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

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

Construction and Building MATERIALS Construction and Building Materials 21 (2007) 1356–1361 www.elsevier.com/locate/conbuildmat Sulfate resistance...

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Construction and Building

MATERIALS

Construction and Building Materials 21 (2007) 1356–1361

www.elsevier.com/locate/conbuildmat

Sulfate resistance of blended cements containing fly ash and rice husk ash P. Chindaprasirt a

a,*

, P. Kanchanda a, A. Sathonsaowaphak a, H.T. Cao

b

Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand b Senior Materials Scientist, Connell Wagner, Sydney, Australia Received 5 April 2005; received in revised form 5 October 2005; accepted 17 October 2005 Available online 14 September 2006

Abstract In this paper, the sulfate resistance of mortars made from ordinary Portland cement containing available pozzolans viz., fly ash and ground rice husk ash (RHA) was studied. Class F lignite fly ash and RHA were used at replacement dosages of 20 and 40% by weight of cement. Expansion of mortar prisms immersed in 5% sodium sulfate solution and the change in the pH values of the solution were monitored. The incorporation of fly ash and RHA reduced the expansion of the mortar bars and the pH values of the solutions. RHA was found to be more effective than fly ash. Examination of the fractured surface of mortar prisms, after a period of immersion, by scanning electron microscopy confirmed that sulfate attack of blended cement mortars was restricted owing to the reductions in calcium hydroxide and C/S ratio of the C–S–H gel in the blended cement mortar. In comparison to Portland cement mortar, less calcium sulfate and much less ettringite formations were found in the mortars made from blended cement containing RHA. The amounts of calcium sulfate and ettringite found in the blended cement mortar containing fly ash were also small but were slightly more than those of RHA mortar. Up to 40% of Portland cement could be replaced with these pozzolans in making blended cement with good sulfate resistance. Ó 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 intensive process and releases a very large amount of green house gas to the atmosphere. It has been reported that 13,500 million ton is produced from this process, which accounts for about 7% of the green house gas produced annually [1]. Efforts have, therefore, been made to promote the use of pozzolans such as fly ash, calcined kaolin, rice husk ash and palm oil fuel ash [2–5] to replace part of Portland cement. This reduces the total amount of the Portland cement used. Fly ash is the most common pozzolan and is being used worldwide. In Asia and many parts of the world, a large amount of rice husk could be obtained as * Corresponding author. Tel.: +66 4320 2846 7x131; fax: +66 4320 2846 7x102. E-mail addresses: [email protected], [email protected] (P. Chindaprasirt).

0950-0618/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2005.10.005

an agricultural by-product. The properly burnt and ground rice husk ash is also suitable for use as a pozzolan [6]. The annual output of lignite fly ash from Mae Moh power station in the North of Thailand is around 3 million tons. The quality of this lignite fly ash has improved drastically over the last 10 years owing to the use of better quality lignite and improved combustion. This fly ash is now classified as class F and is being used quite extensively for construction in Thailand. Up to now the potential use of this fly ash has admittedly not been fully achieved, as almost all the fly ash concretes used is not durability-based. In general, replacement of cement by fly ash reduces the initial strength of concrete, whereas the strength at later age as well as the durability viz. sulfate resistance and acid resistance are improved [4,7]. The incorporation of fly ash increases the porosity of the cement paste but the average pore size is reduced. This results in a less permeable paste which is less susceptible to the ingress of the harmful solution [8,9].

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Rice husk consists of about 40% cellulose, 30% lignin group and 20% silica and hence the ash contains a large amount of silica [3,6]. The silica exists in two forms: amorphous or crystalline silica depending on the temperature and duration of burning. Silica of amorphous form is reactive and suitable for use as a pozzolan to replace part of Portland cement. Amorphous silica is obtained by burning rice husk at temperature lower than 700 °C [10]. With proper burning and grinding, the amorphous reactive rice husk ash could be produced and used as a pozzolan [6]. Even for higher burning temperature with some crystalline formation of silica, good result could be obtained by fine grinding [11]. The reactive RHA can be used to produce good quality concrete with reduced porosity and reduced Ca(OH)2 [12]. Rice husk ash has been used successfully in such applications as concrete with controlled permeability formwork and roller compacted concrete [13,14]. Sulfate attack is one of the most important problems concerning the durability of concrete structures. Under the sulfate environment, cement paste undergoes deterioration resulting from expansion, spalling and softening [15,16]. It is generally recognized that addition of pozzolan reduces the calcium hydroxide in cement paste and improves the permeability of concrete. This helps to increase the resistance of concrete to the attack of sulfate and other harmful solutions [2]. The increase in the service life of the structure made from the blended cement containing pozzolan would further reduce the amount of Portland cement use. The knowledge of the use of lignite fly ash and RHA to increase the resistance of concrete to the harmful solutions, especially sulfate solution, would be beneficial to the understanding of the mechanism and to the applications 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 Thailand and ground rice husk ash (RHA) were used. The rice husk

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ash was obtained from open burning of 20 kg heap of rice husk with a maximum burning temperature of 600 °C. The burnt rice husk ash was whitish gray in color. The rice husk ash was then ground in a laboratory rod mill to reasonable fine particles. Local river sand with S.G. of 2.65 was used for making a mortar. Chemical compositions and Blaine fineness of PC, FA and RHA used in this work are given in Table 1. Morphological features of as-received fly ash and RHA are shown in Fig. 1. The combined amount of SiO2, Al2O3, and Fe2O3 in FA was 78.1% indicating that Mae Moh fly ash is a class F fly ash. The Blaine fineness of the FA was 2600 cm2/g, which was coarser than PC (2900 cm2/g). RHA contained high silica content of 90% and low loss on ignition (LOI) of 3.2%. This suggests that RHA was burnt relatively complete. The Blaine fineness of RHA was 14,000 cm2/g using rod mill grinding. All mortars were made with sand to binder ratio of 2.75 and adjusted water contents to achieve similar flow of 110 ± 5%. The compressive strengths at 7, 28, 90 and 180 days were obtained using 50 mm cubes for normal watercured in accordance with the ASTM C109. The test for sulfate-induced expansion was done following the procedures described in ASTM C1012 with 5% sodium sulfate solution. It is required that the mortar acquires the strength of 20 MPa before the immersion in the sulfate solution. For all mixes except the mixes with high fly ash and rice Table 1 Chemical compositions of Portland cement, fly ash and rice husk ash Oxides

PC

FA

RHA

CaO SiO2 Al2O3 Fe2O3 MgO SO3 Na2O K2O LOI Blaine fineness (cm2/g)

63.4 22.1 3.7 2.9 2.5 2.5 0.1 0.5 1.1 2900

13.0 44.4 23.5 10.2 3.0 1.1 0.1 2.0 1.8 2600

0.8 90.0 0.5 0.9 0.6 0.1 0.1 2.1 3.2 14,000

Fig. 1. Morphological features of as received FA and ground RHA: (a) as-received fly ash, and (b) ground RHA.

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husk ash replacement, the age of the immersion was 1 day. The age of immersion of the FA40 and RHA40 mortars were 2 and 10 days, respectively, because the strength development of these mortars were relatively slow. The solutions were changed weekly for the first month, monthly until six months and three monthly thereafter. The pH values of the sulfate solutions of all immersions were monitored. Scanning electron microscopy was performed for the mortars after immersion in the sulfate solution for 9 months. 3. Results and discussion 3.1. Water-to-binder ratio and compressive strength Table 2 shows water-to-binder (W/B) ratios and compressive strengths of the mortar mixes containing FA and RHA at different replacement dosages. The addition of the lignite fly ash resulted in a reduction in the water requirement of the mortar for similar flow. W/B ratios were 0.55, 0.53 and 0.51 for mortars containing no fly ash (PC), 20% FA (FA20) and 40% FA (FA40), respectively. On the contrary, the incorporation of RHA resulted in marked increases in water demand as W/B ratios increased from 0.55 to 0.68 and 0.80 for PC, 20% RHA (RHA20) and 40% RHA (RHA40) mortars, respectively, owing to the high surface area of RHA. As shown in Table 2, the introduction of fly ash and RHA resulted in a reduction of 7-day compressive strength. At 28 days, the use of fly ash and high level of replacement of RHA also resulted in reduction in the strength as compared to that of PC mortar. The reduction of early strength is typical of the fly ash mixes. For RHA mixes, the low initial strength was due to the high waterto-binder of the mixes. For 20% RHA replacement level, although the W/B ratio was increased, the strength at 28 days was higher than that of PC mix, suggesting that RHA is quite reactive. In general, these results indicated that the lignite fly ash and RHA are pozzolanic materials with different characteristics. Although possessing high fineness, RHA contribution to the strength development was limited by the high water demand associated with its high surface areas. Lignite fly ash showed typical strength development pattern of Class F fly ash, i.e. slow in the first 28 days and continues well after 90 days.

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 evident that the expansion of the PC prism is much larger than those made with the blended cements. The accelerating expansion pattern of the PC mortar is observed from 120 days onward. There is no obvious accelerating expansion pattern shown by blended cement mortar prisms. FA20, FA40 and RHA20 mixes show a ‘‘linear’’ pattern of expansion after about 120 days in sulfate solution. RHA40 mix, however, shows a very small expansion even after immersion for 360 days. Fig. 3 shows the pH levels of the sodium sulfate solution. The highest level of the pH of sulfate solution was approximately 12.5 for all the mortars and was observed within the first 7 days of immersion, indicating that a substantial amount of the calcium hydroxide was leached out and thus increased the pH of the solution. The fresh solution pH is 7.0–7.5. After one day of immersion, the pH level is found to be more than 12. It has been reported that the pH of 12–12.5 was obtained within a few hours of immersion [17]. The pH of the solution is increased slightly as the immersion period continued until the solution is replaced by a fresh solution. After immersion of the mortar bars, the pH of the fresh solution again increases rapidly to a high value but less than the previous highest value owing to the less amount of hydroxide ion. At 90 and 180 days, the pH levels of the sulfate solutions are significantly lower Expansion (microstrain)

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2,500 PC

2,000 1,500 1,000

FA20 RHA20 FA40

500

RHA40

0 0

60

120

180

240

300

360

Immersion (days) Fig. 2. Expansion of mortar bars in 5% sulfate solution.

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

OPC FA20 FA40 RHA20 RHA40

Water-to-binder ratio

Compressive strength, (MPa) 7 day

28 day

90 day

180 day

0.55 0.53 0.51 0.68 0.80

44 32 29 31 17

51 45 46 54 32

57 57 62 61 43

60 57 77 62 53

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

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and different. The pH value of the solution with PC mortar is the highest of 11.0 followed by those of FA20, FA40, RHA20 and RHA40 with 10.7, 10.5, 10.1 and 9.5, respectively. The expansion of the mortar bar is sensitive to the pH level of the solution [18]. At pH 12–12.5 only ettringite formation can take place and at pH of 8.0–11.5 gypsum formation and decalcification occur [17]. It has also been suggested that the dissolution of Ca(OH)2 and calcium sulfoaluminates, and the decalcification of CSH with a high C/S ratio in hardened PC paste resulted in a very porous layer whereas the decalcification of the low C/S CSH resulted in a protective layer of silica

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gel [4]. For FA and RHA blended cement, C/S ratio of CSH would have been lower as a result of the pozzolanic reaction. FA and RHA mortars thus show better resistance to the sulfate attack in comparison to PC mortar with RHA, being more reactive and showing better resistance to sulfate attack. 3.3. SEM examination of the mortar prisms After immersion in sulfate solution for 9 months, the mortar prisms were examined using SEM. The results are shown in Fig. 4. These are microstructures of PC, FA20,

Fig. 4. SEM of mortar exposed to sulfate solution for 9 months: (a) portland cement mortar 1 mm depth, (b) FA20 mortar  1 mm depth, (c) FA40 mortar  1 mm depth, (d) RHA20 mortar  1 mm depth, (e) RHA 40 mortar  1 mm depth, and (f) sulfate rich skin of prism and gypsum lens parallel to exposed surface.

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FA40, RHA20 and RHA40 at a depth of about 1 mm from the surface exposed to sulfate solution. A common feature at the exposed surface of sulfate-rich skin and a lens of sulfate forming parallel to the surface are shown in this figure. The morphologies of the samples suggest that the high expansion of PC mortar prism is associated with the large amount of ettringite observed readily in the first 5 mm from the exposed surface. The ettringite formed in the PC 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 smaller diameter (i.e. no apparent change of aspect ratio) in comparison to those found in PC prism. For the case of fly ash, ettringite formation is observed at depth less than 1 mm for FA40 mortar bars and at deeper depths, about 1–2 mm for FA20. In the case of RHA mixes, the ettringite is only found in the first 100–500 lm from exposed surface. This indicates that the penetration of sulfate ions into RHA prisms is very limited despite their significantly high water-to-binder ratios. These results correlate well with those of the pH levels of the sulfate solutions. The other notable difference between PC prisms and blended cement prisms was the massive precipitation of gypsum easily observed in PC prism especially in the near exposed surface zone. Gypsum was also detected in fly ash blended cement prisms but in much less quantities. The precipitation of gypsum in RHA prism was either sporadic or insignificant. The SEM observations obtained in this work are not different to those discussed in the literature [19–21]. The results, however, suggest there would be a close link between ettringite formation and expansion, particularly for the PC. The ettringite observed is not the sub-micron type intimately mixed in C–S–H. The lack of this type of ettringite is associated with little or no expansion, e.g. in RHA prisms. It is likely that the accelerated expansion pattern shown by PC prism was a result of this type of large ettringite formation. Their morphology suggests further that the formation of large ettringite would be the later stage of sulfate attack by sodium sulfate solution. 4. Conclusions Based on the obtained data, it can be concluded that the incorporation of lignite class F fly ash and ground rice husk ash into normal Portland cement result in a significant improvement in the resistance to attack by 5% sodium sulfate solution. Better dimension stability is obtained with blended cements containing FA and RHA. Despite having higher water demand characteristics, RHA at a dosage of up to 40% cement replacement is very effective in providing sulfate resistance. Class F lignite fly ash is only slightly less effective at both 20% and 40% replacement levels in comparison to that with RHA. The pH levels of the sulfate solutions after immersion for 6 months are significantly different, indicating the dif-

ferent levels of calcium hydroxide. Fly ash and rice husk ash mortar are of lower pH levels and thus less susceptible to sulfate attack. SEM examination confirms that ettringite is not a pronounced feature in the microstructure of the blended cement mortars exposed to sodium sulfate solution, in comparison to that of Portland cement mortar. Up to 40% of Portland cement could be replaced with fly ash and RHA in making blended cement mortar with reasonable strength development and good sulfate resistance. This would reduce the amount of Portland cement use and the greenhouse gas. Service life of the mortar would also be increased owing to the higher sulfate resistance. References [1] Malhotra VM. Introduction: sustainable development and concrete technology. ACI Concrete Int 2002;24(7):22. [2] Malhotra VM, editor. Supplementary cementing materials for concrete, CANMET. Canadian Government Publishing Centre; 1987. [3] Stroeven P, Bui DD, Sabuni E. Ash of vegetable waste used for economics production of low to high strength hydraulic binders. Fuel 1999;78:153–9. [4] Shi C, Stegemann JA. Acid corrosion resistance of different cementing materials. Cement Concrete Res 2000;30:803–8. [5] Tay JH. Ash from oil-palm waste as concrete material. J Mater Cil Eng 1990;2:96–105. [6] Metha PK, editor. The chemistry and technology of cement made from rice husk ash. UNIDO/ESCAP/RCTT proceedings of workshop on rice husk ash cement, Peshawar, Pakistan. Bangalore, India: Regional Center for Technology Transfer; 1979. p. 113–22. [7] Chindaprasirt P, Homwuttiwong S, Sirivivatnanon V. Influence of fly ash fineness on strength, drying shrinkage and sulfate resistance of blended cement mortar. Cement Concrete Res 2004;34:1087–92. [8] Poon CS, Lam L, Wong YL. Effect of fly ash and silica fume on interfacial porosity of concrete. ACI Mater J 1999:197–205. [9] Chindaprasirt P, Jaturapitakkul C, Sinsiri T. Effect of fly ash fineness on compressive strength and pore size of blended cement paste. Cement Concrete Comp 2005;27:425–8. [10] Della VP, Kuhn L, Hotza D. Rice husk ash as an alternative source for active silica production. Mater Lett 2002;57:818–21. [11] Ismail MS, Waliuddin AM. Effect of rice husk ash on high strength concrete. Constr Build Mater 1996;10:521–6. [12] Zhang MH, Malhotra VM. High performance concrete incorporating rice husk ash as a supplementary cementing materials. ACI Mater J 1996;93(6):629–36. [13] Coutinho JS. The combined benefits of CPF and RHA in improving the durability of concrete structures. Cement Concrete Comp 2003;25:51–9. [14] Kajorncheappunngam S, Stewart DF. Rice husk ash in roller concrete. Concrete Int 1992;14:38–44. [15] Lauer KR. Classification of concrete damage caused by chemical attack. Mater Struct 1990;23:223–9. [16] Collepardi M. A state-of-the-art review on delayed ettringite attack on concrete. Cement Concrete Comp 2003;25:401–7. [17] Khatri RP, Sirivivatnanon V, Yang JL. Role of permeability in sulphate attack. Cement Concrete Res 1997;27:1179–89. [18] Cao HT, Bucea L, Ray A, Yozghatlian S. The effect of cement composition and pH of environment on sulfate resistance of Portland cements and blended cements. Cement Concrete Comp 1997;19: 161–171.

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mortar exposed to sulfate attack. In: Diamond S et al., editors. Microstructure of cement-based systems/bonding and interfaces in cementitious materials, vol. 370. Pittsburgh: Materials Research Society; 1995. p. 77–82. [21] Shen Y, Xu Z, Tang M. The process of sulfate attack on cement mortars. Adv Cem Based Mater 1996;4:1–5.