras el tratamiento térmico, ambas
39 arcillas experimentan cambios en sus propiedades físicas.
40 A pesar de la disminución en la porosidad y absorción de agua, así como el aumento
41 en la resistencia a la compresión para ambas arcillas, la permeabilidad de la arcilla gris
42 aumenta, a diferencia de la arcilla roja, que muestra una permeabilidad constante más allá
43 de los 1000 ◦ C.
44 Estos hallazgos resaltan la diversidad y la importancia industrial de las arcillas de las
45 regiones de Safi y Fez para aplicaciones de membranas de filtración. Las características de
46 las arcillas rojas y grises proporcionan información sobre su posible utilización en difer-
47 entes industrias. Explorar el comportamiento de estas arcillas puede conducir a mejores
48 membranas de filtración y nuevas aplicaciones industriales.
© 2024 Los Autores. Publicado por Elsevier España, S.L.U. a nombre de SECV. Este es un
artı́culo Open Access bajo la CC BY-NC-ND licencia (http://creativecommons.org/licencias/
49 by-nc-nd/4.0/).

ing it suitable for indoor and outdoor applications [19–21]. The 74

Introduction availability of high-quality clay reserves in Safi has contributed 75

50Q2 Morocco has diverse clay deposits, each with special qualities to the growth of the local ceramics industry, facilitating eco- 76

51 appropriate for industrial and ceramic uses [1,2]. These clays, nomic development and establishing Morocco as a global 77

52 derived from dissimilar geological formations, have distinct competitor. 78

53 properties, including flexibility, heat resistance, and chemi- A city renowned for its cultural heritage, Fez is home to 79

54 cal composition, making them valuable for different industrial large reserves of gray clay [5,8,22]. The deposits of gray clay of 80

55 uses [3–5]. Moroccan clays are in high demand because of their Fez have unique properties that make them suitable for vari- 81

56 extraordinary flexibility in the pottery industry [6]. They are ous applications [22]. The specific characteristics of gray clay, 82

57 perfect for making tiles, pottery, and other ceramic products such as particle size and chemical composition, make it ideal 83

58 since they can be easily molded and molded into complicated for specific industrial applications. Its use in refractory mate- 84

59 shapes [7,8]. The chemical composition of Moroccan clays also rials contributes to the heat resistance and structural integrity 85

60 contributes to their industrial value [9,10]. These clays con- required in the metallurgy and cement manufacturing indus- 86

61 tain minerals and elements, including silica, aluminum, iron tries [23]. 87

62 oxide, and oligo-elements. These components enhance the In recent years, the use of clays for the preparation of filtra- 88

63 distinctive qualities of clays, making them suitable for var- tion membranes has received considerable attention [24,25]. 89

64 ious industrial applications [11–14]. For instance, iron oxide Clays, including the ones found in Morocco, have fine par- 90

65 gives some clays a distinctive reddish hue highly desired in ticles and a porous structure, which makes them suitable 91

66 the ceramics industry for its aesthetic appeal [15–17]. for creating membranes with high surface area and selec- 92

67 The coastal city of Safi, Morocco, has large deposits of tive permeability [26–28]. Modifying the clay’s properties by 93

68 excellent-quality red clay. Safi’s clay deposits are distin- incorporating pore-forming agents and heat treatment makes 94

69 guished by their purity, consistency, and bright red color, it possible to adapt the membranes to specific filtration 95

70 which results from iron oxide [18]. These characteristics make applications [29]. The technology of clay-based membranes 96

71 it a valuable resource of industrial importance. Safi red clay is provides several advantages for filtration applications: they 97

72 widely used in the industry to produce ceramic tiles, bricks, are cost-effective, readily available, and environmentally 98

73 and handicraft products. Safi red clay is wear-resistant, mak- friendly compared to traditional filtration materials [30]. 99

Please cite this article in press as: K. Elataoui, et al., Exploring the diversity of clays: Impacts of temperature on physicochemical
changes, mechanical characteristics, and permeability, and their relevance to membrane applications, Bol. Soc. Esp. Cerám.BSECV
Vidr. 414
(2024),
1–15
https://doi.org/10.1016/j.bsecv.2024.07.001
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100 Clay-based membranes’ scalability makes them a promis- Expert, Horiba Inc., Toronto, Ontario, Canada). The samples 151

101 ing solution for large-scale filtration processes in water were made as follows: in Teflon digesting vessels, 20 mg of the 152

102 treatment, pharmaceuticals, and food processing industries. clay sample were precisely weighed, and 1 mL of a concen- 153

103 In addition, its unique properties, such as its high surface trated HNO3 solution and 4 mL of an HF solution were added 154

104 area and cation exchange capacity, help improve the perfor- (Sigma-Aldrich, St. Louis, Missouri, USA). For 75 min, the sam- 155

105 mance of filtration membranes [31]. The fineness and porous ples were heated in a microwave. The colorless solutions were 156

106 structure of clays facilitate the retention and removal of quantitatively transferred to 100 mL volumetric flasks after 157

107 contaminants, thus ensuring efficient filtration processes. Fur- cooling, and the volume was then filled to the desired level 158

108 thermore, the ability to modify the properties of clays allows with deionized water. 159

109 for the optimization of membrane characteristics such as per- The thermal analyzer is used to assess the thermal 160

110 meability, selectivity, and mechanical strength to meet specific behavior of both clays (DTA-TG) (STA PT 1600, Linseis, Selb, 161

111 filtration requirements. Germany). At a heating rate of 10 ◦ C/min, the results were 162

112 Moroccan clays have various properties that contribute achieved in the air between 25 and 1050 ◦ C. Using this method, 163

113 to their importance for industrial and ceramic applications. we could ascertain the temperatures at which the clays under- 164

114 The red clay of Safi and the gray clay of Fez are particu- went mineralogical transition and weight loss. 165

115 larly important because of their reserves and suitability for Using a Mastersizer 2000 laser particle size analyzer, the 166

116 specific industries. Moreover, using these clays to prepare fil- size distribution of the clay particles was determined (Malvern 167

117 tration membranes offers a promising solution for efficient Panalytical, Malvern, U.K.). Then, the oversized agglomerates 168

118 and durable systems. The detailed characterization of these were broken up by sonicating the solution for 1 min after 169

119 clays allows us to understand all the phenomena that will adding 40 mg of the powder to 40 mL of water. The plastic- 170

120 take place during the preparation of the membranes, and it ity limit is evaluated according to the Moroccan standard Nm 171

121 will also provide valuable information on their filtration per- iso 17892-12. 172

122 formances, allowing the development of advanced filtration The Bernard technique is a good choice for quickly deter- 173

123 technologies in Morocco and the world. mining the carbonate content % [11]. Five grams of clay powder 174

were combined with 10 mL of concentrated hydrochloric acid 175

(Sigma-Aldrich, Missouri, United States) in a graduated cylin- 176

Material and methods
der after it was first filled with water (1N). The interaction 177

between the acid and clay particles forced water out of the 178
124 Raw materials and pretreatment
graduated cylinder, which created CO2 . The sample’s carbon- 179

ate quantity was then determined by directly weighing the gas 180
125 The two clays investigated in the study have been sourced
emitted. 181
126 from two distinct regions in Morocco. The first clay was
127 obtained from Safi (red clay), a coastal city in western Morocco,
128 while the second clay was procured from Fez (gray clay), Preparation of clay ceramic membrane 182

129 located in the country’s northern central part. The clay sam-
130 ples were collected and then prepared for analysis. This In this study, we aimed to investigate precisely the physico- 183

131 entailed drying the samples and grinding them into a fine chemical, mechanical properties, and workability of the two 184

132 powder, which was then passed through a 100 ␮m sieve to different types of clays as filtration membranes. We prepared 185

133 remove impurities and coarse particles. The resulting powder three different sizes of pellets red and gray clay, to achieve this. 186

134 was subsequently utilized for all subsequent analyses. The powders were carefully prepared by weighing the desired 187

amount of red and gray clay with a precision balance. Next, 188

135 Powder characterization techniques the pellets made from clay were used in a dry method. Finally, 189

the powder was axially compressed by 2.4 tons to produce pel- 190

136 We analyzed the chemical composition of each clay using X- let (a) and by one ton for pellets (b) and (c), which were then 191

137 ray fluorescence spectroscopy (XRF). This technique allowed heated to different temperatures. 192

138 us to determine the elemental composition of each sample,

139 including significant elements. A Panalytical ZETIUM X-ray • (a): 20 mm in diameter and 2 mm in thickness. 193

140 fluorescence instrument (Malvern Panalytical, Malvern, U.K.) • (b): 13 mm in diameter and 17 mm in thickness. 194

141 was used in this study. • (c): 13 mm in diameter and 1.5 mm in thickness. 195

142 X-ray powder diffraction was also used to examine the min-
143 eralogical makeup of the raw materials and ceramics heated to The heat treatment was performed in an electric furnace 196

144 900, 1000, and 1100 ◦ C (XRD). The materials were crushed into (LH 15/12 System, Nabertherm Lilienthal, Germany). The ther- 197

145 a fine powder, and Rigaku SmartLab’s CuK radiation diffrac- mal cycle is explained below: 198

146 tometer was used to gather data across a range of 5–60 2Theta The target temperatures of 900, 1000, and 1100 ◦ C were 199

147 by degrees with a 0.04-degree step size (Tokyo, Japan). The attained by applying a temperature ramp of 5 ◦ C/min. Follow- 200

148 PDF 2004 database’s diffractogram was compared to diffrac- ing 2 h at the predetermined temperature, the samples were 201

149 tion peaks to identify minerals. allowed to cool naturally while the oven was turned off. Fig. 3 202

150 Heavy metals were determined using inductively cou- shows the effect of temperature and composition on sintered 203

pled plasma optical emission spectroscopy (ICP-AES, Ultima pellets. 204

Please cite this article in press as: K. Elataoui, et al., Exploring the diversity of clays: Impacts of temperature on physicochemical
changes, mechanical characteristics, and permeability, and their relevance to membrane applications, Bol. Soc. Esp. Cerám.BSECV
Vidr. 414
(2024),
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https://doi.org/10.1016/j.bsecv.2024.07.001
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Q5 Table 1 – Chemical composition of red and gray clay.

Oxides SiO2 Al2 O3 Fe2 O3 MgO MnO CaO K2 O Na2 O TiO2 P2 O5 LOI
Red clay 52.76 17.34 5.95 2.63 0.03 3.84 4.72 0.42 – 0.17 11.6
Gray clay 47.17 9.54 4.13 4.08 0.035 14.16 1.26 0.78 0.55 – 16.27

205 Characterization of clay ceramic membrane ing approximately 52.76% of the total weight. Al2 O3 is also 248

present in a significant percentage, 17.34%. Fe2 O3 and CaO are 249

206 The Archimedes technique was used to calculate the samples’ in smaller quantities than SiO2 and Al2 O3 , each representing 250

207 bulk density, open porosity, and water absorption values as approximately 5.95% and 3.84%, respectively. K2 O and Na2 O 251

208 part of the investigation into the physical characteristics of the are also in moderate amounts, representing about 4.72% and 252

209 samples. First, the ceramic pieces were submerged in water 0.42%, respectively. MgO, TiO2 , and P2 O5 are in smaller quan- 253

210 for 24 h after being dried until their weight (W1 ) remained tities, representing less than 3% of the total weight. 254

211 constant. Then, the samples’ mass suspended in water was Gray clay displays a different chemical composition with a 255

212 calculated (W2 ). Then, the pieces were taken out of the water. relatively high proportion of SiO2 , 47%. CaO and Al2 O3 are also 256

213 Finally, before weighing them, the water on the surface was in significant quantity, representing approximately 14.16% 257

214 quickly blotted with a paper towel (W3 ). The following three and 9.5% of the total weight, respectively. The material also 258

215 equations were then used to determine the samples’ water contains substantial amounts of Fe2 O3 and MgO, each com- 259

216 absorption, apparent porosity, and bulk density values [29]. prising around 4% of the total weight. Other oxides, MnO, K2 O, 260

Na2 O, and TiO2 , are in smaller amounts, each comprising less 261

217  Water adsorption (%) = ((W3 − W1 )/W1 )×100 than 1%. 262

218  Apparent porosity (%) = ((W3 − W1 )/(W3 − W2 ))×100 An in-depth reading of the chemical composition shows 263

219  Bulk density (%) = ((W1 )/(W1 − W2 ))×100 notable differences between the clay samples “Red” and “Gray” 264

in their oxide percentages. The red clay has a higher ratio of 265

220 Compressive strength was measured using an Instron SiO2 , Al2 O3 , Fe2 O3 , K2 O, Na2 O, and a lower percentage of CaO 266

221 3369 apparatus with a load and loading speed of 50 kN and LOI than gray clay. In contrast, gray clay has higher MgO, 267

222 and 0.1 mm/min, respectively, and the pellet size was TiO2 , and MnO rates. These variations in oxide percentages 268

223 13 mm × 17 mm. The morphology and microstructure of the may be attributed to differences in the two clays’ origin, min- 269

224 membranes were analyzed using the Hitachi SC 2500 scanning eralogy, and geological history. In general, by comparing the 270

225 electron microscope (Hitachi High-Technologies Corporation, chemical composition of the two clays, it is possible to identify 271

226 Japan). An acceleration voltage of 5 kV was used for this exam- their distinct characteristics and the potential consequences 272

227 ination. of these disparities on their properties and uses. For example, 273

red clay’s SiO2 /Al2 O3 ratio is 3.02, while gray clay’s is 4.94. 274

228 Filtration (permeability of membrane) Both percentages are significantly higher than the the- 275

oretical value of 1.18 for pure kaolinite. They suggest that 276

229 The laboratory-scale frontal filtration pilot comprises three both samples contain a significant amount of free quartz, alu- 277

230 components: a 300 mL supply tank, an air compressor, and a minosilicate, and other minerals [9,11]. Finally, the loss on 278

231 pressure gauge. The pressure gauge regulates the pressure of ignition (LOI) percentage represents the weight loss due to 279

232 the fluid on the membrane. Before usage, the membrane was removing water, organic matter, and decomposition of car- 280

233 immersed in distilled water for 24 h and then inserted into bonates during heating. Red clay has a lower LOI percentage, 281

234 the membrane housing, which has an effective filtration sur- indicating a lower carbonate and organic matter content than 282

235 face area of 2 cm2 . All filtration experiments were conducted gray clay. 283

236 at room temperature.

237 A membrane’s permeability (P) is an intrinsic characteristic Elemental analysis 284

238 that depends on its structure. In practical terms, permeability

239 can be defined as the ratio of the permeation flux (JP) to the Table 2 shows the concentration of three heavy metals, As, 285
240
Q3 applied pressure (Pm) (Thomas et al., 2009). Cd, and Pb, measured in the two clays. Red clay has a concen- 286

tration of 129 ppb for arsenic, 53 ppb for cadmium, and 15 ppb 287
JP
241 P= for lead. On the other hand, gray clay has a higher concen- 288
Pm
tration for all three elements, with 172 ppb for arsenic, 41 ppb 289
242 where P is the permeability (in L/h m2 bar), JP is the permeation for cadmium, and 10 ppb for lead. Comparing the two clays, 290
243 flux (in L/h m2 ) and Pm is the applied pressure (in the bar). it is clear that gray clay has a much stronger concentration of 291

Result and discussion

Table 2 – Elemental analysis of red and gray clay.
244 Chemical compositions
Heavy metals Red clay (ppb) Gray clay (ppb)

245 The chemical compositions of red and gray clay are listed in Arsenic (75As) 129 172
246 Table 1. The result of red clay shows that the percentage of Cadmium (112Cd) 53 41
Lead (207Pb) 15 10
247 SiO2 is very high compared to the other oxides, represent-

Please cite this article in press as: K. Elataoui, et al., Exploring the diversity of clays: Impacts of temperature on physicochemical
changes, mechanical characteristics, and permeability, and their relevance to membrane applications, Bol. Soc. Esp. Cerám.BSECV
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Fig. 1 – XRD patterns of raw gray clay powder. ( ) Montmorillonite; (o) calcite; (*) quartz; ( ) dolomite; ( ) talc; ( ) illite.

292 these. Specifically, gray clay has around 33% higher concen- have consequences on their potential use in various indus- 303

293 tration of arsenic, 22% lower concentration of cadmium, and trial and commercial applications. For example, the presence 304

294 31% lower concentration of Lead. of clay minerals may affect the plasticity and workability of 305

the clay, as well as its firing temperature. Upon meticulous 306

295 X-ray diffraction (XRD) examination of the two diffractograms, the intensity of cal- 307

cite peaks is clearly greater than that of red clay, which is 308

296 Figs. 1 and 2 depict the XRD patterns of red and gray clay expected due to the significant proportion of calcium oxide 309

297 powder samples, respectively. Red clay contains minerals, present in the chemical composition of the gray clay. How- 310

298 including quartz, calcite, dolomite, hematite, illite, and kaoli- ever, despite the red clay containing approximately twice the 311

299 nite. In contrast, gray clay contains slightly different minerals: amount of magnesium oxide, the intensity of the dolomite is 312

300 quartz, calcite, dolomite, illite, talc, and montmorillonite. considerably weak in contrast to red clay. This phenomenon 313

301 These results provide valuable information on the mineralog- can be explained by forming another primary phase, talc, by 314

302 ical characteristics of these two raw clay samples, which may this oxide. 315

Fig. 2 – XRD patterns of raw red clay powder. (o) Calcite; (*) quartz; ( ) dolomite; ( ) illite; ( ) hematite; ( ) kaolinite.

Please cite this article in press as: K. Elataoui, et al., Exploring the diversity of clays: Impacts of temperature on physicochemical
changes, mechanical characteristics, and permeability, and their relevance to membrane applications, Bol. Soc. Esp. Cerám.BSECV
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(2024),
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316 The carbonate content percentage Plasticity 350

317 The differences in carbonate content between the two clays The plasticity of clays is primarily determined by miner- 351

318 can be explained by the variance in the clays’ calcium oxide alogical composition, particle size, particle shape, and water 352

319 (CaO) and magnesium oxide (MgO) content. Calcium carbon- content. However, the chemical composition of the clay also 353

320 ate (CaCO3 ) is a principal component of various clays and plays a significant role in deciding its plasticity [41]. First, the 354

321 is often used as an indicator of the clays’ overall fertility red clay with a low carbonate content and high Al2 O3 con- 355

322 and productivity. The red clay has a lower carbonate con- tent has higher plasticity (25%) than the gray clay (15%) with 356

323 tent (4%) than the second clay (16%), implying that the latter a higher carbonate content and low montmorillonite content. 357

324 possesses a higher concentration of calcium carbonate and This is because the Al2 O3 content in the Safi clay can con- 358

325 dolomite. This deduction is supported by the observation that tribute to forming clay minerals, resulting in an increased 359

326 the gray clay exhibits a considerably higher percentage of surface charge and a strong affinity for water molecules, lead- 360

327 CaO (14.16%) than the red clay (3.94%). Magnesium is also ing to high plasticity. Additionally, the Safi clay’s illite and 361

328 noteworthy as it is a significant component of dolomite. The kaolinite clay phases are known to have high plasticity [42]. On 362

329 gray clay contains a higher percentage of MgO (4%) than the the other hand, according to the bibliography, the characteris- 363

330 red clay (2.53%). While the presence of organic matter can tics of the talc include low specific surface areas. Furthermore, 364

331 influence the carbonate content of clay, it was not consid- it is generally accepted that the basal surfaces of talc and pyro- 365

332 ered in this study. This was because the DTA curves of the phyllite are hydrophobic, and the edges are hydrophilic. From 366

333 two clays did not exhibit any exothermic peaks, which would these results, it can be inferred that the plasticity of the gray 367

334 indicate significant thermal decomposition of the organic clay is adversely affected by the presence of talc. 368

335 matter. As a result, the impact of organic matter on the

336 carbonate content was considered negligible for this investi- Thermal treatment analysis 369

337 gation.
DTA-TG 370

Fig. 3 revealed endothermic peaks at different tempera- 371

338 Color of clays tures with corresponding mass loss. The initial peak at 95 ◦ C 372

resulted in a mass loss of about 0.65%, attributed to removing 373

339 The difference in color between the two clays, red and gray, can hygroscopic water. A more prominent endothermic peak with 374
340 be attributed to their mineral compositions. Red clay contains a center temperature of 575 ◦ C caused a significant mass loss 375
341 hematite and kaolinite minerals. Hematite is a reddish-brown likely caused by the dehydroxylation of the kaolinite. A small 376
342 mineral that gives red clay its reddish hue [33]. Conversely, peak observed at 595 ◦ C could be related to the allotropic trans- 377
343 kaolinite is a white or gray mineral typically not colored, but formation of quartz. Around 750 ◦ C, the last peak resulted in 378
344 it can sometimes appear slightly yellow or red [34–37]. Gray a significant mass loss and could indicate the calcite’s and 379
345 clay, on the other hand, contains smectite, montmorillonite, dolomite’s decarbonization. These results suggest that the 380
346 and talc minerals. These minerals do not contain significant material undergoes multiple transformations when heated 381
347 amounts of iron oxide like hematite, responsible for the red- and that each transformation could develop distinct miner- 382
348 dish color. Instead, these minerals are gray, which is likely why alogical phases. The thermal behavior of gray clay (Fig. 4) was 383
349 the clay appears gray [38–40]. similar to that of red clay. Several endothermic peaks were 384

Fig. 3 – DTA-TG curves of red clay.

Please cite this article in press as: K. Elataoui, et al., Exploring the diversity of clays: Impacts of temperature on physicochemical
changes, mechanical characteristics, and permeability, and their relevance to membrane applications, Bol. Soc. Esp. Cerám.BSECV
Vidr. 414
(2024),
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Fig. 4 – DTA-TG curves of gray clay.

385 observed. The initial peak detected at 95 ◦ C resulted in a mass Mineralogical transformation 399

386 loss of around 3%, indicating the removal of hygroscopic water. Figs. 5 and 6 show the phases formed during the treat- 400

387 The significant loss in mass is due to smectite, a type of clay ment at different temperatures. These minerals already 401

388 mineral with a high surface area that can swell in the pres- contain the phases that exist at room temperature, which 402

389 ence of water, which can cause a decrease in the bulk density are calcite (CaCO3 ), quartz (SiO2 ), dolomite (CaMg(CO3 )2 ), illite 403

390 of the material in which it is present. ((K,H3 O)(Al,Mg,Fe)2 (Si,Al)4 O10 ), hematite (Fe2 O3 ), and kaolin- 404

391 A larger endothermic peak was observed at 575 ◦ C, result- ite (Al2 Si2 O5 (OH)4 ). As the temperature increases, various 405

392 ing in a consequent loss of mass, which could signify phase transformations occur. At 900 ◦ C, the minerals react to 406

393 the dehydroxylation of smectite, talc, and montmorillonite. form anorthite (CaAl2 Si2 O8 ), diopside (CaMgSi2 O6 ), and gehli- 407

394 Another minor peak was observed at 595 ◦ C, which could be nite (Ca2 Al2 SiO7 ). At 1000 ◦ C, the minerals present at 900 ◦ C 408

395 linked to the allotropic transformation of quartz. Finally, the continue to form, except for gehlinite (Ca2 Al2 SiO7 ), which dis- 409

396 last two peaks, centered around 700 and 750 ◦ C, resulted in appears. The evolution of the mineral phases present can 410

397 significant mass loss, suggesting the decomposition of calcite be deduced by examining the variation of their intensity on 411

398 and dolomite. the diffractogram. Gehlinite (Ca2 Al2 SiO7 ) is a mineral phase 412

Fig. 5 – XRD patterns of red clay sintered at 900 ◦ C, 1000 ◦ C and 1100 ◦ C. (*) Quartz; ( ) illite; ( ) hematite; (X) anorthite; (+)
gehlenite; (ő) diopside; ( ) magnetite.

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Fig. 6 – XRD patterns of gray clay sintered at 900 ◦ C, 1000 ◦ C and 1100 ◦ C. (*) Quartz; ( ) hematite; (X) anorthite; (+) gehlenite;
(ő) diopside; (±) CaO; (©) augite.

413 that forms at around 900 ◦ C, as shown in Table 3. However, Gray clay is a complicated mixture of several materials at 424

414 it is not present at 1000 ◦ C. This is because, at 1000 ◦ C, the room temperature, including calcite (CaCO3 ), quartz (SiO2 ), 425

415 Ca2 Al2 SiO7 phase undergoes a phase transformation into the dolomite [CaMg(CO3 )2 ], smectite [(Na,Ca)0.33 (Al,Mg)2 (Si4 O10 ) 426

416 CaAl2 Si2 O8 phase when there is a substantial amount of free (OH)2 ·nH2 O], illite [(K,H3 O)(Al,Mg,Fe)2 (Si,Al)4 O10 ], and mont- 427

417 quartz [11,18]. morillonite [Mg3 Si4 O10 (OH)2 ]. At 900 ◦ C, the gray clay is 428

418 Finally, when red clay is subjected to a temperature of primarily composed of quartz (SiO2 ), hematite (Fe2 O3 ), and 429

419 1100 ◦ C, some minerals undergo significant changes. How- a range of high-temperature minerals that form through the 430

420 ever, few transformations occur, except for the appearance decomposition of illite and calcite. The decomposition of illite 431

421 of a new phase, magnetite (Fe3 O4 ), and the disappearance of can result in feldspars, such as CaAl2 Si2 O8 and KAlSi3 O8 , while 432

422 hematite. The intensity of anorthite increases while that of calcite decomposition can lead to wollastonite formation 433

423 quartz decreases. (CaSiO3 ) and lime (CaO). By heating clay minerals to tem- 434

Table 3 – Minerals transformation after thermal treatment.

Treatment temperature Red clay Gray clay

Ambient Calcite (CaCO3 ) Calcite (CaCO3 )

Quartz (SiO2 ) Quartz (SiO2 )
Dolomite (CaMg(CO3 )2 ) Dolomite (CaMg(CO3 )2 )
Illite ((K,H3 O)(Al,Mg,Fe)2 (Si,Al)4 O10 ) Smectite ((Na,Ca)0.33 (Al,Mg)2 (Si4 O10 )(OH)2 ·nH2 O)
Hematite (Fe2 O3 ) Illite ((K,H3 O)(Al,Mg,Fe)2 (Si,Al)4 O10 )
Kaolinite (Al2 Si2 O5 (OH)4 ) Montmorillonite (Mg3 Si4 O10 (OH)2 )

900 ◦ C Quartz (SiO2 ) Quartz (SiO2 )

Hematite (Fe2 O3 ) Hematite (Fe2 O3 )
Anorthite (CaAl2 Si2 O8 ) Lime (CaO)
Diopside (CaMgSi2 O6 ) Wollastonite (CaSiO3 )
Gehlinite (Ca2 Al2 SiO7 ) Anorthite (CaAl2 Si2 O8 )
Diopside(CaMgSi2 O6 )

1000 ◦ C Quartz (SiO2 ) Quartz (SiO2 )

Hematite (Fe2 O3 ) Hematite (Fe2 O3 )
Anorthite (CaAl2 Si2 O8 ) Anorthite (CaAl2 Si2 O8 )
Diopside (CaMgSi2 O6 ) Diopside (CaMgSi2 O6 )

1100 ◦ C Quartz (SiO2 ) Quartz (SiO2 )

Anorthite (CaAl2 Si2 O8 ) Augite (Ca(Mg, Fe)Si2 O6 )
Diopside (CaMgSi2 O6 ) Anorthite (CaAl2 Si2 O8 )
Magnetite (Fe3 O4 ) Gehlenite (Ca2 Al2 SiO7 )

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Fig. 8 – Appearance of clay pellets after thermic treatment

at 1170 ◦ C.

Fig. 7 – Color change of ceramic membranes sintered at

the formation of augite. These results suggest that mineral 471
different temperatures.
composition changes can cause significant alterations in a 472

sample’s color. 473

After subjecting red clay with 3.84% CaO and gray clay 474

435 peratures above 1000 ◦ C, these minerals can develop through with 14.16% CaO to heat treatment at 1170 ◦ C, noticeable dif- 475

436 high-temperature processes like pyro-metamorphism. These ferences were observed in Fig. 8. While the gray clay was 476

437 minerals can arise via the recrystallization of clay minerals completely melted, the red clay remained intact, and the pel- 477

438 at high temperatures or from the disintegration of pre- let appeared to retain its original shape. This suggests that a 478

439 existing minerals like feldspars and pyroxenes. Quartz (SiO2 ), higher calcium oxide (CaO) content in the gray clay caused it 479

440 hematite (Fe2 O3 ), and a variety of high-temperature min- to melt under the given temperature. In contrast, the red clay, 480

441 erals, such as anorthite (CaAl2 Si2 O8 ), diopside (CaMgSi2 O6 ), with a lower CaO content, displayed greater resistance to the 481

442 and gehlenite (Ca2 Al2 SiO7 ), make up the majority of the gray heat and maintained its form [43]. 482

443 clay at 1000 ◦ C. When the temperature increases to 1100 ◦ C,

444 the diffractogram shows no significant alterations, indicating Membranes tests 483
445 that anorthite and gehlinite remain in the dominant phases.
446 However, diopside transforms and is converted into augite Figs. 9–11 provide data on various properties measured at dif- 484
447 Ca(Mg,Fe)Si2 O6 through the process of Mg substitution by Fe. ferent temperatures for two types of clays: red and gray. The 485
448 This change in mineral composition can be attributed to the properties include open porosity, water absorption, and bulk 486
449 high-temperature conditions. density. The open porosity of both red and gray clays was 487

measured at three different temperatures: 900 ◦ C, 1000 ◦ C, and 488

450 Color change and fusion test 1100 ◦ C. 489

451 Fig. 7 illustrates how temperature and mineral transforma- At 900 ◦ C, the red clay exhibited an open porosity of 28%, 490

452 tions influence the color of ceramic membranes. The red clay while the gray clay had a slightly higher open porosity of 35%. 491

453 color variations provide insights into the behavior of the illite When the temperature was increased to 1000 ◦ C, the open 492

454 mineral under different heating conditions. When exposed to porosity decreased for both materials, with the red clay show- 493

455 900 ◦ C, the illite mineral breaks down, releasing iron oxide [11]. ing a value of 28% and the gray clay measuring 33%. At the 494

456 This process results in a brick-red hue that remains even when highest temperature of 1100 ◦ C, the red clay’s open porosity 495

457 the temperature is raised to 1000 ◦ C. Interestingly, at 1000 ◦ C, dropped to 18%, while the gray clay maintained a value of 30%. 496

458 the iron oxide does not undergo significant changes, and the Similarly, the water absorption was assessed under an identi- 497

459 brick-red hue remains unchanged. However, when the tem- cal set of three temperatures. When exposed to a temperature 498

460 perature is increased to 1100 ◦ C, the iron oxide undergoes an of 900 ◦ C, the red clay exhibited a water absorption rate of 15%, 499

461 oxidation process, leading to the formation of magnetite. The whereas the gray clay displayed a slightly higher absorption 500

462 creation of magnetite gives the material a brown hue distinct rate of 19%. As the temperature increased to 1000 ◦ C, the water 501

463 from the original brick-red color. absorption of the red clay decreased to 14%, while the gray 502

464 The color of the sample is gray at room temperature. How- clay experienced a reduction to 18%. Finally, at the highest 503

465 ever, as the sample was heated to 900 ◦ C, the color changed temperature of 1100 ◦ C, the water absorption of the red clay 504

466 to brick red, which was attributed to the release of iron oxide plummeted to 9%, while the gray clay maintained a value of 505

467 resulting from the decomposition of illite. No significant trans- 17%. 506

468 formation was observed for iron oxide when the temperature Furthermore, the bulk density was assessed. At 900 ◦ C, the 507

469 was increased to 1000 ◦ C. However, at 1100 ◦ C, the sample red clay exhibited a bulk density of 2.68, whereas the gray clay 508

470 turned yellow due to the reaction of hematite, resulting in had a slightly higher value of 2.75. The bulk density remained 509

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Fig. 9 – Bulk density of membranes firing at 900 ◦ C, 1000 ◦ C and 1100 ◦ C.

Fig. 10 – Open porosity of membranes firing at 900 ◦ C, 1000 ◦ C and 1100 ◦ C.

510 relatively consistent as the temperature increased to 1000 ◦ C, more excellent compaction at elevated temperatures than the 524

511 with the red clay maintaining a measurement of 2.68 and the red clay. 525

512 gray clay showing a value of 2.76. However, when the temper- Both the mineral composition of the ceramic pastes and 526

513 ature reached 1100 ◦ C, both clays experienced a slight increase the optimum firing temperature play a crucial role in deter- 527

514 in bulk density, with the red clay measuring 2.70 and the gray mining the efficiency of filtration membranes. These two 528

515 clay measuring 2.81. It can be observed that the gray clay con- factors influence the formation of pores in the membrane 529

516 sistently had higher bulk density values compared to the red structure, which has a direct impact on its porosity and, 530

517 clay across all three temperatures. At 900 ◦ C, the difference consequently, on its filtration efficiency. At lower firing tem- 531

518 was minimal, with the gray clay having a slightly higher bulk peratures, the presence of carbonates and pore formation 532

519 density. However, as the temperature increased to 1100 ◦ C, are closely linked. Carbonation can break down during firing, 533

520 the disparity in bulk density between the two clays became releasing gases that create pores in the ceramic matrix. Fur- 534

521 more pronounced, with the gray clay exhibiting a significantly thermore, pore-forming mechanisms such as sintering and 535

522 higher value of 2.81 compared to the red clay’s 2.70. This sug- phase transformations are influenced by firing temperature 536

523 gests that the gray clay may have a higher packing density or and mineralogical composition. However, at higher firing tem- 537

Please cite this article in press as: K. Elataoui, et al., Exploring the diversity of clays: Impacts of temperature on physicochemical
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Fig. 11 – Water absorption of membranes firing at 900 ◦ C, 1000 ◦ C and 1100 ◦ C.

538 peratures, the behavior changes. As temperature increases, The results reveal interesting trends regarding the mechanical 552

539 ceramic materials undergo vitrification, where pores can be properties of these membranes (Fig. 12). 553

540 filled with glassy phases, resulting in a decrease in porosity. Starting with the red clay ceramic membrane, its com- 554

541 This is due to the densification of the ceramic structure during pressive strength was 60.97 MPa at 900 ◦ C. As the temperature 555

542 sintering, which leads to pore closure or consolidation. increased to 1000 ◦ C, there was a notable improvement in 556

543 Overall, these results highlight the variations in bulk den- the compressive strength, which rose to 83.25 MPa. The red 557

544 sity, water absorption, and open porosity between red and gray clay membrane reached its highest compressive strength at 558

545 clays at different temperatures, which indicate differences in 1100 ◦ C, measuring 110.165 MPa. These findings indicate that 559

546 their structural characteristics and responses to temperature as the temperature increased, the red clay ceramic mem- 560

547 changes. This result will influence their mechanical strength brane significantly enhanced its structural integrity and ability 561

548 as well as their filtering capacity. to withstand compressive forces. In contrast, the gray clay 562

ceramic membrane displayed a different pattern in terms 563

549 Compressive strength of compressive strength. At 900 ◦ C, its compressive strength 564

was measured at 38.33 MPa, lower than that of the red clay 565

550 The compressive strength of ceramic membranes made from membrane. As the temperature increased to 1000 ◦ C, the gray 566

551 red and gray clay was investigated at different temperatures. clay membrane experienced a slight decrease in compressive 567

Fig. 12 – Compressive strength of ceramic membranes at different temperatures.

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Fig. 13 – Variation in water flux a function of pressures at 900 ◦ C.

568 strength, measuring 37.925 MPa. However, at 1100 ◦ C, the gray sure. The corresponding red flux ranges from 144 L/h m2 to 587

569 clay ceramic membrane demonstrated a moderate improve- 506 L/h m2 . For 1000 ◦ C, the gray flux varies from around 588

570 ment, reaching a compressive strength of 51.155 MPa. 643 L/h m2 to 2689 L/h m2 as the pressure increases from 0.25 589

571 The red clay ceramic membrane consistently outperformed to 1 bar. The red flux ranges from 233 L/h m2 to 1123 L/h m2 . At 590

572 the gray clay membrane in terms of compressive strength 1100 ◦ C, the gray flux ranges from 1362 L/h m2 to 5283 L/h m2 , 591

573 throughout all temperature ranges, as can be seen from com- while the red flux ranges from 274 L/h m2 to 1138 L/h m2 . 592

574 paring the two types of membranes. Accordingly, it can be Despite the high-temperature heat treatment at 1100 ◦ C, 593

575 inferred that the red clay ceramic membrane has higher the membrane’s permeability still increases. This can be 594

576 mechanical qualities and is more appropriate for uses requir- attributed to several factors. Firstly, creating larger pores than 595

577 ing excellent resistance to compressive forces. those formed at 900 ◦ C contributes to the increased permeabil- 596

ity. 597

578 Water flux These larger pores allow easier water passage and enhance 598

the membrane’s permeability. Secondly, although the over- 599

579 Fig. 13 summarizes the flux values for gray and red clays at all porosity decreases with the high-temperature treatment, 600

580 different temperatures (900 ◦ C, 1000 ◦ C and 1100 ◦ C) and pres- the distribution of pores across the membrane is not uniform. 601

581 sures (ranging from 0.25 to 1 bar). Increased pressure results Some areas may have a higher concentration of pores, lead- 602

582 in higher flux values for both clays at different temperatures. ing to localized regions of increased permeability. This uneven 603

583 The values obtained for gray clay are always higher than those distribution of pores can contribute to the overall permeability 604

584 obtained for red clay. increase despite the decrease in porosity. 605

585 At 900 ◦ C, the gray flux ranges from approximately In conclusion, the combination of larger pore creation, 606

586 598 L/h m2 at 0.25 bar pressure to 2260 L/h m2 at 1 bar pres- uneven pore distribution, decreased porosity, and improved 607

Fig. 14 – SEM image of gray clay after sintering at 900 ◦ C and 1100 ◦ C.

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608 compression resistance due to well-sintered ceramic surfaces

609 collectively contribute to the overall increase in permeability
Competing interests
610 despite the high-temperature heat treatment at 1100 ◦ C.
611 The SEM figure provides visual evidence that supports the The authors have no relevant financial or non-financial inter- 659

612 previously mentioned findings. When clay is sintered at 900 ◦ C, ests to disclose. 660

613 it exhibits a homogeneous pore distribution with small pore

614 sizes that do not exceed a few micrometers. On the other Q4
Uncited reference
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616 significantly different pore structure. The pore size increases
[32]. 661
617 significantly to approximately 25 ␮m, and the pore distribu-
618 tion becomes uneven, leading to dense and well-compacted 662

619 zones. references

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620 This observation from the SEM image Fig. 14 aligns with
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