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Construction and Building Materials 22 (2008) 1601–1606

and Building

MATERIALS

Strength, porosity and corrosion resistance of ternary blendPortland cement, rice husk ash and fly ash mortar

P. Chindaprasirt, S. Rukzon *

Department of Civil Engineering, Faculty of engineering, Khon Kaen University, Khon Kaen 40002, Thailand

Received 5 May 2007; received in revised form 19 June 2007; accepted 28 June 2007Available online 10 August 2007

Abstract

This paper presents a study of the strength, porosity and corrosion resistance of mortars made with ternary blends of ordinary Port-land cement (OPC), ground rice husk ash (RHA) and classified fly ash (fine fly ash, FA). Compressive strength, porosity and acceleratedcorrosion with impressed voltage (ACTIV) were tested. The results show that the use of ternary blend of OPC, RHA and FA producesmortars with improved strengths at the low replacement level with RHA and FA and at the later age in comparison to that of OPCmortar. The porosity of mortar containing pozzolan reduces with the low replacement level of up to 20% of pozzolan, but increases withthe 40% replacement level. The chloride induced corrosion resistance of mortar as measured by ACTIV is, however, significantlyimproved with the use of both single pozzolan and the ternary blend OPC, RHA and FA. The corrosion resistance of ternary blendmortar is higher than that of mortar containing single pozzolan. The use of ternary blend OPC, RHA and FA is very effective in enhanc-ing chloride induced corrosion of mortar.Published by Elsevier Ltd.

Keywords: Compressive strength; Corrosion; Fly ash; Mortar; Porosity; Rice husk ash

1. Introduction

A large number of researches have been directedtowards the utilization of waste materials. For the con-struction industry, the development and use of blendedcements is growing rapidly. Pozzolans from industrialand agricultural by-products such as fly ash and rice huskash are receiving more attention now since their uses gen-erally improve the properties of the blended cementconcrete, the cost and the reduction of negative environ-mental effects.

Rice husk is one of the major agricultural by-productsand is available in many parts of the world. When rice huskis burnt at temperatures lower than 700 �C, rice husk ashwith cellular microstructure is produced. Rice husk ash

0950-0618/$ - see front matter Published by Elsevier Ltd.

doi:10.1016/j.conbuildmat.2007.06.010

* Corresponding author. Tel.: +66 0 4320 2846; fax: +66 0 43202846x102.

E-mail address: 4770400031@kku.ac.th (S. Rukzon).

contains high silica content in the form of non-crystallineor amorphous silica. Therefore, it is a pozzolanic materialand can be used as supplementary cementitious materials[1].

Fly ash is the most common pozzolan and is being usedworldwide in concrete works. It is generally realized thatthe use of fine fly ash improves the properties of mortarand concrete [2,3]. Although the porosity of the paste isincreased as a result of the incorporation of fly ash, theaverage pore size is reduced. This results in a less perme-able paste [4,5]. The interfacial zone of the interfacebetween aggregate and the matrix is also improved as aresult of the use of fly ash [6,7].

The enhancement of the resistance to chloride penetra-tion is one of the benefits of incorporation of pozzolans.It is generally accepted that incorporation of a pozzolanimproves the resistance to chloride penetration and reduceschloride-induced corrosion initiation period of steel rein-forcement. The improvement is mainly caused by the

Table 1Mortar mix proportions

Mix No. Symbol OPC FA RHA SP (%)

1 OPC 100 – – 1.92 10FA 90 10 – 0.63 10RHA 90 – 10 2.04 20FA 80 20 – 0.45 20RHA 80 – 20 2.26 10FA10RHA 80 10 10 1.17 20FA10RHA 70 20 10 1.18 15FA15RHA 70 15 15 1.29 10FA20RHA 70 10 20 1.310 40FA 60 40 – 0.111 40RHA 60 – 40 3.712 20FA20RHA 60 20 20 1.6

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

1602 P. Chindaprasirt, S. Rukzon / Construction and Building Materials 22 (2008) 1601–1606

reduction of permeability/diffusivity, particularly to chlo-ride ion transportation of the blended cement concrete[8–10].

The use of the blend of pozzolan has been shown to beadvantageous owing to the synergic effects [11]. In thiswork, ordinary Portland cement, rice husk ash and flyash are used as base materials for studying the ternaryblended cement. The knowledge in terms of strength,porosity and corrosion resistance would be beneficial tothe understanding of the mechanisms as well as for futureapplications of these materials.

2. Experimental details

2.1. Materials

Fly ash is lignite fly ash from Mae Moh power plant inthe northern part of Thailand. Rice husk ash wasobtained from open burning in small heaps of 20 kg ricehusk with maximum burning temperature of 650 �C.Ordinary Portland cement (OPC), river sand with specificgravity of 2.63 and fineness modulus of 2.82, and type-Fsuperplasticizer (SP) were used. Classified fly ash (fine flyash, FA) and ground rice husk ash (RHA) were used.Fine fly ash (FA) with 1–3% retained on sieve No. 325(opening 45 lm) was obtained from air classification ofas-received coarse fly ash. The ground rice husk ash(RHA) was obtained using ball mill grinding of rice huskash until the percentage retained on sieve No. 325 (open-ing 45 lm) was 1–3% as well. Scanning electron micros-copy (SEM) and grading analysis were used on FA andRHA.

2.2. Mix proportions and curing

OPC is partially replaced with pozzolans at the dosageof 0–40% by weight of cementitious materials. Single poz-zolan and a blend of different weight portions of RHAand FA were also used. Sand-to-binder ratio of 2.75 byweight, water to binder ratio (W/B) of 0.5 and SP contentadjusted to maintain the mortar mixes with similar flow of110 ± 5% were used. The cast specimens were covered withpolyurethane sheet and damped cloth in a 23 ± 2 �C cham-ber and were demoulded at the age of 1 day. For strengthand porosity tests, the specimens were moist cured at23 ± 2 �C until the test ages. For the corrosion test, thesamples were cured in distilled water to prevent chloridecontamination. The mortar mix proportions and abbrevia-tions are given in Table 1.

2.3. Compressive strength

The cube specimen of size 50 · 50 · 50 mm was used forthe compressive strength test of mortar. They were tested atthe age of 7, 28 and 90 days. The test was done in accor-dance with the ASTM C109 [12]. The reported results arethe average of four samples.

2.4. Porosity tests

Cylindrical specimens of 100 mm diameter and 200 mmheight were prepared in accordance with ASTM C39 [13].They were tested at the age of 7, 28 and 90 days. Afterbeing cured in water until the age of 28 days, they werecut into 50 mm thick slices with the 50 mm ends discarded.They were dried at 100 ± 5 �C until constant weightachieved and were then placed in desiccators under vacuumfor 3 h. The set-up was finally filled with de-aired, distilledwater to measure the porosity of the mortar. The porositywas calculated using Eq. (1).

p ¼ ðW a � W dÞðW a � W wÞ

� 100 ð1Þ

where

p is vacuum saturated porosity (%),Wa is specimen weight in air of saturated sample (gm),Wd is specimen dry weight after 24 h in oven at100 ± 5 �C (gm) andWw is specimen weight in water (gm).

This method has been used to measure the porosity ofthe cement-based materials successfully [14–17]. Thereported results are the average of two samples.

2.5. Accelerated corrosion test with impressed voltage

Mortar prisms of dimensions 40 · 40 mm and 160 mmin length with embedded steel of 10 mm diameter and160 mm in length were used. The steel was protected suchthat it protruded from the top surface of the prism by15 mm; thus, provided sufficient mortar cover of 15 mmand 15 mm thick mortar at the end of steel bar at the bot-tom of the prisms as shown in Fig. 1. The mortar was castin two layers and compacted using vibrating table.

At the age of 28 days, the prisms were subjected to theaccelerated corrosion test with impressed voltage (ACTIV)using a 5% NaCl solution and a constant voltage of 12 V dc

Fig. 1. Geometry of the mortar blocks (mm).

Table 2Physical properties FA, RHA and OPC

Sample Medianparticle size(lm)

Retained on asieve No. 325 (%)

Specificgravity

Blainefineness(cm2/gm)

OPC 15.0 NA 3.14 3600FA 4.9 1–3 2.45 5700RHA 10.0 1–3 2.23 11,200

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)

Cum

ulat

ive

Pas

sing

(%

)

FA

RHA

OPC

Fig. 3. Particle size distribution of FA, RHA and OPC.

Table 3Chemical composition of OPC, RHA and FA

Oxides OPC RHA FA

SiO2 20.9 93.2 41.1Al2O3 4.8 0.4 21.6Fe2O3 3.4 0.1 11.3CaO 65.4 1.1 14.4MgO 1.3 0.1 3.3Na2O 0.2 0.1 1.1K2O 0.4 1.3 2.6SO3 2.7 0.9 2.2LOI 0.9 3.7 2.5

P. Chindaprasirt, S. Rukzon / Construction and Building Materials 22 (2008) 1601–1606 1603

as shown in Fig. 2. The condition of prism was continu-ously monitored and the time of initiation of first crackwas recorded. This is used as a measurement of the speci-men’s relative resistance against chloride attack and rein-forcement corrosion.

3. Results and discussions

3.1. Characteristics of OPC, FA and RHA

The fineness characteristics of Portland cement and poz-zolanic materials are given in Table 2. The Blaine finenessof OPC is 3600 cm2/gm and those of the FA and RHA are5700 and 11,200 cm2/gm. The specific gravity of the OPC,FA and RHA are 3.14, 2.45 and 2.23, respectively. Theparticle size distributions shown in Fig. 3 suggest that FAis finest, followed by RHA and OPC. The mean particlesizes of FA, RHA and OPC are 4.9, 10.0 and 15.0 lm,respectively.

The chemical constituents are given in Table 3. Fly ashis a Class F fly ash with 74% of SiO2 + Al2O3 + Fe2O3,2.2% of SO3 and 2.5% of LOI meeting the requirement ofASTM C618 [18]. The CaO content of this fly ash is ratherhigh at 14.4% as it is from lignite. RHA, on the other hand,consists mainly of SiO2and the other components are notsignificant. The SiO2 content of 93% satisfies ASTM

Fig. 2. Accelerated corrosion test with impressed voltage (ACTIV).

SiO2 + Al2O3 + Fe2O3 – 93.7 74.0

C618 [18] requirement as a natural pozzolan and 3.7%LOI indicates complete burning.

The as-received fly ash consists of a large range of par-ticle sizes as indicated by SEM micrograph as shown inFig. 4. The particles are mostly spherical in shape. The fineportion surfaces are relatively smooth and those of thelarge particles are usually rough. The SEM photo revealsthat the rice husk ash still maintains its cellular structure.After grounded, RHA consists of very irregular-shapedparticles with a porous cellular surface.

3.2. Compressive strength

Table 4 shows the results of the compressive strength ofthe blended cements mortar containing FA and RHA. The

Fig. 4. SEM of rice husk ash and fly ash.

Table 4Compressive strength of blended cements mortars

Mix No. Symbol Compressive strength (MPa–normalized)

7 days 28 days 90 days

1 OPC 43.5–100 57.0–100 60.0–100

2 10FA 45.0–103 59.2–104 62.7–105

3 10RHA 44.2–102 58.2–102 62.0–103

4 20FA 44.5–102 59.5–105 63.5–106

5 20RHA 44.5–102 58.5–103 62.5–104

6 10FA10RHA 42.0–97 58.0–102 64.0–107

7 20FA10RHA 42.4–97 58.4–102 63.4–106

8 15FA15RHA 43.1–99 58.5–103 63.0–105

9 10FA20RHA 42.5–98 58.7–103 62.8–105

10 40FA 33.0–76 56.5–99 62.0–103

11 40RHA 33.5–77 55.0–97 62.0–103

12 20FA20RHA 41.0–95 55.5–102 61.5–106

1604 P. Chindaprasirt, S. Rukzon / Construction and Building Materials 22 (2008) 1601–1606

strengths of the mortar containing FA, RHA and the ter-nary blended cement were relatively high. The strengthsof mortar containing 10% and 20% of pozzolans and blendof pozzolans are higher than that of the control at all ages.Only the strength at 7 days of mortar containing 10%FA + 10% RHA (10FA10RHA) is slightly lower than thatof the OPC mortar at the same age. The incorporation ofFA produces filler and dispersing effects and increases thenucleation and precipitation sites [4,5]. At this level ofcement replacement of up to 20%, the filler and dispersingeffects could offset the reduction in strength due to thereduction in the OPC. The incorporation of RHA also

produces the filler effect due to its fine particle size. The dis-persing effect has not been reported for the RHA. How-ever, its reactivity is high due to its high surface area.The increase in the hydration could, therefore offset thestrength reduction as a result of reduced OPC.

The increase in the amount of replacement to 40%reduces the early strength of both FA and RHA mortars.However, the strength at the ages of 28 and 90 days of bothFA and RHA mortars are slightly higher than that of thecontrol. This indicates that both FA and RHA are pozzo-lanic materials and the early pozzolanic reaction rate isthus slow. The pozzolanic reaction of both cases, however,can be seen at the age of 28 days onwards resulting in thehigher strength of both FA and RHA mortar in compari-son to that of the control. The results also suggest that bothFA and RHA in this experiment were quite reactive andthe pozzolanic reaction starts quite early.

For the blend of pozzolans, the strengths of mortar arealso comparable to that of OPC mortars at the same age.The strengths at the age of 7 days blended pozzolan mor-tars range between 95–99% of that of OPC. At the age of28 and 90 days, the normalized strength ranges between102–103% and 105–107%, respectively. The results indi-cate that for the high replacement level of 40%, the useof blend of RHA and FA improves the early strengthdevelopment of mortar in comparison to normal singlepozzolan mortar. The incorporation of blend of finepozzolans improves the strength of concrete due to syner-gic effect [11].

y = 457.63x-0.87

R2 = 0.97

10

12

14

16

18

20

22

24

30 35 40 45 50 55 60 65

Compressive strength (MPa)

Poro

sity

(%

)

7 days

28 days

90 days

Fig. 5. Relationship between porosity and compressive strength.

P. Chindaprasirt, S. Rukzon / Construction and Building Materials 22 (2008) 1601–1606 1605

3.3. Porosity results

The results of the porosity of mortars at 7, 28 and90 days are shown in Table 5. At the age of 7 days, theporosities of mortar containing 10% and 20% of pozzolansand blend of pozzolans are lower than that of the control atall ages. The addition of fine particles of FA and RHAcauses segmentation of large pores and increases nucle-ation sites for precipitation of hydration products incement paste [19]. This results in pore refinement and areduction of calcium hydroxide in paste. The mortar con-taining FA gives slightly less porosity than that of RHA.In other word, FA is slightly more effective in modifyingpore and reducing the porosity of mortar. The porosityof 20FA mortar is 17.3% in comparison with 17.8% of bothOPC and 20RHA mortars.

At high replacement level of 40%, the porosities of themortars containing pozzolans increase in comparison withthat of the control. At the age of 7 days, the porosities of40FA, 40RHA and 20FA20RHA mixes are 21.0%, 21.8%and 19.4% which are significantly larger than 17.8% ofthe OPC mortar. The increases in porosity with a relativelarge amount of pozzolans are resulted from the reducedamount of OPC. This results in less hydration productsespecially at the early age where the pozzolanic reactionis small. It should be pointed out here that althoughthe porosity is increased, the beneficial effect of porerefinement as a result of the incorporation of pozzolanexists.

The porosities of the mortars reduce with an increase inage as expected. This is due to the increase in the hydrationof cementitious materials. At the later age of 90 days, theporosities of the mortars containing pozzolans reduce tosimilar values to that of OPC mortar. The porosities of40FA, 40RHA and 20FA20RHA mixes at 90 days are12.7%, 12.8% and 13.4% as compared to 12.8% of OPCmortar at the same age. The relationship between theporosity and strength of mortar follows the conventionalpattern as shown in Fig. 5.

Table 5Porosity of blended cements mortars

Mix No. Symbol Porosity (%)

7 days 28 days 90 days

1 OPC 17.8 13.7 12.82 10FA 16.7 12.9 12.53 10RHA 17.0 13.3 12.64 20FA 17.3 12.9 12.05 20RHA 17.8 13.1 12.66 10FA10RHA 16.9 13.0 12.57 20FA10RHA 17.9 13.4 12.88 15FA15RHA 17.7 13.8 13.09 10FA20RHA 17.9 13.5 13.110 40FA 21.0 14.5 12.711 40RHA 21.8 15.0 12.812 20FA20RHA 19.4 14.9 13.4

3.4. Results of ACTIV

The results of the time of first crack of mortar subjectedto ACTIV are shown in Fig. 6. The time of first crack ofOPC mortar is lowest at 89 h. The time to initial crack ofmortars is found to increase with the incorporation ofpozzolans. The increase in the time of first crack with theincorporation of pozzolan has been reported by otherresearches [20,21]. The increase in the corrosion resistanceof mortar incorporating RHA and FA measured withrapid coulomb passed (RCPT), immersion in sodium chlo-ride solution and RMT has also been reported [22]. For thesingle pozzolan, RHA is found to be more effective inincreasing the time of first crack as compared to FA. Thetime of first crack of 10RHA, 20RHA and 40RHA mortarsare almost the same at 167, 168 and 166 h, respectively,whereas those of 10FA, 20FA and 40FA mortars are160, 148 and 136 h, respectively. It is interesting to notethat the time of first crack of fly ash mortars reduces withan increase in the fly ash replacement levels. This is theresult of the slow pozzolanic reaction of fly ash.

0

20

40

60

80

100

120

140

160

180

200

OP

C

10F

A

20F

A

40F

A

10R

HA

20R

HA

40R

HA

10F

A10

RH

A

10F

A20

RH

A

15F

A15

RH

A

20F

A10

RH

A

20F

A20

RH

A

Tim

e of

ini

tiat

ion

of c

rack

(h)

Fig. 6. Time to initiation of crack of mortars subjected to ACTIV.

1606 P. Chindaprasirt, S. Rukzon / Construction and Building Materials 22 (2008) 1601–1606

For the blends of pozzolans, the results show that theirincorporation to the mixes improves the resistance to cor-rosion of mortar. The time to cracking of 10FA10RHA,20FA20RHA, 10FA20RHA, 15FA15RHA, 20FA10RHAmortars are very high at 169–173 h. These values areslightly but consistently larger than those of RHA mor-tars indicating that the use of blend of FA and RHA isvery effective in increasing the resistance to corrosion ofmortar.

The incorporation of pozzolan such as fly ash reducesthe average pore size and results in a less permeable paste[4,5]. It has also been shown that reactive RHA alsoreduces the porosity of paste [23]. Test also shows thatthe permeabilities of rice husk–bark ash and fly ash arelower than that of OPC concrete [24]. The improvementof chloride induced corrosion resistance of the ternaryblend OPC, FA and RHA mortar is thus the result ofreduced permeability and reduced calcium hydroxide.

4. Conclusions

The use of ternary blend of OPC, RHA and FA signif-icantly improves the mortar in terms of strength at the lowreplacement level and at the later age. The resistance tochloride-induced corrosion of mortar containing pozzolanas measured by ACTIV is significantly improved in com-parison to that of OPC mortar. Both FA and RHA arevery effective in improving the corrosion resistance of mor-tars. RHA is slightly more effective than FA. The corrosionresistance of the ternary blend mortar is consistently higherthan that of mortar containing single pozzolan. At highreplacement of 40% of pozzolan, although the porosity ofmortar is increased at the age of 28 days as compared toOPC mortar, the corrosion resistance is significantlyimproved. This suggests that pore refinement and reduc-tion in calcium hydroxide play important roles in the cor-rosion resistance of ternary blend OPC, FA and RHAmortar.

Acknowledgement

The authors would like to acknowledge the financialsupports of Rajamangala University of Technology PhraNakhon, School of Graduate Studies, Sustainable Infra-structure Research and Development Center and Facultyof Engineering Khon Kaen University.

References

[1] Metha PK. The chemistry and technology of cement made from ricehusk ash. UNIDO/ESCAP/RCTT. In: Proceeding of work shop onrice husk ash cement, Peshawar, Pakistan. Bangalore, India: RegionalCenter for Technology Transfer; 1979. p. 113–22.

[2] Erdogdu K, Tucker P. Effects of fly ash particle size on strength ofPortland cement fly ash mortars. Cem Concr Res 1998;28:1217–22.

[3] Lee SH, Sakai E, Diamond M, Bang WK. Characterization of fly ashdirectly from electrostatic precipitator. Cem Concr Res1999;29:1791–7.

[4] Poon CS, Wong YL, Lam L. The influence of different curingconditions on the pore structure and related properties of fly ashcement pastes and mortars. Constr Build Mater 1997;11:383–93.

[5] Chindaprasirt P, Jaturapitakkul C, Sinsiri T. Effect of fly ash finenesson compressive strength and pore size of blended cement paste. CemConcr Compos 2005;27(4):425–8.

[6] Wong L, Lam L, Poon CS, Zhou FP. Properties of fly ash-modifiedcement mortar–aggregate interfaces. Cem Concr Res1999;29:1905–13.

[7] Kuroda M, Watanabe T, Terashi N. Increase of bond strength atinterfacial transition zone by the use of fly ash. Cem Concr Res2000;30(2):253–8.

[8] Thomas MDA, Bamforth PB. Modelling chloride diffusion inconcrete: effect of fly ash and slag. Cem Concr Res 1999;29(4):487–95.

[9] Bijen J. Benefits of slag and fly ash. Constr Build Mater1996;10(5):309–14.

[10] Tumidajski PJ, Chan GW. Boltzmann–Matano analysis of chloridediffusion into blended cement concrete. J Mater Civil Eng1996;8(4):195–200.

[11] Isaia GC, Gastaldini ALG, Moraes R. Physical and pozzolanic actionof mineral additions on the mechanical strength of high-performanceconcrete. Cem Concr Compos 2003;25:69–76.

[12] ASTM C109, Standard test method for compressive strength ofhydraulic cement mortars (using 2-in or [50 mm] cube specimens),ASTM C109M-99, Annu Book ASTM Stand 04. 01, 2001, p. 83–8.

[13] ASTM C39, Standard test method for compressive strength ofcylindrical concrete specimens, ASTM C39M-01, Annu Book ASTMStand 04. 02, 2001, p. 18–22.

[14] Papadakis VG, Fardis MN, Veyenas CG. Hydration and carbonationof pozzolanic cements. ACI Mater J Technical Paper 1992;89:2.

[15] Cabrera JG, Lynsdale CJ. A new gas parameter for measuring thepermeability of mortar and concrete. Mag Concr Res 1988;40:177–82.

[16] Rossignolo JA, Agnesini MV. Durability of polymer-modifiedlightweight aggregate concrete. Cem Concr Compos 2004;26:375–80.

[17] Gonen T, Yazicioglu S. The influence of compaction pore onsorptivity and carbonation of concrete. Constr Build Mater2007;21:1040–5.

[18] ASTM C618, Standard specification for coal fly ash and raw orcalcined natural pozzolan for use as a mineral admixture in concrete,ASTM C618–00, Annu Book ASTM Stand 04.02, 2001, p. 310–13.

[19] Mehta PK, Studies on the mechanisms by which condensed silicafume improves the properties of concrete: durability aspects. In:Proceedings of international workshop on condensed silica fume inconcrete. Ottawa; 1987, p. 1–17.

[20] Saraswathy V, Song H-W. Corrosion performance of rice husk ashblended concrete. Constr Build Mater 2007;21:1779–84.

[21] Anand Kuber Parande, Ramesh Babu B, Aswin Karthik M, DeepakKumaar KK, Palaniswamy N. Study on strength and corrosionperformance for steel embedded in metakaolin blended concrete/mortar. Constr Build Mater 2006. doi:10.1016/j.conbuild-mat.2006.10.003.

[22] Chindaprasirt P, Rukzon S, Sirivivatnanon V. Resistance to chloridepenetration of blended Portland cement mortar containing palm oilfuel ash, rice husk ash and fly ash. Constr Build Mater 2007.doi:10.1016/j.conbuildmat.2006.12.001.

[23] Zhang MH, Malhotra VM. High-performance concrete incorporatingrice husk ash as a supplementary cementing materials. ACI Mater J1996;93(6):629–36.

[24] Chindaprasirt P, Homwuttiwong S, Jaturapitakkul C. Strength andwater permeability of concrete containing palm oil fuel ash and ricehusk–bark ash. Constr Build Mater 2007. doi:10.1016/j.conbuild-mat.2006.12.001.