CHEMICAL ENGINEERING TRANSACTIONS VOL. 56, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Jiří Jaromír Klemeš, Peng Yen Liew, Wai Shin Ho, Jeng Shiun Lim Copyright © 2017, AIDIC Servizi S.r.l., ISBN 978-88-95608-47-1; ISSN 2283-9216 Workability, Compressive Strength and Leachability of Coal Ash Concrete Mahdi Rafieizonooz*,a, Mohd Razman Salimb, Mohd Warid Hussinc, Jahangir Mirzac, Salmiati Muhd Yunusb, Elnaz Khankhajea aFaculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia bCentre for Environmental Sustainability and Water Security, Faculty of Civil Engineering, Universiti Teknologi Malaysia cUTM Construction Research Centre (UTM CRC), Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia mahdirafiei1982@yahoo.com One of the essential steps for planning solid waste management towards sustainable development is to reuse solid waste in the construction industry. Coal is one of the world's most important sources of energy, fueling almost 40 % of electricity worldwide. Malaysia is commonly producing electricity through burning millions of tonnes of coal. This process generates around 8.5 Mt of coal ash, which comprised of 80 % Fly Ash (FA), and 20 % Bottom Ash (BA) as waste. The study aims to investigate the workability, compressive strength and leachability of concrete containing BA and FA as replacement of sand and cement. Cement was substituted with 20 % of FA by mass and fine aggregate was replaced with BA at 0, 20, 50, 75 and 100 % in the concrete mix. The results show that the percentage sum of SiO2, Al2O3 and Fe2O3 (the main chemical composition in coal ash) in FA and BA are about 78.82 % and 83.24 %. The leaching test showed that the heavy metal concentrations in leachates are much lower than recommended in the USEPA SW 846. Workability of concrete was reduced by increasing BA content as a sand replacement in the concrete mixture. After 91 d and 180 d curing periods, the compressive strength of both the experimental and control samples of concrete were roughly comparable. It can be concluded that BA and FA can be used as a replacement of sand and cement in normal concrete without any environmental problem. 1. Introduction High production of industrial wastes, as well as excessive consumption of natural resources have resulted in negative impacts on the environment. Reduction in the utilisation of raw materials and energy can lead to resource efficiency enhancement in addition to reduced environmental problems. Subsequently, the increase in resource efficiency would enable a sustainable development. Previous researches have shown that it is possible to produce durable concrete utilising industrial waste materials such as red mug aggregates, silica fume, blast furnace slag, coal fly ash (FA) and coal bottom ash (BA). Decreasing the utilisation of construction materials through the application of industrial by-products wastes in manufacturing concrete is a right step towards a sustainable development (Khankhaje et al., 2015). According to ST (2014), it is predicted that by the year 2024 the production of electricity will be based on usage of coal and gas (58 % and 25 %). This prediction shows that the production of electricity by burning coal will be increased from 43 % in 2014 to 58 % in 2024. It is clear that by increasing the amount of coal, the volume of coal ash will increase so does its environmental problems. In general, two types of ash, fly ash (FA) and bottom ash (BA) are produced at coal power plant (80 % - 20 %). The majority of these kind of ash are being abandoned as the waste materials near the factory. It might result in lots of environmental and disposal problems due to a large space of dump yard is required. The production of electricity in Malaysia resulted in huge amount of BA some 1.7 Mt and FA around 6.8 Mt, generated annually. Even though FA is being used in production of concrete and blended cement, BA is not usually used at all. DOI: 10.3303/CET1756074 Please cite this article as: Rafieizonooz M., Salim M.R., Hussin M.H., Mirza J., Yunus S.M., Khankhaje E., 2017, Workability, compressive strength and leachability of coal ash concrete, Chemical Engineering Transactions, 56, 439-444 DOI:10.3303/CET1756074 439 Several researches have studied the characteristics of concrete containing BA as a fine aggregate replacement (Jang et al., 2016). However, the combined utilisation of FA as partial replacement of OPC and BA as partial or total replacement of sand has been studied by few researches (Rafieizonooz et al., 2016). There is not enough study about leachability of concrete containing BA and FA as sand and cement replacement. Cheriaf et al. (1999) investigated the pozzolanic characteristic of coal ash concrete and found that BA has pozzolanic property and is appropriate to be used in concrete production. Kim and Lee (2011) reported that BA can be utilised as replacement of river sand in the manufacturing of high-strength concrete. Singh and Siddique (2013) stated that BA is a potential substitute waste for fine aggregate in concrete. Singh and Siddique (2015) also studied the characteristics of concrete containing a high volume of BA as a substitute of fine aggregate. Aggarwal and Siddique (2014) studied the microstructure and characteristics of concrete incorporating BA and waste foundry sand as a substitute of fine aggregate in the production of concrete. There are several researches documenting heavy metal concentrations and leaching in coal ash concrete from different countries such as Australia (Jankowski et al., 2006), USA (Heidrich et al., 2013), India (Prasad and Mondal, 2008), South Africa (Musyoka et al., 2013) and Zimbabwe (Gwenzi and Mupatsi, 2016). The majority of this researches concentrate on leaching behaviour of heavy metals from unbound coal ash. The findings of these literature are in contrast with each other: Some researcher (Sushil and Batra, 2006) stated that some heavy metals could be leached out from coal ash; whereas other (Gwenzi and Mupatsi, 2016) reported that the leaching of mentioned elements from coal ash was unimportant. These conflicting findings might be due to differences in leaching method and procedures, variability in type and origin of coal burned, types of boiler, the degree of coal pulverization, firing conditions in the furnace, ash handling practice and processing technologies used in thermal power stations. The objectives of this study were to evaluate the influences of the utilisation of BA and FA as partial or total replacement of fine aggregates and cement on concrete compressive strength and workability as well as to investigate leaching of heavy metals in coal ash concrete. 2. Materials and methods 2.1 Materials Ordinary Portland Cement (OPC) used in this research is in accordance with ASTM C150 (2007). A single source of BA and FA was obtained from Tanjung Bin coal power plant located in Johor, Malaysia. The chemical properties of cement, FA and BA are shown in Table 1. Cement and FA used in this study had a Fineness modulus, specific gravity, and soundness of 3,990 and 3,450 cm2/g, 3.15 and 2.45, 1.0 mm. Moreover, BA was sieved through 4.75 mm sieve before use as a replacement of fine aggregate in this research. Table 1: Chemical properties of OPC, FA and BA Material SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 SO3 BaO LOI OPC 20.4 5.20 4.19 62.3 1.55 - 0.01 - 2.11 - 2.36 FA 47.6 23.8 7.42 10.7 1.50 2.16 1.68 2.92 0.76 0.15 - BA 45.3 18.1 19.84 8.70 0.97 - 2.48 3.27 0.35 0.31 - River sand was obtained from Sungai Sayong quarry located in Johor, Malaysia. The river sand used in this study was graded in accordance with the ASTM C33 / C33M (2013). The coarse aggregate was collected from Bukit Namu quarry. The maximum size of crushed stone coarse aggregate was 20 mm. The results of physical properties (specific gravity, water absorption and fineness modulus) of BA, River sand and coarse aggregate are presented in Table 2. Table 2: Physical characteristics of BA, sand and coarse aggregate Material BA Sand Coarse aggregate Specific gravity 1.88 2.62 2.69 Water absorption (%) 11.61 7.4 0.61 Fineness modulus 3.44 2.67 6.48 2.2 Mix proportion and casting Concrete cubes of 100 mm × 100 mm × 100 mm sizes with 20 % of FA as cement replacement and 0, 25, 50, 75 and 100 % of BA as a fine aggregate were cast to evaluate the workability and compressive strength. Saturated Surface Dry (SSD) BA and sand were utilised in all of the concrete specimens. The BA was utilised by mass in concrete as a substitute of fine aggregate. The water-cement ratio (w/c) was 0.55 for all concrete 440 samples. The constant amount of cement (375 kg/m3) and the range of slump (6–18 cm) were the same for all produced concretes. The British (DOE) (BS EN 206 2013) method was utilised to evaluate the mixture ratios. In addition, after compressive strength test, crushed concrete was collected for leaching test. 2.3 Compressive strength and workability Compressive strength of concrete samples was evaluated at 7 d, 28 d, 91 d and 180 d ages of curing in accordance with BS EN 12390-03 (2009). The 3,000 kN compression testing machine was used for applying constant force with the rate of 5.0 kN/s on SSD cubes. Slump test was done for calculating the workability of concrete mixtures. 2.4 Leaching Test Toxicity Characteristics Leaching Procedure (TCLP) which was carried out by some researcher (Yilmaz 2015) was used in this study. According to TCLP method 1311 (U.S. EPA 2015), liquid-to-solid (L:S) ratio is 20:1. Therefore in this research, crushed concrete was collected and sieved through 2 mm sieve. After sieving, 50 g of the mentioned sample was added into HDPE (high density polyethylene) bottles. Then, 1 L of leaching solution (reagent water with acetic acid and acetic acid-NaOH) was added to the bottles to achieve the initial pH of 2.88 ± 0.05 and 4.93 ± 0.05. Afterwards, the bottles were rotated at 30 ± 2 rpm in a Rotary Agitation Apparatus for around 18 h in 20 ± 2 °C. After rotating, the pH of the eluates wwas recorded subsequently. After all, the eluates were filtered by vacuum filtration using glass filter with a 0.45 µm filter holder and acidified with HNO3 (nitric acid) to a pH lower than 2 and kept in the refrigerator in temperature less than 4 °C. the concentration of elements in samples were evaluated by Inductively coupled plasma mass spectrometry (ICP- MS). 3. Results and Discussion 3.1 Workability Using industrial waste materials in concrete as replacement of fine aggregate by BA combined with a replacement of cement by FA could influence on the workability of coal ash concrete (CAC) mixtures. The slump is a one of the main measurement that is indicating the workability of concrete. Table 3 indicates the mix proportion and the result of slump test on CAC and control concrete mixtures. It is clear that by increasing the content of BA in CAC mixtures, the slump value decreased. According to Table 3, the slump values of control mix C0, CAC1, CAC2, CAC3 and CAC4 were 7.3 cm, 9.2 cm, 7.6 cm, 5.3 cm and 3.7 cm. The water absorption of BA (11.61 %) is higher than river sand (7.4 %) and it could be the reason for decreasing workability of CAC mixtures by increasing the amount of BA. On the other hand, replacement of OPC by FA, caused an increase in slump value and the combination of BA and FA in CAC mixtures result in a different range of slump. Up to 50 % substitute level of BA, CAC mixtures CAC1 and CAC2 presented more increase in a slump when compared to that of normal concrete mix C0. Instead, by increasing BA content in CAC mixtures CAC3 and CAC4, considerable reduction happened in a slump when compared to C0 normal concrete mix. The outcomes of this study are comparable to those stated by Singh and Siddique (2014b). Table 3: Workability and Mix proportion of concrete mixtures Codes Cement Fine aggregate w/c Slump (cm) FA Cement BA Sand Control 0 100 0 100 0.55 7.3 CAC1 20 80 25 75 0.55 9.2 CAC2 20 80 50 50 0.55 7.6 CAC3 20 80 75 25 0.55 5.3 CAC4 20 80 100 0 0.55 3.7 3.2 Compressive Strength The results of compressive strength test are shown in Figure 1. The compressive strength development pattern for CAC mixtures during the entire curing period is nearly the same as that of normal concrete. At the early age of curing (7 d), the compressive strength of CAC mixtures decreased with increase in BA volume as replacement of fine aggregate. By increasing the curing time, the compressive strength of CAC mixtures increased at a higher rate than the normal concrete. At 28 d curing age, CAC mixtures CAC1, CAC2, CAC3 and CAC4 achieved 85.38 %, 84.86 %, 80.62 % and 79.26 % of the compressive strength of the normal concrete mix C0. At the curing age of 91 d, the compressive strength of CAC mixtures CAC1, CAC2, CAC3 and CAC4 were 33.44 MPa, 34.55 MPa, 34.20 MPa and 33.85 MPa as compared to 34.48 MPa of the normal concrete specimen. Rapid 441 increase in compressive strength of CAC mixtures could be attributed to the pozzolanic activity of BA and FA. Previous study (Singh and Siddique, 2015) found that the utilisation of by-product BA results in pozzolanic action and it was notable between 28 d and 91 d. Figure 1: Compressive strength of concrete mixtures versus time At curing period of 91 d and 180 d, the expansion of Calcium Silicate Hydrate (C-S-H) gel and creation of extra C-S-H gel occurred because of utilisation of portlandite by the pozzolanic action of BA and FA particles leads to higher compressive strength of CAC mixtures. At 180 d of the curing period, the compressive strength of CAC mixtures CAC1, CAC2, CAC 3 and CAC4 were 35.85 MPa, 36.04 MPa, 36.64 MPa and 36.12 MPa as compare to 35.95 of the normal concrete mixture. The results of this study are in agreement with the finding by Singh and Siddique (2014a), Kim and Lee (2011) and Ghafoori and Bucholc (1997). They also reported that there is no considerable change in compressive strength with the utilisation of BA as a substitute of fine aggregate in concrete. 3.3 Leaching Test Concentrations of the selected components (As, Ba, Be, Cd, Cr, Co, Cu, Pb, Hg, Ni, Se, Ag, Ti, V, Zn) in both eluates with two different initial pH (2.88 and 4.93) obtained from TCLP method were measured by ICP-MS. Table 4 indicates the result of TCLP method on CAC4 mixture particles (for rest of elements, the amount of leaching were not detected). The maximum amount of BA (100 %) as replacement of fine aggregate is applied for the CAC4 mixture, then CAC4 mix was selected for leaching test. All of the elements showed much lower leaching results than standard regulation as mentioned by US EPA SW-846. All of the elements, except Co and Zn, were leached at higher proportions with initial pH of 2.88. It is clear that on the acetic environment, most of the elements were leached at higher proportion. Due to the acidic environment, the permeability of concrete particles increased and because of increasing in permeability of concrete particles, heavy metals could be released faster and simpler. However, it can be concluded that, there is no significant leaching for TCLP method for both initial pH of 2.88 and 4.93. The results of this research are in agreement with the findings by Drakonaki et al. (1998); Beard (2002) and Gwenzi and Mupatsi (2016). They also found that the leaching of heavy metals from coal ash concrete was insignificant and did not need to be considered. Table 4: Leaching test results and Standard limitation (US EPA) (mg/L) Symbol Ba Cr Co Pb Ni Zn Initial pH: 2.88 0.57 0.03 0.01 0.02 0.02 0.02 Initial pH: 4.93 0.43 0.02 0.01 0.01 0.01 0.03 Standard Limit (mg/L) 100 5.0 80 5.0 20 250 0 5 10 15 20 25 30 35 40 Control CAC1 CAC2 CAC3 CAC4 C o m p re s s iv e S tr e n g th ( M P a ) Sample Mixes 7 28 91 180 442 4. Conclusions Influences of the utilising BA and FA as a substitute of fine aggregate and OPC on workability, compressive strength and leaching of coal ash concrete were studied. The following results can be taken from this study: 1. The workability of CAC mixtures were decreased because of using of BA as replacement of river sand in concrete. In fact, use of BA as a sand substitute, increase the concrete’s structure with many more porous particles, fine-shaped and irregular. Therefore, it increases the particles friction which is the main reason for preventing the flow of fresh concrete. 2. The performance of CAC mixture in relation to the compressive strength at similar curing period was almost comparable to that of the normal concrete. At the curing period of 28 d the compressive strength of normal concrete was higher than that of the coal ash concrete specimens. Nevertheless, by increasing in curing time, the compressive strength of the CAC specimens increased with a higher rate than normal concrete. At the curing time of 91 d, the compressive strength of CAC mixtures was almost close to the normal concrete and after 180 d of curing age, it surpassed the figures obtained by the normal concrete. 3. Generally leaching was higher in the high acidic media (pH 2.88) as compared to the low acidic media (pH 4.93). However, results of leaching of all the elements showed that the concentration values are far below the limit regulation as provided by US EPA. 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