Construction and Building Materials 25 (2011) 2206–2213

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Construction and Building Materials

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Properties of metakaolin geopolymer hardened paste preparedby high-pressure compaction

Vladimír Zivica a,⇑, Svetozar Balkovic b, Milan Drabik b

a Institute of Construction and Architecture of the Slovak Academy of Science, Dúbravska cesta 9, 84503 Bratislava, Slovak Republicb Institute of Inorganic Chemistry of the Slovak Academy of Science, Dúbravska cesta 9, 84536 Bratislava, Slovak Republic

a r t i c l e i n f o

Article history:Received 26 November 2009Received in revised form 12 November 2010Accepted 14 November 2010Available online 18 December 2010

Keywords:MetakaolinGeopolymerCompressive strengthPore structure parametersThermal analysisEDX results

0950-0618/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2010.11.004

⇑ Corresponding author. Tel.: +421 2 59309257.E-mail address: usarziv@savba.sk (V. Zivica).

a b s t r a c t

The paper presents the results of the study on the effects of the combination of the low liquid/solid (l/s)ratio and pressure compaction of the fresh pastes on the properties of the hardened metakaolin geopoly-mer paste. It is well known that the combination gives the possibility to prepare cement compositesresulting in the excellent engineering properties. The results obtained shown the high dense nano- ornear-nano-pore structure with high degree of its homogeneity and high strength of metakaolin geopoly-mer hardened paste prepared under the use of very low l/s ratio and pressure compaction.

Two factors were found responsible for the high positive effects:

(i) Mechanical effect of the densifying of the starting mixture and the decrease of the initial porositydue to action of the applied pressure compaction.

(ii) The well known very positive effect of the l/s ratio decrease on the rate and level of the develop-ment of the properties of cement composites.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction A very attractive solution of this approach represents the

It is known that alkali activated aluminosilicates are able toproduce alumino-silicate polymers – geopolymers. The hardeningmechanism involves the chemical reaction of geopolymericprecursors, such as alumino-silicate oxides, with alkali polysilicatesyielding polymeric Si–O–Al bonds. The production of geopolymericprecursors is carried out by calcinations of aluminosilicates – naturalclay materials. Their source can be also some industrial aluminosil-icate waste materials. The result of the hardening mechanism is athree dimensional zeolitic framework unlike traditional hydraulicbinders in which hardening is the result of the hydration of calciumaluminates and silicates [1–7]. This circumstance is a cause ofsignificant differences in the quality and variety of the engineeringproperties of the composites based on geopolymer and currentcements.

As it is very well known the strength and the level of the otherengineering properties of the cement based materials are signifi-cantly dependent on the w/c ratio used. Also the fact that theirstrengths, durability and the quality of other engineering proper-ties are increased when the w/c values are decreased [8–10]. Asignificant factor limiting the effective use of these positive effectsis the decreasing of the quality of the properties of the compositesdue to the decrease of the workability of the fresh compositemixtures prepared with very low w/c ratios.

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combination of super low w/c ratios, under w/c 0.1 and lower,and the pressure compaction of the fresh composite mixtures.The result is a submicroscopic pore structure and an adequate highincrease of the quality of engineering properties of the composites[11–17]. It appears that low-porosity cement composites have agreat potential of reconsideration and modification of compositionand structure.

The aim of the presented work was to study the possibilities ofthe use of effects of the application of very low w/c ratio and pres-sure compaction in the geopolymer cement systems. As a modelthe alkali activated metakaolin system was used.

2. Experimental

2.1. Materials

The metakaolin used was the product of kaolin heat-treatment at 650 �C for 4 h.The data on the chemical composition and specific weight of the original kaolin andmetakaolin are given in Table 1. With the aim to compare the reactivity of bothmaterials also their solution heat was estimated using the method of the kineticsof its development [18,19].

The comparison of the chemical composition shown an increase in the SiO2,Al2O3 and Fe2O3 contents of the metakaolin as a direct consequence of the decreaseof the ignition loss due to the heat-treatment of kaolin. Further interesting effectscould be observed: a significant (ca. 20 times) increased solution heat and, on thecontrary, a decreased specific weight of the metakaolin. The increased solution heatindicated the increased reactivity of metakaolin opposite to the kaolin. The foundspecific weight decrease indicated the decreased compactness of metakaolin. This

Table 1Chemical composition and properties of kaolin and metakaolin used.

Chemical composition (%)

Components Kaolin Metakaolin

Humidity 0.94 0.35Ignition loss 12.20 1.95SiO2 48.06 53.68A12O3 36.76 42.00Fe2O3 1.42 1.70CaO 0.33 0.21MgO 0.24 0.10Specific weight (kg m�3) 2631 2582Solution heat (J g�1) 278.2 5563

Table 2Pore structure parameters of kaolin and metakaolin used.

Pore structure parameter Kaolin Metakaolin

Pore volume (mm3 g�1) 771 1016 (+31.8%)Macro-pore portion (%) r > 7500 nm 26.8 15.5 (�40.7%)Pore median (nm) 1102 772 (�29.9%)Total porosity (%) 77.8 93.5 (+20.2%)Specific surface area calculated from

porosimetry data (m2 g�1)4.9 38.7 (+689.8%)

Fig. 1. Thermograms

V. Zivica et al. / Construction and Building Materials 25 (2011) 2206–2213 2207

of kaol

effect was confirmed by the porosimetry results showing 31.8% increase of porevolume and 20.2% increase of total porosity. Further, the found 40.7% decrease ofmacro-pore content and 689.8% increase of specific surface area indicating themetakaolin as a material significantly finer than the starting kaolin (Table 2).

All the mentioned effects were undoubtedly the consequence of the process ofthe heat-treatment producing the amorphous or near-amorphous SiO2–Al2O3–Fe2O3 product. Its presence in the metakaolin is confirmed from the DTA-curvesby a very broad endotherm between ca. 200 and 950 �C, instead of the sharp endo-therm peak at 511 �C visible on the DTA-curve of the kaolin (Fig. 1).

According to the results the used metakaolin was the material with SiO2/Al2O3

molar ratio 2.22 showing significantly increased fineness and solution heat bothcontributing to the increase of its reactivity as a partner in the process of the geo-polymer formation [7,20,21].

2.2. Test specimens and their testing

For the study the hardened alkali activated metakaolin pastes prepared withactivator solution/metakaolin ratio (l/s) 0.08 and compacted by the pressure300 MPa were used. The prepared test specimens were 20 mm-edge cubes. The ref-erence test specimens were prepared with l/s 0.70 using the compaction by hand.The alkali activator sodium hydroxide was added to the mixture in the form of solu-tion in the adequate quantity of water. The portion of the activator corresponded to7 wt.% of the weight of the metakaolin.

The preparation of the fresh mixtures represented the addition of the givenactivator solution amount to the metakaolin and intensive mixing for 3 min usingelectrical mixer. The pressure compaction followed after this procedure. The

in and metakaolin used.

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20 � 20 � 20 mm cube forms were filled by the metakaolin mixture and exposed tothe pressure of 300 MPa with the endurance of 1 min. The preparation of thereference test specimens was the same but under the compaction by hand.

The prepared test specimens hardened 24 h in the forms, at temperature 20 �Cand relative humidity 95%. Then, after of the demoulding, their testing followed.The test specimens were dried at 105 �C and these properties were observed: bulkweight and compressive strength, hydration products occurred (DTA, TG, EDX), porestructure (intrusion mercury porosimetry) and morphology of the matrix (scanmicroscopy), water bound and the total ignition loss (TG).

2.3. Methods

For bulk weight and compressive strength tests the current testing methodswere used.

Table 3Bulk weight and compressive strength of metakaolin geopolymer pastes and pore structu

Bulk weight(kg m�3)

Compressive strength(MPa)

Total porosity(%)

Pressurecompaction

1862 146.6 15.7

Reference 814 0.03 54.5

Fig. 2. Histogram and pore size distribution curve o

Thermal analysis (simultaneous TG and DTA-curves) was carried out on theequipment SDT 2960 device T.A. Instruments: the sample mass 20 mg, heating ratewas 10 �C min�1, ramping from ambient temperature to 1000 �C in an air atmo-sphere. TG mass losses and DTA effects were analysed using the T.A.I. Thermal Ana-lyst Package.

Pore structure was analysed using the intrusion mercury porosimeter mod. 2000Erba Science working to the pressure 200 MPa with macro-pore unit 120, under theuse of the contact angle 141.3� and surface tension of mercury 0.48 N m�1.

The morphology of the samples was studied by scan electron microscopy usinga Carl Zeiss – EVO 40 HV microscope. Before the scanning process, all samples werecoated with gold to enhance the electron conductivity. The samples were alsoexamined by energy dispersive X-ray analysis (EDX) with spectrometer Quantax400 to analyse chemical composition.

re parameters after 24 h of the hardening.

Pore median(nm)

Specific surface area(m2 g�1)

Portion of pores

r < 100 nm(%)

r > 100 nm(%)

59 5.5 96 4

814 5.8 7 93

f metakaolin geopolymer pressured composite.

Fig. 3. Histogram and pore size distribution curve metakaolin geopolymer reference composite.

Fig. 4. SEM image of the pressure compacted metakaolin geopolymer compositeafter 24 h of the hardening.

V. Zivica et al. / Construction and Building Materials 25 (2011) 2206–2213 2209

3. Results and discussion

3.1. Mechanical properties and pore structure results

The results of the estimation of bulk weight, compressivestrength and pore structure parameters are summarized in Table 3.

The significant difference between the pressure compacted andreference pastes is evident. An increase in the benefit of pressurecompacted paste is 128.7% at the bulk weight and ca. 500 timesat the compressive strength values.

An adequate relationship values of total porosity and poremedian of the pressure compacted probes opposite to those ofreference paste occured (Table 3). At the same time correspondingcontent of macro-pores (r > 100 nm) is 4% at the pressure com-pacted paste and 93% at the reference one, and a reverse ratio�96% and 4%, at micropores contents (r < 100 nm).

The histograms and pore size distribution curves shown in Figs.2 and 3 provide a suitable illustration about the character of porestructure. At the comparison it can be seen at the pressure com-pacted paste a significant shift of pore size curve and the character

Fig. 5. Composition of the pressure compacted metakaolin composite after 24 h of the hardening analysed by EDX.

Fig. 6. SEM image of the reference metakaolin geopolymer composites after 24 h ofthe hardening.

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of the histogram in the area of nano-pores with radius under100 nm. It shows the pressure compacted paste as a material withthe increased pore structure homogeneity and much finer porestructure. It may have an interesting and significant consequencefor the strength characteristics of the material. In the sense with

Fig. 7. Composition of the reference metakaolin geopolymer

the Griffith’s law it can cause an increase of the critical tensionin the benefit of the pressure compacted paste [22].

The found significantly high compressive strength of pressurecompacted paste is undoubtedly based on its advanced pore struc-ture developed under the use of the low l/s ratio and pressure com-paction. The metakaolin paste prepared under these conditionswas able to overcome that prepared under the current conditions.It means under the use of the current l/s ratio and without thepressure compaction.

As it is reported in the literature the currently prepared metaka-olin pastes reached the compressive strength 5 MPa after 1 day (l/sratio 0.70) and ca. from 15 to 90 MPa after 7 days of the hardening[23,24]. The compressive strength values between 0.40 to 64 MPaand 38 MPa are also reported [25–27].

3.2. SEM and EDX results

The shown difference in the comparison of these values withthat 146.6 MPa reached by the pressure compacted paste thepositive character of pore structure was confirmed by the SEMimages (Figs. 4 and 6). As it can be seen the microstructure forthe reference paste is highly in compact, with the large grainsand pores shown as dark places. The occurrence of flake-like layerstructure of metakaolinite particulates is an evidence of the lowdevelopment of the geopolymerization process. Evidently, only asmall quantity of the geopolymer product is taking the place atthe surface layer of the metakaolin particulates [28,29].

composite after 24 h of the hardening analysis by EDX.

Table 4Composition of the metakaolin geopolymer pastes after 24 h of the hardeninganalysed by EDX.

Element as oxide(wt.%)

Pressure compactedpaste

Referencepaste

Metakaolin

SiO2 37.32 49.71 53.68A12O3 33.41 39.44 42.00Na2O 8.38 6.60 –SiO2/A12O3 molar

ratio1.90 2.14 2.17

Na2O/A12O3 molarratio

0.41 0.26 –

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Due to the use of the different magnification the comparison ofthe SEM results enabled only a limited information on the influ-ence of the combination and the low l/s ratio and high-pressurecompaction on the morphology of the particles of the system.But in principle SEM results shown the same tendency of the influ-ence like it can be seen at the porosimetry results.

The microstructure of the pressure compacted paste is dis-tinctly different. In this material, a ‘grain’ structure is still apparent,but the intervening material is of more homogeneous nature, withsmall pores. This observation was linked to a strong correlation inthe compressive strength increase found but also reported in thepaper [22].

The coupling of the SEM with an EDX provided an analysis ofthe surface of the studied materials. It should be expressed thatthe data obtained have only semiquantitative value (Figs. 5 and7). The presence of the main elements O, Si, Al, Na and minor ele-ments K, Fe and Mg can be seen in the EDX spectra of the geopoly-mers. Table 4 shows the composition of the geopolymers occurringin the studied pastes calculated using EDX data. The decreased SiO2

and Al2O3 contents in both pastes opposite to the starting values

Fig. 8. Thermogravimetric (TG) and differential thermal (DTA) analysis curves of the p

are expressed at the pressure compacted paste. The principle ofthe effect is not clear. It can be only assumed as a consequenceof the effect of the geopolymer formation.

The content of Na2O in the pressure compacted paste is by30 wt.% higher opposite to that of reference paste. One reason couldbe the different concentrations of the used activator solutions givenby the different l/s ratios. These caused that under the use of thesame NaOH quantity of 7 wt.% but at the different l/s ratios the con-centration of the activator solution was 46.7 wt.% at the pressurecompacted paste but only 9.1 wt.% at the reference one.

The significance of the NaOH concentration in the process ofgeopolymer formation is unquestioned. Rowles and OConnor re-ported that the compressive strength of geopolymer depends sig-nificantly also on the Na:Al ratio together with Si:Al. Thecompressive strength is increased as the both ratios are increased[25]. It is well known that variation of the SiO2/Na2O ratio signifi-cantly modifies the degree of polymerization of the dissolved spe-cies in an silicate solution [24]. This plays a significant role indetermining the structure and properties of geopolymer gels. Inthe case of the NaOH activator with low concentration, there wouldbe insufficient amount of OH� ions to completely dissolve Si4+ andAl3+ from metakaolin, and insufficient Na1+ to allow for completepolymerization of the network. These both lead to unreactedmetakaolin part and, therefore, lower strength of material. In thecase the high sodium content, there would be excess OH�1 ionsallowing for a complete dissolution of Si4+ and Al3+ frommetakaolin.

The significance of the concentration of NaOH solution for themechanical properties of the geopolymers is confirmed also bythe results reported in the paper [29]. An increase of a compressivestrength from ca. 35 to 65 MPa has been shown when the NaOHsolution concentration increased from 4 to 12 mol L�1. This effectis attributed to the enhanced dissolution of the metakaolin

ressure compacted metakaolin geopolymer composite after 24 h of the hardening.

Fig. 9. Thermogravimetric (TG) and differential thermal (DTA) analysis curves of the reference metakaolin geopolymer composite after 24 h of the hardening.

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particulates and hence the speeded condensation of the monomerin the presence of NaOH with higher concentration.

3.3. Thermal analysis results

The results of the thermal analysis of studied materials areshown in Figs. 8 and 9. As it can be seen these results enable onlylimited information on the phase composition of system. It is evi-dently the consequence of the amorphous or near-amorphous char-acter of the constituents of the materials studied, thus a use of anymethod is rather restricted. DTA-curves are characterized (i) by theoccurrence of the effects in the region about 100 �C which are due tothe loss of evaporable water, and (ii) by the occurrence of the exo-therms at 970 �C which are caused by the mullite formation. TGcurves shown in both pressure compacted and reference pastesvery near levels of weight loss 7.2 and 7.9 wt.%. Besides the ad-sorbed water, the loss is also due to the chemically bound watertypical for a series of hydraulic hydrates. The thermoanalytical datastrongly favor a qualitative similarity of phase composition of pres-sure compacted and reference probes, thus supporting the datainterrelations of pore structure and mechanical tests on the densi-fication and strength development as a unique effect of pressurecompaction.

4. Conclusion

The combination of the low alkali activator solution:binder ra-tio (l/s) with the pressure compaction of the fresh mixture as a sig-nificant accelerator of the development of the pore structure andon it dependent strength has been shown. At the given case theuse of l/s 0.08 and 300 MPa compaction pressure resulted in thevery dense near-nano-pore structure with the high degree of thehomogeneity and the compressive strength overcoming the refer-ence material ca. 500 times.

The found positive effect of the combination of the low l/s ratioand high-pressure compaction also at the another alkali activatedalumino-silicate materials can be expected.

The Si:Al ratio in the starting alumino-silicate materials, fine-ness, the portion and type of alkali activators used, curing regime,and other parameters are undoubtedly important for the develop-ment of the properties of the geopolymer composites with the po-tential in the quality differences of the systems based on thevarious aluminosilicates. Therefore, these factors are worthy ofthe more detailed study because the pressured geopolymer com-posites seem to be very interesting materials.

Acknowledgement

The authors would like to thank to Slovak Grant Agency VEGAfor its support towards this work (Grant No. 2/0055/08).

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