A. Buchwald, K. Dombrowski, M. Weil: The influence of calcium content on the performance ofgeopolymeric binder especially the resistance against acids. 4th International conference on Geopolymers, 29.6.-1.07.05, St.Quentin, France

The influence of calcium content on the performance of geopolymeric binder especially the resistance against acids A. Buchwald*, K. Dombrowski**, M. Weil**** Bauhaus-University Weimar, Chair of building chemistry, Germany ** Freiberg University of Mining and Technology; Institute of Ceramic, Glass and Construction Materials; Germany *** Forschungszentrum Karlsruhe; Department of Technology-Induced Material Flow (ITC-ZTS); Germany Abstract The influence of calcium content on the performance of fly ash and metakaolin geopolymeric binder has been determined. Therefore fly ash or metakaolin, respectively, was substituted by dif-ferent amounts of calcium hydroxide. The kind of the reaction product and its proportion should mainly influence special properties e.g. acid resistance, and might also influence the hardening behaviour. Strength performance and acid resistance against hydrochloric and sulphuric acid have been determined. The results have been related to the kind and composition of the reaction product which has been detected by 29Si NMR, XRD, and Thermal analysis. Beside calcium containing zeolitic phases the calcium has built CSH using the silicon from the fly ash and the metakaolin, respectively. 1 Introduction Geopolymeric binders or alkali-activated cements are made by means of an alkaline activation of materials reactive in this respect [1]. Concerning the “reactive material” there is a wide range of possible raw materials. Beside primary resources like clay [2] (after thermal activation) and natural pozzolan [3] for instance volcanic ashes, a considerable variety of secondary resources like ashes [4] and slags [5] from different processes can be used. As it is known the reaction products from the alkaline activation differ significantly. Slags which contain a lot of calcium oxide in the reactive glass form CSH- and CAH-phases similar to such formed by the hydration of Portland cement, apart from incorporated tetrahedral aluminium into the dreierketten structure of the calcium silicate hydrates [6]. Pure alumosilicate materials like meta-kaolin or alumosilicate fly ashes form alumosilicious polymer networks. The question occurs: What happens in between these extreme different calcium contents? It could be shown on model mixtures that both reaction products (alumosilicate and CSH/CAH-phases) coexist in different proportions depending on concentration of the alkaline activator [7]. Several raw materials especially clays and natural pozzolana contain about 5-20 % CaO. The kind of the reaction product and their proportion should mainly influence special properties e.g. acid resistance, and might also influence the hardening behaviour. In particular as is known that the acidic resistance is low for normal Portland cement binders and high for geopolymeric binders in general [3, 8]. In this field the authors put their investigation. 2 Investigation Materials, composition of mixture and sample preparation Two sets of mixtures one with an alumosilicate fly ash and the other with metakaolin has been in-vestigated (see Figure 1). The chemical composition of the used materials is documented in Table 1, the phase composition is shown in Figure 2. The composition of each mixture is documented in Table 2. The powders have been activated with 8 mol/l NaOH solution. The calcium content of these model mixtures has been increased by successive exchange of the pure alumosilicate powder against calcium hydroxide up to 40 % per mass. Samples of 1 x 1 x 6 cm³ were prepared and kept inside the moulds at 40°C for three days (fly ash binder) or one day (metakaolin binder). Afterwards the samples were stored in closed boxes over water at room temperature.

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Measurement of strength, density, porosity and reaction degree Flexural and compressive strength as well as density and porosity have been measured after 28 and 111 days of storage. The flexural strength (centre point loading) has been measured on original specimens (geometry: 1 × 1 × 6 cm3), the compressive strength was measured on the broken flex-ural-strength samples with a compression area of 1 × 2 cm2. To determine the envelope density, sample volume was measured geometrically and the weight was taken of the dried samples. The bulk density was measured on the grinded material (< 63 µm) with a helium pycnometer. Porosity was calculated from these two densities. The reaction degree was determined by dissolving the dried and grinded binders in hydrochloride acid following instructions given in German standard DIN 52 170. Acid resistance After the storage of 28 days or 111 days, respectively, the specimen were dried at 40°C in a labora-tory oven for one week. The so prepared samples were immersed in different acid solutions as de-scribed as follows: 1) 6 hours exposure to 70 % sulphuric acid at 100°C (sample age: 28 and 111 days) 2) 28 days exposure to 5 % sulphuric acid at 60°C (sample age: 28 days) 3) 28 days exposure to 5 % hydrochloric acid at 60°C (sample age: 28 days). The pH-value of the acid solution during the long term exposure was kept constant by means of regularly exchange of the acid solution. The mass loss has been measured after the total exposure time. Structural investigation Solid state NMR experiments were performed with a Bruker AvanceTM 400 MHz WB, the reso-nance frequency of 29Si and 27Al is 79,52 and 104,29 MHz, respectively. For the 29Si MAS NMR spectra the samples were packed in 4 mm rotors and spun at 55 kHz under the magic angle. The chemical shifts were recorded relative to external tetramethylsilane (TMS). Single pulse technique was applied, a typical number of scans was 12000. For the 27Al MAS NMR spectra the samples were packed in 4 mm rotors and spun at 15 kHz under the magic angle. The chemical shifts were recorded relative to external Al(H2O)6

3+. Single pulse technique was used and the number of scans was 1000. The signal patterns of the spectra were deconvoluted with the Seasolve PeakFit software. In order to determine the X-Ray diffraction pattern the dried material was mixed with a standard material (ZnO about 10 % per weight) and grinded under addition of iso-propylalcohol in a bar mill (McCrone-Micronizer, McCrone Ltd., UK) about 1 minute. Thermal analysis was determined in a Seteram Setsys 1750 equipment under air using a heating rate of 10 K/min in the temperature range of 30-1000°C. 3 Results and discussion Performance of the binder The calcium content influences the hardening behaviour as it can be seen in Figure 3 and Table 3. The slow hardening of the fly ash can be shortened by the addition of Ca(OH)2. Blends with a con-tent of 20 % and higher showed a similar hardening behaviour like its known from slag binders. The samples reached a strength more than 40 N/mm² after 28 days and the strength after 111 days was not drastically higher. In the beginning the mixtures with a low calcium content performed worse compared to the pure ash but after a longer time of hardening these blends reached a higher strength than the samples with high calcium content (Figure 3). As seen from the density measurements the matrix seams to get more compact and dense with increasing content of calcium hydroxide as also shown in SEM images in Figure 4. It should be mentioned that there is a homogeneous distribution of calcium in the matrix as could be proven by SEM element mapping (Figure 5).

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The same trend can be seen on the blends with metakaolin (Figure 6). An optimal calcium content seems to be about 10 %. Because of the higher reactivity of metakaolin the extreme time depend-ence of the hardening behaviour can not be found like for the fly ash binder. The resistance against acids has to be discussed in dependence of the kind of acid which has been used in the test. Figure 7 shows results of the samples prepared with fly ash at an age of 28 and 111 days which were exposed to 70 % sulphuric acid at 100°C for 6 hours. As it can be seen clearly the mass loss is mainly influenced by the mechanical resistance of the binder due to its porosity rather than by the composition of the binder (see Figure 7, right hand side). The long time exposed sam-ples to 5 % sulphuric acid showed a similar tendency, with a high mass loss of the sample prepared with 8 % calcium hydroxide after 28 days (Figure 8). The corrosion appeared from the volume expansion which has to be related to new built gypsum if calcium is available in the mixture. Sam-ples exposed to 5 % hydrochloric acid showed a steady mass loss in dependence on the calcium content (Figure 8). This can be due to the degradation of several unstable phases like unreacted portlandite, carbonated phases like calcite and sodium carbonate or even due to calcium silicate hydrate phases. Structure The reaction products of the fly ash binders are mainly X-ray amorphous as expected. Some little amount of already crystalline alumosilicate could be detect as sodalith. Some of the added calcium hydroxide remained unreacted so far. After 111 days little amount of poorly crystalline CSH phases could be detected in the samples with 20 and 30 % calcium content. No CAH phases could be found using XRD and Rietveld refinement. SEM element mapping (Figure 5) showed a homogenous distribution of calcium in the matrix. Obviously the reaction products have a good coexistence. This becomes more clear looking on the X-Ray results of the metakaolin binders (Figure 9). Because of their higher reactivity the metakaolin binders ended up with a higher reaction degree containing a higher amount of crystalline phases. The diffraction pattern of the 14 % metakaolin sample shows several peaks between 5 and 23° 2Theta belonging to different zeolitic phases as well as CSH- and CAH-phases. The Rietveld refinement preferred mostly the calcium containing zeolithes for in-stance the faujasite type. Because of the low crystallinity of the material the quantification results is not illustrated and interpreted due to a not satisfying accuracy. Nevertheless it can be concluded that both type of phases – zeolitic phases and CSH phases; partly crystalline – have been formed during reaction. The results of the thermal analysis support this thesis. As it can be seen in Figure 10 in the tempera-ture range of 20-300°C the sample without any calcium shows a single peak with the local mini-mum at 125°C due to water evaporation. An increasing calcium content results in a fanning out of this peak which underlines the thesis of existence of several water containing phases. Beside this the DTA graphs show the degradation of the unreacted calcium hydroxide (around 400°C) as well as of built carbonates around 700°C and some crystallisation effects between 700 and 1000°C. A more clearer answer should be given by using NMR spectroscopy. The 27Al NMR spectra of the fly ash binder showed no more than 4 coordinated aluminium with a sharp peak around 60 ppm which can be either related to the aluminium tetrahedron incorporated in the silicate chains of the CSH-phase or bonded in the alumosilicate network. This difference might be cleared up by using 29Si NMR spectroscopy. Silicon tetrahedron which are bonded in silicate chains can either be situ-ated at the chain ends (Q1; signal at –78 ppm ± 2 ppm) or in the middle (Q2, signal at –85 ppm ± 2 ppm). If aluminium is incorporated into the silicate chain as bridging tetrahedron an additional peak can be seen at -82 ppm ± 2 ppm (Q2(1Al)). Zeolithes are built by a network of sili-con and aluminium tetrahedron which are connected over all four edges to other tetrahedron. There-fore five possible peaks of Q4 which are connected to zero, one, two, three or four aluminium tetrahedron might be detectable in the spectra between –110 and –85 ppm. The results of the 29Si NMR spectroscopy of the 28 day old fly ash binders are given Figure 11. The very low reaction degree (Table 3) can be seen by the broad fly ash hump centred around -100 ppm. The main peak of the reaction products are found between –70 and –90 ppm. With increasing cal-

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cium content the centre of this peak seem to shift from –88 to –86 ppm connected with a spread-ing/broadening. The deconvulation – shown in Figure 12 – clarifies this oddity. Extremely oping peaks of the Q

verlap-ise

Conclusion uctural investigation that both - zeolitic phases and CSH-phases - coex-

hydroxide to fly ash based geopolymers improves the es

y ash binders under the here measured conditions had been not much

cknowledgement erman foundation VolkswagenStiftung for financial support. It has to recip-

eferences its, Geopolymers: Inorganic polymeric new materials, J. of Thermal Analysis, 37,

[2] aps: Property controlling influences on the generation of geopolymeric

[3] co: Geopolymeric Cement S-

[4] lomo: Alkali activated Fly Ashes: Properties and Characteristics.

[5] tate and Future of Alkali-Activated Slag Concretes. In: V.M.

[6] . Eaton: 29Si and 27Al MAS-NMR of NaOH-Activated

[7] "Cal-n

[8] ive Environ-

4(1Al)- and the Q2-signal from zeolitic and CSH-phase respectively summarto this broad peak. An exact quantification has not been done because of the strong overlapping and the low reaction degree of the samples. Nevertheless it can be said that both – zeolitic phases and CSH-phases – can be proved in the matrix of the calcium containing samples especially at high reaction degrees. The molar ratio Si:Al of the zeolitic phase is larger than 1 because of the well detectable signals up to –105 ppm indicating not only aluminium tetrahedron in the neighbourhood. 4It could be seen from the strist as reaction product in samples with increasing calcium content. Even samples of very high cal-cium content contain zeolitic phases. The added calcium hydroxide was still not fully reacted after111 days especially in the case of the fly ash. X-Ray diffraction indicates the use of calcium as charge compensation in the zeolithes. The addition of high amounts of calciumearly age strength and the strength generally. But the addition of a small calcium content improvthe strength performance at later ages as well. In the case of metakaolin based geopolymers only anaddition of a small amount of calcium hydroxide led to an increase in strength. For both cases – fly ash and metakaolin binder – an exchange of about 10 % of the alumosilicate with calcium hydrox-ide seems to be favoured. The acid resistance of the flaffected by calcium content – probably due to the remarkable amount of zeolitic phase even at higher calcium contents. AThe authors thank the Grocate Dr. Erica Brendler, TU Bergakademie Freiberg, for the NMR measurements. R[1] J. Davidov

1633-1656 (1991) A. Buchwald, Ch. Kbinders based on clay. Geopolymer 2002, Melbourne, Australia J. Davidovits, L. Buzzi, P. Rocher, D. Gimeno, C. Marini, S. Tocbased on Low Cost Geologic Materials, Results from the European Research Project GEOCITEM. In: J. Davidovits et al. (ed.): Proceedings of the second international conference geo-polymere ´99. (1999) 83-96 A. Fernandez-Jimenez, A. PaProceedings of 11th International Congress on the Chemistry of Cement (ICCC), 2003 Durban,South Africa. (2003) 1332 -1339 B. Talling, J. Brandstetr: Present SMalhotra (ed): Proceedings fo the 3rd Int. Conf. of Fly Ash, Silica Fume, Slag and natural Poz-zolans in Concrete (1989) 1519-1545 P.J. Schilling, L.G. Butler, A. Roy, H.CBlast-Furnance Slag. Journal of American Ceramic Society. 77 (1994) 9, 2363-2368 A. Palomo, T. Blanco-Varela, S. Alonso, L. Granizo: NMR study of alkaline activatedcium Hydroxide - Meta-kaolin" solid mixtures. Proceedings of 11th International Congress othe Chemistry of Cement (ICCC), 2003 Durban, South Africa. (2003) 425 - 434 T. Bakharev, J.G. Sanjayan: Alkali-activated slag concrete: Durability in aggressment. Geopolymer 2002, Melbourne, Australia

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Table 1 Chemical composition of the alumosilicate raw materials metakaolin fly ash LOI 0,8% 2,0% SiO2 52,1% 51,7% Al2O3 43,0% 27,8% Fe2O3 0,7% 6,6% CaO 0,0% 2,8% MgO 0,3% 3,2% TiO2 0,0% 1,3% K2O 2,5% 2,7% Na2O 0,1% 1,0%

Table 2 Composition of mixtures Set 1 Set 2 name fly ash Ca(OH)2 NaOH H2O name metakaolin Ca(OH)2 NaOH H2O 0% 76,9% 0,0% 5,9% 17,2% 0% 50,0% 0,0% 12,8% 37,2%4% 73,8% 3,1% 5,9% 17,2% 4% 48,0% 2,0% 12,8% 37,2%8% 70,8% 6,2% 5,9% 17,2% 14% 66,2% 10,8% 5,9% 17,2% 14% 43,0% 7,0% 12,8% 37,2%20% 61,5% 15,4% 5,9% 17,2% 28% 36,0% 14,0% 12,8% 37,2%30% 53,8% 23,1% 5,9% 17,2% 40% 30,0% 20,0% 12,8% 37,2%

Table 3 Reaction degree of fly ash binder Reaction degree after 28 days 111 days 0% n.d. 29% 8% 20% 33% 20% 36% 42% 30% 44% 54%

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Figure 1 Usable raw materials and their position in the CaO-SiO2-Al2O3 ternary diagram compared to the generated mixtures

Figure 2 Phase composition of the used raw materials

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Figure 3 Compressive strength of fly ash binders

Figure 4 SEM image of fly ash binders with 0 %, 8 %, and 20 % Ca(OH)2 content

Figure 5 Calcium distribution in the 20 % fly ash binder

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Figure 6 Compressive strength of metakaolin binders

Figure 7 Mass loss of fly ash binder after treatment in 70 % H2SO4 and compared to the po-rosity of the binder

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Figure 8 Mass loss of fly ash binder after treatment in 5 % HCl and 5 % H2SO4

Figure 9 XRD diffraction pattern of metakaolin binders after 28 days

Figure 10 DTA curve of metakaolin binders after 28 days

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Figure 11 Comparison of the 29Si NMR spectra of the fly ash binder after 28 days

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Figure 12 Peak separation of the 29Si NMR spectra of fly ash binders with 0 %, 8 %, and 20 % Ca(OH)2 content

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