Powder Technology 235 (2013) 633–639

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Powder Technology

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Characterization and mechanical properties of phosphate-kaolin clay

Alya Charfi, Rym Dhouib Sahnoun ⁎, Jamel BouazizLaboratory of Industrial Chemistry, National School of Engineering, University of Sfax, BP 1173, 3038 Sfax, Tunisia

⁎ Corresponding author. Tel.: +216 98 25 12 55; fax:E-mail address: rymdhouib@yahoo.fr (R. Dhouib Sah

0032-5910/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.powtec.2012.11.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 February 2012Received in revised form 10 October 2012Accepted 14 November 2012Available online 20 November 2012

Keywords:CeramicsX-ray diffractionThermal analysis (DTA, TGA)Infrared spectroscopyMechanical properties

The effects of phosphate additions on the mechanical andmorphological properties of kaolin were investigat-ed. Phosphate additions were orthophosphoric acid H3PO4, potassium dihydrogen phosphate KH2PO4,potassium monohydrogen phosphate K2HPO4 and potassium phosphate K3PO4. The phosphate additionswere fixed at 10% w/w. The green and sintered phosphate-bonded products were characterized by meansof X-ray diffraction (XRD), infrared spectroscopy (IR), thermal analysis (ATD-ATG), dilatometry, scanningelectron microscopy (SEM) and the Brazilian test. The properties of the phosphate-bonded clay bodieswere affected by new mineral phases resulting from the reaction between phosphates and kaolin, whichsubsequently enhanced the mechanical strength and the densification of sintered ceramic products.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Raw natural clay such as kaolin is usually a heterogeneous mixtureof minerals that, in majority, consist of phyllosilicates [1–4]. Duringheat treatment, these materials undergo physico-chemical transforma-tions that involve both a change in the crystal structure of the differentphases (dehydroxylation, amorphization, crystallization, polymorphictransformation etc.) and a modification of the microstructure of themixture (change of geometry, distribution and orientation of pores,grain or crystal growth…) [5–8]. These transformations are alsoaccompanied by a change in mechanical properties. This last parameteris crucial because the properties of many ceramic materials obtainedfrom raw clay are often related to their strength and morphology.Many parameters such as the composition of the raw material havean influence on the evolution of the microstructure of silicate ceramicsand thus on their properties [9–13].

The properties of a large number of phosphates used as a binder todecrease the sintering temperature have been recognized for manyyears. The results of these studies showed that the addition ofphosphates improves the tensile strength of sintered clay [14–16].The researchers explained this increase in strength by the chemicalreaction that they believe can take place between the aluminumalready existing in clay and the phosphate added to the latter therebyallowing vitrification at low temperature. Therefore, and due to theformation of crystals of T-AlPO4 at low temperature, this reaction

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may significantly reduce the sintering temperature of the clays andincrease their mechanical strength [14,16].

This work is a contribution to the improvement of the mechanicalproperties of kaolin by the addition of phosphates (orthophosphoricacid H3PO4, potassium dihydrogen phosphate KH2PO4, potassiummonohydrogen phosphate K2HPO4 and potassium phosphate K3PO4).A set of considerations motivated the choice of these additives toknow the bonding power of phosphate ions, the basic power ofpotassium ions, the high melting point of basic phosphates and theirlow toxicity.

Therefore, this paper determines the variation of mechanicalstrength and morphological properties of clay samples in relation tothe heating temperature. It also studies the effect of the nature of phos-phate additives on themechanical properties of the kaolin products. Forthis purpose, green and sintered samples were characterized by usingseveral techniques: X-ray diffraction (XRD), infrared spectroscopy(IR), scanning electron microscopy (SEM), thermal techniques(DTA/TGA) and the Brazilian test.

2. Materials and methods

2.1. Raw materials

The raw material powder of kaolin [16] was supplied by BWWMinerals; its chemical composition is listed in Table 1. In this study,the additives used were orthophosphoric acid: H3PO4 85%, and densityof 1.70 g.cm−3 (PROLABO), potassium dihydrogen phosphate KH2PO4

(Sigma Aldrich), density of 2.00 g cm−3, potassium monohydrogen

Table 1Chemical composition (percentage weight: %wt) of kaolin.

Oxide SiO2 Fe2O3 Al2O3 MgO K2O SO3 CaO LOIa

%wt 52.41 3.48 29.83 0.81 0.73 0.07 0.36 12.31

a Loss on ignition at 1000 °C.

634 A. Charfi et al. / Powder Technology 235 (2013) 633–639

phosphate K2HPO4 (Riedel de Haen), density of 2.44 g.cm−3 andpotassium phosphate K3PO4 (Sigma Aldrich), density of 2.546 g.cm−3.

2.2. Preparation of mixtures and ceramic products

A previous study [16] defined the parameters concerning the imple-mentation of specimens such as the amount of added phosphates(10 wt.%) and the maturing time (48 h). In this study, the effects ofthe addition of phosphate to kaolin are investigated in relation to thenature of phosphate and the sintering temperature. For this purpose,kaolin is treated with a fixed volume of phosphate solution (distilledwater+H3PO4 or KH2PO4 or K2HPO4 or K3PO4) [16]. Then it wasdried at 110 °C for 24 h andwas ground to pass through a 60 μmdiam-eter sieve. The obtained powder mixtures were molded in a metalmold and uniaxially pressed at 150 MPa to form cylindrical compactswith a diameter of 20 mm and a thickness of about 6 mm. Thegreen bodies were sintered at various temperatures from 800 °C to1250 °C for 1 hr. The heating and cooling rates were 10 °C min−1

and 20 °C.min−1, respectively.

2.3. Experimental methods

The silica composition of the raw materials was determined bystandard methods of wet chemical analysis [17]. The content ofthe other elements was determined by using atomic absorptionspectrometry (Perkin-Elmer 5000).

10 20 30

0

100

200

300

400

500

)

Q

A

A

A

A

A

A

AA

K

KK3

KK2

KK1

KH

K K+QKI

K

I

2 theta

Inte

nsity

(ab

itrar

y un

its)

Fig. 1. X-ray diffraction spectra (Cu Kα) of kaoli

The identification of the mineralogical composition wasperformed by X-ray diffraction (Philips X'Pert X-ray diffractometer)operating with Cu Kα radiation (λ=1.54056 Å). The crystallinephases were identified from the Powder Diffraction Files (PDF) ofthe International Center for Diffraction Data (ICDD).

The infrared spectra of the samples were recorded in the wavenumber range of 400–4000 cm−1, and the spectral resolution was4 cm−1 using a Perkin Elmer Spectrum BX LX 185255 instrument inKBr.

Differential thermal analysis (DTA) and thermogravimetry (TG)were recorded using a Setaram SETSYS Evolution_1750 apparatuswith a heating rate of 10 °C min−1 in a helium atmosphere. In thesetests, 200 mg of sample mass was employed with α-alumina asreference material.

Dilatometry was used in order to determine linear shrinkage usingthe same thermal cycle as the one intended for DTA and TGA.

Mechanical properties of the compacts were measured by Braziliantest. The optimum rupture strengths σr was offered by the followingequation [18–20]:

σr ¼ 2:P=πD:e

where D and e are the diameter and the thickness of the sample and Pis the maximum applied load. The Brazilian test is officially consideredby the International Society for RockMechanics (ISRM) as a method fordetermining the tensile strength of rock materials [19]. The Braziliantest is also standardized by the American Society for testing materials(ASTM) to obtain the tensile strength of concrete materials [20]. Theexperiments were realized by using a “LLOYD EZ50” device oncylindrical samples of approximately 6 mm in thickness and 20 mmin diameter. At least six specimens were tested for each test conditionand an average of the values was then calculated.

The porosity of ceramic bodies was determined by mercuryporosimeter (MICROMERITICS 9500).

40 50 60

A

KK+I

A

A

A

Q+KKK+QK+Q

(degrees)

K: Kaolinite

Q: Quartz

I: Illite

A: Al(H2PO4)3

n and the mixtures KH, KK1, KK2 and KK3.

050010001500200025003000350040004500

wavenumber (cm-1)

3694 36

50 3618

1112

2310

1002

1010

910

790

748

694

532 47

440

3

K

KH

KK1

KK2

KK3

Fig. 2. IR spectra of kaolin and the mixtures KH, KK1, KK2 and KK3.

-16

-14

-12

-10

-8

-6

-4

-2

0

2

0 200 400 600 800 1000 1200 1400

Temperature (°C)

KK3K

KH

KK1KK2

wei

ght l

oss

(%)

Fig. 4. TG curves of kaolin and the mixtures KH, KK1, KK2 and KK3.

635A. Charfi et al. / Powder Technology 235 (2013) 633–639

3. Results and discussion

3.1. Characterization of the crude powder

In the remainder of our study, we will use the following notationsto denote the materials: K (kaolin alone); KH (mixture kaolin—10 wt.% H3PO4); KK1 (mixture kaolin—10 wt.% KH2PO4); KK2(mixture kaolin—10 wt.% K2HPO4); and KK3 (mixture kaolin—10 wt.%K3PO4). We will point out that all the mixtures were dried at 150 °Cbefore characterization.

-10

-5

0

5

10

0 325 650 975 1300

Temperature (°C)

Hea

t Flo

w (

µV)

K

KH

KK1

KK2

KK3

Fig. 3. DTA curves of kaolin and the mixtures KH, KK1, KK2 and KK3.

The XRD patterns of the dried powders were reported in Fig. 1. Itcan be seen that kaolin contains kaolinite, Si2Al2O5(OH)4, as majorclay minerals and quartz (q) as a non-clay mineral and some illite(i) [21]. The diffractograms RX of the mixtures KH, KK1, KK2 andKK3 show the existence, in addition to kaolinite, illite and quartz, afraction of a new phase of Al(H2PO4)3 coming from the reaction ofphosphate compounds with aluminum [16,22].

Fig. 2 shows the KBr spectra of kaolin and the mixtures KH, KK1,KK2 and KK3. According to this figure, the mixing with phosphatecompounds causes the O\H, Si\O, Si\O\Al and Al\OH bands tobroaden and weaken in terms of intensity [23]. These changes aredue to the distortion of the tetrahedral and octahedral layers ofkaolinite [24,25]. So the addition of phosphate induces a decrease oforder in the network of kaolinite [16]. In addition, we note a changein the 1000–1250 cm−1range. This is owing to the existence of anew stretching asymmetric vibrations of P\O that exist at around1022 cm−1 and 1150 cm−1 [26–28].

To detect the phenomena which emerged during the thermaltreatment of kaolin (K) and the mixtures (KH, KK1, KK2 and KK3),the powders were subjected to thermal analysis (DTA and TGA).The measurements carried out under an atmosphere of helium to atemperature of 1250 °C, for kaolin and the mixtures KH and KK3,and to a temperature of 1150 °C, for the mixtures KK1 and KK2,were allowed to obtain the thermograms shown in Figs. 3 and 4.The DTA curve (Fig. 3) of raw kaolin exhibits characteristic endother-mic peaks at 150 °C, 510 °C and 998 °C [16,29]. The DTA curves ofdifferent mixtures (KH, KK1, KK2 and KK3) (Fig. 3) prove, in additionto the characteristic peaks, the presence of an endothermic peak at175 °C which is relative to the formation of mono-aluminumphosphate [30]. The endothermic broad effect in the temperaturerange of 220–350 °C may correspond to the formations ofaluminum diphosphate at 230 °C and aluminum metaphosphate at340 °C [29]. The endothermic peak, observed in the case of kaolin at510 °C, shifts to lower temperatures. This fact is probably due to thereaction of phosphate with aluminum causing the destabilizationof the kaolinite network. Also the thermograms of the mixturesillustrate the appearance of a series of small endothermic kinksbetween 600 °C and 750 °C corresponding to the processing ofaluminum metaphosphate to aluminum phosphate [30]. The intensityof the peaks of dehydroxylation and mullitization in the case of themixtures is lower compared to that in the case of kaolin. This fact isexplained by the reduction of the proportion of metakaolin in the

KK1

KK2

KKHKK3

Temperature (°C)

(L-L

0)/L

0 (%

)2

0

-2

-4

-6

-8

-10

-12

-14

-16

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

Fig. 5. Linear shrinkage versus temperature of kaolin and the mixtures KH, KK1, KK2 and KK3.

636 A. Charfi et al. / Powder Technology 235 (2013) 633–639

mixtures cause by the formation of new compounds depleting theamount of aluminum in the kaolin.

The thermograms of Fig. 4 indicate a significant decrease in weightloss of the mixtures KK1, KK2 and KK3. This leads us to believe thatthe phosphate and the potassium probably react with aluminumand/or silica to give new compounds, thereby depleting the amountof metakaolin.

Fig. 5 displays the sintering shrinkage behaviors of the kaolin andthe kaolin-phosphate mixture samples. For the kaolin, whatever thetemperature, the linear shrinkage is the lowest (Fig. 5). The sampleswith phosphate additions exhibit some differences in sinteringshrinkage behaviors which are described as follows:

(a) The first sintering shrinkage stage: the kaolin and the mixturesbegan to shrink between 500 °C and 600 °C and then ceased toshrink at 700 °C. This first shrinkage is related to thedehydroxylation of the kaolinite.

Table 2Linear shrinkage at 1100 °C for different mixtures.

Material K KH KK1 KK2 KK3

Linear shrinkage (%) 4.5 6.5 8.5 9.8 12.0

Table 3Variation of mechanical strength versus sintering temperature.

Material 800 °C 1100 °C 1250 °C

K 4.42 7.78 16.07KH 13.00 14.50 24.40KK1 6.57 23.40 10.56KK2 9.17 28.10 14.57KK3 8.72 26.71 12.00

(b) The second sintering shrinkage stage: all the samplescontaining phosphate began to shrink at lower temperaturesthan when the kaolin is heated alone. More especially, theKK1, KK2 and KK3 samples show much higher and quickershrinkage rates.

(c) The third sintering shrinkage stage: the initial temperatures ofthe third sintering shrinkage stage decrease in the case of thephosphate-kaolin mixtures. The sintering temperatures beganat about 1150 °C, 1100 °C, 1030 °C, 1030 °C and 1030 °C forthe raw kaolin, KH, KK1, KK2 and KK3, respectively. It seemsthat, unlike when it is the case of other blends, the mixtureKH exhibits a similar behavior to that of the kaolin. This canbe ascribed to the presence of a glassy liquid phase, formedfrom the reaction of potassium and silica in raw materials.Therefore, the samples containing potassium present highersintering shrinkage percentages than the one without potassi-um (Table 2).

3.2. The effect of phosphate additives on densification, mechanicalproperties and porosities

Table 3 summarizes the mechanical strength of different kaolin-phosphate compounds sintered at various temperatures (800 °C,

Table 4Physical and mechanical properties of samples sintered at 1100 °C.

Materials Mechanical strengthσr (MPa)

Densification(%)

Porosity

K 7.78 63.06 36.90KH 14.50 70.00 32.40KK1 23.40 73.06 26.90KK2 28.10 78.97 21.00KK3 26.71 74.28 25.70

10 20 30 40 50 60

MM

M

M

M

M

M

M

Q

QQ

Q

Q

M

2 theta (degrees)

Q: Quartz

M: Mullite

Fig. 6. X-ray diffraction spectra (Cu Kα) of kaolin sintered at 1100 °C for 1 h.

0 10 20 30 40 50 60

L

A

Q

MA

Q

A

Q

QL

M

L

M

L A

LA

M: Mullite; Q: Quartz

A: AlPO4; L: KAlSi2O6

MM

MKK3

KK2

KK1

2 theta (degrees)

Fig. 8. X-ray diffraction spectra (Cu Kα) of KK1, KK2 and KK3 mixtures sintered at1100 °C for 1 h.

KK3

637A. Charfi et al. / Powder Technology 235 (2013) 633–639

1100 °C and 1250 °C). It appears that the mechanical strengths ofkaolin and KH mix increase with temperature, which is not exactlythe same case of the mixtures KK1, KK2 and KK3, whose mechanicalstrengths at 1100 °C are higher than those at 1250 °C. This supportsthe idea of the formation of a glassy phase, previously explained,causing the degradation of mechanical strength. These results alsoshow that, by including these additives, it is possible to gain morethan 200 °C of sintering temperature while having the same strengthor even a greater one than that of kaolin. For example, a specimenprepared from the mixtures sintered at 1100 °C is stronger than theone from the kaolin sintered at 1250 °C. Although this last behavioris irrespective of the nature of the additives, the mechanical proprie-ties of any mixtures reach their maximum value at 1100 °C.

Table 4 summarizes the porosity, the densification rate and themechanical strength of different kaolin-phosphate pellets sinteredat 1100 °C. The values of the mechanical strength of different samplesare in agreement with those of the porosity. Also porosity decreases

10 20 30 40 50 60

Q

AA

A

A

A

Q

Q

M QQ

M

M: MulliteA: AlPO4Q: Quartz

M

MM

M

2 theta (degrees)

Fig. 7. X-ray diffraction spectra (Cu Kα) of KH mixture sintered at 1100 °C for 1 h.

when mechanical resistance as well as the densification rate increase.However, with KH2PO4, the mechanical strength and the densificationrate reach their maximum values while porosity is the lowest.

3.3. Characterization of samples after the sintering process

3.3.1. Characterization of phosphate-kaolin mixesAfter sintering, the samples have been characterized with different

techniques: X-ray diffraction, FTIR and scanning electron microscopy(SEM).

The diffractogram of the kaolin sintered at 1100 °C (Fig. 6) showsthe existence of two phases: mullite and quartz. In addition, thisdiffractogram highlights the existence of an amorphous phase.Moreover, for the spectra of the mixture KH sintered at the sametemperature, a small amount of berlinite AlPO4 is observed (Fig. 7).

4006008001000120014001600

wavenumber (cm-1)

426

938

962

448

46249

4

828

984

1026

1090

1180

K

KH

KK1

KK2

Fig. 9. IR spectra of kaolin and the mixtures KH, KK1, KK2 and KK3 sintered at 1100 °Cfor 1 h.

638 A. Charfi et al. / Powder Technology 235 (2013) 633–639

The appearance of this new phase was predicted in the thermal studyperformed above. X-ray spectra of Fig. 8 show that the mixturesKK1, KK2 and KK3 include in their mineralogical compositions thesame phases as that in the mixture KH. In addition to these phases,we note the presence of new peaks of leucite (KAlSi2O6) on thesediffractograms.

The IR spectra of kaolin and mixtures, sintered at 1100 °C for 1 h,are shown in Fig. 9. At first sight, we note that the spectra of the mix-tures do not present any differentiations compared to the one of kaolin.However, the presence of the broadening of a strong band is reported,which indicates the presence of a glassy phase. Therefore, it is conve-nient to note that the bands at 1010 cm−1 and 1112 cm−1 assignedin kaolin to the Si\O and Al\O, respectively, are transformed into abroad band between 1090 cm−1 and 1180 cm−1 after sintering. Wealso specify that the characteristic bands of AlPO4 and KAlSi2O6,detected by XRD, were overlapped by the broad peaks. This leads us

-K-

-KH-

-KK2-

a

b c

d e

Fig. 10. SEM micrography of kaolin and the mixtures K

to believe that this broad band is the sum of the bands allocated tothe Si\O, Al\O and PO4

3− groups.The SEM examination of the fracture surfaces of the samples K, KH,

KK1, KK2 and KK3 sintered at 1100 °C for 1 h is reported in Fig. 10. Thefracture surfaces clearly show the distinct crystals of mullite. The sizesand shapes of crystals and pores are quite variable. This phase is prob-ably the primary mullite. By adding 10% H3PO4 to the kaolin, porositydecreases significantly (Fig. 10-b). Small crystals on the surface of thesintered KK1 mixture appear (Fig. 10-c). These crystals are probablythe KAlSi2O6 identified by X-ray analysis above. For the same mixturewe distinguish, the emergence of a glassy phase that fills the majorityof the porosity. The K2HPO4 mixed with kaolin also gives rise tothis new phase, KAlSi2O6, marked by small white crystals in themicrograph 10-d. The existence of cylindrical pores with variabledepths and sizes and with a total absence of grain boundaries, indicat-ing the formation of an important glassy phase during the sintering

-KK1-

-KK3-

H, KK1, KK2 and KK3 sintered at 1100 °C for 1 h.

639A. Charfi et al. / Powder Technology 235 (2013) 633–639

process, is also noted in the case of the KK2 mixture (Fig. 10-d). In thecase of the KK3 sample (Fig. 10-e), the KAlSi2O6 phase is alwayspresent and the surface is covered with craters of different sizes. Theamorphous phase has completely camouflaged the grain boundariesof primary mullite.

4. Conclusion

The effect of phosphate additives in kaolin was observed in differentthermal analyses, dilatometry analysis and in RX and IR analyses beforeand after sintering. These surveys allowed us to define the phasespresent in green and sintered mixtures. The phosphate compoundsreact at 1100 °C with aluminum and silica from kaolin to providenew compounds which are AlPO4 and KAlSi2O6. The effects ofphosphate additives on the densification, on the mechanical propertiesand on the porosity of raw kaolin were studied during the sinteringprocess. We can retain from this study that, by including phosphatecompounds, it is possible to gain more than 200 °C of sintering temper-ature while having the same strength or even a greater one than thatof kaolin. The mechanical proprieties of any mixtures reach theirmaximum value at 1100 °C.

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