Ternary Blends of Portland Cement, Bottom Ash and Silica Fume: Compressive Strength of Mortars and Phase Characterizations

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1 424 Chiang Mai J. Sci. 2014; 41(2) Chiang Mai J. Sci. 2014; 41(2) : Contributed Paper Ternary Blends of Portland Cement, Bottom Ash and Silica Fume: Compressive Strength of Mortars and Phase Characterizations Arnon Chaipanich* and Watcharapong Wongkeo Advanced Cement-based Materials Research Unit, Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand. *Author for correspondence; arnonchaipanich@gmail.com, arnon.chaipanich@cmu.ac.th Received: 3 September 2012 Accepted: 4 February 2013 ABSTRACT Bottom ash, a by-product from coal power plant was blended with Portland cement and silica fume to produce ternary blend mixes. Compressive strength of mortars and phase characterizations were investigated. Bottom ash was used to replace part of Portland cement at 10%, 20% and 30% by weight and silica fume was used as an additional material at 5% by weight. Compressive strength test results of blend cement mortars show an increase in compressive strength when silica fume was used in the mix in addition to that of bottom ash. The benefit of silica fume addition was clearly seen especially for mixes with higher bottom ash content (30% bottom ash), compressive strength was significantly enhanced to exhibit strength similar to that of Portland cement control. Thermogravimetric analysis confirms the pozzolanic reaction of silica fume showing an increase in calcium silicate hydrate (C-S-H) and gehlenite hydrate (C 2 ASH 8 ) phases when silica fume was added. X-ray diffraction traces, in general, showed a reduction in Ca(OH) 2 peaks while Scanning Electron Microscopy showed a denser cement matrix when silica fume was added. Keywords: bottom ash, Portland cement, silica fume, compressive strength, hydration 1. INTRODUCTION Coal power plants produce by-products those are fly ash and bottom ash both of which are pozzolanic materials. Pozzolanic materials (materials those react with calcium hydroxide) are known to enhance cement and concrete properties [1-3]. Fly ash when used as a supplementary cementing material with Portland cement has been reported to enhance long term strength and durability properties of concrete against chloride and sulphate attacks [4-5]. By using fly ash to replace part of Portland cement thereby giving ecological benefits cutting down the carbon dioxide emission and energy used in Portland cement manufacturing process. One of the most recent development regarding incorporation of fly ash in cement and concrete mixes has been on the use of silica fume, (a very fine pozzolanic material and a by-product of the ferro-silicon alloys and silicon metal industries), in combination with fly ash together with Portland cement to

2 Chiang Mai J. Sci. 2014; 41(2) 425 produce a ternary blend that is believed to be more superior both through the combination benefits from physical and chemical characteristics of the two materials giving higher strength than when using fly ash on its own [6-11]. Although the use of fly ash is now well known in Thailand and has been used widely for the past decade, the use of bottom ash (BA) on the other hand unlike fly ash is not as widely established. Bottom ash from Mae Moh power plant, Lampang province, Thailand currently occurs at 20% of the total ash produced (10,000t/day) or 2,000 tonnes per day [12]. Nonetheless, bottom ash can be used as both cement replacement or as a partial sand replacement in concrete [13-15]. For utilization as Portland cement replacement, it is found that bottom ash is also pozzolanic and can give enhancement to the strength when subjected to milling so that it obtains a similar particle size to Portland cement [16]. However, there are still very little research works in term of using bottom ash with another material such as silica fume. Wongkeo et al. [17] investigated the compressive strength and microstructure of Portland-Bottom ash-silica fume cement pastes using 20% bottom ash with additional silica fume at 28 days found that the use of silica fume can improve the 28 day strength of bottom ash-cement paste. Similar improvement was found when silica fume was used with bottom ash in structural lightweight concrete [18]. However, the hydration mechanism of such ternary blend is not yet fully understood. Therefore, the aim of this work is to investigate the effect on the compressive strength and hydration (by mean of thermal analysis) of Portland-Bottom ash-silica fume cement pastes at different BA content. For this research, ternary blend mixes of Portland cement, bottom ash and silica fume were produced as mortars and the compressive strength was investigated at 7, 14 and 28 days. Phase characterizations of separately cast pastes of the same mixes were also investigated using Scanning Electron Microscopy, X-ray diffraction and thermogravimetric analysis. 2. MATERIALS AND METHODS Bottom ash (BA) was obtained from Mae Mao Power Plant, Lampang, Thailand and silica fume (SF) was obtained from Sika (Thailand) Limited. Portland cement type I (PC) was used as the control and for mixing with BA and SF to produce a combination of different blends of Portland-bottom ash-silica fume mixes. Chemical compositions of these materials are given in Table 1. Bottom ash was ground for 6 hours to obtain a similar particle size close to that of Portland cement in order for it to be reactive [16] and used to replace Portland cement at 10%, 20% and 30% by weight to produce Portland-bottom ash cement. Silica fume can be seen to be of amorphous characteristic (Figure 1) and is pozzolanic in accordance with American Concrete Institute Committee 116 Report (ACI 116R) [19]. Silica fume was then used as an additive material (as a minor additional constituent) which was added at 5% by weight of Portland-bottom ash cement. For compressive strength tests, mortars samples were mixed using water: binder (Portland cement, bottom ash and silica fume): sand ratio of 0.5: 1: 2.5. Normal construction river sand was used in the mortar mixes. After mixing, the mixes were cast in oiled moulds ( mm 3 ) and then compacted. The specimens were then surfacesmoothed, and covered with plastic film. All specimens were removed from the moulds 1 day after casting. Thereafter, they were cured in water at room temperature ( 25 C). Compressive strength was tested at 7, 14 and

3 426 Chiang Mai J. Sci. 2014; 41(2) 28 days to determine the strength at early ages (pre 28 days) and at 28 days as standard testing age in accordance with The American Society for Testing and Materials (ASTM C109) standard [20]. Characterizations of raw materials were carried out using X-ray diffraction (XRD) and phase characterizations of the hardened pastes were investigated using Scanning Electron Microscopy (SEM; JEOL JEM-5910LV), room temperature (Ni-filtered CuK radiation) X-ray diffraction (XRD; Philips PW 1729) and thermogravimetric analysis (TGA; Mettler Toledo TG/SDTA 851e). All samples used for characterizations were water cured for 28 days. Ground samples were used for XRD and TGA measurements. Table 1. Chemical compositions of Portland cement, bottom ash and silica fume. Oxide SiO 2 Al 2 O 3 CaO Fe 2 O 3 MgO Na 2 O K 2 O P 2 O 5 TiO 2 MnO SO 3 Loss on ignition (LOI) Portland cement (%) Coal bottom ash (%) Silica fume (%) RESULTS AND DISCUSSION 3.1 Characterization of Raw Materials Crystallography of raw materials was analyzed using X-ray diffraction (XRD) and are shown in Figure 1(a)-(c). The XRD pattern of Portland cement (Figure 1(a)) shows peaks matching tricalcium silicate (C 3 S) in JCPDS file number and amorphous broad characteristic. In Figure 1(b), the XRD pattern of bottom ash is mainly amorphous but crystalline silica peak can also be observed at 26 degree 2θ. XRD pattern of silica fume, Figure 1(c), generally shows amorphous characteristic with negligible crystallinity. 3.2 Compressive Strength The strength development results of Portland-BA-SF mortars are plotted as shown in Figure 2. At early ages (pre 28 days), it can be seen that the strengths of BA mixes without the addition of SF (10BA, 20BA and 30BA mixes) are all lower than the PC control and that the compressive strength can be seen to decrease with increasing BA content. Furthermore, the strength developments of BA mixes at early ages are lower for 30% BA, similar for 20% BA and higher for 10% BA when compared to that of PC control mortar. At 28 days, the compressive strength of 30% BA mix was found to be lower than that of the PC control mix where the compressive strength is 38 MPa compared to the PC mix (48 MPa). In addition, it can be seen that there is a greater rate of strength development for Bottom ash mixes with silica fume, especially at early age (7days), and this increase in the strength development is noticed with increasing SF content.

4 Chiang Mai J. Sci. 2014; 41(2) 427 Figure 1. XRD traces of (a) Portland cement, (b) Bottom ash and (c) Silica fume. Figure 2. Compressive strength development of Portland-bottom ash-silica fume mortars. The effect of SF on the compressive strength of PC-BA-SF mixes for each day can be observed in Figure 3. The benefit of using SF can be noticed at an early age where the compressive strength clearly increased with increasing SF (Figure 3a). At 7 days, PC-BA-SF mixes with up to 20% BA and 5% SF can be seen to give higher strength than PC control while similar strength to PC was found for the mix containing 30%BA with 5%SF. Similar effect can be seen at 14 days (Figure 3b) where there is an increase in compressive strength when SF was used in all PC-BA-SF mixes. At 28 days, the compressive strength reached 54 MPa for 10BA5SF and 20BA5SF. Moreover, a significant increase in strength can be seen for mixes with higher BA content, especially at 30% where interestingly 30BA5SF mix gained similar strength to that of PC control

5 428 Chiang Mai J. Sci. 2014; 41(2) where the compressive strength is 47 MPa when compared to that of PC control at 48 MPa. This is clearly much higher than the reference bottom ash mix without silica fume where the strength was found at 38 MPa. This is due to the greater content of amorphous silica (giving more C-S-H) and very fine particles of SF resulting in a faster pozzolanic reaction [1]. Moreover, silica fume particles act as fillers thereby physically filling in the pore spaces in the cement paste thus making the matrix denser [21, 22]. Figure 3. Effect of SF on the compressive strengths of Portland-bottom ash-silica fume mortars at (a) 7 days, (b) 14 days and (c) 28 days. 3.3 Phase Characterizations Scanning electron microscopy Scanning electron micrographs of typical Portland cement, bottom ash (20%BA) and bottom ash-silica fume (20%BA and 10%SF) pastes after 28 days water curing can be seen in Figure 4. Typical cement hydration products such as C-S-H (fiber like) and ettringite (needles like) can be seen in these SEM micrographs. Moreover, it can be seen that the microstructure of the BA paste with SF is denser than the matrix with BA only.

6 Chiang Mai J. Sci. 2014; 41(2) 429 Figure 4. SEM micrographs of typical (a) Portland cement, (b) bottom ash and (c) bottom ash-silica fume pastes X-ray diffraction From X-ray diffraction (XRD) traces of the PC-BA-SF pastes at different BA and SF content (Figure 5). For all mixes, dominant peaks detected by means of XRD is calcium hydroxide where the Ca(OH) 2 peaks can be seen. The amount of Ca(OH) 2 and other unreacted cement constituents determined by XRD thus can be used to give an indication of the hydration reaction [23]. The Ca(OH) 2 peak can in general be seen to reduce when BA is used as direct weight replacement to PC as a dilution effect of replacing part of PC as well as some pozzolanic reaction of BA itself (Figure 5 a-c) [18]. Moreover, when SF is added and by comparing the Ca(OH) 2 peak it can be seen that the intensity of Ca(OH) 2 can be noticed to be less in intensity compared to XRD traces of BA mixes without silica fume. Similar effect was found with fly ash and silica fume ternary mixes [11]. This is clearly seen in higher BA mixes such as mixes with 20% and 30% BA (Figure 5b and 5c) where a significant reduction in Ca(OH) 2 can be noticed when 5%SF was used (20BA5SF, 30BA5SF). Thus suggests that Ca(OH) 2 was consumed in the pozzolanic reaction forming C-S-H and thereby leading to an increase in strength. The reaction and hydration products are further discussed in the next section by means of thermogravimetric analysis (TGA).

7 430 Chiang Mai J. Sci. 2014; 41(2) Figure 5. XRD traces of Portland-bottom ash-silica fume paste at 28 days Thermogravimetric analysis Thermogravimetric analysis (TGA) was used to analyse the phases which do not appear by the use of XRD technique that only shows the crystalline phases. The results obtained from TGA and also interpreted as derivative thermogravimetric (DTG) are shown in Figures 6-9 for Portland cement paste and Portland cement-bottom ash cement pastes with and without the addition of SF. In all cases, there appear to be three mass loss transitions (from room temperature to 1000 C) with the first being from room temperature up to 175 C, the second from C and the third being C. The first transition detects several hydrated phases at this range which are ettringite, C-S-H and C 2 ASH 8 while the second transition occurred as a result of dehydroxylation of Ca(OH) 2 [24]. The third resulted from decomposition of CaCO 3. TGA and DTG curves show that the mass loss of the hydrated phases (ettringite, C-S-H and C 2 ASH 8 ) for reference Portlandbottom ash mixes without silica fume at 20% and 30% BA (Figs. 8a and 9a) were found to be less than that of Portland cement control mix (Figure 6). Less hydration products i.e. less amount of the C-S-H phase would result in lower strength as found when compared to that of the control Portland cement. On the other hand this mass loss of the hydrated phases was found to increase when silica fume was added (Figure 8b and 9b), meaning that there is more C-S-H phase when silica fume was added [11]. Moreover, it is also clear from the second phase that calcium hydroxide can be seen to reduce when BA is used due to both the dilution and pozzolanic effects [11, 18]. This mass loss is seen to be less when SF is added as compared to the reference mixes without SF, thus less Ca(OH) 2 in the sample

8 Chiang Mai J. Sci. 2014; 41(2) 431 [11]. TGA results therefore agree with the compressive strength results where a significant increase in strength was found with an increase in C-S-H phase in BA-SF mixes. Therefore, from the above results, the benefit of silica fume can be seen when used in the ternary system with Bottom ash and Portland cement. The addition of silica fume resulted in the denser microstructure and higher compressive strength with more C-S-H phase and less Ca(OH) 2. Nonetheless, durability aspects of these materials were not investigated in this research work and will be investigated as part of future research works. Figure 6. TGA and DTG analysis of Portland cement fume paste at 28 days. Figure 7. TGA and DTG analysis of Portland cement-bottom ash-silica fume pastes at 28 day: (a) 10% BA and (b) 10% BA with 5% SF.

9 432 Chiang Mai J. Sci. 2014; 41(2) Figure 8. TGA and DTG analysis of Portland cement-bottom ash-silica fume pastes at 28 day: (a) 20% BA and (b) 20% BA with 5% SF. Figure 9. TGA and DTG analysis of Portland cement-bottom ash-silica fume pastes at 28 day: (a) 30% BA and (b) 30% BA with 5% SF 4. CONCLUSIONS Compressive strength test results of the ternary blends of PC-BA-SF mortars show a significant increase in strength when SF was used in the mix as an addition to that of bottom ash. The benefit of silica fume addition was clearly seen especially for mixes with higher bottom ash content (for 20-30% bottom ash mixes), compressive strength was significantly enhanced to exhibit strength similar to that of Portland cement control. Phase characterizations were also carried out using SEM, XRD and TGA. Calcium silicate hydrate (C-S-H), ettringite, gehlenite hydrate, calcium hydroxide and calcium carbonate phases were detected in all mixes. In the mixes with the use of silica fume addition at 5%, there is a reduction in Ca(OH) 2 compared to that of the reference Portland-bottom ash cement pastes and a corresponding increase in calcium silicate hydrate (C-S-H) was observed. ACKNOWLEDGEMENTS The authors are grateful for financial support funded by the National Research University Project, Commission of Higher Education (Thailand). The authors are also grateful to Ms. Waritha Thawornson for her assistance. REFERENCES [1] Malhotra V.M. and Mehta P.K., Pozzolanic and Cementitious Materials, Advance in Concrete Technology, Vol.1, Gordon and Breach Publishers, 1996.

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