Immobilization of Cr(VI) in bagasse ash blended cement pastes

Construction and Building Materials 30 (2012) 218–223

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Immobilization of Cr(VI) in bagasse ash blended cement pastes M.A. Tantawy ⇑, A.M. El-Roudi, A.A. Salem Chemistry Department, Faculty of Science, Minia University, Minia, Egypt

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Article history: Received 29 September 2011 Received in revised form 30 November 2011 Accepted 4 December 2011 Available online 29 December 2011 Keywords: Bagasse ash Blended cement Pozzolana Cement immobilization Solidification/stabilization Cr(VI)

a b s t r a c t The aim of the present work is to study the hydration characteristics and immobilization of Cr(VI) in bagasse ash blended cement pastes by setting time, compressive strength, bulk density, total porosity and immobilization efficiency measurements as well as XRD, FTIR and SEM techniques. It was investigated that hydration characteristics of bagasse ash blended cement pastes were enhanced especially at later ages of hydration. Immobilization of Cr(VI) in bagasse ash blended cement pastes reduces their setting time, accelerates hydration reactions due to precipitation of CaCrO4 and affects the pore formation mechanism of hydrated cement pastes. The alkalinity of bagasse ash blended cement pastes were reduced due to pozzolanic action. Hence, the rate of precipitation of CaCrO4 decreases with bagasse ash content. FTIR results indicate that C–S–H formed by the pozzolanic action of bagasse ash differs in its nature from that formed by hydration of OPC, encapsulates Cr(VI) ions and refines the microstructure of hydrated cement pastes as indicated from immobilization efficiency, XRD and SEM results. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Chromium is one of the heavy metals that has both beneficial and detrimental properties and is generated from many industrial processes such as electroplating, leather tanning, paints and pigments, textile, steel fabrication and production of chromium chemicals. In the environment, chromium usually exists in its common oxidation states, trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)]. The later is the most important one because of its toxicity, solubility, mobility characteristics, strong oxidizing and carcinogenic nature [1]. Cement-based immobilization treatment technology combines interrelated solidification/stabilization (S/S) processes that occur simultaneously to produce a material that will have reduced environmental impact when disposed or reused. The United States environmental protection agency US EPA has identified S/S as the best demonstrated available technology for many of hazardous wastes [2]. Bagasse ash is the waste generated by the combustion of sugar cane bagasse. Apart from silica which is the major component, it contains other oxides as well as unburned carbon [3]. Up to 0.066 tons ash is generated per ton of sugar cane crushed [4]. Huge amounts of bagasse ash have been produced annually in developing countries. Limited amounts of bagasse ash has been used as soil amendment while the rest of bagasse ash is useless and being disposed to open landfills causing serious environmental impacts. However, bagasse ash can be utilized for production of building bricks [5] and as a cement replacement to improve the mechanical ⇑ Corresponding author. Mobile: +20 1002554075. E-mail address: [email protected] (M.A. Tantawy). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.12.016

properties of cement mortar [6,7], normal concrete [8], self compacting concrete [9], conventional hollow concrete [10] and to improve the durability of concrete under aggressive environment [11]. The use of bagasse ash as a cement replacement for immobilization of heavy metals not only marks a novelty in the immobilization technology but also assists in alleviating disposal problem of bagasse ash heaps in developing countries. Many investigators have analyzed the effect of Cr(III) and (VI) on the different Portland cement phases [12,13] and its solidification in cementitious matrix [14,15]. Immobilization of chromium affects the pore formation mechanism causing a wide variety of pore diameters to appear indicating involvement of chromium in silicate formation in the C–S–H matrix [16]. Addition of Cr(VI) may retard hydration reactions as a result of the reduction in permeability caused by precipitation of calcium chromate and coating on cement grain surfaces [17,18]. Addition of Cr(VI) increases the setting time of cement, inhibits cement hydration and reduces the compressive strength of the cement [19]. On contrast, addition of Cr(VI) may increase the setting time of cement and impair the strength development of blended cement [20]. Another possible mechanism was involved in immobilization of Cr(VI) in cement is the formation of modified Cr-ettringite 3CaO
Al2O3
3CaCrO4
32H2O which may occur due to substitution of SO24 in ettringite structure by CrO24 [21,22]. However, this mechanism may require a high concentration of chromate ions in pore solution. The immobilization of Cr(VI) in cement can be related to the formation of Ca–Cr aluminates compounds such as Ca4Al6O12CrO4 and Ca6Al4Cr2O5 [12,13]. At elevated pH (approximately 12.8) the formation of calcium aluminosilicate hydrates is mainly responsible for Cr(VI) immobilization using quicklime and

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fly ash through surface sorption, inclusion and physical entrapment [23] or through the formation of pozzolanic compounds such as 3CaOAl2O30.5CaCrO40.5CaSO4
nH2O [24]. The mechanism by which Cr(VI) is bound to cementitious matrix influences the leaching mechanism [25]. The leachability of Cr(VI) from solidified cement mixtures depends on the initial Cr(VI) concentration, leaching time [19] and leachant pH [26]. Carbonation of cement mortars increases the leaching of chromium ions and addition of ground granulated blast furnace slag decreases the leaching of Cr(VI) from solidified fly ashes [26]. Cement immobilization of Cr(VI) was improved by blending cement with limestone [15] and marble dust [20]. The aim of the present work is to study the hydration characteristics and immobilization of Cr(VI) in bagasse ash blended cement pastes by XRD, FTIR and SEM techniques.

2. Materials and experimental techniques Raw materials used in the present work are: OPC [CEM I (No. 42.5)] from BeniSuif Cement Co., Egypt and raw sugar cane bagasse ash was collected from the boiler of nearby sugar factory in Minia Governorate, Egypt. Raw bagasse ash was calcined in an electrical muffle furnace with a heating rate 10 °C/min at 700 °C for 3 h, recharged from the muffle furnace, cooled to room temperature in desiccator and ground to pass 90 lm sieve. 5% K2CrO4 solution (weight/volume) was prepared (13,000 ppm of Cr(VI)) in distilled water and used in cement immobilization of Cr(VI). Bagasse ash blended cement was prepared by partial replacement of OPC with 10–25% bagasse ash and their mix composition is shown in Table 1. The water of consistency, initial and final setting times were determined using Vicat apparatus according to ASTM designations [27,28]. In case of hydration characteristics, cement pastes were hydrated with their corresponding water of consistency. In case of immobilization of Cr(VI) in cement pastes, 60 ml of 5% K2CrO4 solution (completed by water to its appropriate water of consistency) was added to 220 g of cement. Hence 1.36% wt/wt of K2CrO4 to dry cement (0.36% Cr(VI)) were used. Freshly prepared cement pastes were molded in stainless steel molds (2  2  2 cm) at about 100% relative humidity, demolded after 24 h and cured for 90 days in closed humidity chamber without mixing of hydrated cubes with water to avoid leaching of chromium from hardened cement pastes. Bulk density was determined using Archimedes principle [29]. The compressive strength was measured using a manual compressive strength machine for a set of three cubes according to ASTM designation [30]. Free water content was determined using domestic microwave oven (Olympic electric model KOR-131G, 2450 MHz, 1000 W) [31]. The combined water content was determined using hydration stopped specimen after being ignited in porcelain crucibles at 1000 °C for 1 h in a muffle furnace. The total porosity of the hardened cement paste was calculated from the values of bulk density, free and total water contents as described elsewhere [32]. The Cr (VI) immobilization efficiency after 28 and 90 days of hydration was determined according to US EPA toxicity characteristic leaching procedure (TCLP) [33]. Cr(VI) concentration of the leachate was determined spectrophotometrically by diphenylcarbazide method using Perkin–Elmer Lambda 35 UV/VIS spectrometer [34]. X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses were carried out by Philips X-ray diffractometer PW 1370, Co. with Ni filtered Cu Ka radiation (1.5406 Å). The Fourier transform infrared (FTIR) analysis was measured by spec-

Table 1 Mix composition in wt.% of bagasse ash blended cements. Symbol

OPC

Bagasse ash

C B1 B2 B3 B4

100 90 85 80 75

0 10 15 20 25

trometer Perkin Elmer FTIR system Spectrum X in the range 400–4000 cm 1 with spectral resolution of 1 cm 1. Scanning electron microscopy (SEM) was investigated by Jeol-Dsm 5400 LG apparatus.

3. Results and discussion 3.1. Characterization of raw materials Table 2 illustrates the chemical composition of the OPC and bagasse ash calcined at 700 °C for 3 h determined by XRF analysis. Bagasse ash contains high amount of SiO2 (61.69 wt.%) in addition to small amounts of CaO, Fe2O3, Al2O3, MgO, Na2O and K2O. The sum of SiO2, A12O3 and Fe2O3 equals to 83.03 wt.% which is in good agreement with the requirements (>70 wt.%) stated in ASTM designation for natural pozzolana [35]. According to Bouge’s calculations, OPC is composed of 47.78 wt.% C3S, 23.83 wt.% b-C2S, 10.73 wt.% C3A and 9.68 wt.% C4AF. Fig. 1 illustrates the XRD patterns of bagasse ash calcined at 700 °C for 3 h. Bagasse ash contains amorphous silica as indicated from the heap in the range 15–35 2h in addition to quartz, tridymite and christobalite. Fig. 2 shows the SEM micrograph of bagasse ash calcined at 700 for 3 h. Bagasse ash contains high amount of silica aggregate which may appear as dark grains (a) and residues which may appear as white small particles (b). 3.2. Hydration characteristics and immobilization of chromium (VI) in blended cement pastes Fig. 3 illustrates the water of consistency, initial and final setting times of bagasse ash blended cement pastes hydrated with water or 5% K2CrO4 solution. Water of consistency increases with bagasse ash content may be due to that bagasse ash markedly increases the amount of surface water which related to the specific surface of the system [36]. Setting time elongates with bagasse ash content may be due to increasing the amount of mixing water [37], decreasing OPC content [38] and formation of a layer of bagasse ash particles around anhydrous cement grains which delays the hydration of C3S. The thickness of this layer increases with bagasse ash content hence the rate of hydration of cement decreases at early ages of hydration [39]. Setting time of OPC paste hydrated with K2CrO4 solution is longer than that hydrated with water due to that addition of Cr(VI) increases the setting time of cement [19,20]. Setting time of bagasse ash blended cement pastes hydrated with K2CrO4 solution are shorter than those hydrated with water due to that the incorporation of pozzolana may improve the hydration of cement in presence of Cr(VI). Addition of Cr(VI) may accelerate hydration reactions in presence of pozzolana due to precipitation of calcium chromate and the increased demand for Ca2+ ions in the cement pore solution. As a result, unhydrated cement fraction satisfies the increased demand of Ca2+ ions and accelerates hydration of C3S [18]. Fig. 4 represents the bulk density and total porosity of bagasse ash blended cement pastes hydrated with water or 5% K2CrO4 solution for 90 days. The bulk density decreases while the total porosity increases with bagasse ash content due to increasing the amount of mixing water and decreasing clinker content. The bulk density decreases with bagasse ash content may be due to that bagasse ash have lower specific gravity than OPC. The bulk density of

Table 2 Chemical composition wt.% of OPC and bagasse ash calcined at 700 °C. Material

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

Na2O

K2O

LOI

P2O5

Total

OPC Bagasse ash

20.88 61.69

6.08 11.56

3.18 9.78

63.00 5.57

1.50 3.24

1.60 0.04

0.22 1.06

0.24 0.49

2.35 1.41

0.15 4.05

99.20 98.89

M.A. Tantawy et al. / Construction and Building Materials 30 (2012) 218–223

(a) Q quartz Cr christobalite T tridymite

T Cr

‘

Q

15

Cr 30

25

20

35

40

50

2.24 2.20

Bulk density, g/cm3

Intenisty, %

Q

45

2 theta

46

2.16 2.12

42

2.08 2.04

38

2.00 1.96

Total porosity, %

220

34

1.92 1.88

30 1

(b)

10

2.24

100

45

Curing time, days

2.20

Bulk density, g/cm3

a

b

43

2.16

41

2.12

39

2.08 37 2.04 35

2.00

33

1.96

Total porosity, %

Fig. 1. The XRD patterns of bagasse ash calcined at 700 °C for 3 h.

31

1.92 1.88

29 1

10

100

Curing time, days

Fig. 2. SEM micrograph of bagasse ash calcined at 700 °C for 3 h.

35

B1 (BD)

B2 (BD)

B3 (BD)

B4 (BD)

C (TP)

B1 (TP)

B2 (TP)

B3 (TP)

B4 (TP)

Fig. 4. The bulk density and total porosity of bagasse ash blended cement pastes hydrated with water (a) or 5% K2CrO4 solution (b) for 90 days.

550

Water of consistency

C (BD)

450

Initial Setting Time Cr VI Final Setting Time Cr VI

350

31 250 29 150

27

C

B1

B2

B3

B4

50

Mixes Fig. 3. The water of consistency, initial and final setting times of bagasse ash blended cement pastes hydrated with water or 5% K2CrO4 solution.

(a) 80 Compressive strength, Mpa

Final Setting Time

Setting time, min.

Water of consistency,%.

Initial Setting Time 33

C 70

B1

60

B2 B3

50 B4 40 30 20

(b) 80 Compressive strength, MPa

bagasse ash blended cement pastes hydrated with K2CrO4 solution are lower than those hydrated with water because addition of chromium affects the pore formation mechanism causing a wide variety of pore diameters [16]. Fig. 5 shows the compressive strength of bagasse ash blended cement pastes hydrated with water or 5% K2CrO4 solution for 90 days. The rate of strength development in blended cement pastes was enhanced at later ages of hydration. This may be due to that the formation of a layer of bagasse ash particles around anhydrous cement grains at early ages of hydration which reduces the rate of hydration. At later ages bagasse ash reacts with portlandite accumulated in the pore of hydrated cement pastes as a result of hydration of clinker phases giving additional calcium silicate hydrates. The compressive strength decreases with bagasse ash content due to decreasing OPC content. The compressive strength of bagasse ash blended cement pastes hydrated with K2CrO4 solution

70 60 50 40 30 20 1

10

100

Curing time, days Fig. 5. The compressive strength of bagasse ash blended cement pastes hydrated with water (a) or 5% K2CrO4 solution (b) for 90 days.

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Q quartz T C3 S D C2 S

P Portlandite A C3A

K K 2 CrO4 C CaCrO4

C Cr VI

C

B1 Cr VI P

P T

D K

B4 Cr VI

C Cr VI

A

C Q

B1 Cr VI

B1

B4

B4 Cr VI

4000 3600 3200 2800 2400 2000 1600 1200 800 C

Fig. 7. The FTIR patterns of bagasse ash blended cements hydrated with water or 5% K2CrO4 solution for 90 days.

B1

B4

26

28

30

32

34

36

38

400

Cm-1

40

2 theta Fig. 6. The XRD patterns of bagasse ash blended cement pastes hydrated with water or 5% K2CrO4 solution for 90 days.

are lower than those hydrated with water because addition of chromium affects the pore formation mechanism and reduces the compressive strength of the solidified cement especially at early ages. The compressive strength decreases with bagasse ash content due to decreasing OPC content and retardation effect of calcium chromate layer coating bagasse ash particles. Fig. 6 illustrates the XRD patterns of bagasse ash blended cement pastes hydrated with water or 5% K2CrO4 solution for

90 days. The amount of unhydrated clinker phases (C2S, C3S) and portlandite decrease with bagasse ash content due to decreasing OPC content and that bagasse ash activates the hydration of cement at later ages of hydration. In case of bagasse ash blended cements hydrated with K2CrO4 solution, the content of CaCrO4 decreases with bagasse ash content. Bagasse ash consumes part of portlandite by its pozzolanic activity, hence the alkalinity of hydrated cement pasts containing bagasse ash decreases. As a result, the rate of precipitation of CaCrO4 decreases with bagasse ash content. Fig. 7 illustrates the FTIR patterns of bagasse ash blended cements hydrated with water or 5% K2CrO4 solution for 90 days. In case of bagasse ash blended cements hydrated with water, the absorption bands of CO23 at 1429 cm 1 (asymmetric stretching vibration v3 of CO23 ) and 893 cm 1 (out-of-plane bending vibration v4 of CO23 ) increase with bagasse ash content. Because the total porosity of hydrated cement pastes increases with bagasse ash content, which in turn may increase the rate of carbonation of hydrated cement pastes.

C

B2

C Cr VI

B2 Cr VI

Fig. 8. SEM micrographs of C and B2 hydrated with water or 5% K2CrO4 for 28 days.

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Effeciency, %

90

85

80 28 days 90 days 75

C

B1

B2

B3

B4

Mixes Fig. 9. The efficiency of Cr(VI) immobilization of bagasse ash blended cement pastes hydrated with 5% K2CrO4 solution for 28 and 90 days.

The absorption band of C–S–H at 978 cm 1 (stretching vibration v3 4 of SiO4 ) shifts to higher wavenumber values with increasing bagasse ash content. This may be due to that C–S–H formed by the pozzolanic action of bagasse ash differs in its nature from that formed by hydration of OPC. The absorption band of silica at 462 cm 1 (bending vibration of O–Si–O) arises from the replacement of OPC with bagasse ash. The ettringite band at 1124 cm 1 disappeared may due to that SO24 ions were probably replaced by CO23 ions in case of bagasse ash blended cement pastes. In case of bagasse ash blended cements hydrated with K2CrO4 solution, the absorption bands of C–S– H at 978 and 1644 cm 1 decrease with bagasse ash content due to decreasing of OPC content and retardation effect of calcium chromate layer. Fig. 8 shows the SEM micrographs of OPC and blended cement paste containing 15% bagasse ash hydrated with water or 5% K2CrO4 solution for 28 days. Excess of portlandite prismatic aggregates fills some pores of OPC paste hydrated with water. Bagasse ash reacts with portlandite forming additional C–S–H. In case of OPC paste hydrated with K2CrO4, CaCrO4 was observed as a result of the reaction between portlandite and K2CrO4. Bagasse ash blended cement paste hydrated with K2CrO4 shows refined microstructure (i.e. have lower porosity) compared to that hydrated with water. As illustrated from total porosity values in Fig. 4. This may be due to that addition of Cr(VI) may accelerate hydration reactions in presence of bagasse ash [18]. Fig. 9 represents the efficiency of Cr(VI) immobilization of bagasse ash blended cement pastes hydrated with 5% K2CrO4 solution for 28 and 90 days. The efficiency of Cr(VI) immobilization was enhanced with up to 15% bagasse ash content then decreased after that. This may be due to that formation of additional C–S–H as a result of the pozzolanic activity of bagasse ash encapsulates Cr(VI) ions as detected in XRD results (i.e. decreasing the content of K2CrO4 with bagasse ash content). The efficiency of Cr(VI) immobilization decreases with higher bagasse ash content due to increasing the amount of mixing water, decreasing OPC content and formation of a layer of bagasse ash particles around anhydrous cement grains which delays the hydration of C3S. 4. Conclusions The main conclusions of this investigation are: 1. Hydration characteristics of bagasse ash blended cement pastes including setting time, compressive strength and total porosity were enhanced especially at later ages of hydration. 2. Immobilization of Cr(VI) in bagasse ash blended cement pastes reduces their setting time, accelerates hydration reactions due to precipitation of CaCrO4 and affects the pore formation mechanism of hydrated cement pastes.

3. The alkalinity of bagasse ash blended cement pastes was reduced due to pozzolanic action. Hence, the rate of precipitation of CaCrO4 decreases with bagasse ash content. 4. FTIR results indicate that C–S–H formed by the pozzolanic action of bagasse ash differs in its nature from that formed by hydration of OPC. 5. XRD results indicate that C–S–H formed by the pozzolanic action of bagasse ash encapsulates Cr(VI) ions. 6. SEM results indicate that bagasse ash blended cement pastes hydrated with K2CrO4 have refined microstructure compared to those hydrated with water. 7. According to this investigation, it was recommended that the blended cement which contain 15–20% bagasse ash can be used effectively for immobilization of concentrated waste Cr(VI) solutions (13,000 ppm Cr(VI)) after being mixed with this blended cement in a water to cement ratio of about 30%.

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