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

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

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Construction and Building

MATERIALS

Construction and Building Materials 22 (2008) 1601–1606

www.elsevier.com/locate/conbuildmat

Strength, porosity and corrosion resistance of ternary blend Portland cement, rice husk ash and fly ash mortar P. Chindaprasirt, S. Rukzon

*

Department of Civil Engineering, Faculty of engineering, Khon Kaen University, Khon Kaen 40002, Thailand Received 5 May 2007; received in revised form 19 June 2007; accepted 28 June 2007 Available online 10 August 2007

Abstract This paper presents a study of the strength, porosity and corrosion resistance of mortars made with ternary blends of ordinary Portland cement (OPC), ground rice husk ash (RHA) and classified fly ash (fine fly ash, FA). Compressive strength, porosity and accelerated corrosion with impressed voltage (ACTIV) were tested. The results show that the use of ternary blend of OPC, RHA and FA produces mortars with improved strengths at the low replacement level with RHA and FA and at the later age in comparison to that of OPC mortar. The porosity of mortar containing pozzolan reduces with the low replacement level of up to 20% of pozzolan, but increases with the 40% replacement level. The chloride induced corrosion resistance of mortar as measured by ACTIV is, however, significantly improved with the use of both single pozzolan and the ternary blend OPC, RHA and FA. The corrosion resistance of ternary blend mortar is higher than that of mortar containing single pozzolan. The use of ternary blend OPC, RHA and FA is very effective in enhancing chloride induced corrosion of mortar. Published by Elsevier Ltd. Keywords: Compressive strength; Corrosion; Fly ash; Mortar; Porosity; Rice husk ash

1. Introduction A large number of researches have been directed towards the utilization of waste materials. For the construction industry, the development and use of blended cements is growing rapidly. Pozzolans from industrial and agricultural by-products such as fly ash and rice husk ash are receiving more attention now since their uses generally improve the properties of the blended cement concrete, the cost and the reduction of negative environmental effects. Rice husk is one of the major agricultural by-products and is available in many parts of the world. When rice husk is burnt at temperatures lower than 700 C, rice husk ash with cellular microstructure is produced. Rice husk ash *

Corresponding author. Tel.: +66 0 4320 2846; fax: +66 0 4320 2846x102. E-mail address: [email protected] (S. Rukzon). 0950-0618/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.conbuildmat.2007.06.010

contains high silica content in the form of non-crystalline or amorphous silica. Therefore, it is a pozzolanic material and can be used as supplementary cementitious materials [1]. Fly ash is the most common pozzolan and is being used worldwide in concrete works. It is generally realized that the use of fine fly ash improves the properties of mortar and concrete [2,3]. Although the porosity of the paste is increased as a result of the incorporation of fly ash, the average pore size is reduced. This results in a less permeable paste [4,5]. The interfacial zone of the interface between aggregate and the matrix is also improved as a result of the use of fly ash [6,7]. The enhancement of the resistance to chloride penetration is one of the benefits of incorporation of pozzolans. It is generally accepted that incorporation of a pozzolan improves the resistance to chloride penetration and reduces chloride-induced corrosion initiation period of steel reinforcement. The improvement is mainly caused by the

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reduction of permeability/diffusivity, particularly to chloride ion transportation of the blended cement concrete [8–10]. The use of the blend of pozzolan has been shown to be advantageous owing to the synergic effects [11]. In this work, ordinary Portland cement, rice husk ash and fly ash are used as base materials for studying the ternary blended cement. The knowledge in terms of strength, porosity and corrosion resistance would be beneficial to the understanding of the mechanisms as well as for future applications of these materials. 2. Experimental details 2.1. Materials Fly ash is lignite fly ash from Mae Moh power plant in the northern part of Thailand. Rice husk ash was obtained from open burning in small heaps of 20 kg rice husk with maximum burning temperature of 650 C. Ordinary Portland cement (OPC), river sand with specific gravity of 2.63 and fineness modulus of 2.82, and type-F superplasticizer (SP) were used. Classified fly ash (fine fly ash, FA) and ground rice husk ash (RHA) were used. Fine fly ash (FA) with 1–3% retained on sieve No. 325 (opening 45 lm) was obtained from air classification of as-received coarse fly ash. The ground rice husk ash (RHA) was obtained using ball mill grinding of rice husk ash until the percentage retained on sieve No. 325 (opening 45 lm) was 1–3% as well. Scanning electron microscopy (SEM) and grading analysis were used on FA and RHA.

Table 1 Mortar mix proportions Mix No.

Symbol

OPC

FA

RHA

SP (%)

1 2 3 4 5 6 7 8 9 10 11 12

OPC 10FA 10RHA 20FA 20RHA 10FA10RHA 20FA10RHA 15FA15RHA 10FA20RHA 40FA 40RHA 20FA20RHA

100 90 90 80 80 80 70 70 70 60 60 60

– 10 – 20 – 10 20 15 10 40 – 20

– – 10 – 20 10 10 15 20 – 40 20

1.9 0.6 2.0 0.4 2.2 1.1 1.1 1.2 1.3 0.1 3.7 1.6

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

2.4. Porosity tests Cylindrical specimens of 100 mm diameter and 200 mm height were prepared in accordance with ASTM C39 [13]. They were tested at the age of 7, 28 and 90 days. After being cured in water until the age of 28 days, they were cut into 50 mm thick slices with the 50 mm ends discarded. They were dried at 100 ± 5 C until constant weight achieved and were then placed in desiccators under vacuum for 3 h. The set-up was finally filled with de-aired, distilled water to measure the porosity of the mortar. The porosity was calculated using Eq. (1). p¼

ðW a  W d Þ  100 ðW a  W w Þ

ð1Þ

where 2.2. Mix proportions and curing OPC is partially replaced with pozzolans at the dosage of 0–40% by weight of cementitious materials. Single pozzolan and a blend of different weight portions of RHA and FA were also used. Sand-to-binder ratio of 2.75 by weight, water to binder ratio (W/B) of 0.5 and SP content adjusted to maintain the mortar mixes with similar flow of 110 ± 5% were used. The cast specimens were covered with polyurethane sheet and damped cloth in a 23 ± 2 C chamber and were demoulded at the age of 1 day. For strength and porosity tests, the specimens were moist cured at 23 ± 2 C until the test ages. For the corrosion test, the samples were cured in distilled water to prevent chloride contamination. The mortar mix proportions and abbreviations are given in Table 1. 2.3. Compressive strength The cube specimen of size 50 · 50 · 50 mm was used for the compressive strength test of mortar. They were tested at the age of 7, 28 and 90 days. The test was done in accordance with the ASTM C109 [12]. The reported results are the average of four samples.

p is vacuum saturated porosity (%), Wa is specimen weight in air of saturated sample (gm), Wd is specimen dry weight after 24 h in oven at 100 ± 5 C (gm) and Ww is specimen weight in water (gm). This method has been used to measure the porosity of the cement-based materials successfully [14–17]. The reported results are the average of two samples. 2.5. Accelerated corrosion test with impressed voltage Mortar prisms of dimensions 40 · 40 mm and 160 mm in length with embedded steel of 10 mm diameter and 160 mm in length were used. The steel was protected such that it protruded from the top surface of the prism by 15 mm; thus, provided sufficient mortar cover of 15 mm and 15 mm thick mortar at the end of steel bar at the bottom of the prisms as shown in Fig. 1. The mortar was cast in two layers and compacted using vibrating table. At the age of 28 days, the prisms were subjected to the accelerated corrosion test with impressed voltage (ACTIV) using a 5% NaCl solution and a constant voltage of 12 V dc

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Table 2 Physical properties FA, RHA and OPC Sample

Median particle size (lm)

Retained on a sieve No. 325 (%)

Specific gravity

Blaine fineness (cm2/gm)

OPC FA RHA

15.0 4.9 10.0

NA 1–3 1–3

3.14 2.45 2.23

3600 5700 11,200

100

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

90

as shown in Fig. 2. The condition of prism was continuously monitored and the time of initiation of first crack was recorded. This is used as a measurement of the specimen’s relative resistance against chloride attack and reinforcement corrosion. 3. Results and discussions 3.1. Characteristics of OPC, FA and RHA The fineness characteristics of Portland cement and pozzolanic materials are given in Table 2. The Blaine fineness of OPC is 3600 cm2/gm and those of the FA and RHA are 5700 and 11,200 cm2/gm. The specific gravity of the OPC, FA and RHA are 3.14, 2.45 and 2.23, respectively. The particle size distributions shown in Fig. 3 suggest that FA is finest, followed by RHA and OPC. The mean particle sizes of FA, RHA and OPC are 4.9, 10.0 and 15.0 lm, respectively. The chemical constituents are given in Table 3. Fly ash is a Class F fly ash with 74% of SiO2 + Al2O3 + Fe2O3, 2.2% of SO3 and 2.5% of LOI meeting the requirement of ASTM C618 [18]. The CaO content of this fly ash is rather high at 14.4% as it is from lignite. RHA, on the other hand, consists mainly of SiO2and the other components are not significant. The SiO2 content of 93% satisfies ASTM

Cumulative Passing (%)

80 70 60 50 40

FA

30

RHA

20

OPC

10 0 0.01

0.10

1.00 10.00 Particle Size (micron)

100.00

1000.00

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

Table 3 Chemical composition of OPC, RHA and FA Oxides

OPC

RHA

FA

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 LOI

20.9 4.8 3.4 65.4 1.3 0.2 0.4 2.7 0.9

93.2 0.4 0.1 1.1 0.1 0.1 1.3 0.9 3.7

41.1 21.6 11.3 14.4 3.3 1.1 2.6 2.2 2.5

SiO2 + Al2O3 + Fe2O3



93.7

74.0

C618 [18] requirement as a natural pozzolan and 3.7% LOI indicates complete burning. The as-received fly ash consists of a large range of particle sizes as indicated by SEM micrograph as shown in Fig. 4. The particles are mostly spherical in shape. The fine portion surfaces are relatively smooth and those of the large particles are usually rough. The SEM photo reveals that the rice husk ash still maintains its cellular structure. After grounded, RHA consists of very irregular-shaped particles with a porous cellular surface. 3.2. Compressive strength

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

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

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

Table 4 Compressive strength of blended cements mortars Mix No.

1 2 3 4 5 6 7 8 9 10 11 12

Symbol

OPC 10FA 10RHA 20FA 20RHA 10FA10RHA 20FA10RHA 15FA15RHA 10FA20RHA 40FA 40RHA 20FA20RHA

Compressive strength (MPa–normalized) 7 days

28 days

90 days

43.5–100 45.0–103 44.2–102 44.5–102 44.5–102 42.0–97 42.4–97 43.1–99 42.5–98 33.0–76 33.5–77 41.0–95

57.0–100 59.2–104 58.2–102 59.5–105 58.5–103 58.0–102 58.4–102 58.5–103 58.7–103 56.5–99 55.0–97 55.5–102

60.0–100 62.7–105 62.0–103 63.5–106 62.5–104 64.0–107 63.4–106 63.0–105 62.8–105 62.0–103 62.0–103 61.5–106

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

produces the filler effect due to its fine particle size. The dispersing effect has not been reported for the RHA. However, its reactivity is high due to its high surface area. The increase in the hydration could, therefore offset the strength reduction as a result of reduced OPC. The increase in the amount of replacement to 40% reduces the early strength of both FA and RHA mortars. However, the strength at the ages of 28 and 90 days of both FA and RHA mortars are slightly higher than that of the control. This indicates that both FA and RHA are pozzolanic materials and the early pozzolanic reaction rate is thus slow. The pozzolanic reaction of both cases, however, can be seen at the age of 28 days onwards resulting in the higher strength of both FA and RHA mortar in comparison to that of the control. The results also suggest that both FA and RHA in this experiment were quite reactive and the pozzolanic reaction starts quite early. For the blend of pozzolans, the strengths of mortar are also comparable to that of OPC mortars at the same age. The strengths at the age of 7 days blended pozzolan mortars range between 95–99% of that of OPC. At the age of 28 and 90 days, the normalized strength ranges between 102–103% and 105–107%, respectively. The results indicate that for the high replacement level of 40%, the use of blend of RHA and FA improves the early strength development of mortar in comparison to normal single pozzolan mortar. The incorporation of blend of fine pozzolans improves the strength of concrete due to synergic effect [11].

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3.3. Porosity results

24

The results of the porosity of mortars at 7, 28 and 90 days are shown in Table 5. At the age of 7 days, the porosities of mortar containing 10% and 20% of pozzolans and blend of pozzolans are lower than that of the control at all ages. The addition of fine particles of FA and RHA causes segmentation of large pores and increases nucleation sites for precipitation of hydration products in cement paste [19]. This results in pore refinement and a reduction of calcium hydroxide in paste. The mortar containing FA gives slightly less porosity than that of RHA. In other word, FA is slightly more effective in modifying pore and reducing the porosity of mortar. The porosity of 20FA mortar is 17.3% in comparison with 17.8% of both OPC and 20RHA mortars. At high replacement level of 40%, the porosities of the mortars containing pozzolans increase in comparison with that of the control. At the age of 7 days, the porosities of 40FA, 40RHA and 20FA20RHA mixes are 21.0%, 21.8% and 19.4% which are significantly larger than 17.8% of the OPC mortar. The increases in porosity with a relative large amount of pozzolans are resulted from the reduced amount of OPC. This results in less hydration products especially at the early age where the pozzolanic reaction is small. It should be pointed out here that although the porosity is increased, the beneficial effect of pore refinement as a result of the incorporation of pozzolan exists. The porosities of the mortars reduce with an increase in age as expected. This is due to the increase in the hydration of cementitious materials. At the later age of 90 days, the porosities of the mortars containing pozzolans reduce to similar values to that of OPC mortar. The porosities of 40FA, 40RHA and 20FA20RHA mixes at 90 days are 12.7%, 12.8% and 13.4% as compared to 12.8% of OPC mortar at the same age. The relationship between the porosity and strength of mortar follows the conventional pattern as shown in Fig. 5.

22

1605

-0.87

40

45

50

55

60

65

Porosity (%)

Compressive strength (MPa) Fig. 5. Relationship between porosity and compressive strength.

3.4. Results of ACTIV

20FA20RHA

20FA10RHA

200 180 160 140 120 100 80 60 40 20 0

15FA15RHA

The results of the time of first crack of mortar subjected to ACTIV are shown in Fig. 6. The time of first crack of OPC mortar is lowest at 89 h. The time to initial crack of mortars is found to increase with the incorporation of pozzolans. The increase in the time of first crack with the incorporation of pozzolan has been reported by other researches [20,21]. The increase in the corrosion resistance of mortar incorporating RHA and FA measured with rapid coulomb passed (RCPT), immersion in sodium chloride solution and RMT has also been reported [22]. For the single pozzolan, RHA is found to be more effective in increasing the time of first crack as compared to FA. The time of first crack of 10RHA, 20RHA and 40RHA mortars are almost the same at 167, 168 and 166 h, respectively, whereas those of 10FA, 20FA and 40FA mortars are 160, 148 and 136 h, respectively. It is interesting to note that the time of first crack of fly ash mortars reduces with an increase in the fly ash replacement levels. This is the result of the slow pozzolanic reaction of fly ash.

10FA20RHA

12.8 12.5 12.6 12.0 12.6 12.5 12.8 13.0 13.1 12.7 12.8 13.4

35

10FA10RHA

13.7 12.9 13.3 12.9 13.1 13.0 13.4 13.8 13.5 14.5 15.0 14.9

30

40RHA

17.8 16.7 17.0 17.3 17.8 16.9 17.9 17.7 17.9 21.0 21.8 19.4

10

20RHA

90 days

12

10RHA

28 days

14

40FA

7 days

16

20FA

OPC 10FA 10RHA 20FA 20RHA 10FA10RHA 20FA10RHA 15FA15RHA 10FA20RHA 40FA 40RHA 20FA20RHA

Porosity (%)

28 days 90 days

10FA

1 2 3 4 5 6 7 8 9 10 11 12

Symbol

R = 0.97

18

OPC

Mix No.

7 days

2

20

Time of initiation of crack (h)

Table 5 Porosity of blended cements mortars

y = 457.63x

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

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For the blends of pozzolans, the results show that their incorporation to the mixes improves the resistance to corrosion of mortar. The time to cracking of 10FA10RHA, 20FA20RHA, 10FA20RHA, 15FA15RHA, 20FA10RHA mortars are very high at 169–173 h. These values are slightly but consistently larger than those of RHA mortars indicating that the use of blend of FA and RHA is very effective in increasing the resistance to corrosion of mortar. The incorporation of pozzolan such as fly ash reduces the average pore size and results in a less permeable paste [4,5]. It has also been shown that reactive RHA also reduces the porosity of paste [23]. Test also shows that the permeabilities of rice husk–bark ash and fly ash are lower than that of OPC concrete [24]. The improvement of chloride induced corrosion resistance of the ternary blend OPC, FA and RHA mortar is thus the result of reduced permeability and reduced calcium hydroxide. 4. Conclusions The use of ternary blend of OPC, RHA and FA significantly improves the mortar in terms of strength at the low replacement level and at the later age. The resistance to chloride-induced corrosion of mortar containing pozzolan as measured by ACTIV is significantly improved in comparison to that of OPC mortar. Both FA and RHA are very effective in improving the corrosion resistance of mortars. RHA is slightly more effective than FA. The corrosion resistance of the ternary blend mortar is consistently higher than that of mortar containing single pozzolan. At high replacement of 40% of pozzolan, although the porosity of mortar is increased at the age of 28 days as compared to OPC mortar, the corrosion resistance is significantly improved. This suggests that pore refinement and reduction in calcium hydroxide play important roles in the corrosion resistance of ternary blend OPC, FA and RHA mortar. Acknowledgement The authors would like to acknowledge the financial supports of Rajamangala University of Technology Phra Nakhon, School of Graduate Studies, Sustainable Infrastructure Research and Development Center and Faculty of Engineering Khon Kaen University. References [1] Metha PK. The chemistry and technology of cement made from rice husk ash. UNIDO/ESCAP/RCTT. In: Proceeding of work shop on rice husk ash cement, Peshawar, Pakistan. Bangalore, India: Regional Center for Technology Transfer; 1979. p. 113–22. [2] Erdogdu K, Tucker P. Effects of fly ash particle size on strength of Portland cement fly ash mortars. Cem Concr Res 1998;28:1217–22.

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