Properties of waste-based geopolymer building blocks

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Properties of waste-based geopolymer building blocks Fenghui Huang a, Zonghui Zhou b, and Xin Cheng c Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, Engineering Center of Advanced Building Materials of Ministry of Education, School of Materials Science and Engineering, University of Jinan, Jinan , China a huangfh1989@126.com, b mse_zhouzh@ujn.edu.cn, c mse_chengxin@ujn.edu.cn Keywords: industrial waste, geopolymer, building blocks, durability Abstract:The geopolymer building blocks were prepared with fly ash, steel slag, pulverized blast furnace slag, sand and activator by the pressure molding process. Effects of activator on the microstructure and properties of the samples were analyzed by SEM-EDS and other methods. The results showed that the samples activated by sodium hydroxide with the best comprehensive performance: the compressive strength and flexural strength at 28days are 89.9MPa and7.9mpa, the water absorption and softening coefficient are 6.99% and 0.93, and its volume density is 2124Kg/m 3. The compressive strength loss and weight loss are 9.20% and 2.44% after 25 freeze-thaw cycles, which meet the requirement JCT/ Non-fired rubbish gangue brick. Introduction Geopolymer is a kind of inorganic mineral polymer prepared by activating aluminosilicate raw material with alkali activator. Its major raw materials are industrial wastes and by-products, and the frequently-used shaping methods are pouring and pressure molding process. Generally speaking, the geopolymerization process includes four steps [1]: (1) dissolution of solid aluminosilicate materials in a strong alkaline solution; (2) formation of silica alumina oligomers; (3) polycondensation of the oligomeric species to form inorganic polymeric material; (4) bonding of un-dissolved solid particles in the final geopolymeric structure. It can be seen that activator plays a very important role during the geopolymerization process. The common activators are caustic alkali, carbonate, sulphate, silicate and the blended activator. These activators can influence the properties of geopolymer on different degree due to their different excitation effect [2]. The unfired and steam-free geopolymer building blocks prepared with industrial by-products of fly ash, steel slag and pulverized blast furnace slag have the potential to replace or partially replace traditional clay bricks due to its high strength and energy saving. In this paper, the effects of different activators on the strength development, water absorption, water resistivity and durability of the geopolymer blocks were studied. Experimental Materials. Steel slag and pulverized blast furnace slag(pbfs)were obtained from Jinan Steel Plant, fly ash was obtained from Jinan Huang Tai Electric Power Plant, and ordinary river sand was used. Potassium hydroxide (KOH), sodium hydroxide (NaOH), sodium sulfate (Na 2 SO 4 ) and sodium carbonate (Na 2 CO 3 ) were obtained from Tianjin Chemical Plant. An industrial water glass(na 2 SiO 3 ) with a modulus of and 40% solid content was used. The chemical compositions of the major materials were shown in Table1. 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-06/03/16,22:07:16)

2 936 Architecture, Building Materials and Engineering Management Table 1 Chemical compositions of raw materials/wt/% Materials CaO MgO Fe 2 O 3 Al 2 O 3 SiO 2 Loss Others Steel slag Fly ash PBFS Sample preparation, testing and characterization. Fly ash, steel slag, pulverized blast furnace slag and sand were continuously mixed with activator solution for 4 minutes according to the given proportion. The samples were cast in steel molds with the dimension mm, and pressed under 30MPa for 2 minutes. Then the specimens were de-molded and kept in a chamber of 20 o C until the testing age. The microstructure and microanalyses of the samples were analyzed by SEM-EDS. Test methods of strength, water absorption, water resistivity and durability of the geopolymer blocks referring to JC/T Non-fired rubbish gangue brick were used. Results and discussion Physical properties of geopolymer building blocks. The specimens were prepared with 30% steel slag, 30% pulverized blast furnace slag, 10% fly ash and 30% sand, and 3% activators were dissolved in 10% water. The test results were shown in table 2. Label Table 2 Physical properties of specimens prepared with different activators Activator Compressive strength/mpa Flexural strength/mpa 3d 28d 3d 28d 28d Water absorption /% Softening coefficient Volume density /Kg/m 3 A1 KOH A2 NaOH A3 Na 2 SiO A4 Na 2 SO A5 Na 2 CO A6 KOH+ Na 2 SiO A7 NaOH + Na 2 SiO A8 Na 2 SO 4 +Na 2 SiO It can be seen from table 2 that specimens activated by KOH obtain the highest compressive strength 66.8MPa and 92.7MPa at 3days and 28days respectively, while its flexural strength is lower than that activated by NaOH. This is due to ionic size difference between K + (1.33 Å) and Na + (0.97 Å), leading to a lower surface charge density in the case of K, which not only impact on structure formation from a physical point of view but also from a chemical and hydration point of view[3]. Both the compressive strength and flexural strength of the specimens activated by sodium carbonate and sodium silicate are low, and this phenomenon is the result of the low activation effect due to the OH - concentration of sodium carbonate and sodium silicate solution is not high enough. The specimens activated by Na 2 SiO 3 were with highest water absorption13.17% and lowest softening coefficient0.82. The volume density of specimens activated by KOH and NaOH is larger

3 Applied Mechanics and Materials Vols than that of other alkaline activators. As we all know, specific gravity of the blocks produced by the same material is a constant, the lower volume density, the higher internal porosity [4]. The experimental results demonstrate that the activation effect of caustic alkalis is the best. Freeze-thaw resistance. Table 3 shows that the specimens activated by Na 2 SiO 3 and Na 2 CO 3 have the largest compressive strength loss and weight loss (>20%) after 25 freeze-thaw cycles. The specimens activated by Na 2 SO 4 and Na 2 SO 4 + Na 2 SiO 3 have low weight loss but break, this is due to Activator Table 3 Physical properties of different specimens after 25 freeze-thaw cycles Compressive Strength loss/% Weight loss /% Appearance characteristic after 25 freeze-thaw cycles KOH Large area fall off NaOH surface layer is little intact Na 2 SiO Large area fall off Na 2 SO surface layer is a little intact but break Na 2 CO Large area fall off and break KOH+ Na 2 SiO Small area fall off NaOH + Na 2 SiO Small area fall off but break Na 2 SO 4 + Na 2 SiO Small area fall off but break its porous structure. The specimens activated by NaOH have the best freeze-thaw resistance, while the specimens activated by KOH have high strength but poor freeze-thaw resistance. Resistance to sulfate and alkali. It can be observed from Fig.1 that the samples exposed to 5wt% NaOH solutions have no visual signs of deterioration. After 28days of exposure to NaOH solutions, the surface of the samples is as smooth as immediately before the test. However, the samples exposed to 5wt% H 2 SO 4 solutions have some changes in appearance: the samples become covered by about 1-mm-thick white cover, and there is cracking along the corners and brims of the specimens. This phenomenon is possible due to the reaction between Ca 2+ and invasive SO 2-4 to generate CaSO 4, leading to volume expansion and cracking. It can be seen from table 4 that all the samples have weight gain due to ion invasion, especially the samples activated by Na 2 SO 4 + Na 2 SiO 3 :1.76% weight gain in NaOH solutions and 9.36% weight gain in H 2 SO 4 solutions. Table 4 Weight changes of the samples exposed to 5% solutions of NaOH and H 2 SO 4 for 28days Sample C1 C2 C3 C4 C5 C6 C7 C8 Solution NaOH/% H 2 SO 4 /% NaOH H 2 SO 4 Fig.1 Appearance of samples exposed to 5% solutions of NaOH and H 2 SO 4 for 28days. Geopolymer microstructure characterization. The fracture surfaces of different samples are shown in Fig.2. The micrograph showed that the samples prepared using sodium silicate and sodium carbonate were loose structure, a mass of unreacted fly ash particles and many pores embedded in the matrix were identified. As a result, the samples were with low strength, high water absorption and poor freeze-thaw resistance. The micrograph showed that the sample prepared using

4 938 Architecture, Building Materials and Engineering Management A a B C 1 D E F b 2 Fig.2 SEM EDS characterization of samples prepared using different activators: (A) NaOH (B) Na 2 SiO 3 (C) Na 2 CO 3 (D) Na 2 SO 4 (E) Na 2 SO 4 + Na 2 SiO 3 (F) NaOH+ Na 2 SiO 3, (a) and (b) are chemical characterization by EDS at point 1 in Fig.2 (A) and point 2 in Fig.2 (E). sodium sulfate was porous structure, in addition to the elements Ca and Si, the elements Na, Al were also distributed in the matrix, which indicated that C-S-H and N-A-S-H were co-existed. SEM observations indicated that the samples prepared using sodium hydroxide had more crystalline appearance than the specimens activated by other activators, and quite dense microstructure in the matrix were found. Combining with EDS analysis (Fig.2 a), chemical formula of these crystals may be (Mg, Ca) n (Si-O-Al-O-Si)[5], which is similar to heulandites on the compositions. Conclusions. (1) Silicate, carbonate, sulfate and caustic alkalis all have certain excitation effect, in which caustic alkalis is the best, sulphate and composite activator is better, and silicate and carbonate is the worst. (2) The compressive strengths of the specimens synthesized with sodium hydroxide activator at 3days and 7days obtained 57.9MPa and 89.9MPa respectively, and the water absorption and softening coefficient is 6.99% and 0.93, which is worse than that synthesized with potassium hydroxide activator. However, the former is with nicer freezing resistance and acid resistance. (3) The major hydration products are C-S-H and N-A-S-H gels co-existed. Acknowledgements. This work is supported by Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province and National Natural Science Foundation of China (No ). References. [1] Saeed Ahmari, L.Y. Zhang: Construction and Building Materials. Vol. 29(2012), P.324 [2] F.S.Niu,Y.M.Nie, and J.R.Zhang: Concrete, Vol. 29(2009), P.326 (In Chinese) [3] J.G.S. van Jaarsveld and J.S.J. van Deventer: Ind.Eng.Chem.Res. Vol. 38(1999), P.3933 [4] W.j. Yang, and Y.S.Ni: The production and application technology of shale perforated brick [M]. BeiJing: China building industry press (2011), P.41 (In Chinese) [5] C.J.Shi and F.Q.He: Journal of the Chinese Ceramic Society. Vol.40 (2012), P.69

5 Architecture, Building Materials and Engineering Management / Properties of Waste-Based Geopolymer Building Blocks / DOI References [3] J.G.S. van Jaarsveld and J.S.J. van Deventer: Ind. Eng. Chem. Res. Vol. 38(1999), P /ie980804b