Construction

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Construction and Building Materials 21 (2007) 1356–1361

and Building

MATERIALS

Sulfate resistance of blended cements containing fly ash andrice husk ash

P. Chindaprasirt a,*, P. Kanchanda a, A. Sathonsaowaphak a, H.T. Cao b

a Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailandb Senior Materials Scientist, Connell Wagner, Sydney, Australia

Received 5 April 2005; received in revised form 5 October 2005; accepted 17 October 2005Available online 14 September 2006

Abstract

In this paper, the sulfate resistance of mortars made from ordinary Portland cement containing available pozzolans viz., fly ash andground rice husk ash (RHA) was studied. Class F lignite fly ash and RHA were used at replacement dosages of 20 and 40% by weight ofcement. Expansion of mortar prisms immersed in 5% sodium sulfate solution and the change in the pH values of the solution were mon-itored. The incorporation of fly ash and RHA reduced the expansion of the mortar bars and the pH values of the solutions. RHA wasfound to be more effective than fly ash. Examination of the fractured surface of mortar prisms, after a period of immersion, by scanningelectron microscopy confirmed that sulfate attack of blended cement mortars was restricted owing to the reductions in calcium hydroxideand C/S ratio of the C–S–H gel in the blended cement mortar. In comparison to Portland cement mortar, less calcium sulfate and muchless ettringite formations were found in the mortars made from blended cement containing RHA. The amounts of calcium sulfate andettringite found in the blended cement mortar containing fly ash were also small but were slightly more than those of RHA mortar. Up to40% of Portland cement could be replaced with these pozzolans in making blended cement with good sulfate resistance.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Sulfate resistance; Fly ash; Rice husk ash; Blended cement

1. Introduction

Manufacturing of Portland cement is an energy inten-sive process and releases a very large amount of greenhouse gas to the atmosphere. It has been reported that13,500 million ton is produced from this process, whichaccounts for about 7% of the green house gas producedannually [1]. Efforts have, therefore, been made to promotethe use of pozzolans such as fly ash, calcined kaolin, ricehusk ash and palm oil fuel ash [2–5] to replace part of Port-land cement. This reduces the total amount of the Portlandcement used. Fly ash is the most common pozzolan and isbeing used worldwide. In Asia and many parts of theworld, a large amount of rice husk could be obtained as

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

doi:10.1016/j.conbuildmat.2005.10.005

* Corresponding author. Tel.: +66 4320 2846 7x131; fax: +66 4320 28467x102.

E-mail addresses: prinya@kku.ac.th, chindaprasirt@yahoo.com (P.Chindaprasirt).

an agricultural by-product. The properly burnt and groundrice husk ash is also suitable for use as a pozzolan [6].

The annual output of lignite fly ash from Mae Mohpower station in the North of Thailand is around 3 milliontons. The quality of this lignite fly ash has improved dras-tically over the last 10 years owing to the use of better qual-ity lignite and improved combustion. This fly ash is nowclassified as class F and is being used quite extensivelyfor construction in Thailand. Up to now the potential useof this fly ash has admittedly not been fully achieved, asalmost all the fly ash concretes used is not durability-based.In general, replacement of cement by fly ash reduces theinitial strength of concrete, whereas the strength at laterage as well as the durability viz. sulfate resistance and acidresistance are improved [4,7]. The incorporation of fly ashincreases the porosity of the cement paste but the averagepore size is reduced. This results in a less permeable pastewhich is less susceptible to the ingress of the harmful solu-tion [8,9].

Table 1Chemical compositions of Portland cement, fly ash and rice husk ash

Oxides PC FA RHA

CaO 63.4 13.0 0.8SiO2 22.1 44.4 90.0Al2O3 3.7 23.5 0.5Fe2O3 2.9 10.2 0.9MgO 2.5 3.0 0.6SO3 2.5 1.1 0.1Na2O 0.1 0.1 0.1K2O 0.5 2.0 2.1LOI 1.1 1.8 3.2Blaine fineness (cm2/g) 2900 2600 14,000

P. Chindaprasirt et al. / Construction and Building Materials 21 (2007) 1356–1361 1357

Rice husk consists of about 40% cellulose, 30% ligningroup and 20% silica and hence the ash contains a largeamount of silica [3,6]. The silica exists in two forms:amorphous or crystalline silica depending on the temper-ature and duration of burning. Silica of amorphous formis reactive and suitable for use as a pozzolan to replacepart of Portland cement. Amorphous silica is obtainedby burning rice husk at temperature lower than 700 �C[10]. With proper burning and grinding, the amorphousreactive rice husk ash could be produced and used as apozzolan [6]. Even for higher burning temperature withsome crystalline formation of silica, good result couldbe obtained by fine grinding [11]. The reactive RHA canbe used to produce good quality concrete with reducedporosity and reduced Ca(OH)2 [12]. Rice husk ash hasbeen used successfully in such applications as concretewith controlled permeability formwork and roller com-pacted concrete [13,14].

Sulfate attack is one of the most important problemsconcerning the durability of concrete structures. Underthe sulfate environment, cement paste undergoes deteriora-tion resulting from expansion, spalling and softening[15,16]. It is generally recognized that addition of pozzolanreduces the calcium hydroxide in cement paste andimproves the permeability of concrete. This helps toincrease the resistance of concrete to the attack of sulfateand other harmful solutions [2]. The increase in the servicelife of the structure made from the blended cement contain-ing pozzolan would further reduce the amount of Portlandcement use. The knowledge of the use of lignite fly ash andRHA to increase the resistance of concrete to the harmfulsolutions, especially sulfate solution, would be beneficial tothe understanding of the mechanism and to the applica-tions of these materials.

2. Materials and experimental details

An ordinary Portland cement (PC), lignite fly ash (FA)from Mae Moh power station in the north of Thailandand ground rice husk ash (RHA) were used. The rice husk

Fig. 1. Morphological features of as received FA and grou

ash was obtained from open burning of 20 kg heap of ricehusk with a maximum burning temperature of 600 �C. Theburnt rice husk ash was whitish gray in color. The rice huskash was then ground in a laboratory rod mill to reasonablefine particles. Local river sand with S.G. of 2.65 was usedfor making a mortar. Chemical compositions and Blainefineness of PC, FA and RHA used in this work are givenin Table 1. Morphological features of as-received fly ashand RHA are shown in Fig. 1.

The combined amount of SiO2, Al2O3, and Fe2O3 in FAwas 78.1% indicating that Mae Moh fly ash is a class F flyash. The Blaine fineness of the FA was 2600 cm2/g, whichwas coarser than PC (2900 cm2/g). RHA contained highsilica content of 90% and low loss on ignition (LOI) of3.2%. This suggests that RHA was burnt relatively com-plete. The Blaine fineness of RHA was 14,000 cm2/g usingrod mill grinding.

All mortars were made with sand to binder ratio of 2.75and adjusted water contents to achieve similar flow of110 ± 5%. The compressive strengths at 7, 28, 90 and 180days were obtained using 50 mm cubes for normal water-cured in accordance with the ASTM C109. The test for sul-fate-induced expansion was done following the proceduresdescribed in ASTM C1012 with 5% sodium sulfate solu-tion. It is required that the mortar acquires the strengthof 20 MPa before the immersion in the sulfate solution.For all mixes except the mixes with high fly ash and rice

nd RHA: (a) as-received fly ash, and (b) ground RHA.

0

500

1,000

1,500

2,000

2,500

0 60 120 180 240 300 360

Immersion (days)

Exp

ansi

on(m

icro

stra

in)

PC

FA20RHA20FA40

RHA40

Fig. 2. Expansion of mortar bars in 5% sulfate solution.

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husk ash replacement, the age of the immersion was 1 day.The age of immersion of the FA40 and RHA40 mortarswere 2 and 10 days, respectively, because the strengthdevelopment of these mortars were relatively slow. Thesolutions were changed weekly for the first month, monthlyuntil six months and three monthly thereafter. The pH val-ues of the sulfate solutions of all immersions were moni-tored. Scanning electron microscopy was performed forthe mortars after immersion in the sulfate solution for 9months.

3. Results and discussion

3.1. Water-to-binder ratio and compressive strength

Table 2 shows water-to-binder (W/B) ratios and com-pressive strengths of the mortar mixes containing FA andRHA at different replacement dosages. The addition ofthe lignite fly ash resulted in a reduction in the waterrequirement of the mortar for similar flow. W/B ratios were0.55, 0.53 and 0.51 for mortars containing no fly ash (PC),20% FA (FA20) and 40% FA (FA40), respectively. On thecontrary, the incorporation of RHA resulted in markedincreases in water demand as W/B ratios increased from0.55 to 0.68 and 0.80 for PC, 20% RHA (RHA20) and40% RHA (RHA40) mortars, respectively, owing to thehigh surface area of RHA.

As shown in Table 2, the introduction of fly ash andRHA resulted in a reduction of 7-day compressivestrength. At 28 days, the use of fly ash and high level ofreplacement of RHA also resulted in reduction in thestrength as compared to that of PC mortar. The reductionof early strength is typical of the fly ash mixes. For RHAmixes, the low initial strength was due to the high water-to-binder of the mixes. For 20% RHA replacement level,although the W/B ratio was increased, the strength at 28days was higher than that of PC mix, suggesting thatRHA is quite reactive.

In general, these results indicated that the lignite fly ashand RHA are pozzolanic materials with different character-istics. Although possessing high fineness, RHA contribu-tion to the strength development was limited by the highwater demand associated with its high surface areas. Lig-nite fly ash showed typical strength development patternof Class F fly ash, i.e. slow in the first 28 days and contin-ues well after 90 days.

Table 2Water cement ratios of mortars at constant flow of 110 ± 5%

Mix Water-to-binderratio

Compressive strength, (MPa)

7 day 28 day 90 day 180 day

OPC 0.55 44 51 57 60FA20 0.53 32 45 57 57FA40 0.51 29 46 62 77RHA20 0.68 31 54 61 62RHA40 0.80 17 32 43 53

3.2. Expansion of mortar bars

The patterns of expansion of mortar prisms in 5%Na2SO4 solution are shown in Fig. 2. It is clearly evidentthat the expansion of the PC prism is much larger thanthose made with the blended cements. The acceleratingexpansion pattern of the PC mortar is observed from 120days onward. There is no obvious accelerating expansionpattern shown by blended cement mortar prisms. FA20,FA40 and RHA20 mixes show a ‘‘linear’’ pattern of expan-sion after about 120 days in sulfate solution. RHA40 mix,however, shows a very small expansion even after immer-sion for 360 days.

Fig. 3 shows the pH levels of the sodium sulfate solu-tion. The highest level of the pH of sulfate solution wasapproximately 12.5 for all the mortars and was observedwithin the first 7 days of immersion, indicating that a sub-stantial amount of the calcium hydroxide was leached outand thus increased the pH of the solution. The fresh solu-tion pH is 7.0–7.5. After one day of immersion, the pHlevel is found to be more than 12. It has been reported thatthe pH of 12–12.5 was obtained within a few hours ofimmersion [17]. The pH of the solution is increased slightlyas the immersion period continued until the solution isreplaced by a fresh solution. After immersion of the mortarbars, the pH of the fresh solution again increases rapidly toa high value but less than the previous highest value owingto the less amount of hydroxide ion. At 90 and 180 days,the pH levels of the sulfate solutions are significantly lower

Fig. 3. pH level of the sodium sulfate solutions immersed with mortarbars.

P. Chindaprasirt et al. / Construction and Building Materials 21 (2007) 1356–1361 1359

and different. The pH value of the solution with PC mortaris the highest of 11.0 followed by those of FA20, FA40,RHA20 and RHA40 with 10.7, 10.5, 10.1 and 9.5, respec-tively. The expansion of the mortar bar is sensitive to thepH level of the solution [18]. At pH 12–12.5 only ettringiteformation can take place and at pH of 8.0–11.5 gypsumformation and decalcification occur [17].

It has also been suggested that the dissolution ofCa(OH)2 and calcium sulfoaluminates, and the decalcifica-tion of CSH with a high C/S ratio in hardened PC pasteresulted in a very porous layer whereas the decalcificationof the low C/S CSH resulted in a protective layer of silica

Fig. 4. SEM of mortar exposed to sulfate solution for 9 months: (a) portlandmortar � 1 mm depth, (d) RHA20 mortar � 1 mm depth, (e) RHA 40 mortar �exposed surface.

gel [4]. For FA and RHA blended cement, C/S ratio ofCSH would have been lower as a result of the pozzolanicreaction. FA and RHA mortars thus show better resistanceto the sulfate attack in comparison to PC mortar withRHA, being more reactive and showing better resistanceto sulfate attack.

3.3. SEM examination of the mortar prisms

After immersion in sulfate solution for 9 months, themortar prisms were examined using SEM. The results areshown in Fig. 4. These are microstructures of PC, FA20,

cement mortar �1 mm depth, (b) FA20 mortar � 1 mm depth, (c) FA401 mm depth, and (f) sulfate rich skin of prism and gypsum lens parallel to

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FA40, RHA20 and RHA40 at a depth of about 1 mmfrom the surface exposed to sulfate solution. A commonfeature at the exposed surface of sulfate-rich skin and alens of sulfate forming parallel to the surface are shownin this figure.

The morphologies of the samples suggest that the highexpansion of PC mortar prism is associated with the largeamount of ettringite observed readily in the first 5 mmfrom the exposed surface. The ettringite formed in thePC mortar prism appears as bundle of long needles.Ettringite is also found in FA and RHA prisms. However,the ettringite appears to be of shorter needles with smallerdiameter (i.e. no apparent change of aspect ratio) in com-parison to those found in PC prism. For the case of flyash, ettringite formation is observed at depth less than1 mm for FA40 mortar bars and at deeper depths, about1–2 mm for FA20. In the case of RHA mixes, the ettringiteis only found in the first 100–500 lm from exposed surface.This indicates that the penetration of sulfate ions intoRHA prisms is very limited despite their significantly highwater-to-binder ratios. These results correlate well withthose of the pH levels of the sulfate solutions.

The other notable difference between PC prisms andblended cement prisms was the massive precipitation ofgypsum easily observed in PC prism especially in the nearexposed surface zone. Gypsum was also detected in flyash blended cement prisms but in much less quantities.The precipitation of gypsum in RHA prism was either spo-radic or insignificant.

The SEM observations obtained in this work are not dif-ferent to those discussed in the literature [19–21]. Theresults, however, suggest there would be a close linkbetween ettringite formation and expansion, particularlyfor the PC. The ettringite observed is not the sub-microntype intimately mixed in C–S–H. The lack of this type ofettringite is associated with little or no expansion, e.g. inRHA prisms. It is likely that the accelerated expansion pat-tern shown by PC prism was a result of this type of largeettringite formation. Their morphology suggests furtherthat the formation of large ettringite would be the laterstage of sulfate attack by sodium sulfate solution.

4. Conclusions

Based on the obtained data, it can be concluded that theincorporation of lignite class F fly ash and ground ricehusk ash into normal Portland cement result in a signifi-cant improvement in the resistance to attack by 5% sodiumsulfate solution. Better dimension stability is obtained withblended cements containing FA and RHA. Despite havinghigher water demand characteristics, RHA at a dosage ofup to 40% cement replacement is very effective in providingsulfate resistance. Class F lignite fly ash is only slightly lesseffective at both 20% and 40% replacement levels in com-parison to that with RHA.

The pH levels of the sulfate solutions after immersionfor 6 months are significantly different, indicating the dif-

ferent levels of calcium hydroxide. Fly ash and rice huskash mortar are of lower pH levels and thus less suscep-tible to sulfate attack. SEM examination confirms thatettringite is not a pronounced feature in the microstruc-ture of the blended cement mortars exposed to sodiumsulfate solution, in comparison to that of Portlandcement mortar.

Up to 40% of Portland cement could be replaced withfly ash and RHA in making blended cement mortar withreasonable strength development and good sulfate resis-tance. This would reduce the amount of Portland cementuse and the greenhouse gas. Service life of the mortarwould also be increased owing to the higher sulfateresistance.

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