Sulfate resistance of blended cements containing fly ash and rice husk ash

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    Abstract

    world, a large amount of rice husk could be obtained as

    rice husk ash is also suitable for use as a pozzolan [6].

    age as well as the durability viz. sulfate resistance and acidresistance are improved [4,7]. The incorporation of y 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].

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

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

    Construction and Building Materia

    ConstructionManufacturing 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]. Eorts have, therefore, been made to promotethe use of pozzolans such as y ash, calcined kaolin, ricehusk ash and palm oil fuel ash [25] 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 the

    The annual output of lignite y ash from Mae Mohpower station in the North of Thailand is around 3 milliontons. The quality of this lignite y ash has improved dras-tically over the last 10 years owing to the use of better qual-ity lignite and improved combustion. This y ash is nowclassied as class F and is being used quite extensivelyfor construction in Thailand. Up to now the potential useof this y ash has admittedly not been fully achieved, asalmost all the y ash concretes used is not durability-based.In general, replacement of cement by y ash reduces theinitial strength of concrete, whereas the strength at laterIn this paper, the sulfate resistance of mortars made from ordinary Portland cement containing available pozzolans viz., y ash andground rice husk ash (RHA) was studied. Class F lignite y 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 y ash and RHA reduced the expansion of the mortar bars and the pH values of the solutions. RHA wasfound to be more eective than y ash. Examination of the fractured surface of mortar prisms, after a period of immersion, by scanningelectron microscopy conrmed that sulfate attack of blended cement mortars was restricted owing to the reductions in calcium hydroxideand C/S ratio of the CSH 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 y 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 an agricultural by-product. The properly burnt and groundSulfate resistance of blended crice hu

    P. Chindaprasirt a,*, P. Kanchandaa Department of Civil Engineering, Faculty of Enginee

    b Senior Materials Scientist, C

    Received 5 April 2005; received in revised foAvailable online0950-0618/$ - see front matter 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2005.10.005ments containing y ash andk ash

    A. Sathonsaowaphak a, H.T. Cao b

    , Khon Kaen University, Khon Kaen 40002, Thailand

    ell Wagner, Sydney, Australia

    5 October 2005; accepted 17 October 2005September 2006

    www.elsevier.com/locate/conbuildmat

    ls 21 (2007) 13561361

    and Building

    MATERIALS

  • 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 ne grinding [11]. The reactive RHA canbe used to produce good quality concrete with reducedporosity and reduced Ca(OH) [12]. Rice husk ash has

    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 reasonablene particles. Local river sand with S.G. of 2.65 was usedfor making a mortar. Chemical compositions and Blaineneness of PC, FA and RHA used in this work are givenin Table 1. Morphological features of as-received y ashand RHA are shown in Fig. 1.

    The combined amount of SiO2, Al2O3, and Fe2O3 in FAwas 78.1% indicating that Mae Moh y ash is a class F yash. The Blaine neness 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 neness 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 ow 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 y ash and rice

    P. Chindaprasirt et al. / Construction and Building Materials 21 (2007) 13561361 13572

    been 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 y ash andRHA to increase the resistance of concrete to the harmfulsolutions, especially sulfate solution, would be benecial tothe understanding of the mechanism and to the applica-tions of these materials.

    2. Materials and experimental details

    An ordinary Portland cement (PC), lignite y ash (FA)from Mae Moh power station in the north of Thailandand ground rice husk ash (RHA) were used. The rice huskFig. 1. Morphological features of as received FA and grouTable 1Chemical compositions of Portland cement, y 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 neness (cm2/g) 2900 2600 14,000nd RHA: (a) as-received y ash, and (b) ground RHA.

  • 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 rst 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 9

    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 rst 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 7.07.5. After one day of immersion, the pHlevel is found to be more than 12. It has been reported thatthe pH of 1212.5 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 to

    1358 P. Chindaprasirt et al. / Construction and Building Materials 21 (2007) 13561361months.

    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 dierent replacement dosages. The addition ofthe lignite y ash resulted in a reduction in the waterrequirement of the mortar for similar ow. W/B ratios were0.55, 0.53 and 0.51 for mortars containing no y 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 y ash andRHA resulted in a reduction of 7-day compressivestrength. At 28 days, the use of y 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 y 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 y ashand RHA are pozzolanic materials with dierent character-istics. Although possessing high neness, RHA contribu-tion to the strength development was limited by the highwater demand associated with its high surface areas. Lig-nite y ash showed typical strength development patternof Class F y ash, i.e. slow in the rst 28 days and contin-ues well after 90 days.

    Table 2Water cement ratios of mortars at constant ow 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 77

    RHA20 0.68 31 54 61 62RHA40 0.80 17 32 43 53a 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 signicantly lower

    0

    500

    1,000

    1,500

    2,000

    2,500

    0 60 120 180 240 300 360Immersion (days)

    Expa

    nsio

    n(m

    icros

    train)

    PC

    FA20RHA20FA40

    RHA40

    Fig. 2. Expansion of mortar bars in 5% sulfate solution.Fig. 3. pH level of the sodium sulfate solutions immersed with mortarbars.

  • and dierent. 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 1212.5 only ettringiteformation can take place and at pH of 8.011.5 gypsumformation and decalcication occur [17].

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

    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,

    and

    P. Chindaprasirt et al. / Construction and Building Materials 21 (2007) 13561361 1359Fig. 4. SEM of mortar exposed to sulfate solution for 9 months: (a) portl

    mortar 1 mm depth, (d) RHA20 mortar 1 mm depth, (e) RHA 40 mortar exposed surface.cement mortar 1 mm depth, (b) FA20 mortar 1 mm depth, (c) FA40

    1 mm depth, and (f) sulfate rich skin of prism and gypsum lens parallel to

  • d BFA40, 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 gure.

    The morphologies of the samples suggest that the highexpansion of PC mortar prism is associated with the largeamount of ettringite observed readily in the rst 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 yash, ettringite formation is observed at depth less than1 mm for FA40 mortar bars and at deeper depths, about12 mm for FA20. In the case of RHA mixes, the ettringiteis only found in the rst 100500 lm from exposed surface.This indicates that the penetration of sulfate ions intoRHA prisms is very limited despite their signicantly highwater-to-binder ratios. These results correlate well withthose of the pH levels of the sulfate solutions.

    The other notable dierence between PC prisms andblended cement prisms was the massive precipitation ofgypsum easily observed in PC prism especially in the nearexposed surface zone. Gy...

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