alkali activated binders and pozzolan cement binder.pdf
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ALKALI-ACTIVATED BINDERS AND POZZOLAN CEMENTBINDERS COMPETE BINDER REACTION OR
TWO SIDES OF THE SAME STORY?
A. Buchwald 1, Ch. Kaps 2, M. Hohmann 3
Bauhaus-Universitt Weimar, Professur Bauchemie, Weimar, Germany1 [email protected] [email protected]
ABSTRACTIncreasingly attention has been paid in the last years on alkali-activated binders such as alkali-activated fly ash and slag. Related binders are known as geopolymers which are at least alkali-activated metakaolin binders. In all cases the activation of the solid is made with different alkalinesolutions containing alkali hydroxides, alkali silicates and/or alkali carbonates. It could be shown byother authors [1, 2] and own investigations [3, 4] that other alumosilicate minerals and thermal acti-vated clays react to stable solids after alkali-activation too. All this materials fly ash, slag, meta-kaolin, thermal activated clays are well known as pozzolanic or latent-hydraulic components incements as well.
Are these different binder reactions or not? It will be discussed the common and the different as-pects of both binder systems based on literature and own investigation. Results are presented forsolubility tests and strength performance of different alkali-activated binders made from fly ash,slag, metakaolin, thermal activated clay and brick powder.
1 BINDER REACTION STEPS
1.1 Alkali-activated binder systemsA reactable material in the light of alkali-activated binders has to be consist of a certain amount ofhigh energetic phases like a glassy phase (fly ash, slag). Clays can be transformed into a reactivematerial by a thermal activation process in which the dehydroxylation of the clay mineral leads toan high energetic, unstable and nearly amorphous state of the solid. The reaction mechanism of al-kali-activated materials can be described so far in two steps,
1) The generation of reactable species (alkaline activation)Alkali hydroxide disintegrates the solid network to produce small reactable silicate and alumi-nate species.
2) The real setting reactionPure alumosilicate materials like metakaolin and some types of fly ash set under a condensationreaction which leads to the formation of alumosilicate polymers [5]. As it could be shown byseveral authors the setting of alkali-activated slag and calcium containing materials can be char-acterised by both a condensation reaction and a more or less hydration reaction forming CSH-(or Al-substituted CSH-) and CAH-phases depending on the calcium content and the alkalinity[6-9].
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A detailed characterisation of the reaction products seems to be quite difficult because of the lowcrystallinity of the reaction products. Depending on curing temperature and alkalinity the alumosili-cate polymer changes into a zeolithic phase detectable by XRD.
1.2 Pozzolan cements and slag cementsThe reaction of pozzolans and slag with OPC is usually described by the following two definition.The pozzolan forms generally spoken additional CSH-phases with the Ca(OH)2 from cement hy-dration. The latent-hydraulic material needs an activation to form CSH-phases.The different performance of pozzolanic cements has been shown in many reports, especially lossof strength if the pozzolan content riches high values or if using slag cements in combination withfly ash. A high pH-value in the pore solution is undoubtedly necessary to get a good pozzolanicperformance of the cement [10, 11]. Obviously the alkali hydroxide is necessary to disintegrate theglassy or amorphous phases to produce reactable silicate and aluminate units. This units react withcalcium hydroxide to CSH- and CAH-phases [12].
1.3 Similarity of the binder reactionsAs one can see the two binder systems are more similar than different. Provocative spoken the poz-zolan cement looks like a special case of alkaline activated systems. The alkaline activation is guar-anteed by the OPC pore solution. Therefore a certain amount of OPC is necessary for a good per-formance. But the first reaction step is the alkaline activation of the reactive solid to producereactable monomers which can afterwards react with calcium ions.
2 EXPERIMENTAL INVESTIGATIONS
2.1 Solubility of SiO4- and AlO4-monomers by alkaline solutionsHow alkali hydroxide disintegrate the reactive phases of the solid material can by seen directly insolubility tests. The usefulness of such tests to determine the reactivity of different raw materialshas been described by several authors [2, 13, 14] and has been widely used in own investigations.
Several test regimes has been used to determine the release of silicate and aluminate monomers inalkaline solution:Regime 1: The solubility of the solid in 10% NaOH-solution has been measured for long period
investigation up to 90 days. The mass ratio solution to solid of 400 was used.Regime 2: The same conditions as in regime 1 (40g of 10% NaOH-solution) with a contact time of
20 h but under variation of the mass of the solid (mass ratio solution to solid between 8and 400, resp. between 2 and 139 mg solid per ml solution).
Regime 3: The influence of concentration and kind of alkaline solution has been investigated withthe following test parameters:- alkaline solution: KOH, NaOH, LiOH- concentration of the solutions: 0.01, 0.03, 0.06, 0.1, 0.3, 0.6, 1 and 2.8 mol/l- solid-to-solution of 2,5 mg/ml- duration of test: 20 h
The concentration of silicate and aluminate monomers are determined by measuring the elementconcentration of Si and Al using Optical Emission Spectroscopy. It has been proven that the silicatespecies are really monomeric units under these conditions. Therefor the small reactive species willbe designate as monomers furthermore.
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2.2 Binder preparation and strength measurementsPure binder specimen were prepared by mixing the solid material with the defined amount on solu-tion containing water and solved sodium hydroxide. The amount of water has been levelled on thesame workability of the paste. No aggregates are added.The flexural strength (centre point loading) has been measured on specimen with an geometry of101060 mm3, the compressive strength on the broken samples with an compression area of1020 mm2. Before testing all samples were cured one day at 35C without drying and the follow-ing days at room temperature under 60% relative humidity.
3 CHARACTERIZATION OF THE RAW MATERIALS
Five different materials were used in our investigation: a commercial metakaolin, a slag, a fly ash.The brick powder and the thermal activated clay base on the raw material which contained of themain phases quartz, illite and kaolinite beside some feldspar and carbonate. Both are heated up to850C, the brick under oxygen reduced, water enriched atmosphere and the clay in a normal labo-ratory oven under normal air. The chemical composition of these materials are shown in Figure 1.
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
metakaolin
slag
fly ash
thermal activated clay
brick powder
chemical composition [w.-%]
LOISiO2Al2O3Fe2O3CaOMgOTiO2K2ONa2O
Figure 1 Chemical composition of the investigated materials
4 RESULTS AND DISCUSSION
4.1 Influence of the contact time of the reactive solid with NaOH solution on the release ofsilicate and aluminate monomers and strength performance of the binder
Figure 2 presents the results getting by measuring the release of silicate and aluminate monomers ina 10% sodium hydroxide solution after different contact time with the solid material (test regime 1).As one can see the metakaolin shows the highest release per weight of both silicate and aluminateeven after few hours. It is followed by the fly ash, but this material needs some time before a certainamount of monomers are detectable in the solution. This is clear because of the smaller surface ofthe particles and the higher stability of the glassy phase compared to the metakaolinit phase. Theamount of released aluminate monomers is lower according to the lower amount of aluminate in thematerial.
The thermal activated clay and the brick powder reach almost the same amount of silicate and alu-minate in the solution which is understandable because of the almost identical chemical and phase
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composition. The differences in the light of the release kinetic are due to the different surface of thematerials.
The results of the slag material depart from these of the other materials. The measured amount ofsilicate and aluminate are quit low and do not change over the contact time. The available calciumions seem to react with the released silicate and aluminate monomers to insoluble CSH- and CAH-phases as precipitate. CAH-phases and partly carbonated CAH-phases could be detected in theeluation residue by XRD. The amount of crystalline C2S and C3S-Phasese were decreasing as well.
0
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metakaolinfly ashslagthermal activated claybrick powder
Figure 2 Influence of contact time on the release of silicate (left side) and aluminate(right side) monomers, measured after regime 1
Will the release behaviour be found again in the strength performance? The results of the compres-sive strength measured after the same curing time steps are presented in Figure 3. Looking only onthe progress of the compressive strength one are forced to correlate the results of both investigation.But the trends are not significant and the distribution of the strength values is quite high. Apart fromthis it is noticeable that the only latent hydraulic material the slag performs at best as it was tobe expected. The different strength levels can not be compared directly as it will discussed later.
Should it be expected to found the same progress of both the release of monomers in the solubilitytest and the strength performance? If looking on the parameter used in the solubility test of regime 1there has been set ideal conditions for analysis and discussion of the reactivity. In contrast to solu-bility test the solid-to-solution-ratio of real binder systems reaches easily values over 1 g/ml poresolution. Thats why we used test regime 2 to measure the influence of solid-to-solution-ratio on thesilicate and aluminate monomer concentration released from metakaolin by keeping the NaOH con-centration constantly. The results are presented at Figure 4.
It can be seen that the concentration of silicate and aluminate monomers reaches a maximumaround a ratio of 20 mg/ml. Either the solution process arrives an equilibrium or released monomerscondensate already to insoluble alumosilicate which will be remain in the eluation residue.
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010
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0 10 20 30 40 50 60 70 80 90 100curing time [d]
com
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slagfly ashthermal activated claybrick powder
Figure 3 Influence of curing time on the strength development of some alkali-activatedbinders (compositions of the same ratio NaOH to solid material are compared)
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3 6 14 28 55 139 mass solid per volume NaOH-solution [mg/ml]
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Figure 4 Influence of the ratio solid to solution on the release of silicate and aluminatemonomers from metakaolin (test regime 2)
4.2 Influence of the concentration of sodium hydroxide on the release of silicate and alumi-nate monomers and strength performance of the binders
The concentration of the alkaline solution plays an important role for the reaction step 1 (disinte-gration of the solid material). The silicate and aluminate monomers released from metakaolin usingdifferent concentration of sodium hydroxide are presented in Figure 5. The lower the pH-value ofthe solution the lower the monomer concentration will be. There are no selectivity between silicateand aluminate.
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00.5
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Si Al
Figure 5 Influence of the NaOH concentration on the release of silicate and aluminatemonomers from metakaolin (test regime 3)
The strength performance of binders prepared with different NaOH amount per solid material arepresented in Figure 6. In general the strength increases by an increasing content of sodium hydrox-ide.
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0% 10% 20% 30% 40%NaOH/solid [w-%]
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sive
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ngth
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metakaolin*fly ashslagthermal activated claybrick powder
Figure 6 Influence of the NaOH content on the compressive strength of the alkali-activatedbinders after 28 days (*14 days)
Because of the different used water contents getting a certain workability the concentration of thepore solution will be different for two binders even with the same NaOH amount per solid material.Therefor the strength values are presented again in dependence of the NaOH concentration in theinitial pore solution (Figure 7). The compressive strength shows the almost same dependence fromthe NaOH concentration for all materials apart from the slag. However in every binder the pore vol-ume which has to be filled by the reaction products is very different. Metakaolin needs much morereactable monomers than fly ash because of its higher surface area. On the other hand the reactivity
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of the materials by means of releasable amount of monomers is quite different too (see results in4.1). A different setting behaviour shows again the slag material. The building of CSH- and CAH-phases leads to good strength performance even at low alkalinity.
0
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0 5 10 15NaOH concentration in pore solution [mol/l]
com
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sive
stre
ngth
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m]
metakaolin*fly ashslagthermal activated claybrick powder
Figure 7 Influence of the NaOH concentration in the pore solution on the compressivestrength of the alkali-activated binders after 28 days (*14 days)
4.3 Influence of the kind of alkaline hydroxide on the release of silicate and aluminatemonomers from metakaolin
The pH-values of the single alkaline solutions varying in concentration and kind of alkali ion areshown in Figure 8. If replacing the sodium ions by other alkaline ions in the solubility test onewould expect an order of released monomers correlated to the hydroxide activity. That means wewould expect a higher solubility performed by KOH than by NaOH because of the higher alkalinity.
11.5
12.0
12.5
13.0
13.5
14.0
0.0 0.1 1.0 10.0
molar concentration [mol/l]
pH-v
alue
KOHNaOHLiOH
Figure 8 Influence of the concentration and kind of the alkaline solution on the pH-value(an electrode with a low alkaline error was used for measurements)
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The results using test regime 3 to determine the influence on the kind of alkaline solution are repre-sented in Figure 9. The diagram shows only the release of silicate from metakaolin. Similar to Fig-ure 5 there are no selectivity between silicate and aluminate. The order of monomer releasing fol-lows our expectation only as regards NaOH and LiOH. The KOH which has a much higherhydroxide activity doesnt show a higher monomer releasing. FRAAY [13] got similar results ifeluating fly ash with KOH and NaOH solution. Obviously this effect cant be described thermody-namically alone. May be there are kinetic effects due to the different ion size (diffusion rate in thereaction zone) playing an important role by the disintegration of glassy and dehydroxylated solidphases.
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Figure 9 Influence of the concentration and kind of alkaline-solution on the on the release ofsilicate monomers from metakaolin (test regime 3)
5 CONCLUSION
It could be shown the disintegration of glassy and dehydroxylated phases by the presented results ofthe solubility tests. The chosen test parameter were optimised for ideal measurement conditions andtherefor they are not identical with those in the real process in the pore solution. Nevertheless onecan extract the following conclusions:
1) Glassy and dehydroxylated alumosilicate phases will be disintegrated by alkaline solution intosilicate and aluminate monomers.
2) The amount of released silicate and aluminate monomers depends on the content of the reactivephase and the kind and concentration of the alkaline solution.
3) The kinetics of disintegration depend on the contact surface and the kind of the reactive phaseand on the degree of disorder too.
4) The maximum on released monomers will not be reached in real binder systems because of thelower amount of alkaline solution. The layer which has been precipitated during alkaline activa-tion can limit the process of further disintegration. The remaining solid grains will than work asaggregates.
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5) If calcium ions are present in the solution as well the building of insoluble CSH- and CAH-phases will be preferred.
These points confirm the assertion that alkali-activated binders and pozzolanic cement binders aretwo sides of the same story. In pozzolanic cement binders exist two connected reaction types, theordinary cement hydration and the alkali-activation of the pozzolanic material by the alkaline poresolution. As reaction products one can find primary CSH and CAH-phases (from the OPC), secon-dary CSH- and CAH-phases (from the reaction of Ca(OH)2 with the released silicate and aluminatemonomers) and may be alumosilicate polymers. Alkali-activated binders which will be activated byalkaline hydroxide leads to alumosilicate polymers partly detectable as crystalline zeolitic phases orto CSH and CAH-phases if calcium ions are present in the solution [9].
6 REFERENCES
[1] Davidovits, J.; Buzzi, L.; Rocher, P.; Gimeno, D.; Marini, C.; Tocco, S.: Geopolymeric cement based onlow cost geological materials. Results from the European research project GEOCISTEM, 2nd interna-tional conference of geopolymere, 1999, pp.83-96.
[2] Xu, H. and van Deventer, J.S.J.: The geopolymerisation of alumino-silicate minerals. Int. J. Miner.Process, vol.59, 2000, pp.247-266.
[3] Kaps, Ch. and Hohmann, M.: Mineralische Polymerbinder aus hydrothermal gebranntem Ton, 4th Ibau-sil, Weimar, 2000, vol.1, pp.415-424.
[4] Kaps, Ch.; Hohmann, M. and Hohmann, H.: Ein mineralisch gebundener Leichtbau-Werkstoff, Materi-als Science and Restoration, MRS V; Esslingen, 1999, pp.1593-1602.
[5] Davidovits, J.: Geopolymers:inorganic polymeric new materials, Journal of Thermal Analysis, vol.37,1991, pp.1633-1656.
[6] Brough, A.R.; Katz, A.; Bakharev, T.; Sun, G.-K.; Kirkpatrick, R.J.; Struble, L.J.; Young, J.F.:Microstructural Aspects of zeolite formation in alkali activated cements containing high levels of flyash, Materials Research Society Symposium Proceeding, vol.370, 1995, pp.199-208.
[7] Brough, A. R. and Atkinson, A.: Sodium silicate-based, alkali-activated slag mortars. Part I. Strenght,hydration and microstructure, Cement and concrete research, vol.32, 2002, pp.865-879.
[8] Alonso, S. and Palomo, A.: Calorimetric study of alkaline activation of calcium hydroxide + metakaolinsolid mixtures, Cement and Concrete Research, vol.31, 2001, pp.25-30.
[9] Granizo, M.L.; Alonso, S.; Blanco-Valera, M.T.; Palomo, A.: Alkaline activation of metakaolin: Effectof calcium hydroxide in the products of reaction, Journal of American Ceramic Society, vol.85, 2002,pp.225-231.
[10] Fraay, A.L.A.; Bijen, J.M .; de Haan, Y.M.; Larbi, J.M.: The reaction of fly ash in concrete A criticalexamination. Fly ash, silica fume, slag an natural pozzolans in concrete, supplementary papers ofCANMET/ACI Int. Conference, 1989, pp.33-48.
[11] ACI commitee (ed.) Use of fly ash in concrete. ACI 232.2R-96[12] Hbert, C.; Wieker, W.; Heidemann, D.: Investigation of hydration products in high volume fly ash
binders, Fly ash, silica fume, slag an natural pozzolans in concrete, 7th CANMET/ACI Int. Confe-rence, Chennai, vol.1, 2001, pp.83-98.
[13] Fraay, A.L.A.: Fly ash a pozzolan in concrete, PhD-Thesis at University Delft, 1990.[14] He, C.; Osbck, B.; Makovicky, E.: Pozzolanic reaktions of six principal clay minerals: Activation,
reactivity assessments and technological effects, Cement and Concrete Research, vol.25, 1995,pp.1691-1702.
ABSTRACTBINDER REACTION STEPSAlkali-activated binder systemsPozzolan cements and slag cementsSimilarity of the binder reactions
EXPERIMENTAL INVESTIGATIONSSolubility of SiO4- and AlO4-monomers by alkaline solutionsBinder preparation and strength measurements
CHARACTERIZATION OF THE RAW MATERIALSRESULTS AND DISCUSSIONInfluence of the contact time of the reactive solid with NaOH solution on the release of silicate and aluminate monomers and strength performance of the binderInfluence of the concentration of sodium hydroxide on the release of silicate and aluminate monomers and strength performance of the bindersInfluence of the kind of alkaline hydroxide on the release of silicate and aluminate monomers from metakaolin
CONCLUSIONREFERENCES