Possible Synergism of High temperature Fly Ash and Fluidized Bed Combustion Fly Ash in Cement Composites

Transcription

1 Advanced Materials Research Submitted: ISSN: , Vol. 1106, pp Revised: doi: / Accepted: Trans Tech Publications, Switzerland Online: Possible Synergism of High temperature Fly Ash and Fluidized Bed Combustion Fly Ash in Cement Composites Denisa Orsakova 1, a *, Rudolf Hela 2,b, Petr Novosad 3,c, Jaroslav Valek 4,d 1,2,3,4 Brno University of Technology, Faculty of Civil Engineering, Veveri 331/95, Brno, Czech republic a orsakova.d@fce.vutbr.cz, b hela.r@fce.vutbr.cz, c novosad.p@fce.vutbr.cz, d valek.j@fce.vutbr.cz Keywords: Fluidized bed combustion fly ash (FBC fly ash), fly ash from high temperature combustion (high temperature fly ash) Abstract. One of the possibilities how to activate fly ash in cement composites is to add calcium oxide as a chemical activator. This addition can improve ph of composites. Because fluidized bed combustion (FBC) fly ash contain around 15% of calcium oxide, we decided to add FBC fly ash into binder system of cement composite with high temperature fly ash. Three mixtures were designed. First one contain in binder system only cement and high temperature fly ash. In second and third mixture was part (25% resp. 50%) of high temperature fly ash replaced by FBC fly ash. Consistency, shrinkage, compressive strength and freeze-thaw resistance were tested. Microstructure was detected by XRD and TG analyses. Introduction High temperature combustion fly ash (high temperature fly ash) is quite common used as additive for concrete. In case of FBC fly ash there are some prejudices because of negative effect on physical-mechanical properties and durability. The highest risk is high volume of sulfates and calcium oxide. [1, 2, 3, 4, 5]. Antiohos [6] in his study deals with chemical activation of high temperature fly ash by calcium oxide addition. High volume of calcium oxide in FBC fly ash led us to idea about combination of these two fly ash types. We can talk about alkali activation of fly ash. To achieve as good results as possible we mechanically activated high temperature fly ash by combination of three fly ash fractions (from three sections of electrostatic separators) in ratio to achieve minimal voids content. We use ratio from our previous studies. Materials and methods Materials. Ordinary portland cement CEM I 42,5 R from Heidelberg production was used. High temperature fly ash from Tušimice power plant was used. As mentioned before we mixed fly ashes from three sections of electrostatic separator in ration 10:4:1 (10 weight parts of the coarsest, 4 parts of middle and one part of the finest one). FBC fly ash was used from Tisová power plant. Chemical composition of both is in Table 1. We used two fractions of aggregate - sand (0/4) from Žabčice location and gravel (8/16) from Olbramovice location. SIKA VISCOCRETE 1035 was used as superplasticizer. Table 1: Chemical composition of used fly ashes SiO 2 [%] Al 2 O 3 [%] CaO [%] MgO [%] Fe 2 O 3 [%] SO 3 [%] Na 2 O [%] K 2 O [%] P 2 O 5 [%] MnO [%] Z.Ž. [%] Tisová 39,5 24,7 15,45 0,695 7,625 3,06 < 1,0 0,49 0,35 0,0575 2,045 Tušimice 49 21,8 4,14 1,3 15,7 1,87 0,63 1, ,76 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/04/16,05:12:14)

2 30 Proceedings from 21st Czech Concrete Day 2014 Practice. : Three mixtures were designed. High temperature fly ash was used as only one additive in first mixture. In second and third mixture was part (25 % resp. 50 %) of high temperature fly ash replaced by FBC fly ash. Dosage of water and superplasticizer was constant in all mixtures. Composition of mixtures is shown in Table 2. Table 2: Mixtures composition mixture REF 75%ETU25%ETI 50%ETU50%ETI poznámka cement CEM I 42,5 R Heidelberg classic fly ash powerplant Tušimice FBC fly ash powerplant Tisová sand 0/ location Žabčice gravel 8/ location Olbramovice water superplasticizer 2,55 2,55 2,55 SIKA VISCOCRETE 1035 CZ From designed mixtures were prepared concretes. Slump test according to EN was made on fresh concrete. Shrinkage was detected in first 45 hours after concreting. Cubes mm and beams mm were prepared for compressive strength and freeze-thaw resistance testing. After compressive strength testing were sampling pieces of binder from all concretes for XRD analyze. Samples were placed in water before testing. Tests and analyzes results. Consistency - Slump test. Table 3: Slump test and density of fresh concrete results slump test [mm] fresh concrete density [kg/m3] REF % ETU 25 % ETI % ETU 50 % ETI Using FBC fly as additive leads to lower slump test. Deterioration of workability is caused by formation of primary ettringite - product of sulphates and cement reaction. Density of fresh concrete was not affected by FBC fly ash addition. Shrinkage. Fig.1: Mixtures shrinkage comparison Shrinkage of mixtures was detected in first 45 hours after concreting. Shrinkage of reference mixture was more gradual and finished in 15 hours after concreting. 50% replacement of high temperature fly ash by FBC fly ash caused faster hardening and shrinkage finished in 5 hours. Total shrinkage was almost the same value around 0,275 mm/m.

3 Advanced Materials Research Vol Compressive strength. Compressive strength was testes after 25 and 90 days. Comparison of strength is shown in Fig. 2. Fig. 2: Comparison of mixtures compressive strength Addition of FBC fly ash caused higher compressive strength in age 28 and also 90 days. 50% replacement of high temperature fly ash by FBC fly ash improved strength in 28 days by 5 N/mm 2 and after 90 days is difference between these two mixtures around 4 N/mm 2. Freeze-thaw resistance. Freeze-thaw resistance was measured on beams in age of 28 days. Two beams from each mixture were placed in freezer and frozen and thawed with 25 cycles. After 25 cycles were beams tested on tensile strength Value of freeze-thaw resistance is showed in Table 3. Table 3: Freeze-thaw resistance weight changes [%] tensile strength [N/mm 2 ] freeze-thaw resistance index [-] REF 7,0-0,02 REF freezed 4,7 75%ETU25%ETI 7,3-0,13 75%ETU25%ETI freezd 7,5 50%ETU50%ETI 8,0-0,10 50%ETU50%ETI freezed 7,3 0,67 1,03 0,91 FBC fly ash has positive influence on concrete freeze-thaw resistance. When used 25 % replacement of high temperature fly ash by FBC fly ash value of freeze-thaw resistance was higher than one. XRD analyze. Obr. 3: XRD graph of REF mixture Obr. 4: XRD graph of 75%ETU25%ETI mixture

4 32 Proceedings from 21st Czech Concrete Day 2014 Obr. 5: XRD graph of 50%ETU50%ETI mixture Graphs above show minimal differences between single mixtures. Ettringite was present in all mixtures in similar amount. Summary. According to tests and analyses results it can be shown, that partial replacement of high temperature fly ash by FBC fly ash leads to improvement of concrete properties especially its freeze-thaw resistance. One problem of FBC fly ash using is negative effect on fresh concrete workability, this problem should be resolved by using higher dosage of right superplasticizer. Secondary ettringite was not detected in high volume. Acknowledgement The authors gratefully acknowledge the financial support from Ministry of Industry and Trade of the Czech republic project MPO TI4/582-Possible industrial use of fly ash from low-temperature fluidized bed combustion for production of building materials and internal project of Brno University of technology, Faculty of civil engineering - FAST-J Possibility of combination of fly ash activations for usage in cement composites References [1] Glinicki, M.A., Zieliński, M. - The influence CFBC fly ash addition on phase composition of air-entrained concrete, Bulletin of the Polish Academy of Sciences, Technical Sciences, Vol. 56, No 1 (2008), str [2] Stevens, W., Robl, T., Mahboub, K. - The Cementitious and Pozzolanic Properties of Fluidized Bed Combustion Fly ash, 2009 World of Cioal Ash Conference - May , Lexington, KY, USA, [3] Brandštetr J., Havlica, J., - Phase Composition of Solid Residues of Fluidized Bed Coal Combustion, Quality Tests, and Application Possibilities, Chemical papers 50 (4), (1996), str [4] Conn, R.E., Sellakumar, K., - Utilization of CFB Fly Ash for Construction Applications - Paper No. FBC , Proceedings of the 15 th International Conference on Fluidized Bed Combustion, (1999) [5] Robl, T., Mahboub, K., Stevens, W., Rathbone, R., - Fluidized Bed Combustion Ash Utilization: CFBC Fly Ash as a Pozzolanic Additive to Portland Cement Concrete, Second International Conference on Sustainable Construction Materials and Technologies - Proceedings, ISBN (2010) [6] Antiohos, S., Activation of fly ash cementitious systems in the presence of quicklime Part I. Compressive strength and pozzolanic reaction rate, Cement and Concrete Research 34 (2004), str

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