STUDIES ON DEVELOPMENT OF GEOPOLYMER BASED CEMENTS BY ALKALI ACTIVATION OF FLY ASH AND GRANULATED BLAST FURNACE SLAG CURED UNDER AMBIENT TEMPERATURE

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1 STUDIES ON DEVELOPMENT OF GEOPOLYMER BASED CEMENTS BY ALKALI ACTIVATION OF FLY ASH AND GRANULATED BLAST FURNACE SLAG CURED UNDER AMBIENT TEMPERATURE R.S. Gupta, S. Vanguri, V. Liju, S.K. Chaturvedi and A. Pahuja National Council for Cement and Building Materials India Abstract The chemical reaction of concentrated alkaline solution on reactive alumino silicate source materials such as low lime fly ash, under temperature curing, results in three dimensional polymeric chain and ring structure consisting of Si O Al O bonds manifested in development of geopolymeric material which is similar to natural zeolites but amorphous in microstructure. These geopolymers have cementitious properties. The present study aims at evolving compositions derived from mixes of fly ash and blast furnace slag suitable for development of strength giving geopolymer based cement by alkali activation under ambient temperature curing. Blends of fly ash with granulated blast furnace slag (GBFS) prepared by replacing fly ash with 20 and 30 mass percent GBFS were treated with solution of sodium hydroxide maintaining 8.25 % Na 2O content and 20.5 % H 2O content in the alkali treated mix. The alkali treated samples were cast and pre-cured at 27 ± 2 C and 95 ± 5% relative humidity in humidity chamber for a period of 24 ± 2 hrs followed by curing at 27 ± 2 C temperature and 50 ± 5% relative humidity. The samples cured at ambient temperature conditions were studied for compressive strength development at 3, 7 and 28 days and dimensional stability. The compressive strength values were found to be higher at all ages for the mix containing 30% GBFS and 70 % fly ash. The samples were found dimensionally stable in both the cases. The mineralogical and microstructure investigations have indicated formation of calcium silicate hydrates along with sodium alumino silicate hydrate polymeric gels resulting in development of compressive strength. Therefore, the addition of slag in fly ash has contributed significantly in development of strength properties under ambient conditions of curing. 1

2 1.0 Introduction Geopolymeric cements are environment friendly construction materials and can be synthesized by alkali activation of reactive alumino silicate source materials like low lime fly ash. Geopolymerization reaction involves dissolution of Al and Si from alumino silicate source materials by concentrated alkaline solution such as sodium hydroxide, potassium hydroxide or their silicates at high ph, transportation of dissolved species followed by polycondensation forming a 3 D network of silico aluminate structures, where silicon and aluminium are surrounded by four oxygen atoms in a tetrahedral configuration. These are zeolitic precursors existing as sodium alumino silicate hydrate gel and termed as geopolymers. Geopolymers have certain special properties like rapid compressive strength gain, good fire resistance, high surface hardness along with smoothness, therefore, they may be used in construction activities as replacement of Portland cement in some applications. It has been reported by earlier workers that temperature curing of paste of fly ash with alkaline solution in the range ~40 to ~ 95 C is essential step for establishing an adequately interconnected lattice of bonds during geopolymer formation. However, preferred range of temperature for optimum curing has been considered in between 60 to 90 C. Hence this technology can be used for making pre cast bodies, bricks, tiles etc in construction activities (1 7). Some workers have studied formation and co existence of CSH gel and geopolymeric gel by alkali activation of metakaolin and slag blends by initial curing at 40 C and then subsequent curing at 25 C or at room temperature and reported development of better strength performance in the products prepared with addition of optimum amount of GBFS in metakaolin than the geopolymers obtained by alkali activation of metakaolin under the similar conditions (8 10). It has been mentioned that the activation energy required for activation of slag is lower than the fly ash, thus slag can be activated with lower heat requirement (11). Puligilla and Mondal studied microstructural development and hardening of fly ash slag geopolymer by alkali activation at ambient curing (12). This paper presents studies on the formation of cementitious materials by alkali activation of fly ash slag mixes cured under ambient temperature conditions. 2.0 Experimental 2.1 Materials The chemical composition of fly ash used in the investigations is shown in Table 1 A indicating that it was a low lime siliceous fly ash. The physical properties have also been evaluated and reported in Table 1 B. The Blaine fineness value of the fly ash sample was 260 m 2 /kg showing its coarser nature. XRD technique was used for its mineralogical evaluation which showed the presence of quartz, mullite and hematite in this fly ash sample (Fig.1).The microstructure study was also carried out by scanning electron microscopy (SEM) which indicated its characteristic morphology consisting of spherical cenospheres and plerospheres of different size (Fig.2). 2

3 Fig 1. XRD pattern of fly ash sample Fig. 2. Scanning Electron Micrograph of fly ash 3

4 Table 1 A : Chemical Composition of Fly Ash and GBFS * Gain on ignition Nature of Sample LOI % SiO2% Fe2O3 % Al2O3 % CaO % MgO % Na2O % K2O % TiO2 % Fly ash GBFS 1.14* Table 1 B : Physical Characteristics of Fly Ash samples Sample type Blaine fineness, m 2 /kg Lime Reactivity (LR), MPa Cem.Reactivity, % Autoclave Expn., % coarse fly ash ground fly ash Fig 3. XRD pattern of B F slag 4

5 Granulated B. F. slag (GBFS) used in the studies was analyzed for its chemical composition as per IS 4032 and the results are reported in Table 1 A. XRD pattern of B.F. slag is shown in Fig. 3 showing presence of considerable quantity of amorphous material indicating good granulation of the slag. Glass content of the GBFS was determined by optical microscopy and was found to be 94 percent. Fly ash and GBFS blends were activated by concentrated sodium hydroxide solution prepared from laboratory grade sodium hydroxide of 97% purity in distilled water. 2.2 Preparation of Specimens for Compressive Strength Study The fly ash sample was ground in a laboratory ball mill to 387 m 2 /kg Blaine fineness and the physical characteristics of ground fly ash are given in Table 1 B. The GBFS sample was also ground to 410 m 2 /kg Blaine fineness. Mixes of ground fly ash with ground GBFS were prepared in laboratory ball mill by blending. Two blends of different proportions were prepared by replacing 20% and 30% fly ash with GBFS. These blends were treated, separately, with concentrated solution of sodium hydroxide maintaining Na 2O content 8.25 percent and H 2O content 20.5 percent in the alkali treated mix. Mixing of blends of fly ash and GBFS with sodium hydroxide solution was carried out in mixing apparatus specified in IS 1727: 1967 to get a homogeneous and uniform mix. The above alkali treated samples prepared in mixing apparatus were filled in 50 mm cube moulds of steel, immediately, in a layer of about 25 mm thickness with tamping by tamping rod and then mould was completely filled, tamped and plane with the help of trowel. These specimens were prepared for compressive strength determination. 2.3 Curing of Specimens and Performance Evaluation The moulded specimens prepared above in 2.2 were kept in humidity cabinet maintained at 27 ± 2 C temperature and 95 ± 5% relative humidity for 24 ± 2 hrs. Then the specimens were removed from the moulds and cured at 27 ± 2 C temperature and at 50 ± 5% relative humidity. These specimens were evaluated for compressive strength development at 3, 7 and 28 days from the day of casting. XRD and SEM techniques were used to study mineralogy and microstructure development. After determination of compressive strength part of some specimens were immersed in acetone to arrest the activation reaction for XRD and SEM studies. The acetone treated samples were then dried in desiccator prior to further analysis. 2.4 Determination of drying shrinkage The alkali treated blends of ground fly ash with ground GBFS were prepared as discussed above in 2.2 and cast in bar moulds as specified in IS 1727:1967 for drying shrinkage study. The test specimens were moulded in 2 3 layers, each layer being compacted by pressing and tamping with tamping rod for uniform and homogenous filling. After the top layer had been compacted, the upper surface of the moulds were made plane with the help of trowel. The moulded specimens were precured at 27 ± 2 C temperature and 95 ± 5% relative humidity for 24 ± 2hrs in a humidity cabinet and then demoulded. The initial length of the bars was measured, using length comparator specified in IS Subsequently, the demoulded bar specimens were 5

6 cured in humidity chamber at 27 ± 2 C and 50 ± 5% relative humidity up to 35 days from the day of casting and then the change in length was determined. 3.0 Results and discussion The compressive strength values, at different ages, of the specimens prepared by alkali activation of fly ash replaced with 20% granulated blast furnace slag i.e. sample D and with 30% granulated blast furnace slag i.e. sample C1, under ambient temperature curing, are shown in Fig 4. It is obvious from Fig 4 that compressive strength was continuously increased from 3 days to 28 days in both the cases. The compressive strength was found to be more at all ages in case of alkali activated fly ash replaced with 30% GBFS i.e. Sample C1. However, the alkali activation in both the samples was carried out under similar conditions and there was only one difference of percentage of GBFS content in sample C1 and sample D indicating that percentage of GBFS content influenced the compressive strength property. The specimens cured for 7 days and 28 days of alkali activated sample C1 were treated with acetone after compressive strength determination to arrest the reaction (13). Mineralogical studies were carried out using X-ray diffraction technique and Scanning electron microscopy was applied to study microstructure development. 60 COMPRESSIVE STRENGTH (MPa) Sample C 1 Sample D CURING PERIOD, DAYS Fig 4. Compressive strength pattern of alkali activated fly ash replaced with (i) 20% GBFS (sample D) and (ii) 30% GBFS (sample C1) 6

7 XRD patterns of 7 and 28 days cured alkali treated specimens of sample C1 are shown in Fig 5. In the studied specimen small peaks of zeolitic crystals viz. hydrotalcite, sodalite, nepheline and ussingite were identified. A diffuse peak at 2θ ~ is attributed to CSH in poor crystalline form (14). Broad hump between 2θ ~ is due to the silicate and alumino silicate gel. Small peak corresponding to pirssonite may be due to carbonation (15). Fig 5. XRD pattern of 7 and 28 days cured alkali treated specimens of sample C1 Fig 6. SEM image of alkali activated fly ash slag system cured for 28 days (Sample C1) 7

8 SEM image of alkali activated fly ash slag system (sample C1) cured for 28 days is given in Fig 6, indicated the presence of CSH gel along with NASH gel. CSH gel formation along with NASH gel might have resulted in strength development even at early stages of reaction. The formation of CSH gel is attributed to the hydration of slag (12). The presence of unreacted fly ash particles were also observed in the SEM image (Fig 6). The drying shrinkage studies revealed that the alkali activated products obtained from both the mixes were dimensionally stable as the bars showed shrinkage percent in case of sample C1 and 0.04 percent in case of sample D at 35 days from the day of casting. 4.0 Conclusion Geopolymer based cement at ambient temperature curing conditions may be prepared by alkali activation of fly ash partly replaced by granulated B. F. slag. The alkali treatment of mix of fly ash and granulated B. F. slag formed CSH gel along with NASH gel resulted in better strength performance at ambient curing. 5.0 Acknowledgement The contents reported in this paper are routine R&D activities carried out in National Council for Cement and Building Materials. The paper is being published with the kind permission of Director General, National Council for Cement and Building Materials 6.0 References: 1. Frantisek Skvara, Jan slosar, Jan Bohunek and Alen Markova, Alkali activated fly ash Geoplymeric materials, Proc. 11 th ICCC, 2003, Vol 3, pp A Fernandez Jimenez, I. Garcia Lodeiro, A.Palamo, Durability of alkali activated fly ash cementitious materials, J. Mater. Sc. 42, 2007, pp M. M. Ali, R. S. Gupta and Ashwani Pahuja, Geopolymeric cements and their salient characteristics, Civil Engg. & Const. Review, Vol 26, No. 1, 2013, pp R. S. Gupta, S. Vanguri, V. Liju, M. M. Ali and A. Pahuja, Investigations on geopolymeric cements based on alkali activation of low lime fly ash, Cement International, Vol 12, No. 5, 2014, pp Frantisek Skvara, Tomas Jilek, Lubomir Kopecky, Geopolymer Material Based on Fly ash, Ceramics Silikaty, 49 (3), 2005, pp Fareed Ahmed Memon, Muhd Fadhil Nuruddin, Samuel Demie and Nasir Shafiq, Effect of curing conditions on strength of fly ash based self compacting Geopolymer concrete pdf 8

9 7. A. Palomo, M. W. Grutzeck, M. T. Blanco, Alkali - activated fly ashes A cement for the future, Cem. and Concr. Res. 29, 1999, pp C. K. Yip, G. C. Lukey, J.S.J. van Deventer, The coexistence of geopolymeric gel and calcium silicate hydrate at the early stage of alkaline activation, Cement and Concrete Research, 35, 2005, pp Christina K. Yip, Grant C. Lukey, John L Provis, Jannie S. J. van Deventer, Effect of calcium silicate source on geopolymerisation, Cement and Concrete Research, 38, 2008, pp A Buchwald, H. Hilbig, Ch. Kaps, Alkali activated metakaolin slag blends performance and structure in dependence of their composition, J. Mater. Sci. 42, 2007, pp T. Bakharev, Geopolymeric materials using Class F fly ash and elevated temperature curing, Cement and Concrete Research, 35, 2005, pp Sravanthi Puligilla, Paramita Mondal, Role of slag in microstructural development and hardening of fly ash slag geopolymer, Cement and Concrete Research, 43, 2013, pp C. Ruiz Santaquiteria, J. Sribsted, A. Fernandez Jimenez, A. Palamo, Alkaline solution/ binder ration as a determining factor in the alkaline activation of aluminosilicates, Cement and Concrete Research, 42, 2012, pp N. Jambunathan et al. The role of alumina on performance of alkali activated slag paste exposed to 50 C, Cement and Concrete Research, 54, 2013, pp Susan A. Bernal, John L. Provis, Brant Walkley, Rackel San Nicolas, John D. Gehman, David G. Brice, Adam R. Kilcullen, Peter Duxjon, Jannie S. J. Van Deventer, Gel nanostructure in alkali-activated binders based on slag and fly ash, and effects of accelerated carbonation. Cement and Concrete Research, 53, 2013, PP