Resistance to chloride penetration of blended Portland cement mortar containing palm oil fuel ash, rice husk ash and fly ash

Resistance to chloride penetration of blended Portland cement mortar containing palm oil fuel ash, rice husk ash and fly ash

Available online at www.sciencedirect.com Construction and Building MATERIALS Construction and Building Materials 22 (2008) 932–938 www.elsevier.c...

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Construction and Building

MATERIALS

Construction and Building Materials 22 (2008) 932–938

www.elsevier.com/locate/conbuildmat

Resistance to chloride penetration of blended Portland cement mortar containing palm oil fuel ash, rice husk ash and fly ash P. Chindaprasirt

a,*

, S. Rukzon a, V. Sirivivatnanon

b

a b

Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand Cement Concrete and Aggregates Australia, Level 6, 504 Pacific Highway, St. Leonards, NSW 2065, Australia Received 23 October 2006; received in revised form 25 November 2006; accepted 1 December 2006 Available online 16 January 2007

Abstract This paper presents a study of the resistance to chloride penetration of blended Portland cement mortar containing ground palm oil fuel ash (POA), ground rice husk ash (RHA) and fine fly ash (FA). Ordinary Portland cement (OPC) is partially replaced with pozzolan at the dosages of 20% and 40% by weight of cementitious materials. The water to cement ratio is kept constant at 0.5 and the flow of mortar is maintained at 110 ± 5% with the aid of superplasticizer (SP). Compressive strength, rapid chloride penetration test (RCPT), rapid migration test (RMT) and chloride penetration depth after 30 days of immersion in 3% NaCl solution of mortars were determined. Test results reveal that the resistance to chloride penetration of mortar improves substantially with partial replacement of OPC with POA, RHA and FA. The resistance is higher with an increase in the replacement level. RHA is found to be the most effective pozzolan followed by POA and FA. The use of FA reduces the amount of SP required to maintain the mortar flow, while the incorporations of POA and RHA require more SP. The use of a blend of equal weight portion of POA and FA, or RHA and FA produces mixes with good strength and resistance to chloride penetration. They also require less amount of SP in comparison to that of normal OPC mortar.  2006 Elsevier Ltd. All rights reserved. Keywords: Fine fly ash; Palm oil fuel ash; Rice husk ash; Chloride penetration; Mortar

1. Introduction The resistance to chloride penetration of mortar and concrete is one of the most important issues concerning the durability of concrete structures. When the chloride concentration of mortar or concrete exceeds a certain threshold value, depassivation of steel occurs and reinforced steel starts to corrode [1,2]. It is generally accepted that incorporation of a pozzolan improves the resistance to chloride penetration and reduces chloride-induced corrosion initiation period of steel reinforcement. This is mainly due to the reduction of permeability/diffusivity, particularly to chloride ion transportation of the blended cement concrete [3–5].

*

Corresponding author. Tel.: +66 4320 2846; fax: +66 4320 2846x102. E-mail address: [email protected] (P. Chindaprasirt).

0950-0618/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.12.001

Pozzolans from agricultural waste are receiving more attention now since their uses generally improve the properties of the blended cement concrete, and reduce the environmental problems. Palm oil fuel ash and rice husk ash are two promising pozzolans and are available in many parts of the world. In Thailand alone, approximately 100,000 ton of palm oil fuel ash are produced annually [6]. It is a by-product obtained from a small power plant using the palm fiber, shells and empty fruit bunches as a fuel and burnt at 800–1000 C. The main chemical composition of palm oil fuel ash is silica which is a main ingredient of pozzolan. Research indicates that ground palm oil fuel ash (POA) can be used as a pozzolan in normal and high strength concrete [7]. In addition, partial replacement of OPC with POA helps improve permeability and sulfate resistance of concrete [6,8]. Rice husk is also abundant in many parts of the world. When properly burnt at temperature lower than 700 C,

P. Chindaprasirt et al. / Construction and Building Materials 22 (2008) 932–938

reactive amorphous silica is obtained [9]. The silica content in rice husk ash is high at approximately 90%. Silica in amorphous form is suitable for use as a pozzolan. With proper burning and grinding, ground rice husk ash (RHA) can be produced and used as a pozzolan. Even for higher burning temperature with some crystalline formation of silica, good RHA can still be obtained by fine grinding [10]. The reactive RHA is used to produce good quality concrete with reduced Ca(OH)2 and higher resistance to sulfate attack [11,12]. An industry by-product which is now being used quite extensively as pozzolan in blended cements is fly ash. The incorporation of fly ash enhances the performance of concrete in terms of durability. The use of fly ash usually leads to a less permeable paste, denser interfacial zone between aggregate and the matrix [13–15]. Concrete containing fly ash is, therefore, less susceptible to the ingress of the harmful solutions. It has been shown that the use of fine fly ash (FA) results in better mechanical properties of concrete than those with the coarser fly ash. It increases strength, resistance to sulfate solution and resistance to chloride penetration of concrete [16,17]. The objective of this research is to study the use of POA, RHA and FA to increase the resistance to chloride penetration of mortar. The knowledge would be beneficial for future applications of the material in increasing the durability of mortar and concrete. 2. Experimental details 2.1. Materials Ordinary Portland cement (OPC), palm oil fuel ash from a thermal power plant from the south of Thailand, local rice husk, lignite fly ash from Mae Moh power plant in the northern part of Thailand, river sand with specific gravity of 2.63 and fineness modulus of 2.82, and type-F superplasticizer (SP) were the materials used. Rice husk ash was obtained from open burning in small heap of 20 kg of rice husk with the maximum temperature of 650 C. Ground palm oil fuel ash (POA) and ground rice husk ash (RHA) were obtained using ball mill grinding until the percentage retained on sieve No. 325 (opening 45 lm) was 1–3%. Fine fly ash (FA) with 1–3% retained on sieve No. 325 was obtained from air classification of original coarse fly ash. The SEM (scanning electron microscopy) and grading analysis were performed on POA, RHA and FA. 2.2. Mix proportions and curing Ordinary Portland cement (OPC) is partially replaced with 20% and 40% of pozzolan. In addition to single pozzolan, a blend of equal weight portions of POA and FA (BPF), and a blend of equal weight portions of RHA and FA (BRF) were also used. Sand-to-binder ratio of 2.75 by weight and water to binder ratio (W/B) of 0.5 were used.

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SP was incorporated in order to obtain mortar mixes with similar flow of 110 ± 5% in accordance with ASTM C109. The cast specimens were covered with polyurethane sheet and damped cloth and placed in 23 ± 2 C chamber. They were demoulded at the age of 1 day and cured in water at 23 ± 2 C. The mortar mix proportions are given in Table 1. 2.3. Compressive strength The 50 mm cube specimens were prepared in accordance with ASTM C109 [18]. They were tested at the age of 7, 28 and 90 days. The reported results are the averages of three samples. 2.4. Rapid test on resistance to chloride penetration The 100 · 200 mm cylinders were prepared in accordance with ASTM C39 [19]. After being cured in water until the age of 27 days, they were cut into 50 mm slices with the 50 mm ends discarded. The 50 mm slices were epoxy-coated around the cylinder. 2.4.1. Rapid chloride penetration test The 100 mm dia. · 50 mm epoxy-coated specimens were conditioned and tested at the age of 28 days for rapid chloride penetration test (RCPT) in accordance with the method described in ASTM C1202 [20]. The reported results are the averages of two samples. 2.4.2. Rapid migration test At the age of 28 days, the 100 mm dia. · 50 mm epoxycoated specimens were conditioned and tested for the chloride penetration depth using the rapid migration test (RMT) as shown in Fig. 1 [21,22]. The solutions employed in migration tests were 3% NaCl (in limewater) in the cathode side and limewater in the anode side. Applied voltage of 30 V dc at 8 h was employed. The chloride penetration depths were determined by breaking the specimens and by applying 0.1 M AgNO3 solution [23].

Table 1 Mortar mix proportions Mix

OPC

POA

RHA

FA

SP (%)

OPC 20POA 40POA 20RHA 40RHA 20FA 40FA 20BPF 40BPF 20BRF 40BRF

100 80 60 80 60 80 60 80 60 80 60

– 20 40

– – – 20 40 – – – – 10 20

– – –

1.9 2.0 3.2 2.2 3.7 0.4 0.1 0.8 1.1 1.1 1.6

– – 10 20 – –

20 40 10 20 10 20

Note: sand-to-binder ratio 2.75, W/B = 0.5, flow 110 ± 5%.

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P. Chindaprasirt et al. / Construction and Building Materials 22 (2008) 932–938 100 Commulative Passing (%)

90 80 70 60 50

FA

40

POA

30

RHA

20

OPC

10 0 0.01

0.10

1.00 10.00 Particle Size (micron)

100.00

1000.00

Fig. 3. Particle size distributions of FA, POA, RHA and OPC.

Fig. 1. Schematic diagram of accelerated chloride depth with rapid migration test (RMT) [21].

2.5. Immersion test To confirm the results, actual chloride penetration of mortar immersed in NaCl solution was performed. The test set-up was similar to that described in RTA T263 [24] with the exception that 100 mm dia. · 50 mm cut cylinders and 3% NaCl solution as shown in Fig. 2 were used. The 100 mm dia. · 50 mm specimens were epoxy-coated at the top surface as well as around the cylinder. At the age of 28 days, the specimens were immersed in 3% NaCl solution and kept immersed for 30 days. Chloride penetration depths were determined using 0.1 M AgNO3 solution [23]. 3. Results and discussions 3.1. Characteristics of OPC, POA, RHA and FA The Blaine fineness of OPC is 3600 cm2/g. The specific gravities of OPC, POA, RHA and FA are 3.14, 2.25, 2.23 and 2.45, respectively. The percentages retained on sieve No. 325 of POA, RHA and FA are 1–3%. The particle size distributions as shown in Fig. 3 suggest that FA is finest, followed by POA, RHA and OPC. The median particle sizes of the material used from the finest to the coarsest are as follows: FA 4.9 lm, POA 7.2 lm, RHA 10.0 lm and OPC 15.0 lm.

Fig. 2. Immersion of mortar specimen in 3% NaCl solution.

The chemical constituents are given in Table 2. The main chemical composition of POA is 63.6% SiO2, 7.6% CaO and 6.9% K2O. The high CaO and K2O are most likely from lime and fertilizer. The loss on ignition (LOI) is 9.6% which is not too high indicating a reasonable burning temperature and time. The sum of SiO2, Al2O3 and Fe2O3 is 66.6% which is slightly less than 70% as required for natural pozzolan according to ASTM C618 [25]. RHA, on the other hand, consists of 93% SiO2 complying with ASTM C618 requirement as a natural pozzolan. The LOI of 3.7% indicates complete burning. The SEM photo as shown in Fig. 4 indicates that palm oil fuel ash consists of irregular-shaped particles with a sizable fraction showing porous cellular structure. After being ground, POA consists mainly of fine irregular-shaped particles. The SEM photo reveals that the rice husk ash still maintains its cellular structure. After being ground, RHA consists of very irregular-shaped particles with porous cellular surface. Fly ash in this experiment is a Class-F fly ash with 74% SiO2 + Al2O3 + Fe2O3, 2.2% SO3 and 2.5% LOI meeting the requirement of ASTM C618. However, as this fly ash is from lignite, the CaO content is rather high at 14.4%. The SEM micrograph of fly ash as shown in Fig. 4 reveals that original fly ash consists of a large range of particle sizes. The particles are spherical in shape but the surfaces of the large particles are usually rough. After being classi-

Table 2 Chemical composition of OPC, POA, RHA and FA Oxides

OPC

POA

RHA

FA

SiO2 A12O3 Fe2O3 CaO MgO Na2O K2O SO3 LOI

20.9 4.8 3.4 65.4 1.3 0.2 0.4 2.7 0.9

63.6 1.6 1.4 7.6 3.9 0.1 6.9 0.2 9.6

93.2 0.4 0.1 1.1 0.1 0.1 1.3 0.9 3.7

41.1 21.6 11.3 14.4 3.3 1.1 2.6 2.2 2.5

SiO2 + A12O3 + Fe2O3



66.6

93.7

74.0

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Fig. 4. SEM photos of POA, RHA and FA.

fied, FA mainly consists of small spherical particles with smooth surface. 3.2. Superplasticizer (SP) content The results of SP requirements of the mortar mixes to produce a similar flow are shown in Table 1. The incorporation of FA reduces the SP content of the mixes. The SP content of OPC mortar is 1.9% and those of 20FA and 40FA mortars are only 0.4% and 0.1%. The reduction of SP requirement is associated with the ball-bearing effect of small spherical particles of FA. On the other hand, the incorporation of POA particles slightly increases the SP requirements. This is due to the fine and irregular surface

of POA particles. The SP contents of 20POA and 40POA mortars are 2.0% and 3.2%. The incorporation of RHA significantly increases the SP requirements owing to its cellular porous surface. The SP contents of 20RHA and 40RHA mortars are 2.2% and 3.7%. For the blended pozzolans, the addition of FA to the other pozzolans which require more SP helps keep the SP requirement low. The SP contents of 20BPF, 40BPF, 20BRF and 40BRF mortars are 0.8%, 1.1%, 1.1% and 1.6%, respectively. 3.3. Results of compressive strength The results of compressive strength of mortars are given in Table 3. The strength development of OPC mortar is

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Table 3 Compressive strength of mortars

40BRF

20BRF

40BPF

20BPF

40RHA

20RHA

40POA

40FA

Fig. 5. Charge passed of rapid chloride penetration test (RCPT).

RHA and FA, the charges of 20BRF and 40BRF mortars are 1100 and 250 C which are slightly higher than those of RHA mortars at the same replacement levels. 3.4.2. RMT results The results of RMT using 30 V dc as shown in Fig. 6 are similar to those of RCPT. The accelerated penetration depths are substantially reduced with incorporation of pozzolans as compared to the depth of 16.0 mm of normal OPC mortar. Incorporations of 20% and 40% of FA reduce the depths to 10.0 and 8.0 mm. Incorporations of 20% and 40% of POA further reduce the depths to 7.0 and 4.5 mm. The penetration depths of RHA mortars are lowest at 3.5 and 3.0 mm at 20% and 40% replacement levels. The blended pozzolans also improve the resistance to accelerated chloride penetration of mortars. Accelerated chloride penetration depths of 20BPF, 40BPF, 20BRF and 40BRF mortars are 5.5, 4.0, 5.0 and 3.5 mm, respectively. 3.4.3. Actual chloride penetration test The results of chloride penetration test of mortar immersed in 3% NaCl solution for 30 days are shown in Fig. 7. The results are almost the same as those of RMT while the results of RCPT are only different in magnitude but the trends are similar. This confirms the results of RCPT and RMT that incorporation of pozzolans improves

20 16 12 8 4

40BRF

20BRF

40BPF

20BPF

0 40RHA

3.4.1. RCPT results Usually there is a risk of temperature rise in using RCPT test for mortar specimens. For this experiment, the temperature rise is not large, as the strength of mortar used is reasonably high at around 53–60 MPa. The results of the RCPT test are shown in Fig. 5. The charge passed is substantially reduced with incorporation of pozzolans as compared to 7450 C of normal OPC mortar. The incorporation of 20% and 40% of FA reduces the charge passed to 3050 and 1950 C. POA is more effective than FA and reduces the charge passed to 1900 and 1050 C at 20% and 40% replacement levels. RHA is the most effective and reduces the charge passed to 750 and 200 C at 20% and 40% replacement levels. For blended pozzolans, the Coulomb charges are very low. The charges of 20BPF and 40BPF mortars are 1250 and 850 C which are lower than those of single pozzolan mortars at the same replacement levels. For the blend of

0

20RHA

3.4. Chloride penetration resistance of mortar

2000

40POA

rather good. The 7, 28 and 90-day strengths are 43.5, 57.0 and 60.0 MPa, respectively. At 20% replacement, the strengths of mortars containing POA, RHA and FA are also high between 100% and 105% of those of OPC mortar at the same age. For 40% replacement level, reductions in strength at 7 days are apparent for mixes containing POA, RHA and FA. Their 7-day strengths are 75–77% of that of OPC mortar at the same age. At the age of 90 days, strengths of POA, RHA and FA mortars are 102– 103% of that of OPC mortar at the same age. The low early strengths and later age strength development are the common feature of pozzolanic materials. For the blended pozzolans, the main improvement in strength is at 7 days and at 40% replacement level. The 7-day strength of blended pozzolan mortars are 95–99% of that of normal OPC mortar while the strength of single pozzolan mortars are only 75–77%. It has been suggested that incorporation of blend of fine pozzolans improves the strength of concrete due to synergic effect [26]. This probably contributes to this early strength improvement.

4000

20POA

60.0–100 62.0–103 61.5–102 62.5–104 62.0–103 63.5–105 62.0–103 63.0–104 60.0–100 64.0–106 61.5–102

20POA

57.0–100 57.5–102 53.5–94 58.5–103 55.0–97 59.5–105 56.5–99 57.5–101 56.5–99 58.0–102 55.5–98

20FA

43.5–100 43.5–100 32.5–75 44.5–102 33.5–77 44.5–102 33.0–76 43.5–99 43.0–98 42.0–97 41.0–95

6000

40FA

90 d

OPC

28 d

20FA

7d

Chlarge passed (Coulomb)

OPC 20POA 40POA 20RHA 40RHA 20FA 40FA 20BPF 40BPF 20BRF 40BRF

Compressive strength (MPa–normalized)

Chloride depth (mm)

Mix

8000

OPC

936

Fig. 6. Chloride depths of rapid migration test (RMT).

Chloride depth (mm)

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937

20

4. Conclusions

16

From the test, it can be concluded that POA, RHA and FA can be used as pozzolans to replace part of Portland cement in making mortar with relatively high strength and good resistance to chloride penetration. Spherical FA particles help reduce the amount of SP to produce mortar with similar flow. On the other hand, POA with fine irregular-shaped particles increases the amount of SP required. RHA with very irregular-shaped particles and porous cellular surface requires a large amount of SP. For the blended pozzolans, the use of FA with the other pozzolans which require more SP helps keep the SP requirement low. The RCPT, RMT and actual immersion in 3% NaCl solution test are effective in detecting the improvement of resistance to chloride penetration of reasonably high strength mortars containing pozzolans. The incorporations of POA, RHA and FA significantly improve the resistance to chloride penetration of mortar by increasing nucleation sites for precipitation of hydration products, reducing Ca(OH)2 and improving the permeability of mortar. RHA is the most effective, followed by POA and FA. Test results also indicate that the use of blended pozzolans of equal portion of POA and FA, and RHA and FA also effectively improves the mortar in terms of strength and resistance to chloride penetration. The improvement is due to dispersing effect of fly ash and synergic effect of the blend of fine pozzolans.

12 8

40BRF

40BPF

20BRF

20BPF

40RHA

20RHA

40POA

20POA

40FA

20FA

0

OPC

4

Fig. 7. Chloride depths after 30 days immersion in 3% NaCl solution.

the resistance to chloride penetration of mortars. RHA is the most effective, followed by POA and FA. The addition of a pozzolan such as fly ash or rice husk ash, whose particles are finer than those of Portland cement causes segmentation of large pores and increases nucleation sites for precipitation of hydration products in cement paste [27]. This increases the hydration and refines the pore structure of paste. The increase in hydration leads to a reduction of calcium hydroxide in paste. For fly ash, RHA and OPC mortars, it has been shown that the base condition is lowest for RHA mortars, followed by fly ash and normal OPC mortar [12]. This suggests that Ca(OH)2 consumption in RHA mortar is highest, followed by fly ash and OPC mortars. With regard to permeability, the incorporation of pozzolan such as fly ash reduces the average pore size and results in a less permeable paste [13,14]. It has also been shown that reactive RHA can be used to produce good quality concrete with reduced porosity [11]. Test also shows that the permeabilities of concretes containing palm oil fuel ash, rice husk–bark ash and fly ash are lower than that of OPC concrete [6]. In summary, the incorporation of POA, RHA and FA increases nucleation sites for precipitation of hydration products, reduces Ca(OH)2, and improves the permeability of mortar. These factors contribute to the improvement in the resistance to chloride penetration of mortar with RHA being the most effective, followed by POA and FA. The excellent improvement in the resistance to chloride penetration using the blended pozzolans is probably due to the synergic effect of the blend of fine pozzolans [26]. Additional physical effect related to the cement grains deflocculation is provided by fly ash [26]. The incorporation of fly ash adds additional dispersing effect of cement grains as well as that of other fine pozzolan particles. The larger number of nucleation sites would result in higher amount of hydration and higher calcium hydroxide consumption. The enhancement of strength and resistance to chloride penetration of mortar using the blends of POA and FA, and RHA and FA is, therefore, obtained.

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