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Cement & Concrete Composites 55 (2015) 348–363

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Cement & Concrete Composites

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Effect of grinding of low-carbon rice husk ash on the microstructureand performance properties of blended cement concrete

http://dx.doi.org/10.1016/j.cemconcomp.2014.09.0210958-9465/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +1 864 633 7114.E-mail addresses: [email protected] (H. Kizhakkumodom

Venkatanarayanan), [email protected] (P.R. Rangaraju).1 Tel.: +1 864 656 1241.

Harish Kizhakkumodom Venkatanarayanan a,⇑, Prasada Rao Rangaraju b,1

a 131 Lowry Hall, Glenn Department of Civil Engineering, Clemson University, Clemson, SC 29634, USAb 220 Lowry Hall, Glenn Department of Civil Engineering, Clemson University, Clemson, SC 29634, USA

a r t i c l e i n f o

Article history:Received 2 October 2013Received in revised form 11 September2014Accepted 28 September 2014Available online 8 October 2014

Keywords:Rice husk ashMicrostructurePozzolanDurabilityChloride permeabilitySustainability

a b s t r a c t

The effectiveness of unground low-carbon rice husk ash (URHA) as a pozzolan and the effect of grindingthe URHA to finer fractions for use in portland cement system were investigated. The properties investi-gated include the setting time and calcium hydroxide depletion of rice husk ash (RHA) pastes; micro-structure and flow behavior of RHA mortars; strength and durability of RHA concretes. Results fromthis investigation suggested that the URHA and ground RHA (GRHA) mixtures performed better thanthe control mixtures in all tests conducted except water demand and setting time. The URHA mixturerevealed denser microstructure compared to the control mixture. The internal porosity created by thecoarse RHA grains in the matrix and their inability to completely participate in pozzolanic reactionmay be the reasons for the poorer performance of the URHA mixture than compared to the GRHA mixture.The effect of grinding the RHA to finer fractions either substantially or slightly improved all propertiesexcept final setting time. With the performance of the GRHA concrete somewhat similar to that of theSF concrete, the use of ground RHA can be concluded to provide acceptable performance in portlandcement systems.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The widespread use of portland cement in construction has ren-dered it as an unsustainable material since the raw materials (limestone and clay) used during the cement production are depletingover time and cannot be recovered. In addition, the production ofcement liberates significant amounts (about 0.625–1 ton for every1 ton of cement produced) of carbon-dioxide into the atmosphere,making the carbon foot-print associated with the constructionquite high [1]. The most commonly adopted methods for reducingthe carbon-footprint and promoting sustainability entail the use ofsupplementary cementing materials (SCMs) in concrete [2]. Overthe past three decades, significant research has been conductedon the use of SCMs such as fly ash, silica fume and slag (by-productwaste from different industries) in concrete. Although these SCMsare industrial by-products and otherwise would be land-filled,their generation is associated with large amount of carbon dioxidereleased into the atmosphere. During the recent years, significant

attention is drawn toward the use of sustainable and environmen-tal-friendly materials in concrete such as rice husk ash.

Rice husk is an agro-waste product that is abundantly available– the worldwide annual production is 70 million tons [3,4]. Ricehusk ash (RHA) is produced by the controlled incineration of ricehulls in order to produce amorphous silica with low carbon con-tent. RHA primarily contains silica, carbon and alkali oxides. Basedon literature review, the silica content of RHA, its carbon contentand its alkali content (Na2O and K2O) ranges from 80% to 96%,0.41% to 5.91% and 0.95% to 4.60%, respectively [5]. Previous stud-ies have indicated that RHA can offer a positive solution when usedat appropriate replacement levels [6–21]. For example, Mehta andFolliard [6] found that the mortars containing RHA possess excel-lent acid and sulfate resistance properties compared to those thatdid not contain RHA. Similarly, RHA can be used as a high-perfor-mance pozzolan to improve the mechanical and durability proper-ties of conventional concretes [7,15,16].

Research conducted by Nehdi et al. [8] has shown that the highcarbon content of RHA can negatively impact the workability andentrained air content of concrete. Some salient points from studiesconcerning the use of high-carbon RHA in concrete are as follows:(i) high-carbon RHA concretes have shown to perform poorer thanthe low-carbon RHA concretes and concretes containing no RHA

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H. Kizhakkumodom Venkatanarayanan, P.R. Rangaraju / Cement & Concrete Composites 55 (2015) 348–363 349

[8], (ii) consequently, RHA containing higher carbon contentrequires higher replacement levels to achieve the same order ofconcrete strength compared to RHA containing lower carbon con-tent [5], and (iii) at constant replacement level of RHA, the durabil-ity of concretes with RHA having higher carbon content wasinferior to concrete with RHA having lower carbon content [8].From the perspective of appearance, the unpleasant black colorof RHA with greater carbon content is not appealing for aestheticapplications [17].

As such, RHA containing higher carbon content and higheramounts of crystalline silica is undesirable for use as an effectivepozzolan. While some research has been conducted to investigatethe production of RHA with low carbon content, little success wasachieved in economically producing RHA with a carbon content ofless than 1% [5]. Other efforts focused on improving the reactivityof crystalline silica in RHA through special treatments [16,22–24].In addition to the high carbon content of RHA, one important eco-nomic concern that restricts its use in portland cement system isemploying higher energy to grind RHA to finer fractions. The useof as-obtained or unground RHA is obviously cost-effective due tothe elimination of the grinding process although their addition(without grinding) in portland cement mixtures is not encouragedby many investigators for the following reasons [6–21]:

(i) Unground RHA has a lower bulk density compared to otherpozzolans and therefore can significantly affect the densityof portland cement mixture when used at large dosage rates[5].

(ii) The coarser particles in the RHA have lesser tendency to uni-formly spread over wide areas in the wet mixture than finerparticles, thereby increasing the particle to particle distancein the cementitious matrix and decreasing the homogeneityof the mixture.

(iii) The particle size distribution of unground RHA particlesranges widely and hence, it is necessary to grind them finerto maintain uniformity in size and shape, and to ensure con-sistency in material performance.

(iv) Mixtures containing unground RHA were lower in theircompressive strength than control concrete (i.e. containingno RHA) probably due to inadequate pozzolanic reaction aswell as additional porosity introduced by the larger RHAgrains [20].

While some studies [8,20,25] have shown poorer performancefor unground RHA, none of them provided reasons for such

(i) Unground RHA

Fig. 1. Photograph of ungro

behavior. In addition, the effect of grinding the RHA to finer frac-tions on the microstructure and properties of portland cement sys-tem is not fully understood and investigated even though improvedproperties have been reported with the use of ground RHA [8].

In the current study, rice husk was burnt in a rotary tube fur-nace under controlled condition using the process developed byVempati et al. [26,27], which involved (i) continuous and con-trolled feeding of rice husks in a furnace by maintaining weightof rice husks to volume of furnace ratio between 0.02 and 0.10,(ii) pyrolyzing the rice husks at a constant temperature of approx-imately 800 �C for 40 min while simultaneously agitating them,and (iii) passing a continuous stream of air into the furnace todraw away carbon dioxide and other gases during incineration,thereby producing off-white RHA containing very low carbonand very high amorphous silica contents. A photograph of theoff-white rice husk ash in the as-obtained and ground form isshown in Fig. 1(i) and (ii), respectively. Since rice husks were usedas fuels during their incineration instead of fossil fuels, the produc-tion process of RHA can be considered environmentally friendlyand economical.

The primary objectives of this paper are to characterize the as-obtained or unground low-carbon RHA (URHA) and study themicrostructure of mixtures containing the URHA. In addition, thebeneficial effects of grinding this RHA on the properties of RHApowder, paste, mortar and concrete were studied by comparingthe performance of mixtures containing the URHA and ground ricehusk ash (GRHA). Comparative studies were also conducted withmixtures containing silica fume (SF), the silica content of whichis approximately equal to that of the RHA.

2. Experimental investigation

The experimental program was divided into two parts. The firstpart involved investigations on the effect of grinding of RHA pow-ders on its particle size and shape, setting time behavior and cal-cium hydroxide depletion of RHA pastes; the microstructure andflow behavior of RHA mortars. The second part dealt with deter-mining the mechanical and durability properties of hardened con-crete containing the URHA, GRHA and SF. The specific propertiesinvestigated include compressive strength, split tensile strength,flexural strength, modulus of elasticity, water absorption and rapidchloride ion permeation (RCP). The cementitious pastes and mor-tars were studied at a constant water-to-cementitious materialratio (w/cm) of 0.485 while the concretes were studied at a con-stant w/cm of 0.40.

(ii) Ground RHA

und and ground RHA.

350 H. Kizhakkumodom Venkatanarayanan, P.R. Rangaraju / Cement & Concrete Composites 55 (2015) 348–363

2.1. Materials and mixture proportions

The materials used in this study include ASTM C150 conformingType I portland cement (having an alkali content of 0.82% by massof total oxides), low-carbon RHA, silica fume, fine aggregate, coarseaggregate, poly-carboxylate ether based superplasticizer and air-entraining agent. The chemical composition of the cement andpozzolans are provided in Table 1 and the specific gravity ofcement, RHA and SF were 3.15, 2.19 and 2.20, respectively. Asper BET analysis, the specific surface area of RHA was�35,000 m2/kg, approximately 1.5–2.0 times the specific surfacearea of silica fume [27]. The X-ray diffractogram of the RHA and sil-ica fume is shown in Fig. 2(i). The diffractogram of the RHA showsbroad hump centered around a 2h angle of 22�, indicating that thenature of silica in it is amorphous. The diffractogram of the SF wassimilar to that of the RHA. The fine aggregate used was conven-tional river sand having a specific gravity of 2.65. The coarse aggre-gate was crushed granitic gneiss having a specific gravity of 2.75.The nominal maximum size of aggregate was 25 mm, which metthe ASTM C33 gradation specification for No. 57 stone as shownin Fig. 2(ii).

Table 1Chemical composition of cement and pozzolans.

Material SiO2 Al2O3 Fe2O3 CaO

Cement 19.78 4.98 3.13 61.84RHAa 92.40 0.26 0.30 1.63SFb 93 0.50 2.10 0.80

a RHA – rice husk ash.b SF – silica fume.

(i) Comparison of X-ray diffractogram of RHA and silica fume

0

250

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750

1000

1250

1500

0 10 20 30 40 50 60 70 80

Inte

nsity

(cou

nt p

er s

econ

d)

Bragg's angle (2θθ)

Low-carbon RHASilica fume

Fig. 2. Material characterization and

Table 2iMixture proportions for the cementitious paste and mortars.

Cementitious systems Mixture Quantity of materials (g)

Cement RHA

Pastes Control 500 0RHA/SF 500 (1 � x⁄/100) 500 (x⁄/

Mortars Control 500 0RHA/SF 500 (1 � x⁄/100) 500 (x⁄/

Note: x – replacement level of pozzolan for cement in %.a CA – coarse aggregates.b SP – superplasticizer.c RQ – required quantity.

The mixture proportions for cementitious paste and mortar areshown in Table 2i, and that for concrete are shown in Table 2ii. Themixing of ingredients followed the standard ASTM C192 test proce-dure. Accordingly, a portion of the RHA or SF was added to watercontaining superplasticizer while the remaining portion was addedto cement, and both mixtures were mixed well separately. The(RHA or SF + cement) mixture was then added to the coarse andfine aggregates, and dry-mixed to obtain uniform color. The (RHAor SF + water + superplasticizer) mixture was then added to thedry mixture and mixed for sufficient time to obtain a wet homog-enous concrete. This mixing process ensured thorough dispersionof all the materials in the wet mixture. After obtaining desiredhomogeneity in the wet mixture, the fresh properties of concretewere measured and recorded and are shown in Table 2iii. Thereplacement level of RHA is varied for different test although themaximum dosage did not exceed 15% (by weight of cement) forinvestigations with concrete. The grinding of URHA particles wasperformed using a Retsch planetary ball mill operated at a speedof 250 rpm for specific durations of 0, 5, 15, 30 and 60 min toobtain GRHA with defined particle size ranges and varying particlesize distributions.

Alkali content as Na2Oeq. (%) SO3 MgO Carbon

0.82 4.15 2.54 –1.24 0.11 0.38 0.540.10 0.20 – –

(ii) Gradation of coarse aggregate

0

10

20

30

40

50

60

70

80

90

100

100 1000 10000 100000

Cum

ulat

ive

volu

me

pass

ing

(%)

Sieve size (microns)

Upper (57)Average (57)Lower (57)Coarse aggregate

properties of different materials.

w/cm

Sand CAa Water SPb

0 – 242 RQc 0.485100) 0 – 242 RQ 0.485

1375 – 242 RQ 0.485100) 1375 – 242 RQ 0.485

Table 2iiMixture proportions for concretes.

Concrete IDs Replacement levels (%) Quantity of materials w/cm

Cement (kg/m3) RHA (kg/m3) Sand (kg/m3) Coarse aggregate (kg/m3) Water (kg/m3)

Control 0 427 0 717 1075 171 0.40URHA-7.5% 7.5 393 32 713 1070 170 0.40URHA-15% 15 361 64 713 1070 170 0.40GRHA-7.5% 7.5 393 32 713 1070 170 0.40GRHA-15% 15 361 64 713 1070 170 0.40SF-7.5% 7.5 393 32 713 1070 170 0.40SF-15% 15 361 64 713 1070 170 0.40

Table 2iiiProperties of freshly mixed concretes.

Concrete IDs Super-plasticizera (%) Slump (mm) Density (kg/m3) Air content (%)

Control 0.50 98 2347 2.50URHA-7.5% 0.98 85 2348 2.15URHA-15% 1.30 80 2339 3.20GRHA-7.5% 0.60 92 2355 2.87GRHA-15% 0.90 95 2329 2.98SF-7.5% 0.70 85 2341 2.12SF-15% 1.00 98 2321 2.90

a Super-plasticizer quantity is indicated as percentage by weight of cementitious material.


2.2. Experimental test methods

2.2.1. Properties of RHA powder, paste and mortarThe particle size distribution of the pozzolans was performed

using a Malvern Mastersizer 2000 analyzer. The microstructureof the URHA and GRHA powders was determined using a variablepressure Hitachi S-3400 scanning electron microscope in back-scatter mode. Normal consistency and setting time tests on cemen-titious pastes were conducted as per the ASTM C187 and ASTMC191 test procedures, respectively. All the three pozzolans in thesepastes were used at a dosage level of 0%, 7.5% and 15% by mass.

To ascertain the pozzolanic reactivity of the URHA, GRHA and SFpowders, thermo-gravimetric analysis (TGA) was conducted oncementitious pastes having a constant w/cm of 0.485. The pasteswere prepared at three cement replacement levels of 0%, 10% and20% by mass. The 7- and 28-day water-cured specimens wereground to a fine powder in a protected environment to avoid car-bonation, before being introduced into the TGA unit. TGA testswere performed using Thermal Analysis 2950 Thermo-GravimetricAnalyzer in which the samples were uniformly heated from ambi-ent temperature to 600 �C at a ramp rate of 10 �C per minute usingnitrogen as purging gas. From the TGA plot, the calcium hydroxide[Ca(OH)2] content between 400 and 500 �C in all paste sampleswas determined by using the TA Advantage software. The normal-ized Ca(OH)2 content in all the samples was calculated by consid-ering the Ca(OH)2 content in the control pastes as 100%.

The workability of the URHA, GRHA and SF mortars wasassessed using flow studies conducted as per the ASTM C1437 testprocedure.

2.2.2. Mechanical and durability properties of RHA concreteThe 3-, 7-, 28- and 90-day compressive strengths and 28-day

modulus of elasticity of concrete cylinder specimens(100 mm � 200 mm or 4 in. � 8 in.) was determined in accordancewith ASTM C39 and C469 methods, respectively. The 28-day splittensile strength of 100 mm � 200 mm (4 in. � 8 in.) concretecylinder specimens and 28-day flexural strength of75 mm � 75 mm � 300 mm (3 in. � 3 in. � 12 in.) concrete prismspecimens were determined according to ASTM C496 and C78 testprocedures, respectively. The water absorption capacity and the

rapid chloride ion permeability of 100 mm � 50 mm (4 in. � 2 in.)concrete disc specimens cured for 56 days was determined in accor-dance with ASTM C642 and C1202 test methods, respectively.

3. Experimental results and discussion

3.1. Particle size of RHA powders

The effect of grinding of RHA on its particle size distribution isshown in Fig. 3(i) and (ii), respectively. As the Fig. 3(i) shows, theURHA is coarsely graded compared to others and the average par-ticle size of the URHA, GRHA and densified SF are �30 lm, �6 lmand �7 lm, respectively. Although the particle size of undensifiedSF is of the order of 0.10 lm and is significantly lower than that ofRHA, commercial SF is supplied in densified form and hence, itsaverage grain size is larger.

Fig. 3(ii) shows that the average particle size of RHA decreaseswith increase in the grinding time from 0 to 15 min. However,the particle size reduction of RHA is minimal after a grinding timeof 15 min and very high grinding energy is required to furtherreduce its particle size. A past study conducted by Mehta and Fol-liard [6] suggested avoiding the high grinding energy for producingRHA as it derives high internal surface area from its cellular micro-structure rather than from its reduced particle size. Additionalresearch in this area indicated that the average particle size ofRHA should be less than that of cement, i.e., �10 lm in order forthe RHA to actively participate in the pozzolanic reaction [8]. Theparticle size of GRHA obtained by grinding for 15 min correspondswell with that of other similar studies [8,15].

3.2. Microstructure of RHA powders

The microstructure of the URHA and GRHA as observed througha scanning electron microscope is shown in Fig. 4(i)-(a) and (i)-(b),respectively. As these figures show, the URHA particles are distrib-uted over a wide size range with varying particle shapes while theGRHA particle size range is narrowly distributed. In addition, theURHA particles are irregular, vesicular and layered in appearancehaving porous microstructure. The GRHA particles are fine and uni-formly sized, with some smaller particles clumped together. An

(i) Particle size distribution (ii) Average particle size, D50

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100

0.1 1 10 100 1000

Cum

ulat

ive

volu

me

(%)

Particle size (μμm)

URHAGRHADensified SF

~6 μm ~7 μm ~30 μm0

5

10

15

20

25

30

35

0 10 20 30 40 50 60

Ave

rage

par

ticl

e si

ze, D

50(μ

m)

Time of grinding (minutes)

250 rpm

~6 μm

15 min

Fig. 3. Effect of grinding of RHA on its particle size distribution.


observation of the microstructure of the GRHA indicates significantdegradation of the URHA structure from the grinding process. It isanticipated that the significantly higher amounts of small RHA par-ticles will ensure an improved participation in the pozzolanicreaction.

3.3. Normal consistency and setting time behavior of RHA pastes

The normal consistency and setting time behavior of differentpastes are shown in Fig. 4(ii)-(a) and (ii)-(b), respectively. As theFig. 4(ii)-(a) shows, an increase in the dosage of URHA from 0% to15% increased the normal consistency of paste from 33% to 49%(�48% increase when compared to that of the control paste). Theincreased water demand for the URHA paste to achieve the samelevel of consistency may be due to the ability of the RHA particlesto quickly absorb the mix water from the paste due to their cellularmicrostructure. At any constant replacement level of RHA, the nor-mal consistency of the GRHA paste was lower than that of the URHApaste. This is because, the rate of water absorption of the URHA par-ticles is higher than that of the GRHA particles due to the presenceof larger pore sizes and wider pore size distribution of the former.From this investigation, the authors hypothesize that the grindingof RHA may significantly reduce the absorption of water by GRHAgrains and hence, can improve the workability characteristics ofconcrete. When compared to the URHA or GRHA paste, the SF pasteregistered lower normal consistency probably due to lower specificsurface area and non-cellular shape of the SF particles.

For the setting time behavior, the pastes were prepared at theirnormal consistency and hence, the w/cm in each paste is not thesame. As the Fig. 4(ii)-(b) shows, the initial setting time (IST) ofURHA and GRHA pastes was approximately equal and increasedlinearly with an increase in the dosage of RHA. In the case of finalsetting time (FST), the GRHA paste registered higher values thanthe URHA paste. This may be due to three reasons. Firstly, the nat-ure of pores in the URHA and GRHA particles are different due tochanges in their pore size distribution and bulk density. The grind-ing process significantly collapses the cellular structure of the ricehusk ash grains and hence, their pore size distribution can beexpected to significantly alter, resulting in a narrow distributionfor the GRHA than for the URHA. Similarly, the URHA particles(�228 kg/m3) have lower bulk density than the GRHA particles(�584 kg/m3) [5] and hence, the individual grain porosity (i.e., vol-ume of voids per unit total volume) and inter-particulate porositymay be greater for the former. These differences in the nature ofpores in the URHA and GRHA particles can significantly changethe rate of water absorption for their pastes.

Secondly, the setting time behavior is a function of the rate ofstiffening of the paste which in turn is influenced by the effectivew/c (i.e., the remaining water available to react with portland

cement after the water absorbed by the RHA grains). For a givendosage level of rice husk ash, the URHA grains can be expectedto proportionately absorb more water due to greater porosity thanthe GRHA grains resulting in a lower effective w/c in pastes. It islikely that the decrease in the porosity of the paste with continuedhydration is faster for the URHA paste than for the GRHA paste,thereby resulting in shorter final set times for the former. In addi-tion, the rate of water absorption for the URHA pastes can beexpected to be faster than for the GRHA paste due to the presenceof larger pores in the URHA grains. Therefore, the rate of depletionof water in the paste is slower for the GRHA grains than for theURHA grains, thus affecting the rate of stiffening of the paste.

Thirdly, the rheological properties (not investigated in thisstudy) for explaining the setting time behavior of pastes such asits yield stress and plastic viscosity may be different for the URHAand GRHA pastes due to changes in their effective w/c ratio at con-stant RHA dosage [28,29].

For the SF paste, a slight decrease in the setting time wasobserved on increasing the SF dosage from 0% to 7.5%. Howeverwith further increase in the SF dosage from 7.5% to 15%, the settingtime of the paste increased. Such trends in the setting time may bedue to the variation in paste stiffness at different SF dosage levels[28]. Of the three pastes, the SF paste registered the lowest settingtime at any dosage level. It may be noted that better and accurateinterpretation of the setting time behavior of cementitious pastecan be obtained only with other methods such as hydraulic pres-sure and rheological methods [29].

3.4. Calcium hydroxide depletion of RHA pastes

The 7- and 28-day normalized Ca(OH)2 content of differentpastes are shown in Fig. 4(iii)-(a) and (iii)-(b), respectively. Thesefigures show that the normalized Ca(OH)2 content of the pastesdecreases with an increase in the dosage of pozzolan. This decreasemay be due to dilution and pozzolanic effects. The dilution effect isdefined as that effect caused by the dilution of the cement contentof the paste as a result of partial replacement of cement with poz-zolan, thereby reducing the quantity of Ca(OH)2 produced bycement hydration. This Ca(OH)2 reduction due to the dilution effectis a direct linear function of the dosage of pozzolan as indicated bya dotted line in the figure. The pozzolanic effect is defined as thateffect caused merely due to the reaction of the pozzolan with theCa(OH)2 produced by the cement hydration (pozzolanic reaction)to form the necessary calcium–silicate–hydrate (C–S–H) gel. Theability of a pozzolan to be effective in the cementitious system isdependent on this effect rather than the dilution effect as the latteris independent on the nature of pozzolan used. From this figure,the pozzolanic effect is the reduction in the normalized Ca(OH)2

observed below the dilution effect (dotted line). If any pozzolan

(a) selcitrapAHRU )b( GRHA particles (i) SEM photographs of unground and ground RHA samples

500 μμm 500 μm

)TSFdnaTSI(emitgnitteslanifdnalaitinI)b(ycnetsisnoclamroN)a((ii) Setting time behavior of paste containing URHA, GRHA and SF

3536.5

32.5

41

49

3741

0

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60

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0

Nor

mal

con

sist

ency

(%)

Replacement levels (%)

SF pasteURHA pasteGRHA paste

0

100

200

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400

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600

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0

Setti

ng ti

me

(min

utes

)

Replacement levels (%)

IST: URHA paste IST: GRHA pasteIST: SF paste FST: URHA pasteFST: GRHA paste FST: SF paste

(a) At 7 days (b) At 28 days (iii) Normalized Ca(OH) 2 content of pastes containing URHA, GRHA and SF

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0 5 10 15 20 25 30 35 40

7-da

y N

orm

aliz

ed C

a(O

H) 2

cont

ent (

%)

RHA Replacement level (%)

Inert pozzolanURHA pasteGRHA pasteSF paste

Dilution effect

Pozzolonic effect of URHA

Effect of grinding of URHA

0

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0 5 10 15 20 25 30 35 40

28-d

ay N

orm

aliz

ed C

a(O

H) 2

cont

ent (

%)

RHA Replacement level (%)

Inert pozzolanURHA pasteGRHA pasteSF paste

Dilution effect

Pozzolonic effect of URHA

Effect of grinding of URHA

Fig. 4. Effect of grinding of RHA on specific properties of RHA powder and paste.


is effective in participating in the pozzolanic reaction, then moreCa(OH)2 is depleted and the normalized Ca(OH)2 content is wellbelow the dotted line. Similarly, if a pozzolan is either less effectiveor ineffective, then less Ca(OH)2 is depleted and correspondinglythe normalized Ca(OH)2 content is closer to the dotted line.

Specifically, the Fig. 4(iii)-(a) shows that the addition of URHAat any dosage level resulted in the 7-day Ca(OH)2 content of thepaste to be below the dotted line, indicating that the URHA effec-tively participates in the pozzolanic reaction. For example, the 7-day Ca(OH)2 content of the URHA paste decreased from 100% to56% when the dosage level increased from 0% to 20%. When com-pared to the URHA paste, the GRHA paste at constant dosage levelshowed higher Ca(OH)2 depletion. In addition, the pozzolonic

effect of the URHA and GRHA was 25% and 33%, respectively. Pre-vious studies have shown that the use of finer particles of RHAresulted in higher strengths than that of their coarser ones proba-bly due to finer particles experiencing much faster pozzolan reac-tion than the coarser particles [8,15]. In addition, the 7-dayCa(OH)2 content of the SF paste was lower than that of the URHApaste. The smaller particle size of SF not only reacts quickly withthe cement hydration products but also occupies and fills the porespresent in the cement paste. Also, the 7-day Ca(OH)2 content of theSF paste was comparable with that of the GRHA paste. As seen inFig. 4(iii)-(b), the trends in the 28-day normalized Ca(OH)2 contentof the pastes was similar to that of their 7-day normalized Ca(OH)2

content.


3.5. Microstructure of RHA mortars

The microstructure of 28-day moist-cured mortar specimenscontaining URHA and GRHA at different dosage levels is shownin Figs. 5–9. The micro-cracks observed in some specimens are pri-marily due to artifacts during the specimen preparation processand hence, may not be given much attention.

As the Fig. 5(i) and (ii) shows, the control mortar is character-ized by large number of pores in its matrix, with significantlylarge-sized and numerous unhydrated cement grains. The URHA-7.5% and URHA-15% mortars shown in Figs. 6 and 7, respectively,are characterized by the presence of embedded coarse RHA grainsand some unhydrated cement grains, with dense paste at differentlocations. The embedded coarse RHA grains may increase thematrix porosity in two ways. Firstly, the coarse RHA grains are cel-lular in nature thereby creating internal porosity within the matrix.This internal porosity may serve as a medium for the external liq-uids or available water to flow through the individual coarsegrains. However, since the coarse RHA grains are isolated fromeach other in the matrix as shown in Fig. 6(iii), their internal poros-ity can increase the matrix permeability only when the regionbetween the individual grains is sufficiently permeable. The per-meability of this matrix region depends primarily on the abilityof fine RHA particles to participate in the pozzolan reaction, whichin turn depends on its amorphous silica and unburnt carbon con-tent. The significant depletion of Ca(OH)2 by the URHA particlesdiscussed in the previous section clearly indicated that the matrixregion contains dense C–S–H gel produced from the pozzolanicreaction of the RHA particles. Hence, the internal porosity createdby the coarse grains can be concluded not to affect the matrix per-meability significantly. Secondly, the RHA grains have the inherentability to absorb moisture since the rice husks themselves are verygood absorbent material [26,27]. During the wet mixing of con-crete ingredients, this absorbing ability of RHA grains can help instoring or holding available water within their internal pores. Atlater ages, the absorbed water in RHA grains could be releasedfor long-term cement hydration or pozzolanic reactivity, therebyincreasing the porosity of the matrix as shown in Figs. 6(ii)–(iv)and 7(ii).

With high amounts of calcium, silica and oxygen present in theembedded RHA grains observed using the EDX analysis shown inFig. 6(v) and (vi), the presence of C–S–H gel in the grain structureof RHA particles indicates that the coarse grains may have under-gone partial pozzolanic reaction. By comparing the Figs. 5(i), 6(i)and 7(i), the porosity in the paste of the URHA mortars (excludingthose pores inherent to the embedded coarse RHA grain) decreaseswith increase in the dosage level from 0% to 15%. With lesseramount of unhydrated cement grains for the URHA mortars, their

(i) Location 1 at X600 magnification

100 μμm

Aggregate

Fig. 5. Microstructure of control mortars at differen

microstructure (excluding the internal porosity) can be concludedto be denser than the control mortars.

The GRHA-7.5% and GRHA-15% mortars shown in Figs. 8 and 9,respectively, are characterized by denser paste with very minimalporosity and fewer unhydrated cement grains compared to controlmortars shown in Fig. 5(i). Due to the absence of coarse RHA grains,the internal porosity created by their cellular microstructure in thematrix of GRHA mortars is also virtually absent. It is hypothesizedthat the fine RHA grains present in the matrix would have com-pletely participated in the pozzolanic reaction to form C–S–H gel,considering the high surface area, small average particle size andamorphous silica content of the GRHA particles. A comparison ofthe Figs. 5(i), 8(i) and 9(i) indicate that the amount of pores andunhydrated cement grains decreases with increase in the dosageof GRHA from 0% to 15%. In addition, a comparison of theFigs. 6(i) and 8(i) at a 7.5% dosage level or the Figs. 7(i) and 9(i)at a 15% dosage level indicate that the internal porosity (createdby the RHA grains), the total porosity and the unhydrated cementgrains for the GRHA mortars are lesser than for the URHA mortars,indicating the refined microstructure of the former and the benefi-cial effects of grinding the RHA particles.

3.6. Flow behavior of RHA mortars

Fig. 10(i) through (iii) shows the effect of dosage of URHA, GRHAand SF on the flow behavior of mortars for varying superplasticizerdosages. The maximum flow achievable with this test method is150% and a flow of 110% as per the ASTM C311 method was consid-ered as the minimum flow that can provide a workable mixture.

As the Fig. 10(i) shows, the control mortar without superplast-icizer registered a flow of 85%, requiring only 0.25% and 0.50%superplasticizer dosage to obtain 110% and 150% flow, respectively.With an increase in the dosage of URHA from 0% to 20%, the flow ofmortars decreased substantially from 150% to 0%, indicating theneed to use superplasticizer for obtaining a workable mixture. Inaddition, the URHA-20% mortars crumbled during a flow test,requiring a minimum superplasticizer dosage of 0.50% to initiatea flow. A minimum flow of 110% is achieved only for the controland URHA-5% mortars.

The overall flow behavior of the GRHA and SF mortars shown inFig. 10(ii) and (iii), respectively, was similar to that of the URHAmortars in the sense that the flow of mortars decreased withincrease in the replacement level of the pozzolans. However, whencompared to the URHA mortars, the GRHA and SF mortarsregistered higher flow at constant dosage levels of pozzolan andsuperplasticizer. This higher flow registered with GRHA mortarsmay be due to the absence of coarse RHA grains and the abilityof the finer grains to release the absorbed water from their cellular

)ii( Location 2 at X1000 magnification

100 μm

Porosity

Unhydrated cement grains

t locations after 28 days of moist curing period.

Aggregate

100 m 100 m

Air pockets around the RHA grains

Unhydrated cement grains

Coarse RHA grains Porosity in the

cementitious paste

Air pockets around coarse RHA grains

100 m 100 m

Well refined and dense cementitious matrix

Coarse cellular RHA grains


Air pockets around coarse RHA grains

100 m

Coarse cellular RHA grains

Spectrum at X-X

(i) noitacifingam006Xta1noitacoL )ii( Location 2 at X1000 magnification

(iii) noitacifingam0001Xta3noitacoL )vi( Location 4 at X1000 magnification

(v) noitacifingam004Xta5noitacoL )iv( EDX at X-X of Location 5

μ μ

μ μ

μ

Fig. 6. Microstructure of URHA-7.5% mortars at different locations after 28 days of moist curing.

Aggregate

Embedded coarse RHA grains

Unhydrated or partially hydrated

cement grains

100 m 100 m

Air pockets around the RHA

grains


μμ μ

Fig. 7. Microstructure of URHA-15% mortars at different locations after 28 days of moist curing.



Aggregate

Porosity


100 μm 100 μm

Unhydated cement grains

Unhydated cement grains

Fig. 8. Microstructure of GRHA-7.5% mortars at different locations after 28 days moist curing.

(i) Location 1 at X600 noitacifingam )ii( Location 2 at X1000 magnification

Well refined and dense cementitious

matrix

100 μm 100 μm

Fig. 9. Microstructure of GRHA-15% mortars at different locations after 28 days of moist curing.


microstructure during mixing operation as discussed in Section3.3, thereby enhancing the flowability of mixture. Ruxon et al.[25] have also indicated that ground RHA gives better workabilitycompared to unground RHA when used in concretes at a constantreplacement level of 15%.

The higher flow of the SF mortars compared to that of the URHAmortars is perhaps because, unlike the shape of URHA particles, theparticle shape of silica fume is spherical and non-cellular. The SFmortars registered lower flow than the GRHA mortars probablydue to larger amounts of fine particles in the former despite theaverage particle size of densified SF and GRHA being comparable.The densified SF has the tendency to disperse to fine micron-sizeparticles during mixing process while the GRHA has already under-gone significant reduction in particle size due to grinding action asalready discussed in Fig. 3(ii) and the mixing process appears notto cause any further reduction in its particle size. The addition ofany material finer than cement when used as a pozzolan has beenfound to affect the workability drastically [3,8]. Similar to theURHA-20% mortars, the SF-25% mortars did not yield a cohesivemix and crumbled prior to flow measurement. A minimum superp-lasticizer quantity of 0.50% was required to initiate a flow.

A comparative flow behavior of the URHA, GRHA and SF mortarsis shown in Fig. 10(iv). Here, the typical dosage of superplasticizerrequired to obtain a minimum flow of 110% for the URHA, GRHAand SF mortars was 2.50%, 0.95% and 1.45% (by weight of thecementitious material), respectively. In order to optimize the max-imum replacement level of pozzolans from flow studies, thesuperplasticizer dosage needed to achieve a flow of 110% wasdetermined for each mixture and plotted against the replacementlevel used as shown in Fig. 10(v). In this figure, a linear trendwas assumed to indicate that the superplasticizer requirement

for each mixture increases with an increase in the dosage level ofpozzolan. Considering the manufacturer’s recommended maxi-mum superplasticizer dosage of 1.25% (by weight of the cementi-tious material) as indicated by the dotted line in the graph, themaximum allowable dosage level of the URHA, GRHA and SF toachieve a flow of 110% was 7%, 18% and 10%, respectively: thesedosage levels, however, are not fixed numbers and may vary withthe efficiency of superplasticizer and variables in the concrete mix-ture proportions. In the rest of this investigation, RHA and SF dos-age levels of 7.5% and 15% representing lower and higher dosageswere used for assessing their performance in concrete.

3.7. Effect of grinding of RHA on the strength and durability propertiesof concrete

3.7.1. Rate of development of compressive strength in concretesThe rate of compressive strength development in concretes con-

taining the URHA, GRHA and SF at different dosage levels is shownin Fig. 11(i–iii), respectively. As the Fig. 11(i) shows, the compres-sive strength of the URHA concretes increases with an increase inthe curing period. A steep increase in the strength of concretewas observed until 7 days of curing period at all dosage levels ofURHA. Between the 7- and 28-day curing period, the increase instrength tends to level off, and specifically after the 28-day curingperiod, further increase in strength is marginal. Considering the 7-day strength as the reference, the respective percentage increase inthe 28-day strength of control, URHA-7.5% and URHA-15% was20%, 17% and 26%. As Fig. 11(ii) and (iii) show, the rate of strengthdevelopment of the GRHA and SF concretes was similar to that ofthe URHA concretes. Compared to the 7-day strength of the con-trol, GRHA-7.5% and GRHA-15% concretes, the percentage increase

(i) Flow behavior of URHA mortars (ii) Flow behavior of GRHA mortars

(iii) Flow behavior of SF mortars (iv) Comparison of flow values of RHA and SF mortars at a constant 15% replacement level

0

25

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150

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50

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ControlURHA-5%URHA-10%URHA-15%URHA-20%

Certain minimum SP is required to initiate flow

v

Minimum flow = 110%

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Minimum flow = 110%

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Certain minimum SP is required to initiate flow

Minimum flow = 110%

0

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0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50

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URHA-15%GRHA-15%SF-15%

Minimum flow = 110%

(v) Effect of replacement level of pozzolan on superplasticizer dosage

y = 0.14x + 0.30R² = 0.95

y = 0.07x + 0.04R² = 0.89

y = 1E-01x + 7E-16R² = 9E-01

0

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6

7

0 5 10 15 20 25 30 35

Supe

rpla

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izer

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age

to o

btai

n 11

0% fl

ow (%

)

Replacement level (%)

URHA mortarsGRHA mortarsSF mortars

Maximum superplasticizer

Fig. 10. Flow behavior of mortars containing URHA, GRHA and SF.


in their 28-day strength was 20%, 30% and 34%, respectively. Sim-ilarly, the corresponding increase in the 28-day strength values forthe control, SF-7.5% and SF-15% was 20%, 19% and 20%,respectively.

A comparison of the 28-day strength of the concretes is shownin Fig. 11(iv). As this figure shows, the 28-day strength of concretescontaining URHA, GRHA and SF at both 7.5% and 15% dosage levelswas higher than that of the control concretes. This increase isattributed to the pozzolanic reaction between the Ca(OH)2 in thepaste and the amorphous silica in the RHA or SF. Compared tothe control concrete, the percentage increase in the 28-daystrength at 7.5% and 15% dosage levels of pozzolan was 15% and21% for the URHA concrete; 32% and 37% for the GRHA concrete;26% and 42% for the SF concrete, respectively. In addition, theGRHA concrete registered a 14% increase in the 28-day strength

compared to the URHA concrete at a 7.5% dosage level. At a 15%replacement level, a 9% improvement in compressive strengthwas observed. These results clearly indicate that the grinding ofRHA to finer fractions improves its reactivity and consequentlyits strength in concrete. At a 7.5% dosage level, the GRHA concretehad slightly higher 28-day compressive strength than the SF con-crete. Conversely, at the 15% replacement level, the SF concretewas slightly stronger than the GRHA concrete. This indicates thata near-similarity of strength performance between the GRHA andSF concretes at replacement levels between 7.5% and 15% exists.

In order to compare the performance of RHA from different lit-eratures, normalized strength values for the RHA concrete wasevaluated by considering the compressive strength of the controlconcrete (no RHA) as 100%. The normalized strength valuesobtained in different studies for concretes containing RHA at a

setercnocAHRG)ii(setercnocAHRU)i(

0

10

20

30

40

50

60

70

80

0 20 40 60 80 100

Com

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URHA-7.5%URHA-15%Control

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GRHA-7.5%GRHA-15%Control

(iii) Silica fume concretes (iv) Comparison of 28-day compressive strength

values of concretes

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42.549.1 51.7

56.3 57.4 53.960.6

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80

Com

pres

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ngth

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)

Concrete ID

Fig. 11. Rate of compressive strength development in different concretes.


constant 15% replacement level is shown in Table 3. In addition,the table shows the incineration conditions adopted during theRHA production by different investigators, its color and form, andits oxide composition (both carbon and silica contents). The follow-ing are the salient observations from this table: (i) the normalizedvalues (121% at 7 days and 135% at 28 days) obtained for groundRHA in this research is significantly higher than the valuesobtained by other investigators [10–21]. (ii) The higher valuesobtained in this research when compared to that of Ferraro et al.[17] for ground RHA indicate that an increase in the combustiontemperature from 700 �C to 800 �C can significantly enhance thereactivity of RHA for the same incineration duration and produc-tion technique adopted. (ii) The higher values for unground RHAin this research when compared to that of Zerbino et al. [20] maybe due to significant variations in the amount of reactive silica inRHA despite its total silica content being equal in the two studies.This comparison signifies the advantage of using RHA obtainedthrough controlled combustion process over open heap burningas also discussed in the past [30]. From reactivity and strengthstandpoints, the very low values obtained by Zerbino et al. [20]clearly indicate the need for grinding the RHA to finer fractionsbefore using it in concrete. (iii) In the investigation performed bySalas et al. [16], the normalized strength value obtained for groundRHA both before and after chemical treatments were below 100,indicating that such treatments could change the total silica con-tent of RHA significantly but cannot change much its reactivity.

3.7.2. Modulus of elasticityThe 28-day modulus of elasticity (MOE) of various concretes is

shown in Table 4. The MOE of concrete containing pozzolans wasabove than that of the control concrete by �9% to �16%. The addi-tion of URHA at a dosage level of 7.5% and 15% increased the 28-day MOE of concrete by �11% and �13%, respectively. In another

study, the addition of unground RHA at 15% dosage level wasshown to reduce the 28-day MOE of concrete by �8% [20]. Theaddition of GRHA at a replacement level of 7.5% and 15% increasedthe 28-day MOE of concrete by �10% and �16%, respectively.While some research showed a slight or significant increase inthe MOE of concrete due to GRHA addition [16], some others haveshown a decrease in its MOE [20,7,15]. The contrast in findingsfrom the different studies is due to the use of RHA having high car-bon content obtained by over-burning of rice hulls without propercontrols or/and containing higher amounts of non-reactive or crys-talline silica. A comparison of the test results of URHA and GRHAconcretes at constant replacement levels indicated that the effectof grinding of RHA has negligible impact on the MOE of concreteas also observed in other research [20]. In addition, significant dif-ference in the MOE values were not observed between the RHA andSF concretes [7].

3.7.3. Split tensile strength and flexural strengthThe 28-day split tensile and flexural strength of various con-

cretes are shown in Table 4. Compared to the control concrete,the percentage increase in the 28-day strength at a 7.5% and 15%dosage level of pozzolan was 16% and 4% for the URHA concretes;21% and 15% for the GRHA concretes; and 16% and 30% for the SFconcretes, respectively. Such higher strengths for ground RHA con-crete were observed in other research also [14,15]. At a constant7.5% and 15% dosage level of pozzolan, the percentage increase instrength due to the grinding of RHA was 5% and 11%, respectively.The strength improvements are less but comparable to thoseobtained from the compression test. In comparison with the SFconcretes, the URHA or GRHA concretes registered comparable orlower strengths at 7.5% and 15% dosage levels, respectively. Whileresults obtained by Sakr [11] directly support our findings, theresults obtained by Zhang and Malhotra [7] demonstrate superior

Table 3Test results of strength-over-control or normalized compressive strength values as obtained by different investigators.

Investigator Form of RHA Incineration condition Color of RHA Replacement of RHA (%) w/cm Oxide content (% by ma ) Normalized compressive strength of concrete

Carbon Silica 7 days 28 days

Present study Unground (D50 = 29 lm) Controlled combustion in arotary tube maintained at 800 �Cfor 40 min

Off-white 15 0.40 0.54 92.40 115 121Ground (D50 = 6 lm) Off-white 15 0.40 0.54 92.40 121 135

Bui et al. [10] Ground (D50 = 5 lm) Controlled incineration in adrum incinerator

– 15 0.34 – 86.98 100 102

Sakr [11] – 600 �C for 180 min – 15 0.40 – 87.00 118 112Sensale [12] Ground Residue – 10–20 0.40 – 87.20 78–114 96–119Saraswathy and Song [13] – – – 15 0.53 – 92.95 116 103Ganesan et al. [14] Ground (D50 = 3.8 lm) 650 �C for 60 min on a

uncontrolled fired husk residueBlack 15 0.53 High 87.32 108 113

Cordeiro et al. [15] Ground (D50 = 3.7 lm) Partially burnt in boilers from650 �C to 800 �C

– 15 0.35 – 82.60 �98 104

Salas et al. [16] Ground 600 �C for 180 min Pink 15 0.45 – 90.00 – �95Ground 600 �C for 180 min (chemical

treatment)White 15 0.45 – 99.00 – �93

Ferraro et al. [17] Ground Controlled combustion in arotary tube maintained at 700 �Cfor 40 min

Off-white 15 0.45 0.24 94.80 106 115

Givi et al. [18] Ground (D50 = 95 lm) Controlled combustion of RHAobtained from a local supplier

– 15 0.40 – 87.86 87 102Ground (D50 = 5 lm) – 15 0.40 – 87.86 95 106

Gastaldini et al. [19] – – – 10–20 035 – 90 – 87–97Zerbino et al. [20] Unground Open field burning – 15 0.45 – 95.04 �75 �76

Ground – 15 0.45 – 94.84 �116 �108Lung et al. [21] Unground 600–800 �C – 10–20 0.35 – 91.00 94 �107–109

H.K

izhakkumodom

Venkatanarayanan,P.R

.Rangaraju

/Cement

&Concrete

Composites

55(2015)

348–363

359

ss

Table 4Modulus of elasticity, split tensile strength and flexural strength of different concretes after 28 days of curing.

Concrete ID Modulus of elasticity Split tensile strength Flexural strength

Experimental

Average (GPa) SDa (GPa) COVb (%) Average (MPa) SDa (MPa) COVb (%) Average (MPa) SDa (MPa) COVb (%)

Control 29.81 0.83 2.79 2.72 0.17 6.31 5.17 0.25 4.91URHA-7.5% 33.00 1.19 3.62 3.14 0.17 5.26 5.53 0.34 6.13URHA-15% 33.70 0.69 2.05 2.82 0.19 6.70 5.87 0.33 5.67GRHA-7.5% 32.65 1.61 4.94 3.29 0.09 2.59 5.61 0.29 5.08GRHA-15% 34.61 0.92 2.66 3.13 0.14 4.38 6.20 0.21 3.31SF-7.5% 33.81 0.95 2.82 3.16 0.14 4.35 5.69 0.22 3.89SF-15% 34.52 0.87 2.52 3.54 0.10 2.86 5.91 0.14 2.39

a SD – standard deviation.b COV – co-efficient of variation.


performance for the RHA concrete than for the SF concrete. In anoverall sense, although RHA concrete is stronger than control con-crete, significant attention may not be drawn toward the trendsobtained as a function of the dosage of pozzolan, which is usuallydone with compressive strength test results. This is because of thesmall range of the split tensile strength (2.72–3.54 MPa) values andhigher variability in the test results arising from the sensitivity inthe fabrication, handling, curing and testing of concrete specimens.

In the case of flexural strength, a definite increasing trend in the28-day strength was observed with an increase in the dosage levelof pozzolan as observed with the compressive strength test resultsdiscussed in Section 3.7.1. The percentage increase in the 28-daystrength at 7.5% and 15% dosage levels of pozzolans was 7% and15% for the URHA concretes; 9% and 20% for the GRHA concretes;and 10% and 14% for the SF concretes, respectively. These testresults comply with those obtained by other investigators[7,11,12]. The effect of grinding of RHA into finer fractions didnot substantially improve the 28-day strength and the percentageincrease in strength was only 1% and 7% at 7.5% and 15% dosagelevels of pozzolan, respectively. In addition, the strength of URHAor GRHA concrete was more or less comparable to that of SF con-crete at any given dosage level of pozzolan as obtained in otherresearch [11].

3.7.4. Rate of water absorptionFig. 12(i) through (iii) shows the rate of water absorption of dif-

ferent concretes after a 56-day curing period. All concretes exhibithigh initial absorption rate during the first 3–4 h. The rate ofabsorption slows down significantly after this initial time periodas most of the available pores become saturated.

As the Fig. 12(i) shows, the rate of water absorption of the URHAconcretes remain unchanged despite an increase in the dosagelevel of RHA from 0% to 15%, indicating that its addition at higherdosage levels is not beneficial in reducing the pores. This phenom-enon is perhaps due to the superimposition of two parallel effects.Firstly, the presence of URHA grains with its cellular structure inthe matrix increases the void content and therefore increases thewater absorption. However, the internal porosity in the URHAgrains present at different locations in the matrix is not continuousas previously discussed and therefore, is not likely to increase thepermeability of the matrix and continue to support the absorptionafter the initial period when the pore structure is saturated. Sec-ondly, the pozzolanic reaction of the URHA grains helps in reducingthe water absorption due to the refinement of the pore structure inthe matrix. The sum of the above two effects appear not to changethe total water absorption of the URHA concrete significantly com-pared to the control concrete. The investigation conducted by Giviet al. [18] supports our findings in that the water absorption ofconcrete containing coarse RHA (D50 = 90 lm) is higher than thatof the control concrete containing no RHA.

As the Fig. 12(ii) and (iii) indicate, the water absorption of theGRHA and SF concretes decreases with an increase in the dosagelevel of pozzolans as also observed in other research [16,18]. Thepercentage decrease in the water absorption values at 7.5% and15% dosage level of pozzolan was �13% and �12% for the GRHAconcrete; �13% and �18% for the SF concrete, respectively. Thisbehavior appears to be similar to that of any blended concretecontaining pozzolan as the addition of pozzolans at appropriatereplacement level tends to decrease the water absorption of con-crete [12]. The GRHA and the SF concretes exhibited lower waterabsorption than the control concretes due to the presence of fineramounts of siliceous particles that hydrate faster and participatecompletely in pozzolanic reaction even at an early evolution,resulting in less permeable matrix. A comparison of the 24-hwater absorption of different concretes is shown in Fig. 12(iv) aftera 28-day curing period. Considering the URHA and GRHA con-cretes, the percentage decrease in the water absorption valuedue to the grinding of RHA was �11% and �12% at a 7.5% and15% dosage level of RHA, respectively. In addition, the waterabsorption of the SF concretes was significantly low compared tothat of the URHA concrete but comparable to that of the GRHAconcretes.

3.7.5. Rapid chloride ion permeabilityThe ASTM C1202 test was performed to provide a measure of

the quality of concrete containing URHA and GRHA. This test mea-sures the electrical conductance of concrete specimen in terms ofcharge passed under a constant applied voltage, which is a functionof its pore structure and the composition of pore solution in it[31,32]. This test may not completely indicate the chloride ion per-meability of concrete containing pozzolans as their addition at lowcement replacement levels (of 5%) has shown to significantlyreduce the pore solution alkalinity and may not necessarilyimprove the pore structure of concrete [31]. Despite some limita-tions, this test has been widely used for quality control and com-parison purposes, especially while selecting an appropriateconcrete [33–35].

The test results of different concretes at a 56-day curing periodare shown in Fig. 13. As this figure shows, the rapid chloride ionpermeability (RCP) of the control concrete was in the ‘‘low range.’’An increase in the dosage level of URHA/GRHA/SF from 0% to 15%steeply decreased the RCP of concrete and brought down the val-ues below 1000 C (‘‘very low range’’). The percentage reductionin the RCP value at 7.5% and 15% dosage level was 33% and 54%for the URHA concrete; 64% and 82% for the GRHA concrete; 59%and 75% for the SF concrete, respectively. By comparing the per-centage reduction values obtained in this research (33%) for theURHA concrete at a 7.5% dosage level with the values obtained ina past research performed by Nehdi et al. [8] (�5%), it is evidentthat the carbon content and amorphous silica content of RHA has

setercnocAHRU)i( (ii) GRHA concretes

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(iii) Silica fume concretes (v) Comparison of 24-hour water absorption values of concretes

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5.12 5.02 5.07 4.47 4.48 4.43 4.20

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our

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Fig. 12. Rate of water absorption of URHA, GRHA and SF concretes over a 24 h immersion period.

1732

1157

804 619309

712

431

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3500

4000

Cha

rge

pass

ed (C

oulu

mbs

)

Concrete ID

Moderate

Low

Very Low

Fig. 13. Comparison of RCP values for RHA and silica fume concretes after a 56 daycuring period.


significant effect on the chloride ion permeability of concrete.Similarly, the percentage reduction value for the GRHA concretesobtained in this study is comparable with that of Nehdi et al. [8]and such trends with GRHA additions are observed in otherresearch studies as well [14,15]. Although a significant reductionin the RCP value of concrete containing the URHA or GRHA additioncould be due to the reduction in the pore solution alkalinity asfound with SF concretes in other studies [31], results presentedin this study (in Section 3.5) have revealed better pore structure

and shown significantly lower calcium hydroxide contents forthe URHA or GRHA mixtures than for the control mixture. It istherefore concluded that the addition of URHA or GRHA could sub-stantially reduce the chloride ion permeability of concrete due tothe formation of dense C–S–H gel (having low C/S ratio) from poz-zolanic reaction. In addition, the internal porosity created by theembedded coarse RHA grain in the matrix of the URHA concreteappears to be localized as discussed earlier (in Section 3.2) anddoes not affect the matrix permeability much due to the disconti-nuity between the individual coarse grains in the matrix. Previousstudies have also confirmed that the chloride ion permeability ofconcrete containing RHA is lower than that of the control concrete[14,19].

By comparing the RCP of GRHA concrete with that of the URHAconcrete at constant dosage levels, the effect of grinding the RHA tofiner particles decreased the chloride ion permeability of concretesubstantially by 46% and 62% at 7.5% and 15% dosage levels,respectively. This reduction may be largely attributed to the densemicrostructure of the paste in the GRHA concrete resulting fromthe pozzolanic reaction of the entire grain of fine RHA particlesto form dense C–S–H gel. Between the GRHA and SF concretes,the former performed slightly better than the later at the tworeplacement levels studied.

From a permeability standpoint, although positive results wereobtained with URHA and GRHA addition, its direct use in portlandcement mixtures as a positive solution to address durability prob-lems needs careful assessment. In a research related to the currentstudy, Harish and Rangaraju [36] have demonstrated superior sul-fate resistance property for the URHA mortars than for the portland


cement mortars. However in another research, Zerbino et al. [37]found that the addition of unground and ground RHA increasesthe expansion due to ASR and causes mechanical degradation: thisunusual behavior depends on the alkali content of cement usedand the dosage level of RHA adopted.

4. Conclusions

In this study, a low-carbon RHA produced from a controlledcombustion process was investigated to determine its effective-ness as a pozzolan. Portland cement mixtures containing URHAand GRHA were prepared separately, and their properties werecompared with that of the control (no RHA) and silica fume mix-tures to understand the beneficial effects of adding the URHAand grinding it to fine particles. In addition, microscopic examina-tion of RHA mixtures was conducted to understand the microstruc-tural features. Results obtained from this study provided thefollowing conclusions:

(1) From the studies conducted with URHA mixtures, the addi-tion of URHA in portland cement mixture at the studiedreplacement levels increased its normal consistency, initialsetting time, final setting time, calcium hydroxide depletion,compressive strength, split tensile strength and flexuralstrength compared to the control mixtures. The flow andRCP of the mixture significantly decreased with the additionof URHA.

(2) From the studies conducted with GRHA mixtures, the addi-tion of GRHA at 7.5% and 15% replacement levels substan-tially improved all properties of portland cement mixturesinvestigated. However for certain properties such as com-pressive strength, modulus of elasticity and water absorp-tion, significant improvements were not observed beyond7.5% replacement level of GRHA.

(3) A comparison of the performance of the GRHA and URHAmixtures suggested the following:
– In all tests conducted, the behavior or trends observed
with an increase in the replacement level of RHA forthe GRHA and URHA mixtures were similar.

– For the materials investigated in this study, the optimumreplacement level of RHA evaluated based on flow behav-ior is substantially higher for GRHA mortars (�18%) thanfor URHA mortars (�7%), indicating the beneficial effectsof grinding the RHA.

– The effect of grinding the RHA to finer fractions resultedin improvement in all properties of mixtures, with excep-tion of final setting time. The final setting time of theGRHA paste is higher than that of the URHA paste, likelyresulting from the differences in moisture absorptioncharacteristics of RHA grains as a function of particle sizeand its effects on effective w/c ratio within the paste andthe rate of stiffening of the paste.

(4) The studies conducted to understand the microstructure ofURHA and GRHA mixtures revealed the following:
– The URHA mixture is characterized by embedded coarse
RHA grains in addition to the unhydrated cement grains,partially or fully hydrated cement paste and pozzolanicreaction products that are usually observed in any mix-ture containing pozzolan. The embedded coarse RHAgrains create internal porosities due to their cellularand porous microstructure, thereby increasing the totalvolume of pores in the matrix; however, the permeabilityof the mixture is not much affected as the internal poresare isolated from each other due to the dispersion of thecoarse RHA grains in the matrix.

– The microstructure of the GRHA mortars is denser thanthat of the control and URHA mortars primarily due tolesser amounts of unhydrated cement grains, air voids,Ca(OH)2 contents and embedded coarse grains, and moreamounts dense cementitious paste anticipated to be richin C–S–H gel.

(5) At a given replacement level of pozzolan, the Ca(OH)2 deple-tion, compressive strength, modulus of elasticity, split ten-sile strength, flexural strength, water absorption and rapidchloride ion permeability of the SF mixtures were superiorto that of URHA mixtures and comparable to that of GRHAmixtures.

From the above conclusions, it is evident that the incorporationof low-carbon RHA in concrete provides a positive solution to pre-vent the disposal of rice husks in land. While the higher strengthsobtained in this study with the mixtures containing URHA pro-duced from the controlled incineration process clearly demon-strated the lack of need to grind it to finer fractions, the effect ofenhanced internal porosity created by the coarse RHA grains inthe URHA matrix on its total porosity, permeability and durabilityneeds detailed investigation. The grinding of RHA to finer particlessignificantly improved the properties of concrete. The strength anddurability of GRHA concretes was comparable to that of the mix-tures containing commercially available pozzolans such as silicafume, thereby creating options for utilizing the GRHA in concreteconstructions involving portland cement.

Acknowledgments

The authors convey their acknowledgement to the NSF financialsupport in the form of SBIR Phase II funding and the Grant Numberfor the project was NSF 0724463 and to ChK Group, Inc. for provid-ing the rice husk ash used in this study.

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