Chiang Mai J. Sci. 2006; 33(3) 271

The Rheological Behavior of Kaolin SuspensionsApinon Nuntiya* and Sitthisak PrasanphanDepartment of Industrial Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand.

*Author for correspondence; e-mail : anuntiya@chiangmai.ac.th

Received: 16 February 2006

Accepted: 27 June 2006.

1. INTRODUCTIONThe structure of the kaolin minerals is

based on the combination of two layerstructures. One layer, known as the silica layer,is composed of silicon and oxygen atoms,and the second layer, known as the gibbsitelayer, is composed of aluminium atoms andhydroxyl groups. The kaolin crystal consistsof a large number of two-layer units heldtogether with hydrogen bonds [1-4]. Kaolinplatelets have negative charges on basalsurfaces, due to substitution, and positive andnegative charges on edge surfaces, due tobroken bond at the edge [3-5]. It is generallyaccepted that most clay particles are plateletswith a negative face charge and a positive edgecharge under the proper pH conditions. Awell-known electron micrograph first

published by Thiessen in 1942 clearly showsnegatively charged gold sol particles attachedto the edge of kaolinite platelets which areroutinely found to have a net negative charge[4]. Clays are therefore thought to set up a gelor thixotropy where the positively chargedparticle edges are attracted to the negative facesin a “house of cards” conformation.

The kaolin group of minerals withexample of ideal formulas for kaoliniteAl2(OH)4.Si2O5 and halloysite Al2(OH)4.Si2O5.2H2O [1,3,4]. The two species also differin particle shape. Kaolinite occurs almostinvariably as hexagonal platelets while halloysitecan assume a tubular and spheroidalmorphology. Particle shape or morphologyis an important factor in influencing the

Chiang Mai J. Sci. 2006; 33(3) : 271 - 281www.science.cmu.ac.th/journal-science/josci.htmlContributed Paper

ABSTRACTThe rheological behavior of three kaolins (Narathiwat, Lampang and Ranong) having

different chemical and mineral compositions were studied. The viscosity and thixotropy ofkaolin suspensions were measured as a function of solid content, pH and electrolyteconcentrations. From rheological measurements, it was found that the viscosity and thixotropyincreased with the increasing of the solid content. The highest viscosity of Narathiwat, Lampangand Ranong kaolins showed at pH 2, 6 and 4, respectively. However, thixotropy of threekaolins tends to increase with decreased pH. Plots of Binghan yield stress against pH at differentionic strengths intersect at about pH 7.1, 6.8 and 4.0, for Narathiwat, Lampang and Ranongkaolins, respectively. These values are identified with the isoelectric point of the particle edgesurface. The rheological behavior of kaolins may be explained in terms of the influence ofsolid content, particle size, morphology, and pH and electrolyte concentrations on particleinteraction.

Keywords: bingham yield stress, isoelectric point, kaolin, rheological behavior, thixotropy,viscosity.

272 Chiang Mai J. Sci. 2006; 33(3)

rheological behavior of suspensions [6-9].The rheological behavior of kaolin

suspensions and their sensitivity to pH and ionicstrength have been interpreted in terms ofheteropolar nature of the particles. Diz andRand [10,11], and Diz et al. [12] showed thata common intersection in the curves ofextrapolated yield stress against pH, atdifferent sodium chloride concentrations,could be interpreted as being the pH of theisoelectric point of the edge surface. Thiscommon intersection point is not the samefor all kaolinite samples and Diz and Rand[10] also suggested that isoelectric point wasdetermined by the alumina:silica ratio at thissurface which, as a result of differentialleaching, could vary according to the aqueousenvironment to which the clay mineral hadbeen exposed. Besides, Theng and Wells [6]found intersection point at pH 6.0 for MatauriBay halloysite and at pH 7.1 for Te Akateahalloysite. These values are identified with thepoint of zero charge of the particle edgesurface. The flow characteristic of halloysitemay be explained in terms of the influenceof pH, electrolyte concentrations, and layercomposition on particle interactions.

The aim of this study is to examine therheological behavior of kaolin suspensions.The rheological behavior of kaolinsuspensions may be explained in terms of theinfluence of solid content, particle size andmorphology, pH and electrolyteconcentrations on particle interactions.

2. MATERIALS AND METHODS2.1 Materials

Kaolin powders were used in theexperiments. Kaolins were obtained fromNarathiwat, Lampang and Ranong (Thailand).The three kaolins samples were sievedthrough 325 mesh screen in a wet state,allowing it to settle, and discarding thesupernatant. The washed samples were driedat 80 OC for over 48 h and all samples weregrounded in a rod mill.

2.2 Particle Size AnalysisParticle sizes of the three kaolins

(Narathiwat, Lampang and Ranong) havebeen determined by a laser light diffractioninstrument (Mastersizers, Malvern). Distilledwater was used as a dispersive medium.

2.3 Chemical Analysis by X-ray Fluorescence(XRF)

The chemical composition of the threekaolin powders were determined by BrukerD8/plus XRF spectrometer. The powders,fused in a pellet, were irradiated with x-rays.The elements in sample absorbed part of thatradiation and emitted their own characteristicx-ray fluorescence spectra, which allow theiridentification and quantification to beperformed.

2.4 Mineralogical Analysis by X-ray Diffraction(XRD)

Panalytical model X’pert Pro MPD X-ray diffractometer was used to determine thephases present in the three kaolins powders.Powders were placed in a flat glass holderand scanning was taken, using CuK radiation,over the 2θ range 5° to 95° at scan speeds0.003 s-1 and 0.008 s-1.

2.5 Transmission Electron Microscopy (TEM)Particle morphology of three kaolins (size

and shape) was assessed by transmissionelectron microscopy (JEOL JEM-2010),operating at 200 keV and fitted with an ultrathin window energy dispersive X-ray (EDX)detector was used. To prepare the sample fortransmission electron microscopy, a powderwas dispersed in acetone with the aid ofultrasonic vibration. A drop of dilutesuspension was placed on a carbon coated400 mesh copper grid, and allowed to dry atroom temperature.

2.6 Rheological Measurements2.6.1 Effect of Solid Content and pH

Brookfield rheometer DVIII+ was usedto determine the rheological behavior of the

Chiang Mai J. Sci. 2006; 33(3) 273

three kaolins samples. The kaolin samples wereprepared in the range of 30-40% (w/w) solidcontent in distilled water, and mixed by electricmixer for 10 minutes. 40% (w/w) suspensionwas found to be the most suitable to observeviscosity and thixotropy, for comparing of theeffects of pH. The suspensions were adjustedto pH 2-10 with 12 M HCl and 12 M NaOH.Viscosity and thixotropy were measured byBrookfield rheometer DV-III+ at 15 rpm andspindle number Sc 29.

2.6.2 Isoelectric Point DeterminationsAt 40% (w/w) kaolin suspensions in

sodium chloride solutions (0.02, 0.04, 0.06 and0.08 M) were adjusted to pH 2-10 with 12 MHCl and 12 M NaOH, and mixed by anelectric mixer for 10 minutes. Bingham yieldstress was determined by Brookfieldrheometer DV-III+ at 15 rpm and spindlenumber Sc 29. The isoelectric point wasestimated from the common intersectionpoints of Bingham yield stress-pH curves atdifferent ionic strengths.

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3. RESULTS AND DISCUSSION3.1 Particle Size Analysis

Figure 1 illustrates particle size of thethree kaolins samples. It was found thatNarathiwat kaolin was smaller particle sizethan those of the Lampang and Ranongkaolins, respectively. The particle size analysisof Narathiwat, Lampang and Ranong kaolinsgave an average median particle size D[4,3]of 9.82, 11.42 and 13.47 µm, respectively.

3.2 Chemical AnalysisChemical constituents of the three kaolins

analyzed by XRF are shown in Table 1. Itwas found that the main components of thethree kaolins are Al2O3 and SiO2 with minorcomponents Fe2O3, TiO2, Na2O, K2O, CaO,MgO, MnO and P2O5.

Table 1. Chemical composition of the threekaolins.

Compounds,% N L R

Al2O3 36.18 31.06 35.87SiO2 44.76 53.05 46.25Fe2O3 0.83 1.33 1.71TiO2 0.84 0.07 0.08Na2O 3.12 1.93 1.27K2O 1.15 4.19 2.06CaO 0.13 0.21 0.40MgO 0.11 0.24 0.25MnO 0.01 0.06 0.05P2O5 0.06 0.01 0.03LOI 12.82 17.86 12.04SiO2 / Al2O3 1.24 1.71 1.29

N: Narathiwat kaolin L: Lampang kaolinR: Ranong kaolin LOI: Loss on ignition

3.3 Mineral AnalysisFrom the diffractograms as shown in

Figures 2-4 and SiO2/Al2O3 ratio, theNarathiwat kaolin consists of kaolinite, illiteand quartz. The major compositions of theLampang kaolin are mostly kaolinite, illite,quartz and albite. The Ranong kaolin iscomposed of mostly halloysite, kaolinite, illite,quartz and microcline.

Figure 1. The particle size distribution ofNarathiwat, Lampang and Ranong kaolins.

274 Chiang Mai J. Sci. 2006; 33(3)

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Figure 2. XRD diffractogram of theNarathiwat kaolin.

Figure 3. XRD diffractogram of theLampang kaolin.

Figure 4. XRD diffractogram of the Ranongkaolin.

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Figure 5. TEM micrograph of Narathiwatkaolin.

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Figure 7. TEM micrograph of Ranongkaolin.

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Chiang Mai J. Sci. 2006; 33(3) 275

3.4 Transmission Electron Microscopy (TEM)Figures 5-7 show TEM images of

Narathiwat, Lampang and Ranong kaolins,respectively. From the literature reviews ofFranco et al., and Xu and Van Deventer, itwas found that the well-ordered kaolinitedisplays the large crystals of clear hexagonalshape [13,14]. Surprisingly, disordered kaolinite[13] and halloysite [15,16] show the irregularshape and tubular morphology, respectively.From experimental results, it was found thatthe TEM image of Ranong kaolin had tubularmorphology which corresponded to thestructure of halloysite and was stronglysupported by X-ray diffractogram. However,the electron micrograph of Narathiwat kaolinhad hexagonal shape matching the main peaksof well-ordered kaolinite. On the other hand,the particles of Lampang kaolin have irregularshape and its diffractogram corresponds tothe disordered kaolinite.

3.5 Rheological Measurements3.5.1 Effect of Solid Content

Figure 8 shows the plastic viscosity forall three kaolins suspensions as a function ofsolid content within the range investigated (30-40% w/w). From the experimental results, itwas found that increasing the solid contentof suspensions resulted in increasing the plasticviscosity of the three kaolins [6,9,17,18]. It canbe explained that the small water inaccessibleto the void resulted in high close-pack ofparticles. Nevertheless, in the suspensions, theNarathiwat kaolin had a higher plastic viscositythan those of Ranong and Lampang kaolinsat the same solid content. It can be explainedthat the Narathiwat kaolin was well-definedhexagonal plate and smaller particle size thanthose of Lampang and Ranong kaolins,respectively. Therefore, it gives the highestcompaction. By comparison of the particlesize between Ranong and Lampang kaolins,it can be seen that Ranong kaolin has largerparticle size than that of Lampang kaolin. Butthe Ranong kaolin showed higher plasticviscosity than that of Lampang kaolin due tothe morphology of Ranong kaolin shows

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Figure 9. Plot of shear rate vs. shear stressfor Narathiwat kaolin suspensions.

Figure 10. Plot of shear rate vs. shear stressfor Narathiwat, Lampang and Ranong kaolinsuspensions.

Figure 8. Plot of solid content vs. plasticviscosity for Narathiwat, Lampang andRanong kaolin suspensions.

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276 Chiang Mai J. Sci. 2006; 33(3)

particle packing. According to Yuan andMurray [7], the effects of particle morphologyon the rheological behavior of high-solidskaolin suspensions were investigated usingHercules viscometer. It was found that thespherical halloysite showed the lowest viscosity,followed by the platy kaolinite and tubularhalloysite. The trends can be best explainedby the change of particle packing which isultimately controlled by particle geometry.According to Yildiz et al. [18], it was foundthat by increasing the kaolinite content ofsuspensions, the area of the hysteresis loopof the flow curve and yield stress increase.The comparison of plastic viscosity andthixotropy as a function of solid content ofkaolins determined by Staneva et al., Theng etal., Yuan and Murray, and Yildiz et al., weresimilar to the three kaolins from Thailand usedin this study.

3.5.2 Effect of pHThe plastic viscosity of 40% w/w kaolin

suspensions were measured at various pHlevels as shown in Figures 11-13. From theexperimental results, it was found that thedecrease of the suspension pH resulted in anincrease in the plastic viscosity [6,9,19-21] ofNarathiwat kaolin as shown in Figure 11. Thiscan be ascribed to either the lower the pH ofthe larger the amount of positive charges onthe edges of kaolin. Therefore, the edge-to-face attraction between the positively chargededges and negatively charged basal surfaces ina “house of cards” conformation. On theother hand, the higher the pH, the largeramount of negative charges on the edges ofkaolin. Therefore, the repulsion between thesame negatively charged particles increases[6,19-21]. The result also indicates that thehighest plastic viscosity of Narathiwat kaolinoccurred at pH 2. According to Michaels andBolger [20] who studied the particleinteractions in aqueous kaolinite dispersions, achange in parameters was found in thestructural of flocculated kaolinite suspensions,as deduced from pH and viscosity data. Thehigh viscosity and pseudoplasticity of acidic

tubular halloysite. Therefore, it gives the highestparticle packing resulting in an increase in theplastic viscosity.

The relation between shear rate and shearstress for known flow curves is illustrated inFigure 9 for the suspensions of Narathiwatkaolin at 30-40% w/w solid content. Byincreasing the solid content of suspensions,the thixotropy (the area of the hysteresis loopof the flow curves) increases [8,17,18] due tothe small water inaccessible to the void resultedin closely packed particles and while van derWaals attraction between particles. For theLampang and Ranong kaolins, the effect ofthixotropy is similar to that found in theNarathiwat kaolin (not shown). From thecomparison of thixotropy of three kaolinsas shown in Figure 10, it can be seen that theNarathiwat kaolin provides the highestthixotropy, higher than those of Lampang andRanong kaolins because it has the smallerparticle size and the highest compaction.

The results clearly demonstrated theimportance of solid content, particle size andmorphology as controlling factors of kaolinrheology. Previous studies have indicated thatthe rheological behavior of kaolin suspensionis governed by the fundamental properties ofkaolin particles: their solid content, particle sizeand morphology. It was reported by Stanevaet al. [17] measured the rheological ofporcelain mixture. It was found that the yieldstress, viscosity and thixotropy increase byincreasing the solid concentration due to theincreased number of particle-particleinteraction. Theng and Wells [6] determinedthe flow characteristics of halloysitesuspensions. The three halloysite from NewZealand used were: a thick long tubules fromMatauri Bay, a short thin lath from Te Akatea,and a spherules from Opotiki. It wasobserved that the increase of solid contentof halloysites resulted in an increase in theplastic viscosity due to the increased numberof particle-particle interaction. Besides, theshort thin lath halloysite had a higher plasticviscosity than those of the thick long tubulesand spherules halloysite because of good

Chiang Mai J. Sci. 2006; 33(3) 277

kaolinite suspensions are a consequence ofstrong electrostatic interaction betweenparticles due to the coexistence of negativelyand positively charged basal surfaces. Elevationof pH by addition of sodium hydroxidecauses gradual weakening of interparticleattractions via neutralization of positive edgecharges resulting in a decrease of the viscosity.Theng and Wells [6] measured the flowcharacteristics of halloysite suspensions. It canbe seen that the plastic viscosity for halloysitesincreases with decreasing suspensions pH dueto the magnitude of the positive charge onparticle edges increases, therefore, edge-to-faceassociation, and vice versa. It was reportedby Tombácz and Szerkeres [21], the lower thepH, the larger the amount of positive chargeson the edges of montmorillonite. Therefore,the edge-to-face attraction between thepositively charged edges and negativelycharged basal surfaces results in an increaseof the viscosity, and vice versa. The plasticviscosity of kaoin and montmorillonite as afunction of pH found by Michaels and Bolger,Theng and Wells, Tombácz and Szerkeres aresimilar to that of Narathiwat kaolinsuspensions used in this study.

Furthermore, the plastic viscosity ofLampang and Ranong kaolins were measuredat 40% w/w solid content at various pH levelsas shown in Figures 12-13. On the basic side(pH 7-10), the plastic viscosity decreases withpH, reaching a minimum at pH 10 due to thepositive charge on edges kaolins reversednegatively charged, therefore, particles getgood dispersion due to repulsion betweennegatively charged on both edge and surfaceof particles [6,19-21]. On the acidic, the plasticviscosity increases with pH, reaching amaximum at pH 6 and 4 of Lampang andRanong kaolins, respectively. As discussedearlier, the kaolin suspensions tend to form ahouse of cards structure due to the edge-to-face interaction between the negatively chargededges and positively charged basal surfaces[19-21]. Besides, the decreases in the plasticviscosity at lower pH suggest that the ionicstrength will be raised. The ionic species

strength may first interfere with the negativeelectrical double layer of the faces reducingthe edge-to-face interactions resulting in adecrease of the system plastic viscosity. Theresult also indicates that the highest of plasticviscosity of Lampang and Ranong kaolinsshowed at pH 6 and 4, respectively, suggeststhat these points are isoelectric and the particleof these points will be associated with vander Waals forces[10]. Similar trends with lowand high pH are shown in the data of Changet al. [19], and Williams and Williams [22].Chang et al. studied effect of pH and ionicstrength on the rheology and stability ofaqueous clay suspensions. It was found thaton the basic side, the viscosity first decreaseswith pH, reaching a minimum near pH 11due to the positive charges of the edges whichmay be reversed at high pH. This will disruptthe house of cards structure and result in alowering of the viscosity of system. On theacidic side, as pH decreases (i.e., addition ofmore HCl) the ionic strength will be raised.The ionic species strength may first interferewith the negative electrical double layer of thefaces reducing the edge-to-face interactionsresulting in a decrease of the system plasticviscosity. This is in agreeable with Williams andWilliams [22] who reported that viscositydecreases at low pH due to the effects ofreduced number of interparticle links.

Figure 14 also shows the hysteresis loop

Figure 11. Plot of pH vs. plastic viscosity forNarathiwat, kaolin suspensions at 40 %w/w.

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278 Chiang Mai J. Sci. 2006; 33(3)

enclosed by the acceleration (‘up’) anddeceleration (‘down’) branches of the flowcurves of Narathiwat kaolin (40% w/w) atvarious pH levels. It was found that thedecrease of the suspensions pH resulted in anincrease in the thixotropy. Here the edge-to-face heterocoagulated network can formbecause of the attraction oppositely chargededges and surface of kaolin-platelets [6,21],and vice versa. For the Lampang and Ranongkaolins, the effect of pH on the thixotropy issimilar to that found in Narathiwat kaolin (notshown). The thixotropy show a trend similarto that found by Theng and Wells [6], andTombácz and Szerkeres [21] at all pH whoreport increasing value of thixotropy with anincrease of the suspension pH due to the edge-to-face association, and vice versa.

3.5.3 Isoelectric Point DeterminationsThe Bingham yield stress measurements

were made as a function of pH and sodiumchloride concentration of three kaolinsuspensions (40% w/w) as shown in Figures15-17. The results are interpreted in terms ofthe heteropolar model of kaolin and edgesurface isoelectric points are estimated fromthe common intersection of Bingham yieldstress-pH curves at different ionic strengths[6,10-12]. From the experimental results, it canbe seen that the isoelectric points ofNarathiwat, Lampang and Ranong kaolinsshowed at about pH 7.1, 6.8 and 4.0,respectively.

According to the heteropolar model, atpH value below the edge surface isoelectricpoint the particles are flocculated in an edge-to-face co-ordination as a result ofelectrostatic attraction between the positiveedges and negatively charged basal surfaces.The effect of increasing the concentration ofcounter ions (sodium chloride) in this pHregime is the compression of the electricaldouble layer on both surfaces so as to reducethe electrostatic attraction (i.e. it reduces thedepth of the primary medium in the totalpotential energy of the interaction curve).Consequently, all those properties that depend

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Figure 13. Plot of pH vs. plastic viscosity forRanong kaolin suspensions at 40 %w/w.

Figure 14. Plot of shear rate vs. shear stressfor Narathiwat kaolin suspensions at variouspH at 40 %w/w.

Chiang Mai J. Sci. 2006; 33(3) 279

on the strength of this interaction are changed.Specifically, the Bingham yield stress is reduced.At pH values above the edge surfaceisoelectric point, both surfaces of the kaolinparticle have the same sign and the system maybe deflocculated. Compression of theelectrical double layers by increasing the ionicstrength (sodium chloride) in this region hasthe effect of increasing the depth of primaryminimum and increasing the attractive forcesdue to the van der Waals effect, so increasingthe Bingham yield stress. It is this differentbehavior above and below the edge surfaceisoelectric point that gives rise to the point ofintersection in the Bingham yield stress-pHcurves. This point of intersection essentiallylocates the pH at which the predominantinfluence of the yield stress changes fromelectrostatic attraction to van der Waalsattraction [6,10-12].

According to Diz and Rand [10], therheological measurements have been made asa function of pH and sodium chlorideconcentration for kaolinite, without washingand washing three times with acidic 1 mol/dm-3 sodium chloride solution. The resultsare interpreted in terms of the heteropolarmodel of kaolinite and edge surface isoelectricpoints in the extrapolate yield stress-pH curves,at different ionic strengths. The isoelectric pointof kaolinite at without washed and washedthree times showed at about pH 5.6 and 6.8,respectively. This is in agreement with Randet al. [12] which reported that kaolinite has anisoelectric point at about pH 6.2. Theng andWells [6] found that Bingham yield value issensitive to variation in ambient electrolyteconcentration. For Matauri Bay halloysite andTe Akatea halloysite the plots of pH vs.Bingham yield value at different concentrationof sodium chloride intersect at about pH 6.0and 7.1, respectively. When this is comparedwith the isoelectric point of kaolin groupdetermined by Diz and Rand, and Rand etal., it is found to be similar to the Narathiwatand Lampang kaolin from Thailand used inthis study. On the other hand, Ranong kaolincomposed of tubular halloysite showed

isoelectric point at about pH 4.0 which wasdifferent from that observed by Theng andWells [6].

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1 2 3 4 5 6 7 8 9 10 11

20

30

40

50

60

70

80

90

100

No addition 0.02 M NaCl 0.04 M NaCl 0.06 M NaCl 0.08 M NaCl

Bing

ham

yie

ld s

tres

s (N

/m2 )

pH

Figure 17. Plot of pH vs. Bingham yield stressat different electrolyte concentrations forRanong kaolin suspensions.

Figure 15. Plot of pH vs. Bingham yieldstress at different electrolyte concentrationsfor Narathiwat kaolin suspensions.

Figure 16. Plot of pH vs. Bingham yield stressat different electrolyte concentrations forLampang kaolin suspensions.

280 Chiang Mai J. Sci. 2006; 33(3)

4. CONCLUSIONSFrom the performed investigation, the

following conclusions can be derived:1. Increased solid content causes the

plastic viscosity and thixotropy ofNarathiwat, Lampang and Ranong kaolinsuspensions to increase because the small waterinaccessible to the void results in closelypacked particles. Besides, the plastic viscosityand thixotropy are dependent on particle sizeand morphology of kaolins.

2. The high viscosity of acidic kaolinsuspensions are a consequence of strongelectrostatic interaction between particles dueto the coexistence of positively charged edgesand negatively charged basal surfaces. Elevationof pH by addition of sodium hydroxidecauses gradual weakening of interparticleattractions via neutralization of positive edgecharges, therefore, the repulsion between thesame negatively charged particles results in adecrease of the system plastic viscosity.Besides, the decreases in the plastic viscosityat lower pH due to the ionic species strengthmay first interfere with the negative electricaldouble layer of the faces reducing the edge-to-face interactions resulting in a decrease ofthe system plastic viscosity.

3. The decrease of the suspensions pHresults in an increase in the thixotropy. Theedge-to-face heterocoagulated network canform because of the attraction of oppositelycharged edges and surface of kaolin-platelets.

4. From the experimental results, it wasfound that the isoelectric points of Narathiwat,Lampang and Ranong kaolins existed at pH7.1, 6.8 and 4.0, respectively.

ACKNOWLEDGEMENTThe authors acknowledge the Thailand

Research Fund - Master Research Grant (TRF-MAG) for the financial support.

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