Eco-friendly utilization of corncob ash as partial replacement of sand in concrete

Construction and Building Materials 195 (2019) 165–177

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Eco-friendly utilization of corncob ash as partial replacement of sand in concrete Shazim Ali Memon a,⇑, Usman Javed b,c, Rao Arsalan Khushnood d a

Department of Civil Engineering, School of Engineering, Nazarbayev University, Astana, Kazakhstan International Islamic University, Islamabad, Pakistan c Department of Civil Engineering, COMSATS Institute of Information and Technology, Pakistan d NUST Institute of Civil Engineering, National University of Sciences and Technology, Islamabad, Pakistan b

h i g h l i g h t s
Corncob has significant energy and is used as energy resource.
Generated ash is disposed to landfill sites or ash ponds locking the useful land.
Evaluated corncob ash as replacement of sand in concrete.
22 MPa strength achieved with 10% CCA and can be used for structural application.
Addressed sustainability issues of ash disposal and natural resources depletion.

a r t i c l e

i n f o

Article history: Received 13 January 2018 Received in revised form 20 July 2018 Accepted 7 November 2018

Keywords: Fine aggregate replacement Corncob ash Compressive strength Pulse velocity Water absorption

a b s t r a c t The natural sand reservoirs are depleting globally due to nonstop consumption of aggregate in concrete. The detrimental effect of uncontrolled fine aggregate extraction from riverbeds is also a major concern. Moreover, the proper disposal of agricultural waste resulting from biomass burning is a major environmental challenge. Hence, in this research, we have proposed an eco-friendly solution by investigating the utilization of corncob ash with 0, 5, 10, 15 and 20% as fine aggregate in concrete. CCA was characterized to determine its suitability as fine aggregate by determining physical and chemical properties as well as investigating its morphology at micro and macro level. Thereafter, in fresh state, the CCA concrete was tested for slump, shrinkage and density while in hardened state; it was tested for compressive strength, water absorption, ultrasonic pulse velocity, and density at the age of 7, 28, 56, and 90 days. The thermal gravimetric analysis was also performed to evaluate the possible pozzolanic potential of CCA composite. Test results showed that CCA was well graded, amorphous, free from organic impurities, and having highly porous morphology due to the presence of micro pores, perforations, and tubules. The slump and shrinkage values increased while the fresh concrete density decreased with the increase in the percentage of CCA. The compressive strength, ultrasonic pulse velocity, and hardened concrete density decreased with the increase in the percentage of CCA while the values of these parameters increased with the age of testing. The values of water absorption were found to decrease with the age of testing. At 28 days, the compressive strength of concrete with 10% CCA as replacement of fine aggregate was found to be 22 MPa. For all mixes, the weight loss in sulfuric acid was more pronounced than hydrochloric acid due to more aggressive and destructive nature of sulfuric acid. Chemical composition of CCA and TGA results of CCA composite showed that CCA has pozzolanic potential when used in concrete as partial replacement of fine aggregate. The utilization of CCA provides eco-friendly solution of ash disposal problem. It also provides viable source of raw materials for construction industry and hence would help in conserving natural aggregate resources. Hence, the benefits of using CCA as fine aggregate in concrete were verified. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding author. E-mail address: [email protected] (S.A. Memon). https://doi.org/10.1016/j.conbuildmat.2018.11.063 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

The natural resources of planet earth are being consumed drastically and the environment is being polluted by the human

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activities. According to Steinberger et al. [1], the global material extraction around the world is 48.5 billion tonnes per year, in which the share of construction material and biomass is 16.2 billion tons/year and 17.5 billion ton/year respectively. The material extraction constantly consumes the natural resources and burning of the residual biomass is affecting the environment. In construction materials, the demand of concrete has increased due to increased population as well as boom in infrastructure development [2,3]. It is known that concrete industry is one of the largest consumers of natural resources (i.e. natural aggregate). However, natural sand reservoirs are depleting globally due to nonstop consumption of aggregate in concrete [4,5]. The detrimental effect of uncontrolled fine aggregate extraction from riverbeds is also a major concern [6]. Hence, constant efforts are put together to fulfill the demand of aggregate by replacing it with artificial and recycled materials and to make the concrete sustainable [7–9]. The burning of biomass is a huge environmental concern especially in agricultural countries [10] and is responsible for deteriorating air quality, haze conditions and serious repercussions on health of humans [11,12]. In agricultural based economies like China and India, open field burning of crop residue is a significant portion of biomass emission [13]. According to Streets et al. [13], out of 750 Tg of biomass burned in Asia, 250 Tg (33.4%) stemmed from burning of open field burning. Out of this, the major contributors were India 84 Tg and China 10 Tg. Corncob is one of the crop residue and agriculture waste obtained from maize crop. According to the food and agricultural organization (FAO) of the United Nations, the worldwide production of corn is around 1.02 billion metric tonnes [14], which in turn, yields significant amount of corncob (approximately 15% of corn grain production) [15]. Mostly, in developing countries, corncob is discarded as waste. As a result, it blocks sewers and drains, pollute the air by open burning, which causes serious socioeconomic and health issues. In addition, due to significant energy content of corncob (19 MJ/kg) well comparable with switch grass and wood pellets [16], it is used as fuel in industries for heat generation and power production [17]. However, burning of corncob produces huge amount of corncob ash (CCA), which either is dumped to landfill sites or ash ponds thus requiring large disposal area. Moreover, ash results in air pollution and may have serious repercussions on health of humans [18]. Various researchers investigated the use of agro-industrial waste ash [19] such as rice husk ash [20], sugar cane bagasse ash [21], waste foundry sand and bottom ash [22] as fine aggregate replacement to conserve natural fine aggregate resources. Singh and Siddique [23] used coal bottom ash as replacement of fine aggregate (0, 30, 50, 75, and 100 wt%) in concrete. The authors found that strength and durability properties are not negatively affected up to the replacement level of 50%. The same team [2] used coal bottom ash as replacement of fine aggregate at different levels (0, 20, 30, 40, 50, 75, and 100%) in concrete. The authors suggested to use up to 30% and 50% coal bottom ash when used with and without superplasticizer respectively. The possibility of using coal bottom ash (0, 10, 20, and 30%) as replacement of fine aggregate in self-compacting concrete (SCC) was evaluated by Siddique [24]. The SCC mixes achieved compressive strength of up to 35.2 MPa at the age of 28 days. At any particular age, water absorption and sorptivity of SCC mixes increased with the increase in the content of bottom ash while abrasion resistance decreased with the increase in the content of bottom ash. Siddique [25] also evaluated the possibility of using fly ash (10, 20, 30, 40, and 50%) as replacement of fine aggregate. The fly ash concrete showed superior performance in terms of mechanical properties when compared with control concrete. Singh and Siddique [26] carried out the mechanical and micro morphological investigations on SCC by replacing iron slag (0, 10, 25, and 40%) as substitute of fine aggregate. The iron slag SCC mixes with 40% replacement level showed 20%

increase in compressive strength when compared with control mix. It was concluded that iron slag can be used as fine aggregate to produce structural SCC. The mechanical and durability properties of crushed bricks as fine and coarse aggregate were evaluated at different replacement levels (0, 25, 75, and 100%) in concrete. Based on test results, the authors suggested that the use of fine and coarse bricks in concrete should be limited to 50% and 25% respectively [27]. Shi-Cong and Chi-Sun [28] investigated furnace bottom ash, crushed fine stone and fine recycled aggregate as fine aggregate in concrete. Based on test results, it was concluded that both furnace bottom ash and fine recycled aggregate are potential candidates to be used as fine aggregate in concrete. Finally, in recently published review papers [6,29–31], the authors have suggested that agricultural and industrial waste are potential candidates for use as a replacement of sand. Conclusively, the depletion of natural sand reservoirs and disposal problem especially related to agricultural wastes have created opportunities for researchers to use agricultural wastes in concrete. In this research, corncob ash was used in concrete as partial replacement of fine aggregate. CCA was characterized at micro and macro level for possible use as fine aggregate in concrete. In fresh state, the CCA concrete was tested for slump, shrinkage and density while in hardened state; it was tested for compressive strength, water absorption, ultrasonic pulse velocity and density up to the age of 90 days. The utilization of CCA in concrete would provide eco-friendly solution of ash disposal problem, which may otherwise create air pollution and may have serious repercussions on health of humans. The utilization of CCA as fine aggregate would also provide viable source of raw materials for construction industry and hence would help in conserving natural aggregate resources. It is pertinent to mention here that some researchers have used corncob ash in blended cement [32–34] and recently the optimum incineration and grinding conditions were evaluated to determine the pozzolanic potential of CCA as replacement of cement in cement based composites [18]. However, in this reseach, CCA has been utilized in concrete for the first time as an aggregate based material to primarily investigate the fresh and hardened properties at varying concentrations with added endorsements by supplementary analysis.

2. Experimental investigation 2.1. Materials Ordinary Portland cement (Type I) conforming to ASTM standard C150 was used. Coarse aggregate having maximum size of 20 mm was collected from Dorr River quarry, Havelian (Pakistan), while fine aggregate was obtained from Lawrencepur river site, Pakistan. The corncob ash, which was used as replacement of fine aggregate, was obtained by open burning (uncontrolled) of 25–75 mm corncob (Fig. 1-a). It is known that organic impurities may retard the hydration reaction, by interfering the hydration reactants chemically [35]. Moreover, the strength gain may get hampered due to mutilation of aggregate with organic impurities. Hence, to determine the acceptability of CCA as fine aggregate, the organic impurity test was conducted according to ASTM C40. It was found that CCA sample does not contain organic impurities. Sieve analysis of sand, corncob ash and coarse aggregate was conducted as per ASTM C-33 specifications while the fineness modulus was calculated as per ASTM C-136. The water absorption, specific gravity and density of coarse and fine aggregate were determined as per ASTM C-127 and ASTM C-128 respectively. The physical properties of CCA, fine and coarse aggregate are presented in Table 1 while the particle size distribution curve of

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Fig. 1. a) Crushed corncob b) CCA retained on #50 sieve.

Table 1 Physical Properties of sand, corncob ash and coarse aggregates.

Water absorption Specific Gravity

Table 2 Chemical (Oxide) composition of corncob ash.

Sand

Corncob Ash

Coarse Aggregate

Composition

Percentage

1.8 2.59

27.24 1.15

1.2 2.64

SiO2 Al2O3 Fe2O3 CaO MgO P2O5 K2O Na2O TiO2 MnO Ash content

63.73 15.08 5.32 6.56 4.56 2.5 2.05 0.1 0.06 0.03 2.3

CCA and fine aggregate is shown in Fig. 2. The fineness modulus of CCA and fine aggregate were found to be 2.35 and 2.69 respectively and lie within the range specified by ASTM C 33 (2.3–3.1). It is known that clay minerals and organic material present in fine aggregate have detrimental effect on concrete. Typically, the finer portion of sieve #200 (75 lm), account for significant increase in water demand (Zongjin, 2011). For CCA and sand, the amount of micro-fines was found to be 1.56% and 3.7% and was below 5% limit set by ASTM C-33. It was also found that percentage of both sand and CCA passing any sieve and retained on next consecutive sieve was less than 45%, which fulfills the requirement of ASTM C33. The specific gravity of CCA (1.15) was found to be lower than sand (2.62) and it would influence the density of concrete mixture while the water absorption of CCA was found to be 27.24%, which is 15 times more than that of sand. It would be shown later that the higher water absorption was due to porous nature of CCA. Based on above analysis, it can be concluded that CCA is a suitable candidate to be used as substitute of sand. The results of chemical composition of CCA determined by X-ray florescence are shown in Table 2. It can be seen that the sum of silica (SiO2), alumina (Al2O3) and iron oxide (Fe2O3) is 84.13% (>70%), which meets one of the requirements of pozzolan as per ASTM C168-05. XRD (X-

ray Diffraction) was conducted to see the mineralogical characteristics of CCA sample by using diffractrometer model JDX-3532 JEOL, Japan with CuKa radiation (1.5418 Å) operated at 40 kV and 25 mA and 2h scan between 20° and 80°. The diffraction pattern from ICDD was used to identify the chemical phases of the specimens by using ‘‘MATCH Phase Identification v3.1” software. The X-ray crystallographic pattern of CCA is shown in Fig. 3. The sharp peaks of quartz and stishovite were observed at 2h value of 28.24° (d = 3.157 Å) and 30.73° (d = 2.907 Å) respectively, while comparatively broadened peak of cristobalite was found at 2h

120

Percent passsing

100 80 60 40 20 0 0.1

1 .0

10.0

Grain Size(mm) on logrithmic scale CCA

Sand

ASTM Upper Limit

ASTM Lower Limit

Fig. 2. Grading curve of sand and Corncob ash along with ASTM Limits.

Fig. 3. X-ray Diffraction of Corncob Ash.

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value of 35.57° (d = 2.83 Å). Quartz and Stishovite were again recorded at 2h value of 38.81° (d = 2.318 Å) and 40.46° (d = 2.228 Å). According to available literature, the intensity of phase diffraction peaks are proportion to the concentration of component producing it and peak phase intensity difference symbolizes difference in its concentration [36]. The XRD pattern clearly supports the abundant existence of amorphous silica in CCA. It is known that amorphous silica being reactive than crystalline silica is preferred for pozzolanic reaction [37]. Finally, the micro-morphological features of typical samples of corncob and CCA were determined by Scanning Electron Microscopy (SEM) using model JSM-5910 JEOL, Japan. 2.2. Mix design and sample preparation The conventional concrete mix with target design strength of 30 MPa (at 28 days) was designed by absolute volume method. The partial substitution of sand with corncob ash by volume was achieved in concrete at 0%, 5%, 10%, 15%, and 20% replacement level. The quantities of cement and coarse aggregate were fixed while the water to cement ratio was kept as 0.45. The details of mix design are shown in Table 3. It is pertinent to mention here that the quantity of water absorbed by CCA was compensated in mix proportioning.

ASTM C-597 respectively, while the water absorption and densities were determined as per ASTM C-642. For thermogravimetric analysis, cement mortar chunks of 10 mm  10 mm size were extracted from crushed concrete of C0 and C5 mixes which were immersed in acetone later for 24 h to stop the hydration as reported in ‘Lea’s Chemistry of Cement and Concrete’[41]. The small chunks were then oven dried at 105 ± 5 °C for 24 h followed by grinding into powder. For acid attack test, the (150 mm  150 mm  150 mm) cubes from each mix were moist cured for 28days. After completion of curing period, these samples were oven-dried for 24-h at a temperature of 100 °C. The samples were then cooled in a desiccator at room temperature and the oven-dry weight was recorded (W1). Thereafter, the samples were immersed in 15% acid solutions, which has been reported effective for acid attack test to capture the maximum damage in short duration of 28 days [42,43]. Samples were immersed for 28 days and solutions were renewed after 14 days to maintain pH as reported by Torgal et al. [44]. Furthermore, sample-solution ratio used was kept constant at 0.30 by volume. After removing the samples from acid solution, they were ovendried for 24-h at 100 °C and their weight was recorded (W2). The percentage of weight loss was determined by using the following formula.

Weight loss ð%Þ ¼ ðW1  W2Þ  100=W1

2.3. Casting and curing of specimens 3. Test results and discussions Concrete cubes having dimensions of 150 mm 150 mm  150 mm were cast in cubical moulds to determine the compressive strength, density, water absorption and Ultrasonic Pulse Velocity. For mixing and casting conditions, the temperature and relative humidity were kept as 23 ± 4.0 °C and 90% respectively in accordance with ASTM C 511 [38]. After demoulding the samples at 24 ± 1 h, they were immersed cured in water at room temperature up to the specified age of testing. 2.4. Testing program Each concrete mixture was tested to determine various properties in fresh and hardened state. Slump and unit weight were determined as per ASTM C 143 M-05 and ASTM C138M-01 respectively. The total shrinkage during the initial period is crucial as such measurement mode has been reported previously to gauge the volumetric stability of resultant concrete formulations [39]. The shrinkage response of cement based composites was determined by using shrinkage apparatus (Shwindrine GmBH, Germany) that follows linear shrinkage protocols of ASTM C1698-09 [40]. The sensitivity, relative humidity and temperature were kept as 0.31 mm, 90% and 23 ± 4.0 °C respectively. Moreover, the samples were covered with polythene sheet during the test to allow minimal loss of water due to evaporation. The hardened concrete tests including compressive strength, density, water absorption and Ultrasonic Pulse Velocity were determined at the age of 7, 28, 56, and 90 days. The compressive strength and Ultrasonic Pulse Velocity were determined as per BS 1881: Part 116: 1983 and

3.1. Macro and micro morphology of corncob and corncob ash The macro and micro morphology of corncob is shown in Figs. 4 and 5 respectively. Corncob consists of three layers i.e. chaff, woody ring and pith. The chaff/ beeswing (Fig. 4-b) is the outermost layer, which is similar to beeswing and forms the honeycomb arrangement. These three portions of corncob are highly absorbent [45]. In particular, pith and chaff are more absorbent than woodyring portion. It is also known that corncob particles are less abrasive than sand [45]. The micro morphological features of corncob ash are shown in Fig. 6. CCA particles contain long, extremely porous and perforated tubules. The size of micro pores and perforations was approximately up to 0.5 lm and 10 lm respectively. It is believed that these micro pores, perforations and tubules are responsible for high water absorption and low specific gravity of corncob ash. The SEM micrograph also shows agglomeration of particles with approximate size of up to 10 mm. Finally, the micrographs of SEM show particles resembling the shape of bacteria. It is believed that these particles may impart adhesion and interlocking to the system. It is pertinent to mention here that the shape and size of ash particles have been found to influence the elemental composition as well as pozzolanic performance of cement based composites [46]. Hence, it is suggested that role of different particles shapes and sizes of CCA and its influence on the properties of cement based composites should be investigated in future research work.

Table 3 Mix proportions. Mixes

Cement (kg/m3)

Water to Cement Ratio

Sand (kg/m3)

Corncob Ash (kg/m3)

Coarse Aggregate (kg/m3)

Water (kg/m3)

C0 (CM) C5 C10 C15 C20

444 444 444 444 444

0.45 0.45 0.45 0.45 0.45

642.22 610.09 577.98 545.87 513.76

0 15.90 31.81 47.71 63.62

1077.45 1077.45 1077.45 1077.45 1077.45

199.10 199.10 199.10 199.10 199.10

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decrease linearly with the correlation factor of 0.972. Finally, the standard deviation of fresh concrete density samples was 2.7%. 3.3. Linear shrinkage response

(a)

Pith (C)

Woody (B)

Chaff/Beeswing (A)

(b) Fig. 4. Morphology of corncob (a) (a) Image of Corncob [45]; (b) SEM image of Corncob (A) Chaff/Beeswing (B) Woody-ring (d) Pith.

The results of early shrinkage response of cement based composites are shown in Fig. 9. In comparison to control mix, the shrinkage of C5, C10, C15 and C20 mixes reduced to 86.9, 68.16, 51.76 and 26.93% respectively. This shows that C5 mix is the most dimensionally stable mix. The lowest shrinkage value of the C5 mix may be attributed to the optimum water retention on adsorptive surface of CCA to reduce self-desiccation and thus reduce the shrinkage [48–50]. With further added contents, there was reduction in the exposed surficial area due to formation of lumps via cohesion developed as a result of inherent moisture in air dry condition as evidenced from SEM micrograph given in Fig. 6. This might have caused reduction in optimal water retention to compensate the total shrinkage with increasing CCA content as the cluster formation dominates at higher replacement levels than C5 mix. However, still the early shrinkage responses of all CCA mixes were lower than control mix. As water compensation for CCA was incorporated in the mix, CCA acted as water reservoir and fulfilled the deficiency of water on concrete hydration [48], hence, porous nature of CCA attributed to reduce total shrinkage, therefore, shrinkage response of the concrete mixes containing CCA was lower than control. Bai et al. [47] evaluated the performance of furnace bottom ash as fine aggregate in concrete and found that at fixed water cement ratio, the total shrinkage response of the furnace bottom ash was lower than control formulation. These results are also in line with the results of Ghafoori and Bucholc [51], Kou and Poon’s [52] and Singh and Siddique [53], who reported that the shrinkage of the ash incorporated mixes remained lower than the control concrete.

3.2. Properties of fresh concrete 3.4. Compressive strength The workability of fresh concrete was determined by slump. The results of slump of freshly prepared concrete at different replacement levels of CCA (0, 5, 10, 15, and 20%) are presented in Fig. 7. The results indicate that the slump values increased with the increase in the percentage of CCA in concrete. The maximum and minimum slump values of 165 mm and 51 mm were found for C20 and C0 mixes respectively. This shows that the increase in the slump value for C20 was 3.2 times higher than that of C0 mix. As mentioned earlier, all CCA mixes were designed based on saturated surface dry condition and hence the water absorbed by CCA was compensated in the mix design. This shows that more water was available for lubrication of the mixes containing higher CCA content, which in turn, increased the fluidity of fresh concrete containing CCA. Shi-Cong and Chi-Sun [28] observed similar behavior, where the slump values increased with the increase in the percentage of furnace bottom ash and fine recycled aggregate in the concrete mix. It is pertinent to mention here that the authors also made water compensation in the mix design for the water absorbed by furnace bottom ash and fine recycled aggregate. For bottom ash having higher 1 h water absorption of 30.4% and at fixed water cement ratios (0.45 and 0.55), Bai et al. [47] also found increase in the slump values with the increase in the percentage of bottom ash in the concrete mix. The results of fresh concrete density are shown in Fig. 8. The fresh concrete density of CCA concrete was lower than control mix and was found to decrease with the increase in the percentage of CCA. In comparison to control concrete, the drop in fresh concrete density for C5, C10, C15 and C20 mixes was found to be 1.5%, 3.65%, 4.41% and 5.5% respectively. The decrease in density was due to the porous nature and lower unit weight of CCA as compared to natural sand. The fresh concrete density was found to

The results of compressive strength for control mix and the mixes incorporating CCA as replacement of fine aggregate in concrete at different ages are shown in Fig. 10. As expected, the compressive strength increased with the increase in the age of testing. It can also be seen at all ages (7, 28, 56, and 90 days), the compressive strength decreased with the increase in the percentage of CCA in the mix. The decrease in compressive strength may be related to the availability of high initial free water content rendering bleeding and poor interfacial bonding between cement paste and aggregate. Another possible reason for the reduction in compressive strength is related to strength of individual components of concrete. Since, the CCA particles are more porous (Fig. 6) and weaker than the natural fine aggregate, hence the compressive strength of CCA concrete was found to be less than that of control concrete. For furnace bottom ash and fine recycled aggregate, Shi-Cong and Chi-Sun [28] found that at fixed water cement ratio, the compressive strength decreased with the increase in the percentage of FBA (Furnace Bottom Ash) and FRA (Fine Recycled Aggregate) content in the concrete mixes. Bia et al. [47] showed that compressive strength decreased when the percentage of bottom ash as fine aggregate increased from 0 to 100%. Debieb and Kenai [27] used crushed brick as fine aggregate and reported that compressive strength decreased with the increase in the content of crushed brick in concrete mixes. However, Singh and Siddique [23,54] showed that compressive strength of coal bottom ash concrete (at 90 days) with high water absorption of 31.48% increased with the increase in the percentage of coal bottom ash as fine aggregate in concrete. The variation in results is due to the reason that the mix design was not compensated for the water absorbed by coal bottom ash [23,54]. This in turn reduced the

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Chaff/Beeswing(A)

Woody(B)

Pith(C)

Fig. 5. SEM images of corncob at different magnifications (A) Chaff/Beewing (B) Woody (C) Pith.

effective water cement ratio and hence improved the compressive strength of bottom ash mixes. It is worth mentioning here that the compressive strength of concrete at 28 days with 10% CCA as fine aggregate was approximately 22 MPa, which according to Turkish earthquake resistant building code (minimum 20 MPa compressive strength) is acceptable for applications in seismically active zones [55]. Moreover, the minimum required compressive strength for concrete is 17 MPa (2500 psi) as per the standard set forth by ACI 318 Section 5.2.1 [56]. Since the analyzed formulations render strength beyond the mentioned thresholds, therefore can be claimed feasible for structural use. The standard deviation of compressive strength samples was 2.3%.

3.5. Density The results of hardened concrete density for control mix and the mixes incorporating CCA as replacement of fine aggregate in concrete at different replacements are shown in Fig. 11. It can be seen that the density of concrete mixes decreased with the increase in the content of CCA as fine aggregate. This was due to the low specific gravity of CCA (1.15) when compared with fine aggregate (2.59). At the age of 7 days, the density varied from 2335.3 kg/m3 for control mix to 2207 kg/m3 for C20 mix, showing 5.5% decrease in hardened concrete density. At the age of 90 days, the density varied from 2459.26 kg/m3 for control mix to 2375.2 kg/m3 for C20

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Fig. 6. SEM of corncob ash (a) Magnification X200 (b) Magnification X1000 (c) Magnification 4000 (d) (c) Magnification 500.

165.1

180 139.7

160

Slump(mm)

140

114.3

120

88.9

100 80

50.8

60 40 20 0

C0

C5

C10 Concrete Mixes

C15

C20

Fig. 7. Comparison of slump of different concrete mixes.

2450

Fresh Concrete Density (kg/m3)

2400

2335.3

2350

y = -32.482x + 2362.5 R² = 0.9718

2300.5 2250.18

2300

2232.2 2207.04

2250

Fig. 9. Early shrinkage response of cement based formulations.

2200 2150 2100 2050 2000 C0

C5

C10

C15

C20

Concrete Mixes Fig. 8. Fresh Concrete Densities of concrete mixes.

mix, showing 3.42% decrease in hardened concrete density. This shows that hardened concrete density improved with age. The standard deviation of hardened concrete density samples was 1.4%. It can also be seen that at every stage of testing, the density of hardened concrete with respect to CCA content was found to decrease linearly with correlation coefficient of 0.9916, 0.9578, 0.9147 and 0.9703 at 7, 28, 56 and 90 days respectively (Fig. 11). For high water absorption bottom ash aggregate, Kim and Lee [57] showed that the density of hardened concrete decreased with the increase in the per-

centage of bottom ash as fine aggregate in concrete mixes. The density was found to decrease linearly with the increase in the percentage of bottom ash in concrete mixes. Bilir et al. [58] also demonstrated that the density of hardened mortar decreased with the increase in the content of bottom ash when used as fine aggregate. According to researchers, the lower density of bottom ash mixes is due to high demand of mixing water, which results in more as well as large size of pore thereby making the structure of concrete porous [31]. The relation between compressive strength and unit weight is presented in Fig. 12. At every stage of testing (7, 28, 56 and 90 days), linear relation exist between compressive strength and hardened concrete density. The correlation coefficient at 7, 28, 56 and 90 days was found to be 0.9818, 0.9608, 0.956 and 0.9668 respectively. Hence, it can be deduced that with the increase in the percentage of CCA as fine aggregate will result in reducing the unit weight and compressive strength of concrete while the

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Compressive Strength (MPa)

45 CO

40

C5

C10

C15

C20

35 30 25 20 15 10 5 0 7

28

56

90

Days Fig. 10. Compressive strength of concrete at different ages.

Hardened Density(kg/m3)

2500

y = -19.571x + 2472.8 R² = 0.9703 (90 days)

2450

y = -16.396x + 2445.9 R² = 0.9147 (56 days)

2400 2350

2300

y = -16x + 2420.2 R² = 0.9578 (28 days)

2250

y = -32.482x + 2364.5 R² = 0.9916 (7 days)

2200 C0

C5

C10

7 Days

Concrete Mixes 28 Days 56 Days

C15

C20

90 Days

Fig. 11. Hardened Density of CCA replaced concrete verses curing ages.

40

Compressive Strength (MPa)

35

due to the formation of necessary portlandite. It is believed that the pozzolanic action of CCA results in the consumption of the lower density portlandite to produce secondary CSH gel of relatively higher density [59] that further adds in the density of CCA mixes at the age of 28 days and beyond [50].

y = 0.0845x - 173.25 R² = 0.9818 (7 days) y = 0.2339x - 532.07 R² = 0.9608 (28 days)

30

y = 104.48x + 1984.2 R² = 0.956 (56 days) 25

20

3.6. Water absorption

y = 0.248x - 571.04 R² = 0.9668 (90 days)

15

10 2150

2200

2250

2300

2350

2400

2450

2500

Density (kg/m³) 7 Days

28 Days

56 Days

90 Days

Fig. 12. Relationship between Compressive strength and density of Hardened concrete.

compressive strength and unit weight increased with the curing age. It can also be seen that the slope of density-compressive strength relationship at 7 days was lower than at 28, 56 and 90 days. This shows that the rate of increase in density is higher at later ages (28-d onward) than 7-days. It may be attributed to the pozzolanic activity of CCA that triggers after the age of 7 days

The results of water absorption of concrete at curing age of 7, 28, 56 and 90 days are graphically shown in Fig. 13. As expected, the water absorption of control and CCA mixes decreased with age. Singh and Siddique [23] showed that for control and bottom ash concrete mixes, the water absorption decreased with curing age due to reduction in voids of concrete mixes. It is believed that the reduction in voids of CCA mixes is related to the pozzolanic activity of CCA as evident from its chemical composition. However, at every age of testing, the water absorption values were found to increase with the increase in the percentage of CCA. The reason for increase in water absorption of CCA mixes is related to the porous microstructure of CCA. Another possible reason for the increase in the water absorption is related to the availability of high initial free water content, which may consequently result in the formation of capillaries as reported by Andrade et al. [60] for concrete mixes containing bottom ash. At 7 days, the minimum and maximum values of water absorption were observed for C0 and C20 concrete mixes and were equal to 1.4% and 3.11% respectively. At the age

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Water Absorption(%)

CO

C5

C10

C15

C20

3.5 3 2.5 2 1.5 1 0.5 0 7

28

56

90

Curing Age(Days) Fig. 13. Water Absorption of CCA replacement concrete verses curing ages.

of 90 days, the minimum and maximum values of water absorption were observed for C0 and C20 concrete mixes and were equal to 1.06% and 1.95% respectively. This shows that initially at the age of 7 days, the difference between C0 and C20 was high i.e. 1.71% but at the age of 90 days, it reduced to 0.89%. Debieb and Kenai [27] found similar results, where they showed that water absorption of concrete mixes containing recycled brick increased with the increase in the percentage of crushed brick (25, 50, 75, and 100%) as replacement of fine aggregate. Siddique [24] found that water absorption of concrete mixes at 7 and 28 days increased with the increase in percentage of bottom ash (10, 20, 30% by weight) as replacement of fine aggregate. Polynomial regression equations are plotted (Fig. 14) for control and CCA concrete mixtures from which the behavior of water absorption can be interpolated. All the trend lines have positive slope indicating that with the increase in percentage of CCA replacement, the amount of water absorption increased. The reason for this behavior has been mentioned in the previous paragraph. The correlation coefficients at 7, 28, 56 and 90 days were found to be 0.989, 0.989, 0.988 and 0.982 respectively. 3.7. Ultrasonic pulse velocity The Ultrasonic pulse velocity test was carried out to predict the concrete quality, which according to BIS:13311-92 code refers to excellent, good, medium and doubtful [61]. It is known that UPV values are directly proportional to the concrete quality, hence higher the pulse velocity values, more compact, durable and less porous the concrete is. The UPV values of control and CCA concrete

3.5

y = 0.0019x2 + 0.0499x + 1.3983 R² = 0.9893 (7 days)

Water absorption (%)

3 2.5

y = 0.003x2 + 0.0338x + 1.1371 R² = 0.9895 (28-day) y = 0.004x2 - 0.0049x + 1.1099 R² = 0.9881 (56-day)

2 1.5 1 y = 0.0027x2 - 0.009x + 1.0693 R² = 0.9822 (90-day)

0.5

7 days

28 days

56 days

90 days

0 C0

C5

C10

C15

C20

C25

Concrete Mixes Fig. 14. Regression equations for water absorption corresponding to CCA replacement in concrete.

mixtures are tabulated in Table 4. The ultrasonic pulse veloctiy values decreased with the increase in the percentage of CCA used as replacement of fine aggregate in concrete mixes. The porous nature of CCA is responsible for the decrease in the value of UPV in CCA mixes. Another possible reason for the decrease in the UPV values is related to the availability of high initial free water content, which may consequently result in the formation of capillaries as reported by Andrade et al. [60] for concrete mixes containing bottom ash. Singh and Siddique [31] suggested that increase in water demand and bleeding in bottom ash (having high water absorption) concrete mixes resulted in the increase in number of pores and their continuity. For bottom ash having high water absorption, Singh and Siddique [31] suggested that the increase in the value of pulse velocity with age is related to the increase in gel/space ratio due to continued hydration process with age. The pulse velocity values obtained in this research were compared with BIS:1331192 (Table 4). It can be seen that mostly the quality of concrete mixes can be categorized as good. It is known that UPV values are used as an indicator of strength of concrete. For cement based composites, the correlation between compressive strength and ultrasonic pulse velocity has been reported in exponential form as follows. Compressive Strength = x.ey.ultrasonic pulse velocity, where x and y are the empirical constants. Hence, the correlation between UPV and compressive strength of concrete mixes was determined. The results of correlation are presented in Fig. 15. The coefficient of determination was 0.9067 indicating good relation between data points and regression curve. The variation between actual and predicted compressive strength values obtained from equation proposed in Fig. 15 is presented in Table 5. It can be seen that the variation between the actual and predicted compressive strength lies within +10.17– 9.06%. It has been reported in literature that for non-destructive testing, the magnitude of variation in result always remain within 20% [62]. The empirical equation and parameters obtained from this research are the same as reported by Singh and Siddique [62], Turgut [63] and Nash’t et al. [64]. However, the determination coefficient (R2) observed was higher than that of Nash’t et al. [64] and Turgut [64]. Furthermore, it is also known that ultrasonic wave is affected by the medium through which the wave is going to travel. In this regard, the unit weight of medium is important. Hence, the relationship between ultrasonic pulse velocity and unit weight is plotted in Fig. 16. The coefficient of determination at 7, 28, 56 and 90 days was 0.9515, 0.936, 0.956 and 0.9429 respectively. It shows good relation between regression curve and data exists.

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Table 4 Ultrasonic Pulse velocity and Concrete quality grading as per BIS 13311-92-Part-I. Mixes

Ultrasonic Pulse Velocity (m/s)

C0 C5 C10 C15 C20

Concrete quality grading as per BIS 13311-92-Part-I

7d

28 d

56 d

90 d

Pulse Velocity (m/s)

Concrete quality grading

3760 3690 3400 3160 3090

3850 3700 3570 3460 3390

4317 4045 3945 3771 3665

4381 4217 4085 3836 3645

Above 4500 3500–4500 3000–3500 Less than 3000

Excellent Good Medium Doubtful

40

Compressive Strength (MPa)

35

Compressive Strength = 1.1678 e0.7954*UPV R² = 0.9067

30

25

20

15

10 2.99

3.19

3.39

3.59

3.79

3.99

4.19

4.39

and weight loss. Hence, the increase in weight loss of CCA mixes (absorption of more acid) may be related to porous microstructure of CCA, the availability of high initial free water content in CCA mixes as well as fluid transport through concrete. The minimum and maximum values of weight loss after exposure to hydrochloric acid were observed for C0 and C20 concrete mixes and were equal to 1.21% and 2.27% respectively. Similarly, the minimum and maximum values of weight loss after exposure to sulfuric acid were observed for C0 and C20 concrete mixes and were equal to 5.7% and 9.28% respectively. This shows that the damage caused by sulfuric acid was more severe than hydrochloric acid. It is also known that sulfuric acid is more aggressive and destructive than hydrochloric acid. This is due to the reason that in case of sulfuric acid, a product called ettringite is formed, which causes expansion and disruption of the set cement paste, whereas, no such product is formed in case of hydrochloric acid [68].

Ultrasonic Pulse Velocity(km/s) Fig. 15. Relation between compressive strength and ultrasonic pulse velocity of concrete.

3.8. Acid attack Alkaline nature of concrete due to presence of portlandite makes it vulnerable to acid attack. Constituents of cement paste get disintegrated when they come in contact with acids. Acid attack was conducted to analyze sulphate resistance of investigated formulations as reported in literature [65–67] for its possible applications in acid rich environments like sanitary sewer systems. Hence, the performance of concrete mixtures, in terms of weight loss, was determined by keeping the samples into hydrochloric and sulfuric acids for 28 days. The results of weight loss of control and CCA mixes after acid exposure are given in Fig. 17. For both hydrochloric and sulfuric acid, the weight loss increased with the increase in the percentage of CCA in the mixes. As shown earlier that at any specific age, the water absorption increased with the increase in the percentage of CCA in the mixes. This shows that with the increase in the content of CCA in the concrete mixes, it absorbed more acid. This, in turn, resulted in more deterioration

3.9. Thermogravimetric analysis (TGA) The thermogravimetric analysis was carried out to determine the possible pozzolanic potential of CCA in concrete mixes. For this purpose, TGA of control and CCA concrete mix containing 5% CCA as fine aggregate was determined at the age of 90 days. The TGA curve along with the details of calculated weight loss for control and CCA concrete mixes are shown in Fig. 18 and Table 6. Three weight loss regions can be observed. The first weight loss located in 110–300 °C region is mainly due to dehydration of C-S-H; the second weight loss in 450–550 °C is related to dehydroxylation of portlandite while the third weight loss in 750–900 °C is related to decarbonation of calcium carbonate [69]. It can be seen that in the first region, CCA mix (C5) showed higher weight loss (42.67%) than control mix (34.24%) hence it has higher hydration reaction degree. In the second region, the CCA mix showed lower weight loss (10.87%) than control mix (11.06%). This shows that Ca(OH)2 was consumed by CCA and hence testifying the presence of pozzolanic activity. It also shows that CCA can enhance the cement matrix activity and accelerate secondary hydration reaction of Ca(OH)2. Although at all testing ages, CCA mixes showed lower compressive strength when compared with control concrete and the reason for this behavior has been mentioned earlier, however, from the TGA

Table 5 Deviation between actual and predicted compressive strength determined from relationship in Fig. 15. Sample

Pulse velocity (m/s)

Predicted compressive strength (MPa)

Actual compressive strength (MPa)

Variation (%)

1 2 3 4 5 6 7 8 9 10

3760 3690 3400 3390 3570 3462 3390 3546 3645 3757

23.24 21.98 17.45 17.31 19.98 18.33 17.31 19.61 21.21 23.19

23.70 22.03 17.84 16.05 21.97 18.50 16.05 18.23 19.25 21.39

1.95 0.23 2.17 +7.88 9.06 0.89 +7.88 +7.56 +10.17 +8.41

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Ultrasonic Pulse Velocity(km/s)

4.39

y = 0.0057x - 9.3974 R² = 0.9515 (7 days)

results it can be deduced that CCA has pozzolanic potential when used in concrete as replacement of fine aggregate.

y = 0.0085x - 16.646 R² = 0.936 (28 days)

4. Conclusions and recommendations

4.19 3.99 3.79 3.59

y = 0.0092x - 17.982 R² = 0.956 (56 days)

In this research, we have suggested an eco-friendly solution for disposing CCA by utilizing it as replacement of fine aggregate in concrete. Some of the important findings of this research are:

y = 0.0091x - 17.948 R² = 0.9429 (90 days)

3.39

a) From material characterization results, it was found that CCA was well graded, free from organic impurities, amorphous in nature, finer portion within ASTM specified limits and having porous morphology due to the presence of micro pores, perforations and tubules. b) The slump values increased with the increase in the percentage of CCA as replacement of fine aggregate. This was probably due to availability of more free water available for lubrication and hence increased the fluidity of CCA mixes. The shrinkage of CCA mixes was found to be less than control mix with C5 mix showing 86.9% reduction in shrinkage value when compared to control mix. The density of fresh concrete decreased with the increase in the percentage of CCA in the mixes due to porous nature and lower unit weight of CCA. c) The compressive strength decreased with the increase in the percentage of CCA in the mixes while the compressive strength increased with the increase in the age of testing. The decrease in compressive strength may be related to the availability of high initial free water content as well as due to the inherent weaker mechanical properties of CCA. The compressive strength achieved at 28 days with 10% CCA as fine aggregate is acceptable for applications in seismically active zones according to Turkish earthquake resistant building code. d) The density of hardened concrete decreased with the increase in percentage of CCA. The reduction in unit weight is desirable in context of reducing cross sectional dimensions of structural elements and dead load. Moreover, reducing dead load is important to decrease the earthquake damage. Linear correlation was found in between unit weight and compressive strength of concrete. e) The water absorption of concrete increased with the increase in percentage of CCA as fine aggregate due to porous nature of CCA as well as availability of high free water content in the mix. f) For all concrete mixes, the pulse velocity values increased with the increase in the age of testing. According to BIS:13311-92 [61], mostly the quality of concrete mixes can be categorized as good. The correlation between pulse velocity and compressive strength was exponential with the value of coefficient of determination higher than 0.98 at all the ages of testing, indicating good relationship between data points and regression curve. The correlation between pulse velocity and unit weight also exist and was found to be linear.

3.19 2.99 2150

2200

2250

2300

2350

2400

2450

2500

Unit Weight(kg/m³) 7 Days

28 Days

56 Days

90 Days

Fig. 16. Relation between ultrasonic pulse velocity and unit weight of concrete.

Weight Loss (%)

10

9.28

9.11

9.00

8.96

8 5.70

6 4

2.08

2

2.04

2.17

2.27

1.21

0

C0

C5

C10

C15

C20

Concrete Mixes HCL

Sulfuric Acid

Fig. 17. Percentage Weight Loss after exposure to acid environment.

Fig. 18. TGA and derivative weight loss curves of control and CCA mixes.

Table 6 Weight loss of CCA mortar at 90 days. Concrete

C0 C5

Weight loss (%)

Weight loss with respect to total weight loss (%)

Stage 1

Stage 2

Stage 3

Stage 1

Stage 2

Stage 3

4.51 4.16

1.457 1.06

3.96 2.44

34.24 42.67

11.06 10.87

30.06 25.02

Note: Stage 1: dehydration occurred due water loss from C–S–H; Stage 2: Dehydroxylation of portlandite; Stage 3: Decarbonation of calcium carbonate.

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g) For both hydrochloric and sulfuric acid, the weight loss increased with the increase in the percentage of CCA in the mix. The weight loss in sulfuric acid was more pronounced than hydrochloric acid due to more aggressive and destructive nature of sulfuric acid h) TGA results showed that CCA has pozzolanic potential when used in concrete as replacement of fine aggregate. Conclusively, the utilization of CCA provides eco-friendly solution of ash disposal problem, which may otherwise create air pollution and may have serious repercussions on health of humans. It also provides viable source of raw materials for construction industry and would help in conserving natural aggregate resources. The authors recommend to further analyze the role of replacement percentage on the portlandite consumption, identification of hydrates alongside long term drying shrinkage and durability prospects endorsed with essential nano/micro supplementary investigations in future research work. Conflict of interest The authors declare no conflict of interest. Acknowledgement This research was supported by Nazarbayev University Faculty development competitive research grants (090118FD5316). References [1] J.K. Steinberger, F. Krausmann, N. Eisenmenger, Global patterns of materials use: a socioeconomic and geophysical analysis, Ecol. Econ. 69 (5) (2010) 1148–1158. [2] M. Singh, R. Siddique, Effect of coal bottom ash as partial replacement of sand on workability and strength properties of concrete, J. Clean. Prod. 112 (Part 1) (2016) 620–630. [3] R.V. Silva, J. de Brito, R.K. Dhir, Establishing a relationship between modulus of elasticity and compressive strength of recycled aggregate concrete, J. Clean. Prod. 112 (Part 4) (2016) 2171–2186. [4] J.N. Farahani, P. Shafigh, B. Alsubari, S. Shahnazar, H.B. Mahmud, Engineering properties of lightweight aggregate concrete containing binary and ternary blended cement, J. Clean. Prod. 149 (2017) 976–988. [5] B.M. Mithun, M.C. Narasimhan, Performance of alkali activated slag concrete mixes incorporating copper slag as fine aggregate, J. Clean. Prod. 112 (Part 1) (2016) 837–844. [6] A. Tiwari, S. Singh, R. Nagar, Feasibility assessment for partial replacement of fine aggregate to attain cleaner production perspective in concrete: a review, J. Clean. Prod. 135 (2016) 490–507. [7] A. Hanif, Y. Kim, Z. Lu, C. Park, Early-age behavior of recycled aggregate concrete under steam curing regime, J. Clean. Prod. 152 (2017) 103–114. [8] C. Shi, K. Zheng, A review on the use of waste glasses in the production of cement and concrete, Resour. Conserv. Recycl. 52 (2) (2007) 234–247. [9] B.S. Thomas, R. Chandra Gupta, Properties of high strength concrete containing scrap tire rubber, J. Clean. Prod. 113 (2016) 86–92. [10] S. Memon, I. Wahid, M. Khan, M. Tanoli, M. Bimaganbetova, Environmentally friendly utilization of wheat straw ash in cement-based composites, Sustainability 10 (5) (2018) 1322. [11] H. Zhang, S. Wang, J. Hao, X. Wang, S. Wang, F. Chai, M. Li, Air pollution and control action in Beijing, J. Clean. Prod. 112 (2016) 1519–1527. [12] H. Zhang, J. Hu, Y. Qi, C. Li, J. Chen, X. Wang, J. He, S. Wang, J. Hao, L. Zhang, L. Zhang, Y. Zhang, R. Li, S. Wang, F. Chai, Emission characterization, environmental impact, and control measure of PM2.5 emitted from agricultural crop residue burning in China, J. Clean. Prod. 149 (2017) 629–635. [13] D.G. Streets, K.F. Yarber, J.H. Woo, G.R. Carmichael, Biomass burning in Asia: Annual and seasonal estimates and atmospheric emissions, Global Biogeochem. Cycles 17 (4) (2003). n/a-n/a. [14] F.a.A.O.o.t.U. Nations, Food Outlook Biannual Report on Global Food Markets, Italy, 2016. [15] M. Bagby, N. Widstrom, Biomass uses and conversions, 1987. [16] D. Zych, The viability of corn cobs as a bioenergy feedstock, A report of the West Central Research and Outreach Center, University of Minnesota, 2008. [17] O. Ioannidou, A. Zabaniotou, E.V. Antonakou, K.M. Papazisi, A.A. Lappas, C. Athanassiou, Investigating the potential for energy, fuel, materials and chemicals production from corn residues (cobs and stalks) by non-catalytic and catalytic pyrolysis in two reactor configurations, Renew. Sustain. Energy Rev. 13 (4) (2009) 750–762.

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