modification of clay properties by aging

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Modi cation of clay properties by aging: Role of indigenous microbiota andimplications for ceramic processingRoberta Gaidzinski a ,⁎ , Patrícia Osterreicher-Cunha b , Jamil Duailibi Fh.c, Luís Marcelo Tavares aa Department of Metallurgical and Materials Engineering, COPPE, Universidade Federal do Rio de Janeiro, UFRJ. Cx. Postal 68505, CEP 21941-900, Rio de Janeiro, RJ, Brazilb Department of Civil Engineering, Pontifícia Universidade Católica do Rio de Janeiro, PUC-Rio. Rua Marquês de São Vicente 225-301 L, CEP 22451-900, Rio de Janeiro, RJ, Brazilc Department of Processing and Characterization of Materials, Instituto Nacional de Tecnologia (INT), Av. Venezuela 82, CEP 20081-312, Rio de Janeiro, RJ, Brazil

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

Article history:Received 18 September 2007Received in revised form 30 June 2008Accepted 3 July 2008Available online 15 July 2008

Keywords:AgingClaysTechnological properties

Storing clays for a period of time before their use in ceramic processing is recognized to improve theirtechnological properties as compared with the corresponding freshly-mined materials. Although themechanisms underlying this ‘ aging ’ process are not well understood, the improvements in quality have oftenbeen attributed to biological factors. The paper investigates the role of indigenous microorganisms and theiractivity in clay aging. Two clays from Brazil were sterilized by autoclaving and gamma-ray irradiation, andtheir chemical, physical, and technological properties were measured after storing for 6 months. Forcomparison, the same measurements were carried out on non-sterilized samples that were subjected to thesame conditions. The clays did not present the same response to sterilization, since aging was bene cial forthe ceramic properties of one clay and detrimental to the other. A comparison of data from sterilized andnon-sterilized aged samples showed that the changes in chemical properties could not be directly related tothe presence of microorganisms, since the clays responded to aging independently of sterilization. Further,the changes in the technological properties of the clays with aging were probably related to their initialphysical properties and not to the presence of microorganisms.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Storing freshly-mined clays in stockpiles and subjecting them to theaction of environmental elements during a reasonable period of timebefore entering the production process is a practice often used in theCeramic Industry to improve the technologicalpropertiesof clays ( Abajo,2000 ). This process, called “ aging ” , generally results in the improvementof therheological behaviorof theclays.The improvementin rheology, inparticular, plasticity, results in better clay workability during ceramicprocessing stages, such as drawing and pressing ( Abajo, 2000).

Investigations presented in the literature have suggested that thebiological action is the most signi cant mechanism responsible for theimprovements during aging ( Vaiberg et al., 1980; Groudeva andGroudev, 1995; Velde, 1995; Abajo, 2000 ). Organic acids, mainly citric,gluconic and oxalic acids, released during bacterial growth throughoxidation of inorganic sulfur or nitrogen compounds, are capable of solubilizing Fe +3 and Al+3 ions from clay mineral structures ( Groudevaand Groudev,1995 ). These modify the clay mineral charge, the speci csurface area, and also the pH of the dispersions, which could contributeto increased plasticity ( Abajo, 2000). Some microorganisms are

responsible for secretion of polysaccharides, which can bridge theparticles in aggregates and promote an increase in plasticity and areduction in drying shrinkage ( Abajo, 2000).

Investigations to assess the in uence of microorganisms in theprocess of clay aging ( Vaiberg et al., 1980; Baranov et al., 1985 )demonstrated that processing clays using bacteria increased bothplasticity and strength after drying and sintering and reduced waterabsorption. Besides improving the ceramics technological and physico-chemical properties, the treatment also reduced the required time foraging. Work from Groudeva and Groudev (1995) demonstrated thecontribution of extra-cellular polysaccharides in this process. Theyshowedthat silicate bacteria cultures were themost effective to improvethe ceramic properties of kaolins from Bulgaria. The improvement bythese bacteria was attributed mainly to the mucilaginous exopolysac-charides produced during their growth. Organic acids produced bybacteria were considered to be responsible for the reduction in the sizeof kaolin particles, which increased plasticity. Another key factoridenti ed to be responsible for the increase in plasticity was theformation of colloidal particles as a result of the partial degradation of the structure of clay minerals, mainly aluminum and iron hydr(oxides)and silica.

The present work aims to assess the role of indigenous soilmicroorganisms in clay aging, when isolated or bioaugmented micro-organisms and/or populations have not been introduced in the clays.The role of these microorganisms was investigated by comparing the

Applied Clay Science 43 (2009) 98 – 102

⁎ Corresponding author. Present address: Centro de Tecnologia Mineral (CETEM) Av.Pedro Calmon, 900, Cidade Universitária, Rio de Janeiro, Brazil. Tel.: +55 21 38657256;fax: +55 21 25903047.

E-mail address: [email protected] (R. Gaidzinski).

0169-1317/$ – see front matter © 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.clay.2008.07.007

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Applied Clay Science j o u r n a l h o me p ag e : ww w. el sev ie r. c om / l o ca t e / c l ay

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characteristicsof clays thatwere sterilized using different techniques tothose of non-sterilized clays both before and after storage for 6 months.

2. Materials and methods

2.1. Clay samples

Samples from Itaboraí (State of Rio de Janeiro) and from Santa

Gertrudes (State of São Paulo) were collected for the study.The freshlycollected sampleswere placed in sealed plastic bags, the samples wereair-dried, crushed in a laboratory smooth double roll crusher, mixedand then coned and quartered using a conical heap ( Herbst andSepulveda,1985 ) to guarantee that the samples were as representativeas possible. Each lot of clay was then split into four representativesamples for testing: one sample was used for initial materialcharacterization, two were sterilized using irradiation and autoclav-ing, respectively, and the last was left non-sterilized and used as acontrol sample.

2.2. Characterization

Mineralogical analyses were carried out by X-ray diffraction. Theseanalyses were conducted with samples of the clay fraction (natural,heated and intercalated with ethylene glycol) of each raw material, inpulverized form, using Cu – Kα radiation with sweeping angles 2 θvarying from 3 to 60° ( Jackson, 2005 ). The chemical composition of the raw materials was determined by X-ray uorescence spectro-metry. The loss on ignition and themoisture content were determinedby gravimetric methods. The plasticity of the clays was determinedaccording to Associação Brasileira de Normas Técnicas (1984a,b) fromcalculation of Atterberg indices: lower plasticity limit (LPL), upperplasticity limit (UPL) and plasticity index (PI).

2.3. Sterilization

Sterilization consisted of rst conditioning a representative sampleof each clay in an Erlenmeyer ask. Autoclaving was carried out usingan autoclave, set at 121 °C and 1 kgf/cm 2 (1 atm). Followingrecommendations from Wolf and Skipper (1994) , samples weresubjected to three consecutive 1-hour sessions of autoclaving with 2-day incubation periods in-between sessions.

Selected samples were irradiated using gamma radiation with a60 Co isotope source, in a Gammacell irradiator (MDS Nordion, modelGC220E). The radiation dosage used was 28 kGy, with 4.0 kGy/h for atotal time of 7 h. This dosage was chosen following the literature(Brown, 1981; Lessard and Mitchell, 1985 ), which suggests that adosage in the range from 20 to 40 kGy would be suf cient for theinhibition of bacterial activity in soils and clays.

After three autoclaving sessions and one 28 kGy-irradiationsession with samples of the two different clays, the effectiveness of the sterilization process was assessed by analyzing the survival of

culturable bacterial populations. Pour-plating was used as describedby Lorch et al. (1995) . Brie y, 1 g of clay was used for successivedilutions down to a concentration of 10 − 4 , and 1 mL of this dilutedsample was added to the culture medium (Tryptone Soy Broth 1:10with 1.8%of agar) in Petri dishes (plates). Triplicate samples were thenincubated at 30 °C for a period of 10 days, at the end of which, it was

veri ed if any growth of microorganism cultures occurred in theplates.

For autoclaved samples, no growth of microorganisms wasobserved in the plates. However, for irradiated samples growth of microorganisms in the plates was observed for both clays studied.Therefore, an additional irradiation session at a dosage of 28 kGy wascarried out for each clay. The additional irradiation session wasconducted 3 days after the last irradiation, during which period the

samples were kept at 30 °C. A subsequent new plating of the samplesdemonstrated no growth of microorganisms. The overall dosage of irradiation for each sample was equal to 56 kGy(two 28 kGysessions).

2.4. Experiments after sterilization

After sterilization, samples were kept at room temperature for6 months in sealed containers to simulate the process of aging. Inaddition, clay samples that were not subject to sterilization were alsostored for the same period underthe same conditions. After the end of this storage period, chemical, biological and technological character-ization tests were carried out. Chemical characterization consisted of pH measures in water ( Thomas, 1996 ), of redox potential usingpotentiometric methods ( Patrick et al., 1996 ) and measurement of the amount of organic matter with the Walkley and Black method(Embrapa, 1997 ). The cation exchange capacity was determined byadsorption of methylene blue ( American Standard for Testing andMaterials, 1999 ). The speci c surface area of particles was determinedbyadsorptionof N 2 gasat −195°C,and applyingthe BETequation usinga Quantachrome instrument.

Biological characterization was conducted by measuring the totaldegrading enzymatic activity of the clays ( Adam and Duncan, 2001 ).This consisted of adding 2 g of clay to 15 mL of potassium phosphatebuffer and incubating the suspension with 200 μ L of uoresceindiacetate (FDA) for 20 min at 30 °C using an orbital shaker at 100 rpm.After incubation, the samples were centrifuged during 3 min at2000 rpm, ltered and the amount of uorescein formed by microbialactivity was determined in the ltrate with a Gensis 2 spectro-photometer (Spectronics Instruments) at 490 nm. For each analyzedsample, a blank with no FDA and eight replicates were prepared. Thestatistical treatment used for these results was the test Q withreliability level at 90% con dence ( Ohlweiler, 1984 ).

Technological characterization of the samples consisted of rstpreparing test bodies measuring 11.4×2.5×1.0 cm by applying a 30-MPauniaxialpressure. After dryingat 110 °C, the test bodies were thensintered in the laboratory at 1050 °C, with a heating/cooling ramp of 10 °C/minandmaintainingat theset temperature for1 h.The followingparameters were then measured: apparent density, linear retraction,

exural strength ( American Association of Testing Materials, 1988b ),water absorption ( American Association of Testing Materials, 1988a )and loss on ignition.

3. Results and discussion

3.1. Clays characterization

Table 1 summarizes data on the particle size distributions andmeasurements of plasticity of the two clay samples. Itaboraí claypresented the largest proportion of material b 2 µm and was classi ed

Table 1Physical properties of the clays

Clay Mass (%) Plasticity (%)

Coarse sand (2 – 0.2 mm) Fine sand (0.2 – 0.05 mm) Silt (0.05 – 0.002 mm) Clay ( b 0.002 mm) LPL UPL PI

Itaboraí 8.0 4.6 21.4 66.0 36.24 69.00 32.8Santa Gertrudes 30.6 21.8 25.6 22.0 22.30 34.20 11.9

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as highly plastic (PIN

15%). Santa Gertrudes clay contained lowerproportion of material b 2 µm and intermediate plasticity (7% bPIb 15%).

The mineralogical characterization of the b 2 μ m material by meansof X-ray diffraction and semiquantitative analyzes ( Santos, 1998 )showed the presence of 24% illite and smaller amounts of kaoliniteand smectite for the Itaboraí clay; and illite (47%), and smalleramounts of kaolinite, quartz and interstrati ed smectite – vermiculitefor the Santa Gertrudes clay.

Table 2 summarizes the chemical composition of the two clays.Itaboraí clay presented low silica (SiO 2 ) and high alumina (Al 2O3 )content, which are indicative of the high content of clay mineralspresent. In the case of Santa Gertrudes clay the opposite occurred,with a high content of silica and low alumina content, which areindicative of the low proportion of clay minerals and large amounts of free quartz in the sample. Santa Gertrudes clay showed a highpotassium oxide content, which is probably due to the presence of illite, already identi ed by X-ray diffraction. The alkaline earth oxide(CaO andMgO) contents arealso higher, which maybe an indication of the presence of carbonates.

Itaboraí clay presented a higher moisture content and speci csurface area than Santa Gertrudes clay ( Table 3). Santa Gertrudes claywas mildly alkaline (pH in H 2 O), whereas Itaboraí clay was acid.Negative ΔpH (Table 3) indicates that both samples are cation-retainingclays. They present redox potentials in the interval of −100 to+100 mV, which characterizes them as reducing clays ( Patrick et al.,1996 ). The lower absolute value for Santa Gertrudes clay indicatedpredominance of metabolism of strictly anaerobic microorganisms.The redox potential for Itaboraí clay is characteristic of metabolism of strictly aerobic microorganisms. Further, Itaboraí clay presented ahigher cation exchange capacity than Santa Gertrudes clay due to thepresence of smectite. The enzymatic activity of Itaboraí clay washigher than that of Santa Gertrudes clay.

3.2. Aging on non-sterilized samples

No signi cant change in moisture content and organic matteroccurred, whereas a reduction was observed in cation exchangecapacity ( Table 3). Aging at the conditions studied strongly reduced

the enzymatic activity for Itaboraí clay, compared to a modestreduction for Santa Gertrudes clay. Particularly for Itaboraí clay,aging was detrimental to the microbiota.

3.3. Aging of sterilized samples

For the autoclaved sample of Santa Gertrudes clay, the enzymaticactivity was zero ( Table3 ) indicating that no microorganisms survivedin the sample, at least in active state, or evolved during aging. Theirradiated sample showed an 86% reduction in the enzymatic activityin comparison to the initial sample. As in this case the culture of clayssamples was negative immediately after sterilization (Section 2.3),non-culturable microorganisms and/or exoenzymes probably sur-vived, resisting the irradiation treatment. Exoenzymes, released bymicroorganisms to actoutside thecells,do not survive autoclaving, butareable to resistirradiation.Therefore, in the irradiated samples, activeenzymes present in the claycan exist and hydrolyze FDA, while wholebacterial cells that form colonies on the culture media have beeneliminated. This does not occur with autoclaving. The positiveenzymatic activity observed is only related to the non-culturablemicroorganisms that resisted sterilization. This is in accordance withthe activity results of all sterilized samples, with the activity inautoclaved samples being lower than those of irradiated ones ( Table 3 ).In the case of Itaboraí clay neither autoclaving nor irradiation resultedin an enzymatic activity of zero after aging, which indicated thesurvival of microorganisms in these samples. These microorganismsthat resisted sterilization were also non-culturable, since they did notappear active upon plating conducted immediately after sterilization(Section 2.3). Still, signi cant reduction in the enzymatic activity inrelation to the non-sterilized samples was observed.

Larger amounts of organic carbon and ner soil particles canprotect themicrobiota from harsh environmental conditions as well asfrom predation. Finer materials that create smaller pores also promotethe formationof aggregates that provide deepprotection formicrobialcells. However, a less ef cient circulation of air and water inside thesepores and aggregates depletes those populations of appropriateamounts of oxygen and nutrients for intensive activity. Also, thehigher speci c surface area of ner soils provides better support forthe adhesion of microorganisms ( Ladd et al., 1996; Krumholz, 2000;

Table 2Chemical composition of the clays (mass %)

Clay Al2 O3 SiO2 TiO2 Fe2 O3 CaO Na2 O MgO K2 O P2 O5 Mn2 O3 aLOI

Itaboraí 25.46 47.87 1.11 7.62 0.06 0.13 1.25 1.92 0.08 0.04 14.46Santa Gertrudes 15.05 69.62 0.70 5.28 0.81 1.98 2.55 3.78 0.19 0.04 8.20

a Loss on ignition.

Table 3Changes of the chemical and biological properties of the clays after sterilization and aging

Clay Aging Sterilization Moisture(%)

a E.A.(μ g/min g)

b Eh(mV)

pH cO.M.(g/kg)

d CEC(meq/100g)

e SSA(m2 /g)

H2O KCl ΔpH

Itaboraí None None 7.12 (0.4) 0.0744 (30.9) 98.9 4.8 3.3 −1.5 1.7 9.0 30.7Yes None 7.14 (0.8) 0.0012 (27.2) 90.7 4.4 3.1 −1.3 1.5 6.8 NaYes Autoclaving 4.43 (Na) 0.0023 (2.9) 86.3 4.3 2.9 −1.4 1.7 4.8 25.8Yes Irradiation 7.09 (Na) 0.0177 (12.9) 143.3 4.7 3.2 −1.5 1.7 7.0 33.6

SantaGertrudes

None None 3.70 (1.5) 0.0378 (10.5) −43.2 7.8 5.9 −1.9 1.9 5.5 10.9Yes None 3.53 (0.9) 0.0232 (22.8) −85.0 8.9 7.3 −1.6 1.9 3.0 NaYes Autoclaving 2.81 (Na) 0.0000 (0.0) −35.6 6.9 4.9 −2.0 1.8 2.3 9.7Yes Irradiation 3.35 (Na) 0.0053 (0.0) −66.6 7.3 5.4 −1.9 1.5 1.8 12.3

Coef cients of variation in parentheses. Na: Measurement not available.a Enzymatic activity.b Redox potential.c Organic matter.d Cation exchange capacity.e

Speci c surface area.

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Ranjard and Richaume, 2001; Holden and Fierer, 2005 ). Sandy mediapresent higher porosity and therefore better air, water and nutrientscirculation and input, favoring microbial growth to some extent. Onthe other hand, in such soils humid heat from autoclaving has greaterpenetration capacity thus providing a more ef cient sterilization, asseen in the case of the Santa Gertrudes clay. As for irradiation, soilswith a large proportion of material b 2 µm (such as Itaboraí clay)present better protection for microorganisms, hindering and/or

obstructing the ef ciency of sterilization.A reduction in the amount of organic matter was observed forirradiated samples of Santa Gertrudes clays samples. This may berelated to the consumption by remaining microorganisms during thelong time (6 months) of incubation after sterilization with no increasein biomass and also to organic matter denaturation after sterilization(McNamara et al., 2003 ). As the organic matter includes biomass, thisreduction also could have occurred as a result of the death of themicroorganisms. The measurement of organic matter carried out inthe present study gives the total carbon content. However, the organiccarbon can be present in the biomass and/or the clay. Since for theaged non-sterilized sample no consumption of organic matter wasobserved, it is likely that the reduction is associated to denaturationof organic matter.

No changes in the organic matter were observed after sterilizationfor the Itaboraí clay sample. The ner particle size could be, at least inpart, responsible for these results. The organic matter could also becon ned in the small pores of the clay, being thus protected againstdenaturation or consumption by microorganisms that survivedsterilization. It is still possible that the biomass associated with theorganic substance was not totally eliminated by sterilization, sincesome activity was detected. Since no change in organic matter wasobserved for the non-sterilized sample, it may be concluded that noconsumption of organic matter was produced by the microorganismspresent in the sample during aging.

Some variations have been observed in the redox potential, bothfor sterilized and non-sterilized samples, but they remained withinthe expected range of redox reactions of all clays ( Table 3).

The pH values of both clays in water changed after sterilization byautoclaving. Salonius et al. (1967) , Brown (1981) , Shaw et al. (1999)and Gaidzinski (2006) observed a decrease in pH of soil samples afterautoclaving. This reduction was attributed to the solubilization of organic acids during autoclaving. However, Wolf et al. (1989) did notobserve changes in the pH of soil samples after three autoclavingsessions. Thus, different soils may behave differently, demonstratingthe importance of the speci c properties of each soil.

Sterilization reduced the cation exchange capacity. One of the mostlikelyfactors is the reductionin negative charge densityin the surface of clay minerals as a result of the deadpopulation of microorganismsaftersterilization. Another factor would be the dead biomass associated toorganic matter.

On the other hand, the reduction in cation exchange capacity of both non-sterilized clays could be related to the reduction in theirenzymatic activity and the detrimental effect of aging. As the result of the death of microorganisms by the reduction of oxygen intake, waterand nutrients to the clays (the samples were stored in sealed plasticbags duringaging) there was a reduction in negative charge density inthe surface of clay minerals as a result of the dead population of microorganisms.

Only a marginal change of speci c surface area was observed aftersterilization and aging, with a reduction in the case of autoclaved andan increase in thecaseof irradiated samples ( Table3 ). The reductioninthe speci c surface area during autoclaving is consistent with resultsby Jenneman et al. (1986) , who observed the same trend forautoclaved soil samples. Based on results of scanning electronicmicroscopy after autoclaving the authors demonstrated that the claymineral particles were changed into rounded-off shapes with theincrease in the amount of aggregates. These aggregates thenpresented a lower surface area for adhesion of bacterial cells incomparison to the original untreated clay. On the other hand, theincrease in speci c surface area in comparison to the initial sample forthe irradiated samples is consistentwith results from Pushkareva et al.(2002) . These researchers concluded that the increase in radiationdosage caused an increase in theamountof centers of radiation withinthe sample. An increase in the amount of different types of inducedradiation defects can result in the observed increase in the speci csurface area and, consequently, a higher protection againststerilization.

3.4. In uence of aging and sterilization on technological properties

Aging of non-sterilized samples did not signi cantly in uenceneither the density of the green body or the sintered product nor thelinear retraction ( Table 4). An improvement in some technologicalproperties was observed for Itaboraí clay: increase in exural strengthin both green and sintered test pieces and the reduction in waterabsorption. The opposite response was found for Santa Gertrudes clay.

Thedifferent responses of thetwo clays toagingmaybe explainedonthe basis of their distinct initial physical and chemical characteristics.Aging under the conditions studied, that is, with no intake of air ormoisture,wasfavorable forsome ceramic propertiesof Itaboraí clay.Thismaybe attributed to thecomparativelyhigherinitial plasticity, moisturecontent and clay mineral content. The higher water-retention capacitymay have helped in wetting the clay mineral particles, as well ashydrating theinterlayercationsduringtheperiodof6 monthsof storage.This may have been responsible for improvement in clay rheology and,in turn, exural strengthof the testpiecebefore sintering,as well as aftersintering. The opposite response of Santa Gertrudes clay to aging couldbe associated to its comparatively lower initial moisture content andplasticity, and coarser particle size, which resulted in an unfavorable

Table 4Technological properties of the clays after sterilization and aging

Clay Aging Sterilization Unsintered aLOI (%) Sintered

Density(g/cm 3 )

Flexural strength(MPa)

Density(g/cm 3 )

Flexural strength(MPa)

Water absorption(%)

Linear retraction(%)

Itaboraí None None 2.19 (3.1) 2.24 (23.0) 14.73 (0.4) 2.22 (3.0) 6.52 (23.1) 9.93 (1.1) 5.42 (1.9)Yes None 2.19 (1.0) 2.72 (8.4) 16.88 (0.3) 2.24 (0.01) 7.89 (14.8) 8.94 (5.0) 5.50 (1.9)Yes Autoclaved 2.12 (2.9) 2.66 (10.0) 15.04 (1.3) 2.08 (2.5) 7.02 (6.0) 10.67 (4.8) 5.13 (1.8)Yes Irradiated 2.10 (1.5) 3.00 (12.5) 15.56 (1.5) 2.06 (2.2) 9.04 (8.8) 10.01 (4.2) 5.05 (3.6)

Santa Gertrudes None None 2.07 (2.5) 3.15 (13.3) 8.76 (3.8) 2.33 (1.5) 28.03 (7.1) 0.35 (12.7) 6.75 (1.1)Yes None 1.99 (2.2) 1.86 (3.0) 7.62 (1.1) 2.25 (1.1) 25.45 (1.2) 1.86 (13.8) 6.53 (0.3)Yes Autoclaved 2.05 (1.5) 2.00 (5.0) 6.84 (1.1) 2.30 (2.1) 23.16 (0.7) 2.15 (4.3) 6.79 (1.0)Yes Irradiated 2.04 (1.7) 1.88 (4.6) 7.02 (0.6) 2.29 (2.6) 21.86 (3.0) 2.13 (0.6) 6.64 (1.7)

Coef cients of variation in parenthesis.a

Loss on ignition.

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effect of aging under the conditions of little oxygen intake and wateravailability.

Essentially the same response to aging has been found for samplesthat were subject to sterilization, in spite of the signi cantly lowerenzymatic activity and cation exchange capacity of the clays subjectedto autoclaving and irradiation. This suggests that, under the agingconditions studied, the change in ceramic properties observed maynot be attributed to the microbiology.

4. Conclusions

Three sessions of autoclaving at 121 °C and 1 atm were capable of sterilizing the clays, as measured from the lack of activity in bacterialcultures in plates. Sterilization by irradiation was only successful witha gamma irradiation dosage of 56 kGy, which is well above 40 kGy assuggested in the literature ( Brown, 1981; Lessard and Mitchell, 1985 ).

Measurement of enzymatic activity after storage for a period of 6 months demonstrated that complete sterilization was probably onlyachieved for Santa Gertrudes clay subjected to autoclaving. Thedifferences observed in the behavior of the clays after sterilization maybe an indication of the role of mineralogy and diversity of microorgan-isms present.

Besides a reduction in enzymatic activity, aging and sterilizationalso reduced the cation exchange capacity.

In respect to the technological ceramic properties, the samplesstudied presented opposite responses to aging: while an improvementin some properties was observed for Itaboraí clay, aging for 6 monthswas detrimental to some ceramic properties, such as exural strengthand water absorption, of Santa Gertrudes clay. These changes wereobserved for both sterilized and non-sterilized samples after aging for6 months without direct contact with the environmental elements,and are not likely to be related to microbiology. Indeed, the distinctresponse of the clays to aging may be associated to the different initialproperties of the clays, including plasticity, particle size and moisturecontent. The higher plasticity and moisture content, and ner sizedistribution of Itaboraí clay make it amenable to aging even withoutdirect contact with fresh airor additional moisture.On the other hand,the lowerplasticity andmoisture content andcoarser size distributionof Santa Gertrudes clay, coupled to exposure to conditions of pooroxygen and no water intake studied resulted in a detrimental effect of aging.

Acknowledgment

The authors would like to thank the nancial support from theConselho Nacional de Desenvolvimento Cientí co e Tecnológico.

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