# Characteristic Investigations on Ethiopian Kaolinite: Effect of Calcination Temperature on Pozzolanic Activity and Specific Surface Area

Authors: Adamu Esubalew Kassa; Nurelegne Tefera Shibeshi; Belachew Zegale Tizazu; S. Venkatesa Prabhu
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Kaolinite, a clay mineral, is an important industrial mineral which was gainfully utilized with several industrial significances. Towards mineral beneficiation, the present study aimed to investigate the effect of calcination temperature on the pozzolanic activity and specific surface area of Ethiopian kaolinite (Ek). The raw Ek and calcined Ek were characterized using different sophisticated techniques, such as XRD, FTIR, DSC, TGA, dynamic light scattering, electrophoretic light scattering, and SEM analysis. The pozzolanic activity and specific surface area of Ek particles were ascertained using Chapelle’s and Brunauer–Emmett–Teller methods, respectively, with some modifications. The results revealed that the calcination temperature had a great influence on pozzolanic activity and specific surface area of Ek. It was observed that the pozzolanic activity linearly increased with calcination temperature until 700°C and showed a maximal value of 1235 mg Ca(OH)2 g−1. On the other hand, beyond 700oC, the pozzolanic activity was inferred to be declined to 445 mg Ca(OH)2 g−1 at 1000°C due to agglomeration of calcined kaolinite (metakaolinite) and formation of spinel phase. A similar phenomenon was observed for specific surface area which increased with temperature until 700°C to exhibit the maximal values of 47.794 m2·g−1. Further, the investigation on zeta potential analysis showed that the magnitude value of the metakaolinite was observed to be less than the zeta potential value of the raw kaolinite, which indicated that the flocculation tendency of metakaolinite was greater than the kaolinite flocculation affinity in water.


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## Body

## 1. Introduction

Pozzolan refers a fine material containing silicon or aluminum silicon in amorphous phases and reacts with calcium hydroxide in the presence of water to produce compounds with appreciable binder properties [1]. Due its cheaper cost, improved chemical resistance, and lower environmental impacts, pozzolan has gained a potential significance among the industrial minerals, especially substitute materials for cement-based concrete. Kaolinite is one of the clay minerals, which can be exploited potentially to produce pozzolan through calcination [2]. Normally, kaolinite consists of single tetrahedral silica and octahedral alumina sheets merged together with hydrogen bonding [3]. In view of beneficiation of minerals, calcination is widely applied that can significantly improve physical and chemical properties of any minerals. In such a way, calcination of kaolinite has been proven as a triumphant approach for improving the desirable physicochemical properties for cement industry [4]. The calcination of kaolinite promotes formation of metakaolinite via the dehydroxylation reaction [5]. The calcined kaolinite (metakaolinite) reacts with the calcium hydroxide in the presence of water by the cement hydration to form compounds, such as hydrated calcium silicates and hydrated calcium aluminosilicates that have substantial cementitious properties [6].During conventional cement production, huge amounts of CO2 are released into the atmosphere which harms the environment [7]. It is evident that the Ordinary Portland Cement (OPC) manufacturing emits around 0.8 tons of CO2 per ton of cement, which accounts more than 5% of the total CO2 emissions worldwide [8]. Recently, some appropriate supplementary cementitious materials are used to replace a part of the clinker in a cement or mixture of cement-concrete in order to sense lower environmental impact and associated remediation costs [9]. Keeping in view, metakaolinite can be an appropriate material since it has potential cementitious properties. The OPC manufacturing demands a lot of energy and gives off huge amounts of CO2. Alternatively, calcined kaolinite requires less energy and emits water instead of CO2, which has drawn a significant attention on it [10]. The production energy and cost of the kaolinite based cement have been 70% and 45%, respectively, which is less than those of OPC [11].However, the chemical and physical properties of the intermediate phases of metakaolinite produced during various calcination stages determine the industrial interest on calcined kaolinite [12]. One of the most ponderable characteristics of cementitious materials is pozzolanic activity, which measures the ability of material to react with calcium hydroxide in the presence of water [13]. The magnitude of pozzolanic activity indicates the quality and effectiveness of the cementitious materials. Hence, pozzolanic activity plays a vital role for any supplementary cementitious materials such as metakaolinite as a pozzolan [14]. The calcined kaolinite has favorable pozzolanic activity which could be used to substitute a part of the CO2 intensive clinker in cement production [15].So far, several studies have been carried out on different kaolinite samples to investigate on synthesis of zeolite [16, 17], fabrication of ceramic membrane [18], and synthesis of adsorbent [19, 20]. However, Ethiopian kaolinite was not undertaken till now to evaluate for its pozzolanic activity. Hence, the present study focused to investigate the effect of calcination temperature on the pozzolanic activity of metakaolinite produced from Ethiopian kaolinite. In addition, the influence of calcination temperature on specific surface area of the resulted metakaolinite particles was also studied. Further, in order to get an understandings of calcination process, the raw kaolinite and calcined kaolinite were characterized using Brunauer–Emmett–Teller (BET), X-ray diffractometry (XRD), Fourier transformer infrared spectrometry (FTIR), differential scanning calorimetry (DSC), thermogravimetry (TGA), dynamic light scattering (DLS), electrophoretic light scattering (ELS), and scanning electron microscope (SEM). Such a study will be helpful for attaining a clear picture on Ethiopian kaolinite properties towards its use of different industrial applications.


## 2. Materials and Methods

### 2.1. Ethiopian Kaolinite Sample Preparation and Determination of Pozzolanic Activity

The raw kaolinite sample was collected from Awash Melkasa Aluminum Sulphate and Sulfuric Acid Share Company, Ethiopia. The raw kaolinite was soaked in ultrapure water to remove impurities and debris. Further, it was suspended with water to separate from the insoluble solid residues. This suspension was left for settling under the action of gravity to single out the wet kaolinite from supernatant liquid. The wet kaolinite was allowed to dry overnight at 105°C. Then, the dried sample was milled using a mortar and pestle followed by sieving the sample using a sieve analyzer (Elettronica veneta S.P.A, CE IC–205/EV, Italy) to single out the 106μm particles. The predetermined sample sizes (5 g) of samples were undertaken for calcination at different temperatures, such as 575, 600, 625, 650, 700, 800, 900, and 1000°C for 180 min using a muffle furnace (MF 106, Turkey).The pozzolanic activity of the metakaolinite was determined with some modification as outlined by Chappelle’s method [21]. 1 g of metakaolinite sample was well-mixed with 2 g of Ca(OH)2 along with in 250 mL distilled water. The suspension was boiled at 90°C for 16 h with continuous stirring (1000 rpm). Then, the suspension was allowed to cool for reaching room temperature. A 250 mL sucrose solution (0.24 g·mL−1) was added to the suspension and stirred with magnetic bar for 30 min. The resulted solution was filtered and then 25 mL of this solution was titrated against 0.1 M HCl. Phenolphthalein (0.1%w/v in ethanol 50%v/v) was used as an indicator. The titration reactions of the process are represented as in equations (1) and (2):(1)CaO+2HCl⟶CaCl2+H2O(2)CaOH2+2HCl⟶CaCl2+2H2OThe pozzolanic activity of the metakaolinite (PAMK) was determined using the following equation:(3)PAMK=2V1−V2V1M2M11000,where PAMK is the pozzolanic activity of metakaolinite (mg of Ca(OH)2 g−1); V1 is the volume (mL) of 0.1 M HCl consumed by the blank solution; V2 is the volume (mL) of 0.1 M HCl consumed by the sample solution; and M1 and M2 are molar masses of CaO and Ca(OH)2 (g mol−1), respectively.


### 2.2. Determination of Specific Surface Area

The specific surface area for the kaolinite and metakaolinite samples was determined by surface area analyzer (HORIBA SA–9600 series, model: SA–9603, USA) using the BET method through nitrogen adsorption at the temperature of 77 K liquid nitrogen. The samples were degassed at 200°C for 60 min. In each BET experiment, the quantity of kaolinite and metakaolinite in each sample was 0.75 g. The BET was operated at room temperature and atmospheric pressure (25°C and 700 mm) [22, 23]. The specific surface areas of the samples were determined using the BET equation:(4)1VPo/P−1=C−1VmC×PPo+1VmC,where P and Po are the equilibrium and saturation pressure of the adsorbates at the temperature of adsorption, respectively; V is the volume of adsorbed gas (cm3·g−1); Vm is the monolayer adsorbed gas volume (cm3·g−1); and C is BET constant.


### 2.3. Particles Size and Zeta Potential Determination

The particles size distribution (PSD) and Zeta potential distribution (ZPD) of kaolinite and metakaolinite were characterized using Malvern Zetasizer Nano-ZS instrument (ZEN 3600, UK) using the dynamic light scattering (DLS) and electrophoretic light scattering (ELS) techniques, respectively. The samples were measured three times at 25°C. The dispersion medium was ultrapure water (refractive index = 1.330, viscosity = 8.872 × 10−4 Pa·s, and dielectric constant = 78.5). The samples were prepared with similar procedure as described by Tan et al. [24] and Chorom and Rengasamy [25] for PSD and ZPD, respectively. Each sample (0.50 g) was distributed and homogenized in 50 mL ultrapure water with sonication (Soniprep 150, UK) for 15 min. The homogenized dispersion of the sample was allowed to settle for 180 min. Further, the supernatant liquid from each sample was used for determination of the PSD and ZPD.


### 2.4. X-Ray Diffractometer (XRD) Analysis

The crystalline structures of kaolinite and metakaolinite were characterized by X-ray diffraction (SHIMDZU XRD–7000, Japan) at cool temperature (–45°C). Using 1.5406 nm wavelength of kαCu anode with X-ray tube voltage of 40 kV, the specimens were step-scanned at the rate of 3° min−1 with 2θ angles from 5 to 70°. The XRD diffraction peaks were analyzed by XPowder software (XPowder, Version 2010).


### 2.5. Fourier Transformer Infrared Spectrometer (FTIR) Analysis

The functional group bands in kaolinite and metakaolinite were characterized by Fourier transformer infrared spectrometer with attenuation total reflection (FTIR–ATR) detector of DTGS KBr (iS50 ABX smart iTX, USA). The middle infrared reflection was set in the range of 4000–400 cm−1 wavenumber absorption bands with 32 numbers of scans and resolution of 16 cm−1.


### 2.6. Thermal Analysis

The thermal phase change of kaolinite sample was characterized by differential scanning calorimeter (DSC) (SKZ1052B, China). The scanning tests were conducted at the heating rates of 10, 15, and 20°C min−1 from 25 to 700°C under nitrogen gas atmosphere with flow rate of 120 mL·min−1. For each heating rate, 20 mg of samples were loaded into the furnace with ceramic crucible, and an empty ceramic crucible was also used for reference. The specific enthalpies of endothermic peaks were obtained from DSC peak area.The mass loss and thermal property of kaolinite was investigated using thermogravimetry analyzer (TGA) (SHIMDZU DTG-60H, Japan). The TGA experiments were determined from 25 to 1100°C with the heating rate of 15°C min−1 under nitrogen gas atmosphere. The nitrogen gas flow rate was 50 mL·min−1. The samples mass of 15 mg was loaded into the furnace with ceramics crucible. An empty ceramic crucible was considered as a reference.


### 2.7. Scanning Electron Microscope (SEM) Analysis

The surface morphologies of kaolinite and metakaolinite samples were analyzed using scanning electron microscope (SEM) (FEI Quanta 200F, USA). The SEM images of the samples were taken at 30μm.


## 2.1. Ethiopian Kaolinite Sample Preparation and Determination of Pozzolanic Activity

The raw kaolinite sample was collected from Awash Melkasa Aluminum Sulphate and Sulfuric Acid Share Company, Ethiopia. The raw kaolinite was soaked in ultrapure water to remove impurities and debris. Further, it was suspended with water to separate from the insoluble solid residues. This suspension was left for settling under the action of gravity to single out the wet kaolinite from supernatant liquid. The wet kaolinite was allowed to dry overnight at 105°C. Then, the dried sample was milled using a mortar and pestle followed by sieving the sample using a sieve analyzer (Elettronica veneta S.P.A, CE IC–205/EV, Italy) to single out the 106μm particles. The predetermined sample sizes (5 g) of samples were undertaken for calcination at different temperatures, such as 575, 600, 625, 650, 700, 800, 900, and 1000°C for 180 min using a muffle furnace (MF 106, Turkey).The pozzolanic activity of the metakaolinite was determined with some modification as outlined by Chappelle’s method [21]. 1 g of metakaolinite sample was well-mixed with 2 g of Ca(OH)2 along with in 250 mL distilled water. The suspension was boiled at 90°C for 16 h with continuous stirring (1000 rpm). Then, the suspension was allowed to cool for reaching room temperature. A 250 mL sucrose solution (0.24 g·mL−1) was added to the suspension and stirred with magnetic bar for 30 min. The resulted solution was filtered and then 25 mL of this solution was titrated against 0.1 M HCl. Phenolphthalein (0.1%w/v in ethanol 50%v/v) was used as an indicator. The titration reactions of the process are represented as in equations (1) and (2):(1)CaO+2HCl⟶CaCl2+H2O(2)CaOH2+2HCl⟶CaCl2+2H2OThe pozzolanic activity of the metakaolinite (PAMK) was determined using the following equation:(3)PAMK=2V1−V2V1M2M11000,where PAMK is the pozzolanic activity of metakaolinite (mg of Ca(OH)2 g−1); V1 is the volume (mL) of 0.1 M HCl consumed by the blank solution; V2 is the volume (mL) of 0.1 M HCl consumed by the sample solution; and M1 and M2 are molar masses of CaO and Ca(OH)2 (g mol−1), respectively.


## 2.2. Determination of Specific Surface Area

The specific surface area for the kaolinite and metakaolinite samples was determined by surface area analyzer (HORIBA SA–9600 series, model: SA–9603, USA) using the BET method through nitrogen adsorption at the temperature of 77 K liquid nitrogen. The samples were degassed at 200°C for 60 min. In each BET experiment, the quantity of kaolinite and metakaolinite in each sample was 0.75 g. The BET was operated at room temperature and atmospheric pressure (25°C and 700 mm) [22, 23]. The specific surface areas of the samples were determined using the BET equation:(4)1VPo/P−1=C−1VmC×PPo+1VmC,where P and Po are the equilibrium and saturation pressure of the adsorbates at the temperature of adsorption, respectively; V is the volume of adsorbed gas (cm3·g−1); Vm is the monolayer adsorbed gas volume (cm3·g−1); and C is BET constant.


## 2.3. Particles Size and Zeta Potential Determination

The particles size distribution (PSD) and Zeta potential distribution (ZPD) of kaolinite and metakaolinite were characterized using Malvern Zetasizer Nano-ZS instrument (ZEN 3600, UK) using the dynamic light scattering (DLS) and electrophoretic light scattering (ELS) techniques, respectively. The samples were measured three times at 25°C. The dispersion medium was ultrapure water (refractive index = 1.330, viscosity = 8.872 × 10−4 Pa·s, and dielectric constant = 78.5). The samples were prepared with similar procedure as described by Tan et al. [24] and Chorom and Rengasamy [25] for PSD and ZPD, respectively. Each sample (0.50 g) was distributed and homogenized in 50 mL ultrapure water with sonication (Soniprep 150, UK) for 15 min. The homogenized dispersion of the sample was allowed to settle for 180 min. Further, the supernatant liquid from each sample was used for determination of the PSD and ZPD.


## 2.4. X-Ray Diffractometer (XRD) Analysis

The crystalline structures of kaolinite and metakaolinite were characterized by X-ray diffraction (SHIMDZU XRD–7000, Japan) at cool temperature (–45°C). Using 1.5406 nm wavelength of kαCu anode with X-ray tube voltage of 40 kV, the specimens were step-scanned at the rate of 3° min−1 with 2θ angles from 5 to 70°. The XRD diffraction peaks were analyzed by XPowder software (XPowder, Version 2010).


## 2.5. Fourier Transformer Infrared Spectrometer (FTIR) Analysis

The functional group bands in kaolinite and metakaolinite were characterized by Fourier transformer infrared spectrometer with attenuation total reflection (FTIR–ATR) detector of DTGS KBr (iS50 ABX smart iTX, USA). The middle infrared reflection was set in the range of 4000–400 cm−1 wavenumber absorption bands with 32 numbers of scans and resolution of 16 cm−1.


## 2.6. Thermal Analysis

The thermal phase change of kaolinite sample was characterized by differential scanning calorimeter (DSC) (SKZ1052B, China). The scanning tests were conducted at the heating rates of 10, 15, and 20°C min−1 from 25 to 700°C under nitrogen gas atmosphere with flow rate of 120 mL·min−1. For each heating rate, 20 mg of samples were loaded into the furnace with ceramic crucible, and an empty ceramic crucible was also used for reference. The specific enthalpies of endothermic peaks were obtained from DSC peak area.The mass loss and thermal property of kaolinite was investigated using thermogravimetry analyzer (TGA) (SHIMDZU DTG-60H, Japan). The TGA experiments were determined from 25 to 1100°C with the heating rate of 15°C min−1 under nitrogen gas atmosphere. The nitrogen gas flow rate was 50 mL·min−1. The samples mass of 15 mg was loaded into the furnace with ceramics crucible. An empty ceramic crucible was considered as a reference.


## 2.7. Scanning Electron Microscope (SEM) Analysis

The surface morphologies of kaolinite and metakaolinite samples were analyzed using scanning electron microscope (SEM) (FEI Quanta 200F, USA). The SEM images of the samples were taken at 30μm.


## 3. Results and Discussion

### 3.1. Effect of Calcination Temperature on the Pozzolanic Activity

The variation in pozzolanic activity with respect to different calcination temperatures for metakaolinite is presented in Figure1. The pozzolanic activity of raw kaolinite was observed to be 580 mg Ca(OH)2 g−1. It was apparent that the pozzolanic activity linearly increased from 1020 to 1235 mgCa(OH)2 g−1 during the calcination temperature of 600 to 700°C which revealed that the calcination temperature increased the reactivity of the kaolinite with calcium hydroxide in the presence of water. On the other hand, beyond 700°C, further increment in calcination temperature, the pozzolanic activity of metakaolinite exhibited a declined manner of profile. It was found that the pozzolanic activity significantly reduced from 1235 to 445 mgCa(OH)2 g−1 during the temperature range of 700–1000°C. This decline in pozzolanic activity might be occurred beyond 700°C due to the agglomeration of metakaolinite and formation of spinel phase. From the results, it is clear that the highest pozzolanic activity of metakaolinite can be found while the calcination is carried out at 700°C. Hence, the metakaolinite with highly reactive to Ca(OH)2 can be obtained as pozzolan that could be a potential cementitious material [6, 13, 14].Figure 1
Effect of calcination temperature on the pozzolanic activity of Ethiopian kaolinite.


### 3.2. Specific Surface Area (SSA) Analysis

The specific area has the key role in chemical reaction, which promotes the reactivity of calcined kaolinite with calcium hydroxide in the presence of water. The specific surface area of the kaolinite at various calcination temperatures is shown in Table1. The specific surface area (SSA) proportionately increased from 32.150 to 47.794 m2·g−1 as the temperature increased from 575 to 700°C. The SSA increased when the calcination temperatures were increased, which revealed that the surface activation could be increased due to dehydroxylation reaction. The specific surface area (SSA) of the uncalcined kaolinite was 30.545 m2g−1. However, as the calcination temperature increased from 900 to 1000°C, the SSA was declined below the uncalcined kaolinite SSA, which might be due to agglomeration of metakaolinite and formation of spinel at these temperatures [21].Table 1
The specific surface area (SSA) of the kaolinite samples at various calcination temperature.
Calcination temperature (°C)Vm (cm3·g−1)BET constantSSA (m2g−1)Raw kaolinite0.0016.09830.5455750.0018.86332.1506000.0019.78034.3696250.00111.43139.2306500.00113.64343.5107000.00114.34747.7948000.0018.97233.3609000.0015.79326.30810000.0015.65017.807


### 3.3. Particles Size and Zeta Potential Analysis

The particles size distribution (PSD) of kaolinite and metakaolinite is depicted in Figure2(a). The PSD of the kaolinite was from 0.30 to 1.70 μm; and the PSD of metakaolinite was also from 0.21 to 1.30 μm. The kaolinite and metakaolinite PSD mode values were 0.71 and 0.45 μm, respectively. The metakaolinite particles size was less than the kaolinite particles size, which confirmed that the kaolinite structure could be broken down; and constituent water molecules was removed during the calcination process [26].Figure 2
(a) PSD and (b) ZPD of kaolinite and metakaolinite.
(a)(b)Figure2(b) illustrates the zeta potential distribution (ZPD) of kaolinite and metakaolinite. The kaolinite and metakaolinite zeta potential (ZP) mode values were −21.81 mV and −0.58 mV (pH = 6.6), respectively. The electrophoretic mobility values were also −1.71 and −0.05 μm cm·V−1 s−1, respectively. The magnitude of ZP value of the metakaolinite was less than that of the ZP value of the kaolinite, which indicated that the flocculation tendency of metakaolinite was greater than the kaolinite flocculation affinity in water. The smaller zeta potential values could result in aggregation and flocculation of particles due to van der Waals interparticles force of attraction [27].


### 3.4. X-Ray Diffractometer (XRD) Analysis

The XRD diffraction peaks of kaolinite and metakaolinite are revealed in Figure3. The crystal pattern of kaolinite was monoclinic with space group and unit cell axes of a = 5.15 Å, b = 8.94 Å, and c = 7.40 Å and angles of α = 90°, β = 104.862°, and γ = 90°. The high intensity crystalline system Miller indices were (001), (020), 1−10, 1−11, (302), 22−1, (240), and 3−2−3 at the 2θ angles = 12.15, 20.52, 24.56, 35.67, 38.81, 45.74, 55.28, and 62.72°, respectively. The quartz peak at the highest intensity was also observed at the 2θ angle = 26.78°. The XRD diffraction peaks of kaolinite were crystalline, and metakaolinite was amorphous. The XRD diffraction peaks disappeared, and quartz peak survived in calcined kaolinite which showed that kaolinite was transformed into metakaolinite during calcination. This result was in agreement with results described in literature [28].Figure 3
(a) XRD diffraction peaks of kaolinite and metakaolinite (calcined kaolinite).


### 3.5. Fourier Transformer Infrared Spectrometer (FTIR) Analysis

Figure4 depicts the FTIR curves of kaolinite and metakaolinite. In kaolinite, the functional group bands at 3688 and 3620 cm−1 was due to Al–OH stretching. The function group bands at 1000, 750, and 523 cm–1 were silica peaks caused by Si–O stretching, Si–O–Si stretching, and bending, respectively. The functional group band at 908 cm–1 was attributed to Al–O– bending vibration. Moreover, in metakaolinite, the bands at 3688 and 3620 cm−1 disappeared which revealed that constituent water was removed and metakaolinite was formed during calcination. Besides, the silica peaks at 1000, 750, and 523 cm−1 were survived, but the transmittance intensity was reduced. The Al–O– group of the octahedral AlO6 disappeared at 908 cm–1. The FTIR result was in agreement with the results of previously published reports [21, 29].Figure 4
FTIR curves of kaolinite and metakaolinite.


### 3.6. Thermal Analysis

The kaolinite samples DSC and TGA peaks are depicted in Figures5(a)–5(c). Figure 5(a) describes the DSC peaks at the heating rates of 10, 15, and 20°C min−1. Three endothermic peaks were detected due to adsorbed water removal, impurities decomposition, and dehydroxylation reaction of kaolinite. The adsorbed water removal was detected below 100°C, and the dehydroxylation reaction was from 450 to 650°C. Figure 5(b) depicts the specific enthalpy of the dehydroxylation reaction of kaolinite. The specific heat required of dehydroxylation reaction increased as the heating rate was increased. The specific heats were 268, 271, and 274 J·g−1 at 10, 15, and 20°C min−1, respectively. The endothermic peak of the dehydroxylation reaction was shifted to the right at the heating rate of 10°C min−1, which revealed that more time was required to scan the samples. Moreover, decomposition specific heats of the impurities were 76, 78, and 79 J·g−1 at 10, 15, and 20°C min−1, respectively.Figure 5
(a) DSC peaks, (b) specific enthalpy, and (c) TGA peaks of kaolinite.
(a)(b)(c)The TGA endothermic and exothermic peaks of the kaolinite are shown in Figure5(c). The first (I), second (II), and third (III) endothermic peaks were detected in the TGA curve due to removal of adsorbed water, impurities decomposition, and dehydroxylation reaction of the kaolinite, respectively. The total mass loss was 11.16%, in the temperature till 700°C. The exothermic peak was obtained from 980 to 1050°C, which might be due to phase transformation from metakaolinite to spinel formation, and there was no mass loss on the phase transformation process. The TGA analysis values were in agreement with the previous literature [6].


### 3.7. SEM Analysis

The SEM micrographs of the kaolinite and the metakaolinite are shown in Figures6(a) and 6(b). The sample SEM micrograph resolutions were taken at 30 μm. Both the kaolinite and the metakaolinite surface morphologies were heterogeneous size and spongy-like porous shape. The metakaolinite surface morphology heterogeneous size change was not significant, which revealed that thermal treatment could not significantly modify surface morphology of kaolinite [13].Figure 6
SEM micrographs of (a) kaolinite and (b) metakaolinite.
(a)(b)


### 3.8. The Comparison of This Study with the Literature

Table2 depicts the comparison of this study (Ethiopian kaolinite) pozzolanic activity and specific surface area with the literature. In the investigation of the maximum pozzolanic activity of various origin kaolinites, the appropriate calcination temperatures have been reported from 650 to 850°C [1, 6, 13, 14, 21, 30–32]. The maximum pozzolanic activity of Ethiopian kaolinite in the present study was obtained at 700°C, which was within the range of the calcination temperatures in the literature. The maximum pozzolanic activity of Ethiopian kaolinite in the present study was the largest over the pozzolanic activities of other origins of kaolinite, except the South African kaolinite. Moreover, the specific surface area of the kaolinite in the present study was the largest over the other origins kaolinite in the mentioned literature. The results of the present study confirmed that the calcined Ethiopian kaolinite has favorable pozzolanic activity which might be used to substitute a part of the CO2 intensive clinker in cement production.Table 2
The comparison of Ethiopian kaolinite pozzolanic activity and specific surface area with the literature.
Origins of kaoliniteCalcination temperature (°C)Suitable temperature (°C)Test methodSSA (m2·g−1)PAMK (mg of Ca(OH)2 g−1)Strength (MPa)ReferenceArgentina700–800700Frattini——52[6]Greece550–950650Chapelle—800—[13]Serbia550–700650Chapelle9.18650—[14]South Africa475–1200650Chapelle—2100—[21]Switzerland600–800600Frattini24.7—60[30]Turkey550–850750Frattini——66.6[31]France—650Chapelle171121—[32]Portugal750–940850Chapelle10.91180—[1]Ethiopia575–1000700Chapelle47.81235—This study


## 3.1. Effect of Calcination Temperature on the Pozzolanic Activity

The variation in pozzolanic activity with respect to different calcination temperatures for metakaolinite is presented in Figure1. The pozzolanic activity of raw kaolinite was observed to be 580 mg Ca(OH)2 g−1. It was apparent that the pozzolanic activity linearly increased from 1020 to 1235 mgCa(OH)2 g−1 during the calcination temperature of 600 to 700°C which revealed that the calcination temperature increased the reactivity of the kaolinite with calcium hydroxide in the presence of water. On the other hand, beyond 700°C, further increment in calcination temperature, the pozzolanic activity of metakaolinite exhibited a declined manner of profile. It was found that the pozzolanic activity significantly reduced from 1235 to 445 mgCa(OH)2 g−1 during the temperature range of 700–1000°C. This decline in pozzolanic activity might be occurred beyond 700°C due to the agglomeration of metakaolinite and formation of spinel phase. From the results, it is clear that the highest pozzolanic activity of metakaolinite can be found while the calcination is carried out at 700°C. Hence, the metakaolinite with highly reactive to Ca(OH)2 can be obtained as pozzolan that could be a potential cementitious material [6, 13, 14].Figure 1
Effect of calcination temperature on the pozzolanic activity of Ethiopian kaolinite.


## 3.2. Specific Surface Area (SSA) Analysis

The specific area has the key role in chemical reaction, which promotes the reactivity of calcined kaolinite with calcium hydroxide in the presence of water. The specific surface area of the kaolinite at various calcination temperatures is shown in Table1. The specific surface area (SSA) proportionately increased from 32.150 to 47.794 m2·g−1 as the temperature increased from 575 to 700°C. The SSA increased when the calcination temperatures were increased, which revealed that the surface activation could be increased due to dehydroxylation reaction. The specific surface area (SSA) of the uncalcined kaolinite was 30.545 m2g−1. However, as the calcination temperature increased from 900 to 1000°C, the SSA was declined below the uncalcined kaolinite SSA, which might be due to agglomeration of metakaolinite and formation of spinel at these temperatures [21].Table 1
The specific surface area (SSA) of the kaolinite samples at various calcination temperature.
Calcination temperature (°C)Vm (cm3·g−1)BET constantSSA (m2g−1)Raw kaolinite0.0016.09830.5455750.0018.86332.1506000.0019.78034.3696250.00111.43139.2306500.00113.64343.5107000.00114.34747.7948000.0018.97233.3609000.0015.79326.30810000.0015.65017.807


## 3.3. Particles Size and Zeta Potential Analysis

The particles size distribution (PSD) of kaolinite and metakaolinite is depicted in Figure2(a). The PSD of the kaolinite was from 0.30 to 1.70 μm; and the PSD of metakaolinite was also from 0.21 to 1.30 μm. The kaolinite and metakaolinite PSD mode values were 0.71 and 0.45 μm, respectively. The metakaolinite particles size was less than the kaolinite particles size, which confirmed that the kaolinite structure could be broken down; and constituent water molecules was removed during the calcination process [26].Figure 2
(a) PSD and (b) ZPD of kaolinite and metakaolinite.
(a)(b)Figure2(b) illustrates the zeta potential distribution (ZPD) of kaolinite and metakaolinite. The kaolinite and metakaolinite zeta potential (ZP) mode values were −21.81 mV and −0.58 mV (pH = 6.6), respectively. The electrophoretic mobility values were also −1.71 and −0.05 μm cm·V−1 s−1, respectively. The magnitude of ZP value of the metakaolinite was less than that of the ZP value of the kaolinite, which indicated that the flocculation tendency of metakaolinite was greater than the kaolinite flocculation affinity in water. The smaller zeta potential values could result in aggregation and flocculation of particles due to van der Waals interparticles force of attraction [27].


## 3.4. X-Ray Diffractometer (XRD) Analysis

The XRD diffraction peaks of kaolinite and metakaolinite are revealed in Figure3. The crystal pattern of kaolinite was monoclinic with space group and unit cell axes of a = 5.15 Å, b = 8.94 Å, and c = 7.40 Å and angles of α = 90°, β = 104.862°, and γ = 90°. The high intensity crystalline system Miller indices were (001), (020), 1−10, 1−11, (302), 22−1, (240), and 3−2−3 at the 2θ angles = 12.15, 20.52, 24.56, 35.67, 38.81, 45.74, 55.28, and 62.72°, respectively. The quartz peak at the highest intensity was also observed at the 2θ angle = 26.78°. The XRD diffraction peaks of kaolinite were crystalline, and metakaolinite was amorphous. The XRD diffraction peaks disappeared, and quartz peak survived in calcined kaolinite which showed that kaolinite was transformed into metakaolinite during calcination. This result was in agreement with results described in literature [28].Figure 3
(a) XRD diffraction peaks of kaolinite and metakaolinite (calcined kaolinite).


## 3.5. Fourier Transformer Infrared Spectrometer (FTIR) Analysis

Figure4 depicts the FTIR curves of kaolinite and metakaolinite. In kaolinite, the functional group bands at 3688 and 3620 cm−1 was due to Al–OH stretching. The function group bands at 1000, 750, and 523 cm–1 were silica peaks caused by Si–O stretching, Si–O–Si stretching, and bending, respectively. The functional group band at 908 cm–1 was attributed to Al–O– bending vibration. Moreover, in metakaolinite, the bands at 3688 and 3620 cm−1 disappeared which revealed that constituent water was removed and metakaolinite was formed during calcination. Besides, the silica peaks at 1000, 750, and 523 cm−1 were survived, but the transmittance intensity was reduced. The Al–O– group of the octahedral AlO6 disappeared at 908 cm–1. The FTIR result was in agreement with the results of previously published reports [21, 29].Figure 4
FTIR curves of kaolinite and metakaolinite.


## 3.6. Thermal Analysis

The kaolinite samples DSC and TGA peaks are depicted in Figures5(a)–5(c). Figure 5(a) describes the DSC peaks at the heating rates of 10, 15, and 20°C min−1. Three endothermic peaks were detected due to adsorbed water removal, impurities decomposition, and dehydroxylation reaction of kaolinite. The adsorbed water removal was detected below 100°C, and the dehydroxylation reaction was from 450 to 650°C. Figure 5(b) depicts the specific enthalpy of the dehydroxylation reaction of kaolinite. The specific heat required of dehydroxylation reaction increased as the heating rate was increased. The specific heats were 268, 271, and 274 J·g−1 at 10, 15, and 20°C min−1, respectively. The endothermic peak of the dehydroxylation reaction was shifted to the right at the heating rate of 10°C min−1, which revealed that more time was required to scan the samples. Moreover, decomposition specific heats of the impurities were 76, 78, and 79 J·g−1 at 10, 15, and 20°C min−1, respectively.Figure 5
(a) DSC peaks, (b) specific enthalpy, and (c) TGA peaks of kaolinite.
(a)(b)(c)The TGA endothermic and exothermic peaks of the kaolinite are shown in Figure5(c). The first (I), second (II), and third (III) endothermic peaks were detected in the TGA curve due to removal of adsorbed water, impurities decomposition, and dehydroxylation reaction of the kaolinite, respectively. The total mass loss was 11.16%, in the temperature till 700°C. The exothermic peak was obtained from 980 to 1050°C, which might be due to phase transformation from metakaolinite to spinel formation, and there was no mass loss on the phase transformation process. The TGA analysis values were in agreement with the previous literature [6].


## 3.7. SEM Analysis

The SEM micrographs of the kaolinite and the metakaolinite are shown in Figures6(a) and 6(b). The sample SEM micrograph resolutions were taken at 30 μm. Both the kaolinite and the metakaolinite surface morphologies were heterogeneous size and spongy-like porous shape. The metakaolinite surface morphology heterogeneous size change was not significant, which revealed that thermal treatment could not significantly modify surface morphology of kaolinite [13].Figure 6
SEM micrographs of (a) kaolinite and (b) metakaolinite.
(a)(b)


## 3.8. The Comparison of This Study with the Literature

Table2 depicts the comparison of this study (Ethiopian kaolinite) pozzolanic activity and specific surface area with the literature. In the investigation of the maximum pozzolanic activity of various origin kaolinites, the appropriate calcination temperatures have been reported from 650 to 850°C [1, 6, 13, 14, 21, 30–32]. The maximum pozzolanic activity of Ethiopian kaolinite in the present study was obtained at 700°C, which was within the range of the calcination temperatures in the literature. The maximum pozzolanic activity of Ethiopian kaolinite in the present study was the largest over the pozzolanic activities of other origins of kaolinite, except the South African kaolinite. Moreover, the specific surface area of the kaolinite in the present study was the largest over the other origins kaolinite in the mentioned literature. The results of the present study confirmed that the calcined Ethiopian kaolinite has favorable pozzolanic activity which might be used to substitute a part of the CO2 intensive clinker in cement production.Table 2
The comparison of Ethiopian kaolinite pozzolanic activity and specific surface area with the literature.
Origins of kaoliniteCalcination temperature (°C)Suitable temperature (°C)Test methodSSA (m2·g−1)PAMK (mg of Ca(OH)2 g−1)Strength (MPa)ReferenceArgentina700–800700Frattini——52[6]Greece550–950650Chapelle—800—[13]Serbia550–700650Chapelle9.18650—[14]South Africa475–1200650Chapelle—2100—[21]Switzerland600–800600Frattini24.7—60[30]Turkey550–850750Frattini——66.6[31]France—650Chapelle171121—[32]Portugal750–940850Chapelle10.91180—[1]Ethiopia575–1000700Chapelle47.81235—This study


## 4. Conclusion

This study aimed to produce pozzolan as supplementary materials for cement production from Ethiopian kaolinite. Different characteristic investigations were carried out on metakaolinite obtained at different calcination temperatures. From the results, the maximum pozzolanic activity was found to be 1235 mg of Ca(OH)2 g−1 when the kaolinite was calcined at 700°C. The outcomes revealed that the appropriate calcination temperature of 700°C for anticipated properties of required product can save substantial amount energy. The specific surface area of metakaolinite was determined as 47.79 m2·g−1 at 700°C. This result showed that the surface activation could be increased due to dehydroxylation reaction at this temperature.The XRD and FTIR results indicated that the resulted metakaolinite crystalline was monoclinic, and the hydroxide functional group stretch band was detected at greater than 3600 cm–1. From the TGA result, three endothermic peaks and one exothermic peak of the kaolinite were due to adsorbed water removal, impurities decomposition, metakaolinite formation, and phase transformation of metakaolinite into spinel. The results clear that the Ethiopian kaolinite can be a promising raw material for pozzolan production at appropriate processing condition that can be a substitute part of the CO2 exhaustive clinker in cement production process.



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*Source: 2481066-2022-05-11.xml*