1 Introduction

Kaolinite-containing clay as a gift from nature has been applied for various uses due to its physicochemical characteristics. Kaolin clays contain kaolinite, one of the clay minerals, that has been widely used for various purposes due to its high content of silica and alumina. From the past, its uses include industrial applications in paints, ceramics, electrical insulators, cosmetics and rubber production among others [1,2,3]. Lately, the application of kaolinite (or in form of metakaolinite) for catalyst production, carbon sequestration, supplementary cementitious and geopolymer starting materials are receiving global attention. Kaolin clays both in the raw and thermally treated forms have been used to synthesize aluminosilicate materials for catalytic applications [4, 5]. It was also applied to produce kaolin-based cement plug as an alternative to OPC plugs for upstream carbon sequestration [6]. Its current use as supplementary cementitious materials is on the increase [7,8,9,10,11,12,13]. This is due to global demand for cement by construction and building industry. Supplementary cementitious materials are used to replace part of OPC so as to reduce energy requirement and enormous emission of CO2 during cement manufacturing. The CO2 is one of the leading causes of climate change, which results in global warming. To reduce this emission, local kaolin clays due to their availability and the reactive nature of their thermally treated forms, are being considered as supplementary cementitious materials to replace parts of cement. Thermal treatment of kaolin clays gives rise to formation of amorphous material which can produce reactive silicate and aluminate species. These species are capable of reacting with calcium hydroxide solution that forms during cement hydration to generate stable insoluble compounds (calcium-silicate-hydrate and calcium-aluminate-hydrate) which possesses cementitious properties [8, 12, 14, 15]. Apart from CO2 emission reduction, other advantages of substituting some portion of cement with metakaolin have been documented to include lower heat of hydration, higher density, improved strength and resistance to degradation mechanisms [11].

Pure metakaolin or thermally treated local kaolin clay is one of the main materials used for production of geopolymers. Geopolymer is an inorganic aluminosilicate material that similarly functions as a binder used for various purposes. It is also undergoing consideration as an alternative to ordinary Portland cement. Authors have reported the use of pure metakaolin and locally available kaolin clays as suitable materials for producing geopolymers [16,17,18,19,20]. Kaolin clays are useful as geopolymer materials because they contain high silica and alumina species which react with alkaline silicate solution to form inorganic binders called geopolymer.

The alumina and silica in kaolin clays exist in crystalline unreactive states. This creates necessity for thermal treatment which transforms the kaolinite into amorphous reactive metakaolinite. This expands the scope of usefulness of kaolin clays. Thermal treatment of kaolinite at temperatures within 400–750 °C has been documented to result in its dehydroxylation to metakaolinite [21, 22]. Sometimes kaolinite is accompanied by other clay and associated minerals depending on its geological origin. These minerals give each kaolin clay its unique properties which also affect its reactivity [15] and functionality. The properties that affect functionality of kaolin clays include physical characteristics, chemical composition, mineralogical phases, structural functional groups and thermal behavior [23]. X-ray fluorescence (XRF) spectroscopy, X-Ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are usually used for the determination of the chemical composition, mineralogical phases, structural functional groups and structural changes upon thermal reaction.

Investigations are ongoing globally on the suitability of using locally available kaolinite-containing clays for various purposes [1, 3, 4, 7, 9, 16, 18]. Nigeria is blessed with deposits of kaolin clays, although with varied purity. However, the potentials of these kaolin clays have not been widely studied due to lack of scientific knowledge about their properties. In the past, kaolin clay has been used primarily in Nigeria for production of pottery, molding bricks and sculpture. Recently, researchers have been experimenting with Nigerian kaolin as ceramic materials, cement replacement [24, 25] and also as geopolymer materials [16]. Kaolin clays deposits are widely spread in Nigeria and if their properties are known, they can serve as good sources of ceramics, supplementary cementitious and geopolymer materials, and for other uses.

The focus of this study therefore, was to characterize both raw and thermally treated five Nigerian kaolinite-containing clays from different deposits in South West Nigeria using various characterization techniques. These clays have been underutilized due to lack of knowledge about their properties which are scarce in literature. The scientific understanding of the properties of these clays will enhance their applications. In order to investigate the chemical composition, thermal reactions, mineralogical phases, structural functional groups, the clays in their raw and thermally treated states were subjected to X-ray fluorescence (XRF) spectroscopy, thermogravimetric analysis coupled with differential thermal analysis (TGA/DTA), X-ray diffraction (XRD) and Fourier Transform infrared (FTIR) spectroscopy.

2 Materials and methods

2.1 Materials

Five natural kaolin clays were obtained from their deposits in two states in South-western Nigeria. The clays named Ijero-Ekiti (IJ), Ikere-Ekiti (IK) and Isan-Ekiti (IS) were collected from deposits in Ijero, Ikere and Oye Local Governments of Ekiti state, while Abusoro (AB) and Odigbo (OD) were collected from deposits in Okitipupa and Odigbo Local Governments in Ondo state. The clays were physically differentiated by their colors (Fig. 1). The clays were air-dried for 3 days, crushed and sieved to 212 µm with a standard sieve.

Fig. 1
figure 1

Colors of the raw clays

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2.2 Sample characterisation

2.2.1 Simultaneous thermal analysis

Simultaneous thermal behaviors of the raw clays which included thermogravimetric analysis (mass loss changes) and differential thermal analysis (TGA/DTA) were measured with NETZSCH STA 449 F3 Jupiter thermal analyzer. The temperature range of the analyzer was set at 40–900 °C with a heating rate of 10 °C per minute under nitrogen atmosphere.

2.3 Production of thermally treated clays

The clays were thermally treated in a programmable furnace for 6 h according to Ogundiran and Kumar [16] at their various maximum dehydroxylation temperatures. The IJ and OD clays were heated at 650 °C, while AB, IK and IS clays were heated at 700 °C as shown by their respective DTA curves. The raw clays were designated as RIJ, RIK, RIS, ROD and RAB while the thermally treated clays were labelled as TIJ, TIK, TIS, TOD and TAB. These samples were analyzed with the selected characterization techniques as shown below.

2.4 Surface area (BET method) and porosity analysis

The surface area and pore size distribution of the raw and thermally treated clays were measured based on physical adsorption–desorption of N2 gas on the surface of the clays using Surface Area and Porosity Analyzer (Micromeritics ASAP 2020-V4.03). The raw and thermally treated clays were degassed by oven drying at 105 °C for 6 h to remove moisture. Specific surface area was determined using Brunauer-Emmet-Teller (BET) method and volume of pores in the samples by Barrett-Joyner-Halenda (BJH) method.

2.5 X-ray fluorescence analysis of the clays

Oxide composition of the thermally treated clays was determined using Energy Dispersive X-Ray Fluorescence Spectrometer (EDX3600B) with a filament current of 350µA and an accelerating voltage of 40 kV.

2.6 X-ray diffraction analysis of the clays

The XRD patterns of mineral phases in the raw and mineral phase transformations in the thermally treated clays were obtained from D2 Phaser Bruker XRD difractometer using X-ray powder diffraction technique. The sample conditions were 2θ range from 5° to 80° with a scan speed of 0.02°/0.5 s. The CuKα radiation was generated with an accelerating voltage of 30 kV and filament current of 10 mA and the detector used was Lynxeye.

2.7 Fourier transform infra-red analysis of the clays

Structural functional groups of the raw and thermally treated clays were identified by FTIR spectrometer (FT/IR-4700typeA) set to scan frequencies across a range of 4000 to 400 cm−1 with a resolution of 4 cm−1. Sample preparation was done by KBr pellet technique.

3 Results and discussion

3.1 Thermogravimetric and differential thermal analyses (TGA/DTA) of the clays

The results of thermogravimetric and differential thermal analysis of the clays (Fig. 2) showed that the weight loss and the temperature at which thermal reaction occurred differed for the different clays. The DTA curves for the clays were characterized with three mass losses and three endothermic peaks. The initial portion of TGA and DTA (temperatures of 100–200 °C) curves showed an insignificant weight loss of 0.04 (IK and IS), 0.06 (IJ), 0.12 (OD), 0.26 (AB) % during the first endothermic reaction which signified loss of adsorbed and interlayer water (nH2O). The higher water loss demonstrated by OD and AB were possibly due to the presence of hydrated 2:1 clay mineral which contains interlayer water. Clays are differentiated by their chemical and mineralogical compositions as well as their structure. Clay mineral structure consists of silicon tetrahedrons (four-coordinated Si) and alumina octahedrons (six-coordinated Al) primarily. There are different clay group minerals depending on the ratio of silica to alumina (SiO2/Al2O3) and how their sheets are layered. The 1:1 clay minerals consist of tetrahedral-octahedral sheet where silica and alumina sheets form a layer by shared oxygen atoms and the layers are held together by hydrogen bonding; kaolinite is an example in this group. On the other hand, the sheet may be layered in a 2:1 to form tetrahedral-octahedral-tetrahedral where silica, alumina and silica sheets form a layer by shared oxygen atoms with interlayer space where adsorbed water molecules and or ions are found. Montmorillonite, illite and chlorite are examples of clay minerals in this group [26, 27].

Fig. 2
figure 2

TGA/DTA curves of raw clays

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The oxide composition and XRD patterns of OD and AB confirmed that they contained 2:1 clay group {illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10(OH)2,(H2O), and montmorillonite (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·n(H2O)}which are hydrated. A more rapid and major weight loss was observed at 390–650, 430–650, 440–700, 450–700 and 440–700 °C for IJ, OD, IS, IK and AB, respectively. This represents the beginning and completion of dehydroxylation of kaolinite in the clays. However, it has been documented that apart from kaolinite, other hydroxylated clay minerals also show endothermic reactions within these temperature ranges due to dehydroxylation of their octahedral sheet. Some authors reported overlap of dehydroxylation of other aluminosilicate clay minerals like chlorite, illite and montmorillonite within these temperatures [12, 28,29,30,31]. The mass losses at these temperatures were 2.9%; 7.9%, 8.1%; 11.9%; 12.2%, for IJ, AB, OD, IS and IK clays, respectively. The high mass losses for the clays except IJ at the dehydroxylation temperature are indications that the clays contained highly reactive kaolinite. The extent of mass loss showed the degree of dehydroxylation for each clay and this was higher for IK and IS. This is also consistent with their XRD pattern, where IK and IS indicated the formation of large proportion of amorphous material (Fig. 7).

The endothermic reaction of IJ at 390–650 °C may be attributed to dehydroxylation of small amounts of kaolinite and other clay minerals contained in the clay overlapping with transition of α-quartz (SiO2) to β-quartz (SiO2). As already documented [32, 33], the endothermic reaction of quartz at around 573 °C corresponds to structural transition of trigonal (alpha) to hexagonal (beta). Based on this fact, if IJ contained only quartz, mass change was not expected to occur at these temperatures for IJ. However, a small mass change of about 2.9% occurred. This might be an indication of dehydroxylation of clay minerals. The presence of trace amounts of kaolinite and illite was later supported by XRD pattern and FTIR of IJ. The XRD pattern of IJ also revealed the presence of feldspar in the form of albite (NaAlSi3O8). Nevertheless, albite also remained stable to heat at this temperature [33]. Additional mass losses (0.03, 1.11, 1.22 and 2.12%) were observed at the third endothermic peaks between 700 and 900 °C for IJ, OD, AB, IS and IK. This third broad and low intensity endothermic peak has been associated with the presence of 2:1 clay minerals [12]. This temperature interval was documented as the temperature at which the final structural breakdown of 2:1 clay minerals (montmorillonite and illite) occurs [12, 18, 31, 34]. This is in agreement with the results of XRD analysis where Illite was detected in IK, IJ, IS and OD, while montmorillonite and illite were detected in AB (Fig. 6) at their maximum dehydroxylation temperatures.

3.2 Surface area and porosity analysis

The BET surface area of the raw clays were in the range of 2.30–16.2m2/g, while that of thermally treated clays increased from 2.78 to 17.5m2/g in Table 1. Raw Ijero clay (RIJ) and its thermally treated form had the lowest value compared to raw Abusoro clay and its thermally treated form. These results indicated that the BET surface areas increased with thermal treatment of the raw clays [35]. The t-plot micropore areas decreased from the raw clays to the thermally treated clays except for ROD and TOD in Table 1. Furthermore, the values of the micropore areas were smaller than the external surface areas except for RIJ and RIS, indicating that the external surface area was the major active surface for the clays [36].

Table 1 BET surface area and pore sizes of raw and thermally treated clays
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According to International Union of Pure and Applied Chemistry (IUPAC), the adsorption–desorption isotherms of the raw and thermally treated clays correspond to Type IV isotherm in Fig. 3, these isotherms are illustrated by a limited uptake over a series of high relative pressure and a hysteresis loop [36,37,38]. The first part of the adsorption isotherms in the range of low relative pressures (0.02–0.7) was due to monolayer-multilayer adsorption, while the second section was associated to adsorption by capillary condensation in the mesopores [36, 37]. The hysteresis of the adsorption isotherms of the raw and thermally treated clays in Fig. 3 corresponds to H3 hysteresis pattern, which suggests that their pores are narrow and slit-shaped having average pore diameter of 118–213 Å for the raw clays and 125–187 Å for the thermally treated clays as shown in Table 1 [36].

Fig. 3
figure 3

Isotherm linear plot of raw and thermally treated clays

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The pore size distribution and the cumulative pore volume of the clays were analyzed using Barrett-Joyner-Halenda (BJH) model on the adsorption isotherms and presented in Figs. 4 and 5. The volume of pores available for interaction with the N2 gas increased from the raw to the thermally treated clays. The presence of mesopores was observed in the raw and thermally treated clays. However the volume of pores in the region of 590–1770 Å (59–177 nm) suggests that macropores are the major pores present in the raw clays. Consequently, in the thermally treated clays, macropores in the range of 630–2018 Å (63–202 nm) as shown in Fig. 4 indicates that the distribution of macropores is higher in the thermally treated clays than in the raw clays. Therefore it can be concluded that high BET surface area, macropores present in the thermally treated clays are desirable properties for its suitability as precursors for supplementary cementitious and geopolymer materials.

Fig. 4
figure 4

Barrett–Joyner–Halenda (BJH) pore size distribution of raw and thermally treated Ijero clays (V- pore volume; D-pore diameter)

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Fig. 5
figure 5

BJH cumulative pore volume of raw and thermally treated Ijero clay

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3.3 Color and x-ray fluorescence (XRF) analysis of the clays

The colors of the raw clays (Fig. 1) revealed that Ijero-Ekiti clay is white impacted by albite. Ikere-Ekiti is golden yellow but turned brownish red when thermally treated. This brownish red color is due to the presence of high amount of Fe2O3 impurity. Isan-Ekiti is creamy white; Abusoro is light pink perhaps due to the presence of montmorillonite as shown by XRD (Fig. 6). Odigbo is greyish white, probably due to the presence of illite in the clay (Fig. 6). The oxide composition (XRF) results of the thermally treated clays (Table 2) showed that the clays consist mainly of SiO2 (46.4–62.1%) and Al2O3 (19.7–41.0%). Alumina and silica are the building blocks for geopolymer formation. Consequently, these clays may be suitable as geopolymer starting materials. The oxide composition indicated that none of the clays is pure kaolinite-containing clays but contained traces of other minerals. The clays contained low content of CaO which is an indication of absence of significant amount of calcite [39]. Ikere-Ekiti and IS have the highest composition of Al2O3 which suggested the existence of high amount of kaolinite in the clays [30]. Odigbo and Abusoro have higher amount of silica when compared with IJ, IK and IS, signifying the presence of other clay minerals in addition to kaolinite. Usually, 2:1 clays (e.g. smectite and mica groups) have more silica content and lower alumina than 1:1 clay kaolinite group as reported in previous studies [2, 3, 7, 9, 18]. Odigbo clay on comparison with others has considerable amount of K2O, signifying the presence of illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]. The presence of this mineral was confirmed in the XRD diffractograms of the clays, particularly in OD (Fig. 6). Furthermore, a significant amount of Fe2O3 was observed in IK and IS, indicating presence of hematite and was also found in their XRD patterns. The presence of this oxide resulted in the characteristic color displayed by IK and IS (Table 2). These clays may require purification to remove or reduce Fe2O3 if they are intended for industrial uses. Ijero-Ekiti has the least percentage composition of Fe2O3 (0.10%) compared to Ikere clay (8.51%). This is also revealed in the whitish colour of the clay. Ikere-Ekiti, AB and OD contained anatase; the presence of it was also confirmed by their XRD patterns in thermal state (Fig. 7).

Fig. 6
figure 6

X-ray diffraction patterns of raw kaolin clays (RIJ = Raw Ijero clay, RIK = Raw Ikere, RIS = Raw Isan clay, RAB = raw Abusoro clay and ROD = Raw Odigbo clay

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Table 2 X-ray fluorescence analysis of thermally treated clays
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Fig. 7
figure 7

X-ray diffraction patterns of thermally treated clays

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Additionally, the clays except Ijero-Ekiti can be classified as Class N natural pozzolans category of cementitious materials. ASTM C618 (2014) as cited by Lorentz et al. [23] specifies that for natural pozzolans, the sum of SiO2, Al2O3 and Fe2O3 must be ≥ 70% by mass and that SO3 content must not be > 4%. The sum of these oxides in the clays except IJ (66.2%) ranged from 91.9% (IS) to 97.4% (AB), thereby eliminating IJ as pozzolanic material. The SO3 was only detected in IJ, OD and AB and were less than the stipulated bench mark. Therefore, these clays can be applied as supplementary cementitious materials.

3.4 X-ray diffraction (XRD) analysis

The clay and associated minerals in the raw clays and their transformation in the thermally treated clays were identified using XRD (Figs. 6 and 7). It can be observed that the thermal treatment at the maximum dehydroxylation temperatures caused kaolinite peaks to disappear with appearance of humps at 2θ = 20°–30° while others were retained. In raw IJ, feldspar, in form of albite (NaAlSi3O8) was predominant with traces of quartz, illite and kaolinite. This Na-feldsper was confirmed by the highest composition of Na2O in IJ (Table 2). The absence of other forms of feldspar [Anorthite (CaAl2Si2O8), orthoclase (KAlSi3O8)] was confirmed by low amount of CaO and K2O in IJ (Table 2). Kaolinite peak disappeared in the thermally treated clay, but albite and quartz peaks were retained, indicating that quartz and albite were more resistant to heat treatment than kaolinite. Albite is a sodium feldspar, an aluminosilicate mineral which melts over a wide range of temperatures (1100–1134 °C) as reported in literature [40]. Its color is commonly white, and its uses commercially include ceramic production when mixed with kaolin and quartz. When it is heated at high temperatures, it fuses and acts as a binder, binding quartz and kaolin. This presents IJ as a potential material for ceramic production since it naturally contains these materials. The XRD peaks of raw IK and IS demonstrated crystalline kaolinite as the dominant clay mineral with traces of illite, quartz and hematite. The presence of hematite in the clay was also confirmed by high amount of Fe2O3 (Table 2). It is worthy to note that, unlike IJ, AB and OD, large amount of amorphous materials (metakaolinite) revealed by diffused big humps between 20 and 30° 2θ were observed in the XRD patterns of thermally treated IK and IS (Fig. 7). The amorphous materials suggest the presence of higher proportion of metakaolinite from the thermal transformation of kaolinite in the clays. The high kaolinitic properties of IK and IS are in agreement with the results of TGA/DTA of the clays where the two clays showed higher mass losses at the dehydroxylation temperatures (Fig. 2). The mineralogical composition is also in good agreement with their high silica and alumina content (Table 2). The XRD pattern of raw AB showed the presence of kaolinite, montmorinollite and illite as the clay minerals with traces of quartz and hematite. However, in the thermally treated state, the peaks of kaolinite disappeared and were replaced with amorphous materials due to the transformation of kaolinite to metakaolinite, while those of illite, montmorillonite and quartz are still observable.

The XRD patterns of OD in the raw state indicated the presence of kaolinite, and traces of illite, quartz and hematite. In the thermally treated state, it showed the presence of illite and an amorphous material, which is demonstrated by the hump between 20° and 30° 2θ. It also contained traces of quartz and hematite. The observed amorphous material can be associated with the presence of metakaolinite from the thermal transformation of kaolinite in the clay. The appearance of illite peaks in the XRD patterns of the clays and montmorillonite in AB at the dehydroxylation temperatures suggests that the structures of these clay minerals were still retained at the temperature. Zhou et al. [11] and Wang et al. [31] reported that illite was not fully dehydroxylated until around 850 °C. To improve the amorphous status of these clays, future investigations should include thermal treatment at higher temperatures.

The presence of amorphous materials in the thermally treated IK, IS, AB and OD suggest that when they are used as supplementary cementitious materials, they will show pozzolanic reactivity which can be used in cement and concrete industry. Similarly, when they are used as geopolymer materials they will display alkaline reactivity. Quartz and hematite in the clays showed little or no change with thermal treatment.

3.5 Fourier transform Infrared (FTIR) spectroscopy

Infrared spectroscopy was also used to determine the functional groups of minerals that were present in the raw and thermally treated clays. The FTIR spectra of raw and thermally treated clays (Fig. 8) and their major functional groups in the 4000–3000 cm−1 and 1300–500 cm−1 spectra regions as identified in literature are discussed. For all the raw clays, the OH stretching vibrations typical of the presence of kaolinite occurred at absorption bands 3698-3694 cm−1, and of 2:1 clay minerals (montmorillonite and illite) occurred at absorption bands 3626–3620 cm−1 in all the clays [2, 16, 28, 31, 41]. These OH vibrations are due to hydroxyl groups that are attached to aluminium octahedron sheet of the clay minerals. An OH broad stretching vibration at 3437 cm−1 and 3434 cm−1 was identified in RIK and ROD and this has been attributed to OH vibration of water of hydration on 2:1 clay minerals, particularly illite [31]. Outside this region, bands around 1640 and 1630 cm−1 were identified in the spectra of raw IJ, IK, IS, AB and OD clays. These bands were linked with OH bending vibrations of adsorbed water [31, 41]. The bands between 1339 and 1249 cm−1 have been assigned to stretching vibrations of Si–O and Al–O bonds in albite (NaAlSi3O8) confirming the presence of albite in IJ [41].

Fig. 8
figure 8

FTIR spectra of raw and thermally treated clays

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In the 1300–600 cm−1 region, absorption bands associated with Si–O–Si stretching vibrations, which are typical of kaolinite, illite and montmorillonite were observed in the raw IJ, IK, IS, AB, and OD clays at wave numbers 1041, 1032,1033, 1032 and 1031 cm−1 respectively [31, 42]. Al(VI)–OH bending vibrations typical of the clay minerals were observed at bands between 914 and 911 cm−1 [16, 19, 31] in the clays. The Si–O bending vibrations in the region 600 to 400 cm−1 in the clays have been attributed to a couple of stretching-bending vibrations of the cation-oxygen octahedra lying on a similar spectral region [31].

Si–O–Al(VI) bending vibrations due to kaolinite were also observed at bands between 697 and 688 cm−1 [16]. Si–O vibrations at around 535 cm−1 and between 471 and 469 cm−1 were observed and associated with the presence of quartz [16, 19]. These observations confirm the presence of kaolinite in all the clays. The absorption bands at 792, 697, 695, 796 and 692 cm−1 were observed for RIJ, RIK, RIS, RAB and ROD, respectively and these have been associated with the Si–O stretching frequencies of quartz [11, 32]. These bands are typical of quartz in kaolinite, montmorillonite and illite.

Remarkable changes were observed in the FTIR spectra of the thermally treated clays compared to the raw clays. The broad bands of the OH groups at 4000–3000 cm−1 spectra region in the raw clays that indicated the presence of crystalline kaolinite disappeared at the dehydroxylation temperature of each clay. This was replaced by low intensity broad bands typical of 2:1 clay minerals [12]. These authors documented that the absorption band of montmorinollite decreased with increasing temperature above 600 °C and could not be detected at temperature up to 925 °C, confirming that at the dehydroxylation temperature of kaolinite, the structure of montmorillonite is less severely disturbed. The size of this band was peculiar for each of the five clays. It was observed that it was biggest for IK and least for IJ. This was also reflected in the amount of weight loss at the third endothermic reaction of these clays. Mass losses of 0.03, 0.07, 1.11, 1.22 and 2.12% were observed at the third endothermic peaks for IJ, OD, AB, IS and IK. The XRD patterns confirmed the presence of illite in all the clays together with montmorillonite in AB only.

Three prominent absorption bands between 1116 and 1031 cm−1 were replaced with broad bands in the thermally treated clays at wave number between 1092 and 1046 cm−1 [16, 19]. Al (VI)–OH bending vibrations at bands between 914 and 911 cm−1 in the raw clays disappeared and new bands which indicating that tetrahedral coordination Al(IV)–O stretching vibrations appeared at 797, 822, 819 and 796 cm−1 for TOD, TIS, TIK and TAB respectively. The Si–O–Al bending vibrations bands between 541 and 535 cm−1, were replaced by Si–O in-plane bending vibration at 463 and 469 cm−1 in all the thermally treated clays [16, 19].

The FTIR results confirmed the presence of kaolinite in the raw clays and transformation of kaolinite to metakaolinite in the thermally treated clays. The XRD patterns were consistent with FTIR spectra of both raw and thermally treated clays.

4 Conclusion

Thermal, surface area, oxide and mineralogical composition, and structural properties of raw and thermally treated five local kaolin clays from South-West Nigeria were characterized. The BET surface area values increased from the raw clays to the thermally treated clays. Combining the results of TGA/DTA, XRF, XRD and FTIR, Ijero-Ekiti clay was identified as feldspar (albite)-quartz containing clay with traces of kaolinite and illite, Ikere-Ekiti and Isan-Ekiti clays were classified as kaolinite-dominated clays with traces of hematite, illite, quartz and anatase. Abusoro clay contained kaolinite primarily with small amounts of illite, montmorillonite, hematite and quartz and Odigbo clay contained kaolinite and trace amounts of illite, hematite and quartz. There were good agreements among TGA/DTA, XRD and FTIR results which confirmed the transformation of kaolinite in the raw clays to metakaolinite in the thermally treated clays. Based on the properties displayed, it can be concluded that when clays from Ikere-Ekiti, Isan-Ekiti, Abusoro and Odigbo deposits are thermally treated, they have potentials to form useful supplementary cementitious and geopolymer materials to make binders in building and construction while clay from Ijero-Ekiti deposit can serve as ceramic material. Further investigations are required to determine the pozzolanic activity, alkaline silicate reactivity and ceramic formation potentials of the thermally treated clays.