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Construction and Building Materials 22 (2008) 932–938

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MATERIALS

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

P. Chindaprasirt a,*, S. Rukzon a, V. Sirivivatnanon b

a Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailandb 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 2006Available online 16 January 2007

Abstract

This paper presents a study of the resistance to chloride penetration of blended Portland cement mortar containing ground palm oilfuel ash (POA), ground rice husk ash (RHA) and fine fly ash (FA). Ordinary Portland cement (OPC) is partially replaced with pozzolanat 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 ofmortar 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 withPOA, RHA and FA. The resistance is higher with an increase in the replacement level. RHA is found to be the most effective pozzolanfollowed by POA and FA. The use of FA reduces the amount of SP required to maintain the mortar flow, while the incorporations ofPOA 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 goodstrength 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 andconcrete is one of the most important issues concerningthe durability of concrete structures. When the chlorideconcentration of mortar or concrete exceeds a certainthreshold value, depassivation of steel occurs and rein-forced steel starts to corrode [1,2]. It is generally acceptedthat incorporation of a pozzolan improves the resistanceto chloride penetration and reduces chloride-induced cor-rosion initiation period of steel reinforcement. This ismainly due to the reduction of permeability/diffusivity,particularly to chloride ion transportation of the blendedcement concrete [3–5].

0950-0618/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2006.12.001

* Corresponding author. Tel.: +66 4320 2846; fax: +66 4320 2846x102.E-mail address: prinya@kku.ac.th (P. Chindaprasirt).

Pozzolans from agricultural waste are receiving moreattention now since their uses generally improve the prop-erties of the blended cement concrete, and reduce the envi-ronmental problems. Palm oil fuel ash and rice husk ashare two promising pozzolans and are available in manyparts of the world. In Thailand alone, approximately100,000 ton of palm oil fuel ash are produced annually[6]. It is a by-product obtained from a small power plantusing the palm fiber, shells and empty fruit bunches as afuel and burnt at 800–1000 �C. The main chemical compo-sition of palm oil fuel ash is silica which is a main ingredi-ent of pozzolan. Research indicates that ground palm oilfuel ash (POA) can be used as a pozzolan in normal andhigh strength concrete [7]. In addition, partial replacementof OPC with POA helps improve permeability and sulfateresistance of concrete [6,8].

Rice husk is also abundant in many parts of the world.When properly burnt at temperature lower than 700 �C,

Table 1Mortar mix proportions

Mix OPC POA RHA FA SP (%)

OPC 100 – – – 1.920POA 80 20 – – 2.040POA 60 40 – – 3.220RHA 80 20 2.240RHA 60 40 3.720FA 80 – – 20 0.440FA 60 – – 40 0.120BPF 80 10 – 10 0.840BPF 60 20 – 20 1.120BRF 80 – 10 10 1.140BRF 60 – 20 20 1.6

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

P. Chindaprasirt et al. / Construction and Building Materials 22 (2008) 932–938 933

reactive amorphous silica is obtained [9]. The silica contentin rice husk ash is high at approximately 90%. Silica inamorphous form is suitable for use as a pozzolan. Withproper burning and grinding, ground rice husk ash(RHA) can be produced and used as a pozzolan. Evenfor higher burning temperature with some crystalline for-mation of silica, good RHA can still be obtained by finegrinding [10]. The reactive RHA is used to produce goodquality concrete with reduced Ca(OH)2 and higher resis-tance to sulfate attack [11,12].

An industry by-product which is now being used quiteextensively as pozzolan in blended cements is fly ash. Theincorporation of fly ash enhances the performance of con-crete in terms of durability. The use of fly ash usually leadsto a less permeable paste, denser interfacial zone betweenaggregate and the matrix [13–15]. Concrete containing flyash is, therefore, less susceptible to the ingress of the harm-ful solutions. It has been shown that the use of fine fly ash(FA) results in better mechanical properties of concretethan those with the coarser fly ash. It increases strength,resistance to sulfate solution and resistance to chloride pen-etration 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 penetra-tion of mortar. The knowledge would be beneficial forfuture applications of the material in increasing the dura-bility of mortar and concrete.

2. Experimental details

2.1. Materials

Ordinary Portland cement (OPC), palm oil fuel ash froma thermal power plant from the south of Thailand, localrice husk, lignite fly ash from Mae Moh power plant inthe northern part of Thailand, river sand with specific grav-ity of 2.63 and fineness modulus of 2.82, and type-Fsuperplasticizer (SP) were the materials used. Rice huskash was obtained from open burning in small heap of20 kg of rice husk with the maximum temperature of650 �C. Ground palm oil fuel ash (POA) and ground ricehusk ash (RHA) were obtained using ball mill grindinguntil the percentage retained on sieve No. 325 (opening45 lm) was 1–3%. Fine fly ash (FA) with 1–3% retainedon sieve No. 325 was obtained from air classification of ori-ginal coarse fly ash. The SEM (scanning electron micros-copy) and grading analysis were performed on POA,RHA and FA.

2.2. Mix proportions and curing

Ordinary Portland cement (OPC) is partially replacedwith 20% and 40% of pozzolan. In addition to single poz-zolan, a blend of equal weight portions of POA and FA(BPF), and a blend of equal weight portions of RHA andFA (BRF) were also used. Sand-to-binder ratio of 2.75by weight and water to binder ratio (W/B) of 0.5 were used.

SP was incorporated in order to obtain mortar mixes withsimilar flow of 110 ± 5% in accordance with ASTM C109.The cast specimens were covered with polyurethane sheetand damped cloth and placed in 23 ± 2 �C chamber. Theywere demoulded at the age of 1 day and cured in water at23 ± 2 �C. The mortar mix proportions are given in Table1.

2.3. Compressive strength

The 50 mm cube specimens were prepared in accordancewith ASTM C109 [18]. They were tested at the age of 7, 28and 90 days. The reported results are the averages of threesamples.

2.4. Rapid test on resistance to chloride penetration

The 100 · 200 mm cylinders were prepared in accor-dance with ASTM C39 [19]. After being cured in wateruntil the age of 27 days, they were cut into 50 mm sliceswith the 50 mm ends discarded. The 50 mm slices wereepoxy-coated around the cylinder.

2.4.1. Rapid chloride penetration test

The 100 mm dia. · 50 mm epoxy-coated specimens wereconditioned and tested at the age of 28 days for rapid chlo-ride penetration test (RCPT) in accordance with themethod described in ASTM C1202 [20]. The reportedresults are the averages of two samples.

2.4.2. Rapid migration test

At the age of 28 days, the 100 mm dia. · 50 mm epoxy-coated specimens were conditioned and tested for the chlo-ride penetration depth using the rapid migration test(RMT) as shown in Fig. 1 [21,22]. The solutions employedin migration tests were 3% NaCl (in limewater) in the cath-ode side and limewater in the anode side. Applied voltageof 30 V dc at 8 h was employed. The chloride penetrationdepths were determined by breaking the specimens andby applying 0.1 M AgNO3 solution [23].

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

0

10

20

30

40

50

60

70

80

90

100

0.01 0.10 1.00 10.00 100.00 1000.00Particle Size (micron)

Com

mul

ativ

e P

assi

ng (

%)

FA

POA

RHA

OPC

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

Table 2

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

2.5. Immersion test

To confirm the results, actual chloride penetration ofmortar immersed in NaCl solution was performed. The testset-up was similar to that described in RTA T263 [24] withthe exception that 100 mm dia. · 50 mm cut cylinders and3% NaCl solution as shown in Fig. 2 were used. The100 mm dia. · 50 mm specimens were epoxy-coated at thetop surface as well as around the cylinder. At the age of28 days, the specimens were immersed in 3% NaCl solutionand kept immersed for 30 days. Chloride penetrationdepths 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 specificgravities of OPC, POA, RHA and FA are 3.14, 2.25,2.23 and 2.45, respectively. The percentages retained onsieve No. 325 of POA, RHA and FA are 1–3%. The parti-cle size distributions as shown in Fig. 3 suggest that FA isfinest, followed by POA, RHA and OPC. The median par-ticle sizes of the material used from the finest to the coars-est are as follows: FA 4.9 lm, POA 7.2 lm, RHA 10.0 lmand OPC 15.0 lm.

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

The chemical constituents are given in Table 2. Themain chemical composition of POA is 63.6% SiO2, 7.6%CaO and 6.9% K2O. The high CaO and K2O are mostlikely from lime and fertilizer. The loss on ignition (LOI)is 9.6% which is not too high indicating a reasonable burn-ing temperature and time. The sum of SiO2, Al2O3 andFe2O3 is 66.6% which is slightly less than 70% as requiredfor natural pozzolan according to ASTM C618 [25]. RHA,on the other hand, consists of 93% SiO2 complying withASTM C618 requirement as a natural pozzolan. The LOIof 3.7% indicates complete burning.

The SEM photo as shown in Fig. 4 indicates that palmoil fuel ash consists of irregular-shaped particles with a siz-able fraction showing porous cellular structure. After beingground, POA consists mainly of fine irregular-shaped par-ticles. The SEM photo reveals that the rice husk ash stillmaintains its cellular structure. After being ground, RHAconsists of very irregular-shaped particles with porous cel-lular surface.

Fly ash in this experiment is a Class-F fly ash with 74%SiO2 + Al2O3 + Fe2O3, 2.2% SO3 and 2.5% LOI meetingthe requirement of ASTM C618. However, as this fly ashis from lignite, the CaO content is rather high at 14.4%.The SEM micrograph of fly ash as shown in Fig. 4 revealsthat original fly ash consists of a large range of particlesizes. The particles are spherical in shape but the surfacesof the large particles are usually rough. After being classi-

Chemical composition of OPC, POA, RHA and FA

Oxides OPC POA RHA FA

SiO2 20.9 63.6 93.2 41.1A12O3 4.8 1.6 0.4 21.6Fe2O3 3.4 1.4 0.1 11.3CaO 65.4 7.6 1.1 14.4MgO 1.3 3.9 0.1 3.3Na2O 0.2 0.1 0.1 1.1K2O 0.4 6.9 1.3 2.6SO3 2.7 0.2 0.9 2.2LOI 0.9 9.6 3.7 2.5

SiO2 + A12O3 + Fe2O3 – 66.6 93.7 74.0

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

P. Chindaprasirt et al. / Construction and Building Materials 22 (2008) 932–938 935

fied, FA mainly consists of small spherical particles withsmooth surface.

3.2. Superplasticizer (SP) content

The results of SP requirements of the mortar mixes toproduce a similar flow are shown in Table 1. The incorpo-ration of FA reduces the SP content of the mixes. The SPcontent of OPC mortar is 1.9% and those of 20FA and40FA mortars are only 0.4% and 0.1%. The reduction ofSP requirement is associated with the ball-bearing effectof small spherical particles of FA. On the other hand, theincorporation of POA particles slightly increases the SPrequirements. This is due to the fine and irregular surface

of POA particles. The SP contents of 20POA and 40POAmortars are 2.0% and 3.2%. The incorporation of RHA sig-nificantly increases the SP requirements owing to its cellu-lar porous surface. The SP contents of 20RHA and 40RHAmortars are 2.2% and 3.7%. For the blended pozzolans, theaddition of FA to the other pozzolans which require moreSP helps keep the SP requirement low. The SP contents of20BPF, 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 givenin Table 3. The strength development of OPC mortar is

Table 3Compressive strength of mortars

Mix Compressive strength (MPa–normalized)

7 d 28 d 90 d

OPC 43.5–100 57.0–100 60.0–100

20POA 43.5–100 57.5–102 62.0–103

40POA 32.5–75 53.5–94 61.5–102

20RHA 44.5–102 58.5–103 62.5–10440RHA 33.5–77 55.0–97 62.0–103

20FA 44.5–102 59.5–105 63.5–105

40FA 33.0–76 56.5–99 62.0–103

20BPF 43.5–99 57.5–101 63.0–104

40BPF 43.0–98 56.5–99 60.0–100

20BRF 42.0–97 58.0–102 64.0–106

40BRF 41.0–95 55.5–98 61.5–102

0

2000

4000

6000

8000

OP

C

20FA

40FA

20PO

A

40PO

A

20R

HA

40R

HA

20B

PF

40B

PF

20B

RF

40B

RF

Chl

arge

pas

sed

(Cou

lom

b)

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

0

4

8

12

16

20

OP

C

20FA

40FA

20PO

A

40PO

A

20R

HA

40R

HA

20B

PF

40B

PF

20B

RF

40B

RF

Chl

orid

e de

pth

(mm

)

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

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

rather good. The 7, 28 and 90-day strengths are 43.5, 57.0and 60.0 MPa, respectively. At 20% replacement, thestrengths of mortars containing POA, RHA and FA arealso high between 100% and 105% of those of OPC mortarat the same age. For 40% replacement level, reductions instrength at 7 days are apparent for mixes containingPOA, RHA and FA. Their 7-day strengths are 75–77%of that of OPC mortar at the same age. At the age of 90days, strengths of POA, RHA and FA mortars are 102–103% of that of OPC mortar at the same age. The low earlystrengths and later age strength development are the com-mon feature of pozzolanic materials.

For the blended pozzolans, the main improvement instrength is at 7 days and at 40% replacement level. The7-day strength of blended pozzolan mortars are 95–99%of that of normal OPC mortar while the strength of singlepozzolan mortars are only 75–77%. It has been suggestedthat incorporation of blend of fine pozzolans improvesthe strength of concrete due to synergic effect [26]. Thisprobably contributes to this early strength improvement.

3.4. Chloride penetration resistance of mortar

3.4.1. RCPT results

Usually there is a risk of temperature rise in using RCPTtest for mortar specimens. For this experiment, the temper-ature rise is not large, as the strength of mortar used is rea-sonably high at around 53–60 MPa. The results of theRCPT test are shown in Fig. 5. The charge passed is sub-stantially reduced with incorporation of pozzolans as com-pared to 7450 C of normal OPC mortar. The incorporationof 20% and 40% of FA reduces the charge passed to 3050and 1950 C. POA is more effective than FA and reducesthe charge passed to 1900 and 1050 C at 20% and 40%replacement levels. RHA is the most effective and reducesthe charge passed to 750 and 200 C at 20% and 40%replacement levels.

For blended pozzolans, the Coulomb charges are verylow. The charges of 20BPF and 40BPF mortars are 1250and 850 C which are lower than those of single pozzolanmortars at the same replacement levels. For the blend of

RHA and FA, the charges of 20BRF and 40BRF mortarsare 1100 and 250 C which are slightly higher than those ofRHA mortars at the same replacement levels.

3.4.2. RMT results

The results of RMT using 30 V dc as shown in Fig. 6 aresimilar to those of RCPT. The accelerated penetrationdepths are substantially reduced with incorporation ofpozzolans as compared to the depth of 16.0 mm of normalOPC mortar. Incorporations of 20% and 40% of FA reducethe depths to 10.0 and 8.0 mm. Incorporations of 20% and40% of POA further reduce the depths to 7.0 and 4.5 mm.The penetration depths of RHA mortars are lowest at 3.5and 3.0 mm at 20% and 40% replacement levels.

The blended pozzolans also improve the resistance toaccelerated chloride penetration of mortars. Acceleratedchloride penetration depths of 20BPF, 40BPF, 20BRFand 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 mortarimmersed in 3% NaCl solution for 30 days are shown inFig. 7. The results are almost the same as those of RMTwhile the results of RCPT are only different in magnitudebut the trends are similar. This confirms the results ofRCPT and RMT that incorporation of pozzolans improves

0

4

8

12

16

20

OP

C

20FA

40FA

20PO

A

40PO

A

20R

HA

40R

HA

20B

PF

40B

PF

20B

RF

40B

RF

Chl

orid

e de

pth

(mm

)

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

P. Chindaprasirt et al. / Construction and Building Materials 22 (2008) 932–938 937

the resistance to chloride penetration of mortars. RHA isthe most effective, followed by POA and FA.

The addition of a pozzolan such as fly ash or rice huskash, whose particles are finer than those of Portlandcement causes segmentation of large pores and increasesnucleation sites for precipitation of hydration productsin cement paste [27]. This increases the hydration andrefines the pore structure of paste. The increase in hydra-tion leads to a reduction of calcium hydroxide in paste.For fly ash, RHA and OPC mortars, it has been shownthat the base condition is lowest for RHA mortars, fol-lowed by fly ash and normal OPC mortar [12]. This sug-gests that Ca(OH)2 consumption in RHA mortar ishighest, followed by fly ash and OPC mortars. Withregard to permeability, the incorporation of pozzolan suchas fly ash reduces the average pore size and results in a lesspermeable paste [13,14]. It has also been shown that reac-tive RHA can be used to produce good quality concretewith reduced porosity [11]. Test also shows that the per-meabilities of concretes containing palm oil fuel ash, ricehusk–bark ash and fly ash are lower than that of OPCconcrete [6].

In summary, the incorporation of POA, RHA and FAincreases nucleation sites for precipitation of hydrationproducts, reduces Ca(OH)2, and improves the permeabil-ity of mortar. These factors contribute to the improve-ment in the resistance to chloride penetration of mortarwith RHA being the most effective, followed by POAand FA.

The excellent improvement in the resistance to chlo-ride penetration using the blended pozzolans is probablydue to the synergic effect of the blend of fine pozzolans[26]. Additional physical effect related to the cementgrains deflocculation is provided by fly ash [26]. Theincorporation of fly ash adds additional dispersing effectof cement grains as well as that of other fine pozzolanparticles. The larger number of nucleation sites wouldresult in higher amount of hydration and higher calciumhydroxide consumption. The enhancement of strengthand resistance to chloride penetration of mortar usingthe blends of POA and FA, and RHA and FA is, there-fore, obtained.

4. Conclusions

From the test, it can be concluded that POA, RHA andFA can be used as pozzolans to replace part of Portlandcement in making mortar with relatively high strengthand good resistance to chloride penetration. SphericalFA particles help reduce the amount of SP to produce mor-tar with similar flow. On the other hand, POA with fineirregular-shaped particles increases the amount of SPrequired. RHA with very irregular-shaped particles andporous cellular surface requires a large amount of SP.For the blended pozzolans, the use of FA with the otherpozzolans which require more SP helps keep the SPrequirement low.

The RCPT, RMT and actual immersion in 3% NaClsolution test are effective in detecting the improvement ofresistance to chloride penetration of reasonably highstrength mortars containing pozzolans. The incorporationsof POA, RHA and FA significantly improve the resistanceto chloride penetration of mortar by increasing nucleationsites for precipitation of hydration products, reducingCa(OH)2 and improving the permeability of mortar.RHA is the most effective, followed by POA and FA. Testresults also indicate that the use of blended pozzolans ofequal portion of POA and FA, and RHA and FA alsoeffectively improves the mortar in terms of strength andresistance to chloride penetration. The improvement isdue to dispersing effect of fly ash and synergic effect ofthe blend of fine pozzolans.

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