Rice husk ash

Rice husk ash

Rice husk ash Bhupinder Singh Indian Institute of Technology Roorkee, Roorkee, India 13.1 13 Introduction Rice husks are the hard protective cover...

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Rice husk ash Bhupinder Singh Indian Institute of Technology Roorkee, Roorkee, India

13.1

13

Introduction

Rice husks are the hard protective coverings of rice grains which are separated from the grains during milling process. Rice husk is an abundantly available waste material in all rice producing countries, and it contains about 30%50% of organic carbon. In the course of a typical milling process, the husks are removed from the raw grain to reveal whole brown rice which upon further milling to remove the bran layer will yield white rice. Current rice production in the world is estimated to be 700 million tons. Rice husk constitutes about 20% of the weight of rice and its composition is as follows: cellulose (50%), lignin (25%30%), silica (15%20%), and moisture (10%15%). Bulk density of rice husk is low and lies in the range 90150 kg/m3. Sources of rice husk ash (RHA) will be in the rice growing regions of the world, as for example China, India, and the far-East countries. RHA is the product of incineration of rice husk. Most of the evaporable components of rice husk are slowly lost during burning and the primary residues are the silicates. The characteristics of the ash are dependent on (1) composition of the rice husks, (2) burning temperature, and (3) burning time. Every 100 kg of husks burnt in a boiler for example will yield about 25 kg of RHA. In certain areas, rice husk is used as a fuel for parboiling paddy in rice mills, whereas in some places it is field-burnt as a local fuel. However, the combustion of rice husks in such cases is far from complete and the partial burning also contributes to air pollution. The calorific value of rice husks is about 50% of that of coal, and assuming that husks have about 8%10% of moisture content and zero bran, the calorific value is estimated to be 15 MJ/kg. Under controlled burning conditions, the volatile organic matter in the rice husk consisting of cellulose and lignin are removed and the residual ash is predominantly amorphous silica with a (microporous) cellular structure (Fig. 13.1). Due to its highly microporous structure, specific surface area of RHA as determined by the BrunauerEmmettTeller (BET) nitrogen adsorption method can range from 20 to as high as 270 m2/g, while that of silica fume, for example is in the range of 1823 m2/g. The chemical composition of RHA is significantly dependent on combustion conditions, and the burning temperature must be controlled to keep silica in an amorphous state. The ash obtained from uncontrolled combustion (as in open-field burning or in industrial furnaces at temperatures greater than 700 C800 C) will contain significant amounts of cristobalite and tridymite which are nonreactive silica minerals. In order to develop pozzolanic activity, such ashes will be required to be ground to a very fine particle size which is likely to make their use financially Waste and Supplementary Cementitious Materials in Concrete. DOI: https://doi.org/10.1016/B978-0-08-102156-9.00013-4 Copyright © 2018 Elsevier Ltd. All rights reserved.

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Figure 13.1 Scanning electron micrograph of RHA. (A) 500 3 magnification and (B) 1000 3 magnification (Huang et al., 2017).

unviable. Under controlled combustion (burning temperatures in the range of 500 C700 C for a period of about 1 hour), amorphous silica is the major constituent of ash whose reactivity is attributed to the presence of this form of silica and to its very large surface area resulting from the microporous structure of ash particles. Although reactivity of a pozzolanic material improves upon increasing its fineness, Mehta and Monteiro (1997) reckon that grinding RHA to a high degree of fineness is not advisable since this material derives its pozzolanic activity from the internal surface area of its microporous particles which is already very high. When obtained from controlled combustion, the specific surface (as measured using nitrogen adsorption) of RHA can be as high as 50,000 m2/kg even though the particle size may be in the range of 1075 μm, which is large when compared to that of silica fume for example. The average composition of well-burnt RHA is 90% amorphous silica, 5% carbon, and 2% K2O. The applications of RHA include its use as a pozzolan in the construction industry, as a filler, additive, abrasive agent, oil adsorbent, sweeping component, and as a suspension agent for porcelain enamels. In the construction industry, RHA can be used as a partial replacement for cement. According to Chandrasekhar et al. (2006), each application requires specific properties such as reactivity for cement and concrete, chemical purity for synthesizing advanced materials, whiteness, and proper particle size for filler applications and high surface area and porosity for use as an adsorbent and catalyst. If used as a supplementary cementitious material in concrete, for example, RHA particles may have a high water demand due to their porous microstructure. This can be controlled by intergrinding the RHA particles with clinker during the process of cement manufacture so as to breakdown the porous structure and thereby reduce water demand. If intergrinding is not possible, then RHA may be used by blending it with cement at site. RHA in the blended cement will fix free lime released by clinker silicates during their hydration. The amorphous silica in the RHA can react with Ca(OH)2 in the secondary hydration reaction to form a kind of C-S-H gel, which has a floc-like morphology with a porous structure and large specific surface. The formation of the additional C-S-H contributes to both strength development and enhanced durability of concrete since in the secondary

Rice husk ash

419

Ca++ Cement

0–3 hours

Ca++

SI Cement

Ca++

Ca++ RHA

Ca++

Ca++

Ca++

SI

Ca++

C-S-H

C-S-H

CH

3–8 hours CH

RHA

CH CH

C-S-H

8–40 hours

CH

CH RHA

C-S-H CH

CH

After 40 hours C-S-H

Figure 13.2 Schematic illustration of the hydration mechanism of cement paste containing RHA (Hwang and Chandra 1997).

hydration reaction the free lime is converted into C-S-H gel which is insoluble in water. Fig. 13.2 presents a schematic illustration of the hydration mechanism proposed by Hwang and Chandra (1997) of cement paste containing RHA. RHA in blended cement is known to contribute to concrete strength from as early as 13 days of maturing. In addition to its contribution to strength, even at relatively small replacement dosages of 10% by weight of cement, RHA can produce a strong transition zone and very low permeability in hardened concrete in addition to significant reduction of bleeding in fresh concrete. Since Portland cement (PC) is typically the most expensive constituent of concrete, replacement of a part of it with RHA offers improved concrete affordability, particularly for developing countries.

13.2

Effect of combustion conditions

Burning of rice husks can be carried out by using any of the following methods: (1) Open-field burning: This process produces poor quality ash and is polluting in

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Energy requirement for production of different pozzolanic materials (Ramachandran 1995)

Table 13.1

Materials

Total energy required in kJ/kg of material

Cement Lime Hydrated lime Burnt clay pozzolan RHA Surkhi (burnt brick) Fly ash

372 173 142 68 12 12 0

nature; (2) Fluidized-bed furnace burning: In this process, combustion of rice husk takes place in a controlled environment; and (3) Industrial furnace: Combustion conditions can be controlled in modern industrial furnaces to produce high quality ash with silica in the amorphous and highly cellular form. Table 13.1 gives the energy requirements for the production of different pozzolanic materials. It has been pointed out earlier that the form of silica (crystalline or amorphous) in RHA will depend upon temperature and combustion duration of rice husks. Completely burnt rice husk is grey to white in color, whereas partially burnt rice husk is blackish. The two phases in the decomposition of rice husk under heat treatment are carbonization and decarbonation. Mehta (1979) reports that rice husk with amorphous silica can be produced by maintaining the combustion temperature below 500 C under oxidizing conditions for prolonged periods or up to 680 C with a hold time less than 1 minute. According to Yeoh et al. (1979), for combustion times less than 1 hour, RHA will remain in the amorphous form for temperatures of up to 900 C, while crystalline silica is produced if the temperature is held at 1000 C for more than 5 minutes. Chopra et al. (1981) observed that at burning temperatures of up to 700 C, the silica in the ash remains in the amorphous form. Hwang and Wu (1989) noted that during burning, at 400 C, the polysaccharides in the husk begin to depolymerize. Above 400 C, dehydration of sugar units occurs, and at 700 C, the sugar units decompose. At temperatures above 700 C, unsaturated products react together and form a highly reactive carbonic residue. On the basis of X-ray diffraction data and chemical analysis of their RHA samples produced under different burning conditions, Hwang and Wu (1989) state that the higher the combustion temperature, the greater is the percentage of silica in the ash. Della et al. (2002) found that RHA with a 95% silica content could be produced after combusting partially burnt RHA at 700 C for 6 hours. The carbon content in the RHA which was 18.6% before burning was reduced to 0.14% after burning. The average particle size in the fully burnt RHA varied in the range 310 μm. The specific surface area of the partially burnt RHA reduced from 177 to 54 m2/g after completion of burning with this figure increasing to 81 m2/g following wet-grinding after completion of burning. Nair et al. (2008) state that husk samples burnt for more than 12 hours at temperatures in the range of 500 C700 C produce high reactivity ash, whereas shorter

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421

duration burning in the range of 15360 minutes resulted in RHAs with high carbon content. Effect of temperature on chemical composition of RHAs burnt for 2 hours as reported by Al-Khalaf and Yousif (1984) and Hwang and Wu (1989) is presented in Table 13.2. Fig. 13.3A shows changes in the mineralogical composition of RHA with burning temperature with the silica in the ash being in the amorphous state for burning temperatures in the range of 500 C600 C and the crystalline phases beginning to show up at 700 C (Xu et al., 2012). At 800 C, the ash sample has a sharp peak at 2θ 5 22 degrees indicative of the presence of cristobalite which represents crystallization of silica in the ash. Similarity in the X-ray diffractograms of silica fume and RHA with amorphous silica may be seen in Fig. 13.3B. For the purpose of comparison, Fig. 13.3C presents X-ray diffractogram of PC as reported by Chopra et al. (2015). According to Chandra (1997), the mean diameter of pores in the microporous structure of RHA is the highest when rice husk is burnt at temperatures between 600 C and 700 C, and therefore the pozzolanic activity of the ash formed at this temperature should be the highest. At temperatures in excess of 800 C, an increase in the burning temperature, burning time, and the burning conditions tends to sinter or coalesce the RHA particles together. In the context of burning conditions, any change in the rate of oxidation from a predominantly CO2 environment to a predominantly oxygen environment is likely to result in a significant drop in both microporosity as well as surface area of the ash particles. The results of Chandra (1997) on the effect of burning conditions on RHA quality are reproduced in Table 13.3 and illustrated in Fig. 13.4.

13.3

Applications of RHA

Swamy (1986) has traced the earliest use of RHA in concrete to two German patents filed in the year 1924. Significant advancements in the use of RHA in concrete occurred after the 1970s when it was reported that the use of RHA can improve compressive and flexural strengths, improve durability, reduce the effects of alkalisilica reaction, and reduce shrinkage of concrete. Blending of cement with RHA speeds up setting time although water requirements of the blended cement may be greater than that for the unblended (Portland) cement. Blended cements with RHA replacement levels of more than 10% have improved compressive strength due to higher content of reactive silica. Compared to PC, RHA cement has improved resistance to acid attack. This property is attributed to the presence of silica in the RHA which when in combination with calcium hydroxide (CH) reduces the amount of CH susceptible to acid attack. RHA is useful in the manufacture of concrete for marine applications. Replacing 10% of PC in such concrete with RHA improves resistance to chloride penetration. Concrete made with ternary cement consisting of a blend of PC, fly ash, and RHA has been shown to have significantly higher strength when compared to concrete made with binary cements consisting of either RHA or fly ash. RHA is also a potential key constituent of the hybrid cementitious precursors used in alkali activated cementitious materials to produce durable

Table 13.2

(A): Effect of temperature on chemical composition of RHA burnt for 2 hours (Al-Khalaf and Yousif

1984) Temperature and time of burning 

450 C2 h 500 C2 h 550 C2 h 600 C2 h 700 C2 h 850 C2 h

LOI

3.49 3.30 2.89 2.69 2.38 1.89

Percentage oxide composition SiO2

K2O

SO3

CaO

Na2O

MgO

Al2O3

P2O5

Cl

Fe2O3

MnO

85.88 86.89 87.19 86.02 85.81 87.72

4.10 3.84 4.10 3.76 4.10 3.96

1.24 1.54 1.54 1.82 1.88 1.25

1.12 1.40 1.30 1.12 1.40 1.43

1.15 1.15 1.05 1.15 1.22 1.11

0.46 0.37 0.43 0.39 0.40 0.36

0.47 0.40 0.37 0.36 0.38 0.40

0.34 0.35 0.32 0.30 0.30 0.30

0.39 0.45 0.33 0.27 0.14 0.16

0.18 0.19 0.17 0.16 0.17 0.16

0.091 0.087 0.091 0.086 0.091 0.090

(B): Effect of temperature on chemical composition of RHA (Hwang and Wu 1989) Temperature ( C)

Element (%)

Oxide (%)

Si K Ca Na Mg S Ti Fe SiO2 MgO SO3 CaO K2O Na2O Fe2O3

, 300

400

600

700

1000

81.90 9.58 4.08 0.96 1.25 1.81 0.00 0.43 88.01 1.71 1.12 2.56 5.26 0.79 0.29

80.43 11.86 3.19 0.92 1.20 1.32 0.00 1.81 88.05 1.17 1.12 2.56 5.26 0.79 0.74

81.25 11.80 2.75 1.33 0.88 1.30 0.00 0.68 88.67 0.84 0.81 1.73 6.41 1.09 0.46

86.71 7.56 2.62 1.21 0.57 1.34 0.00 0.00 92.15 0.51 0.79 1.60 3.94 0.99 0.00

92.73 2.57 1.97 0.91 0.66 0.16 0.00 0.68 95.48 0.59 0.09 1.16 1.28 0.73 0.43

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Intensity (cps)

(A) 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

800°C–2 h 700°C–2 h 600°C–2 h 500°C–2 h 10

15

20

25

30

35

40

45

50

55

60

2θ (°) (B)

Intensity (count per second)

1500 Low–carbon RHA Silica fume

1250 1000 750 500 250 0 0

10

20

30

40

50

60

70

80

Bragg’s angle (2θ) (C) 1600 1400 3–CS 3–CS

1200 Intensity

2– CS - Di calcium silicate oxide 3–CS - Tri calcium silicate oxide

2–CS 3–CS

1000 600

2–CS 3–CS

600

3–CS

400

3–CS

2– CS 3–CS 3–CS

2– CS 3–CS

200 0 0

10

20

30

40

50

60

70

80

Position (°2 theta)

Figure 13.3 (A) X-ray diffractogram of RHA obtained from different calcination temperatures (Xu et al., 2012). (B) Comparison of X-ray diffractograms of silica fume and RHA containing amorphous silica (Venkatanarayanan and Rangaraju, 2015). (C) X-ray diffractogram of PC (Chopra et al., 2015).

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Waste and Supplementary Cementitious Materials in Concrete

Table 13.3 Effect of burning conditions on crystal structure and surface area of RHA (Chandra 1997) Burning

Hold Time

Environment

Properties of Ash

Temperature 500 C600 C

7008000C .8000 C

Crystalline? 1 min 30 min 2h 15 min1 h .1 h .1 h

Moderately oxidising Non-crystalline Highly oxidizing

Partially crystalline Crystalline

Surface area, m2/g 122 97 76 100 610 ,5

1000 900

Crystalline structure

800 Temperature (ºC)

700

Microporous and cellular structure

600 500 400 300 200

Contain more unburn carbon

100 0 0

1

3 2 Time (hours)

4

5

Figure 13.4 Effect of burning conditions on RHA quality (Chandra, 1997).

building products. Siddique and Khan (2011) have identified the following applications for RHA: G

G

G

G

G

G

G

G

G

Blended cements Green concrete High-performance concrete Refractory Roofing shingles Ceramic glaze Insulator Waterproofing chemicals Oil spill absorbent

Rice husk ash

13.4

425

Properties of RHA

According to Malhotra and Mehta (1996), differences in the chemical composition of mineral admixtures like fly ash, silica fume, or RHA do not have any significant effect on their properties unless accompanied by significant mineralogical changes which in turn are also dependent on the conditions of processing or formation. They further add that it is the mineralogical and granulometric characteristics which determine how a mineral admixture would influence engineering properties of concrete like workability, strength, and durability. Physical and chemical characteristics of RHA are discussed in the following sections.

13.4.1 Physical properties RHA is a fine material with particle sizes being generally less than 45 μm and the average particle size being in the range of 610 μm. As Fig. 13.1 shows, RHA particles are highly cellular and have a microporous character with a high internal surface such that RHA particles (with a mean size of as much as 45 μm) can have a specific surface area which is three times higher than that of silica fume particles (having a mean size in the range of 0.11 μm). It has been mentioned earlier that since RHA derives its pozzolanicity from its high internal surface area, grinding this ash to a high degree of fineness will serve no useful purpose. According to Sugita et al. (1997), specific pore volume of coarse RHA may be as high as 0.16 cm3/g with the pore sizes being distributed in the range of 240 nm, the average radius being 12.3 nm. Because of its high porosity, RHA will have high water absorption which may adversely affect the fresh properties of mortar or concrete containing RHA. Although fine grinding of RHA may not improve its reactivity, it has the potential to alter the pore structure so that water absorption is reduced. Sugita et al. (1997) and Real et al. (1996) have reported that pore size in ground RHA lies in the range of 25 nm with the average radius being 4 nm. Physical properties of RHA reported by selected authors are presented in Table 13.4. Table 13.4

Physical properties of RHA

Property

Value Mehta (1992)

Mean particle size (μm) Specific gravity Fineness (passing 45 μm sieve) (%) Specific surface (m2/g)

2.06 99

Bui et al. (2005)

Ganesan Zhang and Feng et al. et al. (2008) Malhotra (1996) (2004)

5

3.80

2.10

2.06 99 36.47

7.4 2.06 99

2.10

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Waste and Supplementary Cementitious Materials in Concrete

Variation of particle size with grinding time (Habeeb and Mahmud 2010)

Table 13.5

Material Grinding time (minutes) Average particle size (μm) BET nitrogen adsorption (m2/g) Blaine’s surface area (m2/kg)

Ordinary Portland cement

RHA F0

RHA F1

RHA F2

RHA F3

22.1

90 63.8

180 31.3

270 18.3

360 11.5

25.3

27.4

29.1

30.4

351.4

Figure 13.5 Samples of (A) rice husk, (B) burnt rice husk as ash, (C) RHA after grinding (Habeeb and Mahmud 2010).

The effect of grinding (carried out using a Los Angeles mill) time on physical properties of RHA as reported by Habeeb and Mahmud (2010) is summarized in Table 13.5. This table shows that increase in grinding time from 90 to 270 minutes resulted in a reduction in the average particle size of about 70% such that the size of the RHA particles was comparable to the average size of the cement grains. Fig. 13.5 presents samples of rice husk and its ash with Part (c) of this figure showing the ash in its ground state as reported by Habeeb and Mahmud (2010). Results obtained by Al-Khalaf and Yousif (1984) on the effect of grinding time on fineness of RHA burnt at various temperatures for 2 hours are presented in Fig. 13.6. The grinding was carried out using a modified Los Angeles machine in which the grinding media consisted of steel balls and a steel chain of length 2 m and weighing 450 g/m.

13.4.1.1 Particle size distribution According to Della et al. (2002), the mean particle size of RHA after rice husks were burnt at 700 C for 6 hours was about 33 μm with all the particles in the sample being smaller than 112 μm. When the sample was ground for 80 minutes, the mean size was reduced to 0.68 μm with all particles being smaller than 6 μm.

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20,000

Fineness of RHA - cm2/gm

15,000

10,000 Burning temperature

500ºC 450ºC 550ºC 600ºC 700ºC

5000

1

2

3 4 Grinding time - hours

5

6

7

Figure 13.6 Relationship between grinding time and fineness of RHA burnt at various temperatures for 2 hours (Al-Khalaf and Yousif 1984).

%

100

50

0 1

10 100 Particle size (μM)

1000

Figure 13.7 Particle size distribution of a RHA sample (Agarwal 2006).

Improperly burnt RHA can have specific surface area values of about 177 m2/g, and when such samples are subject to controlled burning at temperatures of 700 C for 6 hours, the specific surface area can decrease to values as low as 54 m2/g. Della et al. (2002) state that this decrease in the specific surface area is proportional to heating temperature and time, and prolonged heating at elevated temperatures cause an agglomeration effect which leads to decreased porosity. When the burnt samples were further subjected to wet grinding for 80 minutes, the specific surface area increased to 81 m2/g. Particle size distribution obtained by Agarwal (2006) of an RHA sample collected from a paper mill using rice husk as a fuel is presented in Fig. 13.7.

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Waste and Supplementary Cementitious Materials in Concrete

13.4.2 Chemical composition and pozzolanic activity After burning at a temperature in the range of 600 C700 C for 2 hours, RHA will consist of 90%95% SiO2, 1%3% K2O, and about 5% of unburnt carbon. Provided it is burnt properly, the quality of ash from different sources will not vary significantly. Table 13.6 shows a comparison of the chemical composition of rice husk and RHA reported by Ikpong and Okpala (1992). The most significant effect of the conversion of rice husk into RHA is on the amount of silica (5-fold increase) and on the loss on ignition (LOI) (an almost 10-fold decrease). Chemical composition reported by some other authors is compiled in Table 13.7. If RHA is proposed to be used as a pozzolan in cement or in concrete, then it should satisfy the

Chemical composition of rice husk and RHA (Ikpong and Okpala 1992) Table 13.6

Constituent

Percent present

SiO2 Al2O3 Fe2O3 Na2O K2O CaO MgO LOI P2O5

Table 13.7

Rice husk

RHA

15.30 0.56 0.07 0.082 0.18 0.018 0.37 83.12 0.30

82.13 4.27 0.38 0.14 1.23 0.16 1.65 8.60 1.44

Chemical composition of RHA reported by selected

authors Constituent

Silica (SiO2) Alumina (Al2O3) Iron oxide (Fe2O3) Calcium oxide (CaO) Magnesium oxide (MgO) Sodium oxide (Na2O) Potassium oxide(K2O) Sulfur oxide(SO3) LOI

Percentage Mehta (1992)

Zhang and Malhotra (1996)

Bui et al. (2005)

87.2 0.15 0.16 0.55 0.35

87.3 0.15 0.16 0.55 0.35

86.98 0.84 0.73 1.40 0.57

1.12 3.68 0.24 8.55

1.12 3.68 0.24 8.55

0.11 2.46 5.14

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chemical composition requirements given in ASTM C618 which inter alia states that the combined proportion of silicon dioxide, aluminum oxide, and iron oxide in the ash should not be less than 70% and LOI should not exceed 12%. The hydration of PC involves reactions of the anhydrous calcium silicates (C3S and C2S) and aluminates (C3A and C4AF) phases with water to form hydrated phases. Of interest are the hydration reactions of C3S and C2S since it is these reactions which produce calcium hydroxide with which the amorphous silica in RHA can react in the secondary hydration reaction. In primary hydration, C3S and C2S react with water as follows: 2ð3CaO:SiO2 Þ 1 6H2 O ! 3CaO:2SiO2 :3H2 O 1 3 CaðOHÞ2 2ð2CaO:SiO2 Þ 1 4H2 O ! 3CaO:2SiO2 :3H2 O 1 3 CaðOHÞ2 In the secondary hydration reaction, the amorphous silica in RHA reacts with the Ca(OH)2 liberated in the primary hydration reaction, and according to James and Rao (1986), the product of this reaction is a type of C-S-H gel. The possible reaction between silica and Ca(OH)2 in the presence of water has been described as follows: 2SiO2 1 3CaðOHÞ2 1 H2 O ! 2Ca1:5 SiO3:5 :2H2 O Zhang and Malhotra (1996) have concluded that incorporation of RHA in cement and in concrete has the following effect on microstructure, on interfacial zone between aggregates and paste and on hydration: (1) Ca(OH)2 and calcium silicate hydrates (C-S-H) are the major hydration and reaction products in the RHA paste, and because of the pozzolanic reaction the paste incorporating RHA had a lower Ca(OH)2 content than the control PC paste; (2) incorporation of the RHA in concrete reduced porosity and the amount of Ca(OH)2 in the interfacial transition zone (ITZ); and (3) the width of the interfacial zone between the aggregate and the cement paste when compared with that in the control PC composite was also reduced. However, the porosity of the RHA composite in the interfacial zone was higher than that of a composite containing silica fume. It is interesting to note that Jauberthie et al. (2000) have found that concentration of amorphous silica is high on the external face of the husk and significantly smaller on the internal face and practically nonexistent within the husk. Pozzolanic activity of RHA was investigated by Agarwal (2006) in terms of the 7- and the 28day compressive strength of moist mortar cubes. Accelerated pozzolanic activity index (PAI) of the control mortar consisting of a mix of 1 part of cement and 2.75 parts of sand was 310 kg/cm2 (control value), whereas this value for the RHA received from the boilers of a paper mill was 260 kg/cm2 (16% lower). When the as-received RHA sample was processed by sieving, the PAI for the sample passing through the 150-μm sieve and retained on the 75-μm sieve was 110% of the control value. The PAI values for the samples retained on the 45-μm and for the sample passing through the 45-μm sieves were 135% and 148% of the control value,

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Waste and Supplementary Cementitious Materials in Concrete

Accelerated pozzolanic index values of RHA as per ASTM 124098 and at Room Temperature (RT) (Agarwal 2006)

Table 13.8

System

Accelerated pozzolanic index (kg/cm2)

Percentage of control

7 day (RT) kg/cm2

28 day (RT) kg/cm2

43 Grade Ordinary Portland cement RHA (1.2% LOI) (as received) 150 passing retaining 75 μm 45 μm retaining Less than 45 μm RHA (13% LOI) ,75μm RHA(20% LOI) ,45 μm

310

100

250

360

260

84

140

260

340

110

180

340

420 460 390

135 148 126

240 240 260

390 415 400

380

123

260

410

respectively. The test results of Agarwal (2006) are reproduced in Table 13.8, which also shows the effect of carbon content on pozzolanic activity index. X-ray diffractograms typical of RHA containing silica in the amorphous and in the crystalline form are shown for the purpose of illustration in Fig. 13.8. The broad humps centered at 2θ  22 degrees in Fig. 13.8A are indicative of the presence of silica in the amorphous form, whereas the sharp peaks in Fig. 13.8B show the presence of crystalline silica in the form of cristobalite and tridymite. The presence of cristobalite and tridymite is attributed to burning of rice husks at high temperatures in the range of 800 C1000 C which resulted in crystallization of silica in the ash.

13.5

Properties of fresh concrete containing RHA

13.5.1 Workability In general, addition of any fine mineral admixture to concrete can be expected to increase its cohesiveness and make it slightly stiffer. RHA is no exception and its use in concrete will reduce initial slump and therefore, the use of a higher watercement ratio or a workability enhancing admixture in concrete containing RHA may be required. It may be noted that at a given water content, small additions of RHA (less than 3% by weight of cement) will be helpful in improving the workability and stability of concrete by suppressing the tendency towards bleeding and segregation. The effect on slump and compaction factor of concrete in which cement was partially replaced with 20%, 25%, and 30% of RHA was investigated by Ikpong and Okpala (1992) for concretes of Grades 20, 25, 30, and 40 MPa. Their results are reproduced in Table 13.9 and indicate that to attain the same level of

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(A)

RHA500-6Q

RHA500-12Q

RHA500-12S

0

10

20

30 Angle

40

50

60

(B) Cristobalite Tridymite

0

20

40

60

80

100

Angle

Figure 13.8 XRD patterns of RHA samples: (A) Sample with amorphous silica. (B) Sample with crystalline silica (Nair et al., 2008).

workability, the mixes with the binary binder (cement 1 RHA) will require a higher water content when compared to the concrete in which PC is the only binder. Table 13.10 presents the findings of Ismail and Waliuddin (1996) on the effect of RHA replacement levels in the range of 10%30% on the workability of highstrength concrete. The results in this table show that both slump as well as density decreased with increasing substitution of cement with RHA though the fineness of RHA did not have any significant effect on either the slump or the density. Bui et al. (2005) studied the influence of RHA replacement levels (10%, 15%, and 20% by mass of PC) on the slump of concrete mixtures considering the following variables: (1) two types of PCs with specific surface areas of 2700 and 3759 cm2/g and (2) water-to-binder (w/b) ratios of 0.30, 0.32, and 0.34. Although a superplasticizer (SP) was used in all the mixes to obtain high workability, in the

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Waste and Supplementary Cementitious Materials in Concrete

Slumps and compaction factors of concrete containing various replacement levels of RHA (Ikpong and Okpala 1992)

Table 13.9

Strength (MPa)

RHA (%)

W/C ratio

Slump (mm)

Compaction factor

20

0 30 40 0 30 40 0 30 40 0 30 40

0.80 0.83 0.85 0.69 0.72 0.75 0.64 0.64 0.65 0.54 0.57 0.61

40 33 35 35 35 37 50 55 35 50 45 45

0.93 0.93 0.92 0.89 0.92 0.92 0.90 0.90 0.89 0.90 0.87 0.91

25

30

40

Slumps and densities of concrete containing various replacement levels of RHA (Ismail and Waliuddin 1992)

Table 13.10

Mix type

RHA (%) #200

A Aa-10 Aa-20 Aa-30 Ab-10 Ab-20 Ab-30

W/(C 1 RHA)

Slump (mm)

Density (kg/m3)

0.24 0.31 0.33 0.36 0.30 0.32 0.34

70 30 60 30 30 45 32

2425 2405 2400 2398 2403 2396 2390

#325

10 20 30 10 20 30

mix with the w/b of 0.34, the dosage of this admixture was kept constant so that influence of RHA replacement levels on workability could be monitored. Fig. 13.9 shows that the SP dosage remaining unchanged, the workability of this concrete decreased with increasing substitution of cement with the RHA. The effect of fineness (27.4, 29.1, and 30.4 m2/g) of RHA at a replacement level of 20% by mass of cement on slump and density of concrete has been reported by Habeeb and Fayyadh (2009). Their results show that in order to obtain a slump which was about 90% of that of the control concrete having cement as the only binder, the SP dosage had to be increased from 0.63% in the control concrete to 2% in the concrete in which relatively the finest RHA had been used to substitute 20% by mass of cement. It may also be noted in this table that concrete containing the finest RHA (20F3) had the highest fresh density, which is attributed to superior particle packing due to the finer material.

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250 Cement PC 30 Cement PC 40

Slump (mm)

200

150

100 50

0 Control

10% RHA

15% RHA

20% RHA

Figure 13.9 Effect of RHA replacement level on the slump of gap-graded concrete made with a w/b ratio of 0.34 (Bui et al., 2005).

13.5.2 Setting time It has been reported by Chandra (1997) that addition of RHA tends to shorten the setting time as is illustrated in Fig. 13.10. This has been attributed to the water absorption ability of RHA due to its microporous structure as a result of which the w/c ratio is the surrounding matrix is reduced. The results of 0%, 5%, 10%, 15%, 20%, 25%, 30%, and 35% replacement levels of cement with RHA on initial and final setting times noted by Ganesan et al. (2008) are presented in Fig. 13.11. The following observations were made by them: consistency of the control paste having cement as the only binder was about 32%, and this value increased in an almost linear manner with an increase in the RHA content with the standard consistency at cement replacement level of 35% being 44%. The results in Fig. 13.11 show that initial setting time increased up to 15% cement replacement level beyond which there is a gradual decrease, whereas the final setting time decreased with increase in the cement replacement level.

13.6

Properties of hardened concrete containing RHA

13.6.1 Compressive strength In concretes with cement as the binder, the ITZ is usually less dense than the bulk cement mortar, and the transition zone contains a large amount of plate-like crystals of Ca(OH)2. In such concretes, the transition zone is the weakest link and is the site of microcracks induced by thermal and humidity changes in concrete. Hence, the transition zone has a significant influence on the mechanical properties of concrete. In general, the addition of pozzolanic materials affects both strength and permeability of concrete

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Waste and Supplementary Cementitious Materials in Concrete

12 (a) W/B=0.35

10 Final setting

8

Setting time (Hours)

6 4

Inital setting

2 0 (b) W/B=0.47

8

Final setting

6 4 Inital setting

2 0 0

5

10 RHA (%)

15

20

Figure 13.10 Setting time of cement paste containing RHA (Chandra 1997).

350 Initial Final

Setting times (minutes)

300 250 200 150 100 50 0 0

5

10 15 20 25 Cement replacement level (%)

30

35

Figure 13.11 Initial and final setting times of RHA blended cement (Ganesan et al., 2008).

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since the products of the secondary hydration reaction not only improve packing of the transition zone but also serve to block the large voids in the hydrated cement paste. As a result the porosity of the cement paste is reduced and pore refinement takes place. Due to its microporous character, RHA adsorbs a large amount of water which not only reduces bleeding but also helps in limiting the entrapment of bleed water on the underside of aggregate particles. As a result the aggregate-paste bond is improved. Fig. 13.12 adapted from the work of Chandra (1997) shows however that for achieving high strength of paste it is important to add the correct amount of RHA and large dosages can have an adverse affect on strength. It may be noted that the early age strength of concrete is a function of the w/c ratio and as long as this ratio remains unchanged, the early strength of concrete is unlikely to be affected by the presence of pozzolans with only the ultimate strength being enhanced due to pozzolanic action. Table 13.11 presents the effect of cement replacement levels of 0%, 30%, and 40% with RHA on compressive strengths of concretes having 28-day designed strengths of 20, 25, 30, and 40 MPa. The results in this table have been adapted from Ikpong and Okpala (1992) and show that for each of the designed strengths, the control mix with cement as the only binder attained a higher compressive strength than the mixes with the binary cement (cement 1 RHA). Furthermore, except for the mixes with blended cement having 28-day designed strength of 40 MPa, all the mixes attained their 28-day designed strengths. For this design strength, the mix containing 30% RHA achieved 98.5% of the design strength, whereas the mix with 40% RHA achieved 86.5% of the target strength. It is interesting to note in Table 13.11 that a majority of the mixes attained more than 60% of their designed strength at 7 days.

Compressive strength (kg/cm2)

800 700 600 Ground

500 400 300 200

Unground

100 0 0

5

10 15 20 RHA replacement level (%)

25

30

Figure 13.12 Effect of RHA replacement level on compressive strength of paste (Chandra 1997).

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Waste and Supplementary Cementitious Materials in Concrete

Table 13.11 Compressive strength development with age (Ikpong and Okpala 1992) Design strength (N/mm2) 20

25

30

40

Compressive strength (N/mm2)

RHA (%)

0 30 40 0 30 40 0 30 40 0 30 40

7 day

14 day

28 day

90 day

17.5 15.8 3.27 22.9 18.7 14.8 28.4 22.6 16.7 32.4 24.0 1.5

23.9 18.2 15.2 27.2 23.8 21.0 37.6 32.0 27.5 38.8 33.5 29.3

33.1 29.1 27.2 35.4 31.1 28.6 43.5 38.3 31.8 44.2 39.4 34.6

37.8 36.1 33.9 39.2 37.5 34.0 46.3 43.2 37.1 47.6 44.9 38.9

Zhang and Malhotra (1996) compared the compressive strength of concrete made with two types of blended cements. One of these cements was blended with 10% RHA and the other was blended with an equal fraction of silica fume. In all the concretes the w/b ratio was kept constant at 0.40, and Fig. 13.13 shows compressive strength results up to the age of 730 days. The results in Fig. 13.13 show that in general, RHA concrete achieved higher strengths relative to the control concrete though these strengths were lower than the strengths of the concrete containing silica fume as one of the binders. For example, at 28 days, the compressive strength of the RHA concrete was 38.6 MPa compared to a value of 36.4 MPa for the control concrete and 44.4 MPa for the concrete containing silica fume. The corresponding figures for the age of 180 days are 48.3, 44.2, and 50.2 MPa, respectively. Ismail and Waliuddin (1996) studied the effect of cement replacement with RHA (passing #200 and #325 sieves) in the range of 10%30% on the compressive strength of high-strength concrete. The measured compressive strengths are reproduced in Table 13.12, and the authors concluded that (1) it is possible to design high-strength concrete with blended cement containing RHA; (2) the optimum replacement of cement with the finely ground RHA was in the range of 10%20%; and (3) during the initial 3 days of concrete maturing in particular, the hydration rate in the concretes made with the blended cement was slower compared to that in the control concrete made with the PC and this effect was noted at all the maturing ages under investigation, Table 13.12. Bui et al. (2005) studied the compressive strength of concrete made with two type of PC having Blaine’s specific surface area of 2700 cm2/g (PC30) and 3759 cm2/g (PC40) with 10%, 15%, and 20% cement replacement levels (by mass) with RHA. The following conclusions were drawn by them: (1) RHA has the potential to be used as a highly reactive pozzolanic material to improve the

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Compressive strengh (MPa)

60

40

20 Control 10% RHA 10% SF 0

0

200

400 Age (days)

600

800

Figure 13.13 Compressive strength development of concretes versus age (Zhang and Malhotra 1996).

Compressive strength of RHA concrete (Ismail and Waliuddin 1996)

Table 13.12

Mix type

RHA (%) #200

Aa Aa-10 Aa-20 Aa-30 Ab-10 Ab-20 Ab-30

W/(C 1 RHA)

#325

10 20 30 10 20 30

0.24 0.31 0.33 0.36 0.30 0.32 0.34

Compressive strength (MPa) 3 day

7 day

28 day

150 day

54.3 46.2 35.3 31.5 47.0 46.7 43.1

62.3 56.0 46.8 39.3 61.0 56.0 51.7

72.4 68.1 57.3 47.7 71.0 70.2 63.0

85.0 71.1 57.4 48.8 72.4 70.3 63.2

microstructure of the ITZ; (2) blending efficiency of the binders was significantly affected by the particle size distribution of cement and RHA; and (3) the strength increase relative to the control concrete made with cement as the only binder was higher when the RHA was blended with the coarser cement, and this observation was validated with the help of numerical simulation which repeated the favorable results obtained in the case of the coarser cement. It was noted that the coarser cement, which was a gap-graded binder, resulted in improved particle packing because of which the porosity of the matrix decreased. Sensale (2006) investigated the influence of two types of RHA on the compressive strength of concrete with one of the ashes, UY-RHA (39.55% reactive silica), being obtained from a rice processing unit in Uruguay and the other, USA-RHA (98.5%

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Waste and Supplementary Cementitious Materials in Concrete

reactive silica) being a product of controlled incineration in the United States. The strength results for the two ashes are given in Table 13.13, and the authors report that: (1) at early age (7 days), concretes containing UY-RHA had a higher compressive strength when compared to concretes containing USA-RHA; (2) at later ages (91 days), the RHA concretes had higher compressive strength compared to the control concrete made with PC, and the highest compressive strength was obtained with the concrete containing 20% cement replacement with USA-RHA; (3) long-term compressive strength of concrete containing UY-RHA was not as high as that of concrete made with USA-RHA and the 91-day strengths of concrete containing USA-RHA increased with replacement levels of cement with RHA. The effect of RHA on the mechanical properties of heavyweight concrete has been investigated by Saker (2006) by using RHA having a specific surface area of 5.6 3 106 mm2/g and a unit weight of 2.06 3 103 kg/m3 and a chemical composition as follows: SiO2: 87%, Al2O3: 1.75%, Fe2O3: 2.5%, CaO: 2.5%, MgO: 2.3%, and K2O: 2.5%. Replacement levels of cement with RHA by weight were 0%, 5%, 10%, 15%, and 20%. Besides parameters related to RHA, the other parameter under investigation was the type of aggregate used in the concrete. Table 13.14 presents the measured mechanical properties of the concretes, and the authors have reported that: (1) in general, as the RHA content increased to 15%, there was an improvement in all the mechanical properties though beyond this concentration, a reduction in strength was noted and (2) concrete made with ilmenite had relatively the highest compressive strength when compared to the concretes made with other two aggregate types, and this has been attributed to the shape, surface area, and hardness of the ilmenite aggregates. Table 13.13

Compressive strength results of RHA concrete (Sensale

2006) w/(c 1 RHA)

RHA Type

0.32 UY USA 0.40 UY USA 0.50 UY USA

Compressive strength(MPa) %

7 day

28 day

91 day

0 10 20 10 20 0 10 20 10 20 0 10 20 10 20

48.4 5.11 44.3 39.5 30.5 35.8 41.1 27.9 29.7 23.6 24.6 24.1 24.9 22.7 20.8

55.5 60.4 54.8 51.4 47.4 42.3 50.4 40.7 40.8 39.4 32.9 31.5 34.9 34.5 35.9

60.6 64.3 62.7 64.5 68.5 45.6 54.9 51.4 51.5 57.3 35.9 35.5 37.9 44.4 52.9

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The effect of RHA on mechanical properties of concrete was studied by Saraswathi and Song (2007) using a control concrete mix containing PC as the only binder and having mix proportions of 1:1.5:3 with a w/c ratio of 0.53. Cement replacement levels with RHA by mass were 0%, 5%, 10%, 15%, 20%, 25%, and 30%, and the measured 28-day compressive strengths together with other mechanical properties are reproduced in Table 13.15. The authors concluded that compressive strength of concrete increased with increase in RHA content, and at 28 days all the RHA concretes showed strengths which were higher than that of the control concrete. Gastaldini et al. (2007) investigated the effect of chemical activators on compressive strength of concrete in which 20% of PC had been replaced with RHA. The following were the parameters considered in the experimental program: w/b ratios of 0.35, 0.50 and 0.65 and binderaggregate ratios of 1:3.75, 1:5.25, and 1:6.9. The chemical activators used at a dosage of 1% by weight of cement consisted of K2SO4, Na2SO4, and Na2SiO3. The measured compressive strengths are compiled in Table 13.16, and the authors have found that: (1) the concrete with 20% RHA and a w/b of 0.50 had a 7-day compressive strength which was equal to that of the control concrete, and the 28-day as well as the 91-day compressive strengths were higher than those of the control concrete for all w/b ratios; (2) each of the three chemical activators lead to significant increase in 7-day strength relative to the control concrete without any activator with K2SO4 being the most effective in strength enhancement.

Table 13.14 28-day mechanical properties of heavyweight concrete containing RHA (Saker, 2006) Type of concrete

Replacement Compressive (%) strength (MPa)

Indirect tensile strength (MPa)

Flexural strength (MPa)

Bond strength (MPa)

Elastic modulus (GPa)

Gravel

0 5 10 15 20 0 5 10 15 20 0 5 10 15 20

2.7 2.9 3.1 3.4 3.4 2.9 3.1 3.3 3.6 3.6 3.7 3.9 4.0 4.5 4.0

4.9 5.5 6.0 7.5 7.4 4.9 6.0 6.8 7.9 7.8 5.9 7.1 8.5 8.9 8.5

5.5 6.7 7.0 7.5 7.4 6.2 7.1 7.7 8.0 7.9 6.9 7.9 8.0 8.3 8.1

21.5 22.0 23.0 24.2 23.8 29.6 30.1 32.0 33.5 31.5 35.5 37.0 37.0 38.0 36.5

Baryte

Ilmenite

44.5 45.5 49.5 50 43.0 45.0 46.0 52.0 54.0 52.0 46.5 47.0 50.0 52.0 51.0

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Waste and Supplementary Cementitious Materials in Concrete

28-day mechanical properties of concrete containing RHA (Saraswathi and Song, 2007)

Table 13.15

RHA (%)

Compressive strength (MPa)

Splitting tensile strength (MPa)

Bond strength(MPa) at 0.25 mm slip

0 5 10 15 20 25 30

36.45 36.49 37.43 37.38 37.71 39.55 37.80

4.49 4.57 4.65 4.92 4.60 4.58 3.67

3.32 4.11 4.31 3.79 3.43 4.07 3.87

Table 13.16 Compressive strength of RHA concrete containing chemical activators (Gastaldini et al., 2007) Mixture

REF (control)

20 RHA

20 RHA1% Na2SO4

20 RHA 1% K2SO4

20 RHA1% Na2SiO3

W/B ratio

0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65

Compressive strength (MPa) 7 day

28 day

91 day

58.3 36.4 24.6 54.2 36.4 17.7 59.2 39.5 25.3 65.9 36.4 28.9 54.2 37.4 29.1

64.2 47.7 28.0 69.7 48.1 27.0 73.3 50.7 36.8 77.4 48.1 38.9 74.4 48.7 40.3

76.1 53.5 31.9 83.4 53.9 33.6 82.8 56.5 42.5 91.0 71.8 48.3 77.5 53.8 45.1

Chindaprasirt et al. (2007) investigated the effect of 20% and 40% substitution of PC with RHA on the compressive strength of blended cements. The following were the characteristics of the RHA: silica content: 90%, LOI: 3.2%, and Blaine fineness: 14,000 cm2/g. The mortar mixes were made using a sand-to-binder ratio of 2.75, and the water content across the mortars was so adjusted to obtain a flow of 110% 1 / 2 5%, and the measured compressive strengths at various curing ages are reported in Table 13.17. This table shows that although the mix RHA 20 had a relatively lower initial (7 day) strength, its 28-, 90-, and 180-day strength was higher than that of the control mortar which is indicative of high reactivity of the RHA.

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On the other hand, the mix RHA 40 showed strengths lower than that of the control mortar at all curing ages, and this is attributed to the very high w/b ratio of this mortar which was required to obtain the desired flow behavior. Trends in compressive strengths of concretes containing 0%, 7%, 10%, and 15% replacement of cement with RHA noted by Ramezanianpour et al. (2009) up to the age of 90 days are presented in Fig. 13.14. This figure shows that the 3, 7, 28, and 90 day strengths of the RHA concretes were higher than those of the control concrete, and further at each of these ages the compressive strength increased with increase in the amount of RHA in the concretes. Table 13.18 shows trends in 28- and 91-day compressive strength measured by Gastaldini et al. (2009) for concrete in which 10%, 20%, and 30% of cement had been replaced with RHA. This table shows that for concrete containing 10% RHA, the increase in 28-day compressive strength relative to the control concrete made with cement as the only binder for the w/b ratios of 0.35, 0.50 and 0.65 ranged between 15% and 27%, and this value for the 91-day strength was 10%21%. For concrete made with 20% RHA, the corresponding figures were 11%34% and

13.17 Compressive strength (Chindaprasirt et al., 2007)

of

Table

Mix

W/B ratio

Ordinary Portland cement RHA 20 RHA 40

0.55 0.68 0.80

blended

cements

Compressive strength (MPa) 7 day

28 day

90 day

180 day

44 31 17

51 54 32

57 61 43

60 62 53

70

Compressive strength (MPa)

60

Control 10% RHA

7% RHA 15% RHA

50 40 30 20 10 0 3 days

7 days

28 days

90 days

Figure 13.14 Effect of cement substitution with RHA on compressive strength of concrete (Ramezanianpour et al., 2009).

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Waste and Supplementary Cementitious Materials in Concrete

19%26% and for concrete containing 30% RHA increase in the 28- and 91-day strength ranged between 6%26% and 7%27%, respectively.

13.6.2 Tensile strength and modulus of elasticity The results of Zhang and Malhotra (1996) presented in Table 13.19 show that the splitting tensile strength, the flexural strength, and the modulus of elasticity of their RHA concrete were comparable to that of their control concrete made with cement as the only binder. Tashima et al. (2005) investigated the effect of cement substitution (by mass) with 5% and 10% of RHA (which had a SiO2 content of 92.99% and Blaine specific surface of 16,196 cm2/g) on the splitting tensile strength and modulus of elasticity of concrete. Their results summarized in Table 13.20 show that at the 5% replacement levels, the splitting tensile strength of the RHA concrete was higher than that of the control concrete at all ages, whereas the modulus of elasticity values across all the ages was comparable. The splitting tensile strength as well

Compressive strength of concretes containing RHA (Gastaldini et al., 2009) Table 13.18

Mixture

W/B ratio

Ref

0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65

10 RHA

20 RHA

30 RHA

Compressive strength (MPa) 28 day

91 day

54 47 28 68 47 32 72 51.5 32 67 50 30

67.9 51.4 35 76.4 62.1 38.6 85.6 62.9 41.7 78.9 65.1 37.3

Mechanical properties of concretes containing RHA (Zhang and Malhotra 1996)

Table 13.19

Mix

RHA (%)

1 2 3

0 10

SF (%)

W/C ratio

Splitting tensile strength (MPa)

Flexural strength (MPa)

Modulus of elasticity (GPa)

0

0.40 0.40 0.40

2.7 3.5 2.8

6.3 6.8 7.0

29.6 29.6 31.1

10

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Table 13.20 Splitting tensile strength and elastic modulus of concretes containing RHA (Tashima et al., 2005) Mixture

Control (0% RHA) 5% RHA 10% RHA

Splitting tensile strength (MPa)

Modulus of elasticity (GPA)

7 day

2 day

91 day

7 day

28 day

91 day

4.85 4.94 4.82

5.37 5.79 5.78

5.41 5.9 5.4

30.08 40.72 40.23

40.85 40.76 40.21

45.04 41.84 40.03

as the modulus of elasticity of the concrete with 10% RHA was marginally lower than that of the control concrete at all ages. Cizer et al. (2006) investigated the flexural strength of mortars made with RHA blended cement up to the age of 120 days. The control mortar was prepared with cement and sand in the ratio of 1:3 by weight. In the mortars with RHA blended cement, the cement replacement levels were 30%, 50%, and 70%. The results of the flexural tests carried out on prismatic specimens of size 40 3 40 3 160 mm were: (a) at all ages, the control mortar had higher flexural strength than any of the mortars containing the blended cement; (b) at all ages, flexural strength decreased with increase in the RHA dosage in the mortars such that the 28-day flexural strength of the mortar in which cement replacement level with RHA was 70% was only about 16% of the strength of the control mortar; (c) the rate of flexural strength increase between 7 and 28 days of hardening was the highest in the case of the control mortar and this rate gradually decreased with increasing RHA concentration in the mortars. In the mortar with the highest RHA dosage, RHA-C.7-3, the rate of strength increase between 7 and 120 days was almost uniform unlike the spike seen in strength gain between 7 and 28 days in the other mortars. Ganesan et al. (2008) have presented results of 28-day splitting tensile strengths of concrete in which cement had been partially replaced (by mass) to the tune of 0%, 5%, 10%, 15%, 20%, 25%, and 30% with RHA. It has been reported that the splitting tensile strength of the control concrete having a w/b ratio of 0.53 was 4.5 MPa with this strength increasing marginally with increase in RHA content to 20% and thereafter decreased slightly when the RHA content was further increased to 25% and 30%. However, irrespective of the RHA dosage, the splitting tensile strengths of the RHA concretes were never lower than that of the control concrete. The results of the investigations of Ramezanianpour et al. (2009) on the influence of RHA on splitting tensile strength and modulus of elasticity of concrete are shown in Figs. 13.15 and 13.16 respectively. Concrete mixes of Ramezanianpour et al. (2009) were made with cement replacement levels of 0%, 7%, 10%, and 15%, and the following conclusions were drawn: (1) at all ages, concretes containing RHA had a higher splitting tensile strength when compared to that of the control concrete (Fig. 13.15B); (2) at both 28 days as well as at 90 days, the modulus of elasticity marginally increased with increase in

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Waste and Supplementary Cementitious Materials in Concrete

Splitting tensile strength (MPa)

6 5

Control

7% RHA

10% RHA

15% RHA

4 3 2 1 0 7 day

28 day

90 day

Figure 13.15 Splitting tensile strengths of RHA concretes (Ramezanianpour et al., 2009).

34

Modulus of elasticity (GPa)

Control 10% RHA

7% RHA 15% RHA

32

30

28

26 28 day

90 day

Figure 13.16 Moduli of elasticity of RHA concretes (Ramezanianpour et al., 2009).

the RHA content in the concretes (Fig. 13.16). This increase was more pronounced for the RHA dosage of 7% and was marginal when the amount of RHA was further increased to 10% and 15%. These results are along expected lines since the material parameter most likely to affect elastic modulus are the coarse aggregate characteristics with the effect of binders being secondary. Habeeb and Fayyadh (2009) Investigated the effect of cement substitution with RHA of three fineness levels on flexural strength, splitting tensile strength, and elastic modulus of concrete. The three fineness levels were 27.4 (F1), 29.1 (F2),and 30.4 (F3) m2/g, and the following conclusions were drawn by the authors: (1) At

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each of the three hardening ages of 28, 90, and 180 days, there was only a marginal increase in flexural strength with RHA fineness though the increase in the splitting tensile strength with increasing RHA fineness was more significant; (2) Moduli of elasticity of all concretes were in the range of 29.632.9 GPa which is indicative of the insignificant influence of RHA fineness on elastic modulus of concrete.

13.6.3 Drying shrinkage of concrete containing RHA Relatively few investigations have been carried out on drying shrinkage of concrete containing RHA. Zhang and Malhotra (1996) compared the drying shrinkage strains of concretes in which 10% of the cement had been replaced with RHA and silica fume. Fig. 13.17 shows results of the shrinkage strain up to the age of 448 days with the measurements having being started after 7 days of initial curing in limesaturated water. The results in this figure show that across all ages, the drying shrinkage strains of the RHA concrete (638 3 1026) were comparable to that of the control concrete as well as the concrete containing SF. Autogenous shrinkage of cement pastes with a w/b ratio of 0.30, and 5% and 10% substitution levels of cement with RHA have been reported by Sensale et al. (2008). Two types of RHAs were used in the investigations with one being sourced from a controlled incineration plant in the United States (CRHA) and the other from a paddy milling industry in Uruguay (RRHA). The SiO2 contents in the two ashes were in the range of 85%90%. Autogenous deformation measurements were made up to 28 days and the results are plotted in Fig. 13.18. This figure shows that: (1) in the control concrete the autogenous deformations were about 600 μm/m during 4 weeks of sealed hardening; (2) an addition of 10% of

Drying shrinkage strain ×10–6

800

600

400

200

Control 10% RHA 10% SF

0

0

100

200 300 Age (days)

400

500

Figure 13.17 Drying shrinkage of RHA concrete and silica fume concrete (Zhang and Malhotra 1996).

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Waste and Supplementary Cementitious Materials in Concrete

(A)

100

Deformation (microstrain)

0 –100

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

Control RRHA 5% RRHA 10%

–200 –300 –400 –500 –600 –700 100

(B)

Age (days) 0 0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

Deformation (microstrain)

–100 –200

Control CRHA 5% CRHA 10%

–300 –400 –500 –600 –700

Figure 13.18 Autogenous shrinkage of pastes containing cements blended with (A) RRHA and (B) CRHA (Sensale et al., 2008).

RRHA reduced this value by almost 50%, whereas in the case of the mix containing CRHA, the reduction was slightly higher. The effect of RHA fineness on drying shrinkage of concrete has been investigated by Habeeb and Fayyadh (2009) at hardening ages of 7, 14, 28, 42, 56, 90, and 180 days. The test samples were moist cured for the first 7 days and were then left exposed to ambient conditions till the date of test. The RHA samples were ground to the following three fineness values: 27.4 m2/g (F1), 29.1 m2/g (F2), and 30.4 m2/g (F3). The significant effect of particle size on drying shrinkage strains measured at various ages with the concrete containing the finest RHA particles (F3) showing the highest shrinkage values at all the test ages.

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In the literature, there are conflicting opinions on the effect of pozzolanic materials, most of which have a pore refinement effect, on drying shrinkage of concrete containing such materials. According to Mehta and Monteiro (2006), concrete containing pore refinement additives will show higher shrinkage and creep which explains the results of Habeeb and Fayyadh (2009) for the concrete containing the F3 RHA. On the other hand, Chindaprasirt et al. (2004) and Zhang and Malhotra (1996) are of the opinion that substitution of cement with pozzolanic materials will reduce drying shrinkage.

13.7

Durability properties of concrete containing RHA

Being a versatile material, concrete is used in structures located in different environment conditions, i.e., hot and arid, arctic, tropical, as well as in structures made for different purposes like bridges, water retaining structures, nuclear power plants, thermal power plants, marine structures, and buildings. During its design life, concrete is exposed to various external and internal factors which can compromise its durability properties and make it vulnerable to chemical attack. It has been shown in the past that mineral admixtures have the potential to enhance durability properties of concrete, and this section discusses the performance of RHA in mitigating durability-related distress in concrete. The discussion focuses on the effect of RHA on the following attributes related to durability: G

G

G

G

G

Permeability Corrosion and carbonation Freezing and thawing resistance Acid and sulfate resistance Alkalisilica reactivity

13.7.1 Permeability Permeability of concrete is closely related to its pore size, and the results of Sugita et al. (1999) reproduced in Table 13.21 show that increasing levels of RHA addition (10%, 20%, and 30%) in their concretes with w/c of 0.65, 0.75, and 0.80 lead to a significant decrease in pore size at both the hardening ages of 28 and 91 days with the RHA dosage of 10% being particularly effective at w/c ratio of 0.65. Incorporation of RHA affects permeability of concrete more significantly than even its compressive strength. Porosity and water absorption of concrete containing RHA was investigated by Saraswathi and Song (2007) by mass replacement of cement with 0%, 5%, 10%, 15%, 20%, 25%, and 30% of RHA. The control concrete in which cement was the only binder had mix proportions of 1:1.5:3 with a w/c ratio of 0.53. The porosity and water absorption tests were carried out as per ASTM C642-97, and the measured results are presented Table 13.22. The following conclusions were drawn by the authors: (1) Porosity values consistently decreased with increase in RHA content, and this has been attributed to

448

Table 13.21

Waste and Supplementary Cementitious Materials in Concrete

Pore size distribution of RHA concrete (Sugita et al.,

1999) W/C (%)

65

75

80

RHA addition (%)

Average pore radius (nm)

0 10 30 0 10 30 0 10 30

Total pore volume (mm3/g)

28 day

91 day

28 day

91 day

35.2 28.2 19.2 29.3 17.9 17.5 32.9 41.2 26.4

40.0 23.1 14.4 47.7 24.9 16.9

82.9 91.6 99.5 92.3 81.8 97.1 87.2 95.4 82.1

77.5 86.9 99.5 74.9 94.0 104.1

22.9

82.3

Porosity and water absorption of RHA concretes (Saraswathi and Song 2007)

Table 13.22

RHA (%)

Porosity (%)

Water absorption coefficient (m2/s)

0 5 10 15 20 25 30

18.06 18.18 18.82 13.82 13.54 13.04 11.89

3.5571 3 10210 6.7587 3 10211 1.0302 3 10211 1.0644 3 10211 1.2122 3 10210 1.4548 3 10210 1.3030 3 10210

improved particle packing with increasing amounts of RHA; (2) Coefficient of water absorption of all the RHA concretes was less than that of the control concrete. This again is attributed to the pore refinement role played by the RHA particles in the concrete matrix. Chloride ion penetration resistance of concrete made with 10% replacement of cement with silica fume and with RHA was investigated by Zhang and Malhotra (1996) using the ASTM C1202 test protocol. Results of the investigation, which are given in Table 13.23, show that at both 28 days as well at 90 days the charge passed through the concretes containing the silica fume and containing RHA was less than 1000 C which as per the classification given in ASTM C1202 corresponds to the category “very high resistance to chloride ion penetration.” Velocity of water absorption (a measure of the capillary forces exerted by the pore structure) and coefficient of water absorption (a measure of permeability of water) were measured by Givi et al. (2010) as per ASTM C1585 on concrete samples containing cement blended with RHA. Significant reduction in

Rice husk ash

449

Table 13.23 Chloride ion penetration results of concretes containing silica fume and containing RHA (Zhang and Malhotra 1996) Mix no

CO-D R10-D SF10-D

Type of concrete

Control 10% RHA 10% SF

W/C ratio

0.40 0.40 0.40

Unit weight (kg/ m3) 2320 2340 2310

Compressive strength (MPa)

36.5 45.5 42.8

Chloride ion resistance (C) 28 day

90 day

3175 875 410

1875 525 360

percentage of water absorption, velocity of water absorption, and also coefficient of water absorption was observed with ultra-fine RHA particles (size B5 μm) at all ages, though for the average sized particles’ (size B95 μm), reduction in water permeability was observed only after 90 days moist curing. Ferraro and Nanni (2012) have also reported reduction in coefficient of water absorption of concrete containing off-white RHA with particles size similar to those in white PC. Permeability properties (water absorption, sorptivity, and chloride penetration) of self-compacting concrete (SCC) blended with RHA and metakaolin (MK) at varying percentages were investigated as per the protocols in ASTM C642 and ASTM C1202 standards by Kannan and Ganesan (2014). Table 13.24 presents variation in permeability properties of concrete containing RHA in the range 5%30%, MK in the range 5%30%, as well as for combination of both the pozzolans in the ratio 1:1. Results in this table show that there was an almost 80% decrease in chloride ion permeability of SCC at the RHA dosage of 15%, and when the pozzolan dosage was changed to 15% RHA 1 15% MK, the total charge passed decreased by almost 95%. It is interesting to note in this table that water absorption increased when cement replacement with RHA exceeded 15%. This is attributed to the relatively poor workability of concrete containing higher RHA replacement levels. Lower compaction levels associated with poor workability may have left voids in the hardened concrete which will be responsible for the increased water absorption. Chopra et al. (2015) have also reported the beneficial impact of 15% cement replacement with RHA on concrete permeability when tested as per ASTM C1202. Ramezanianpour et al. (2009) studied the influence of 0%, 7%, 10%, and 15% cement replacement with RHA on the chloride ion penetration of concrete up to the age of 90 days. Rapid chloride permeability tests (RCPT) were carried out on concrete slices of size 100 3 50 mm sawn from 100 3 200 mm cylinders, and the results are presented in Fig. 13.19. The authors have reported that inclusion of RHA significantly enhanced resistance to chloride ion penetration with this value at the hardening age of 7 days being about four times higher for the concrete containing 15% RHA when compared to the control concrete.

450

Waste and Supplementary Cementitious Materials in Concrete

Table 13.24 Permeability-related properties of concretes containing RHA, MK, and their combination (Kannan and Ganesan 2014) Mix designation

MIX/RHA/ MK 1 RHA (%)

Ordinary Portland cement (100%) RHA 05 RHA 10 RHA 15 RHA 20 RHA 25 RHA 30 MK 5 MK 10 MK 15 MK 20 MK 25 MK 30 RHA 5 1 MK 5 RHA 10 1 MK 10 RHA 15 1 MK 15 RHA 20 1 MK 20

0 5 10 15 20 25 30 5 10 15 20 25 30 10 20 30 40

Water absorption

Sorptivity 3 1026 (m/s1/2)

Total charge passed by RCPT (C)

45.4

3.56

1486.28

3.64 3.43 4.06 6.41 9.2 1.75 1.75 2.69 2.5 2.29 2.5 2.89 3.38 2.71 2.64 2.88

438.62 389.18 306.22 876.96 904.7 1089 431.23 299.32 292.98 59.74 34.67 28.23 286.72 173.6 86.3 25.43

4.53 4.1 3.93 3.92 4.47 4.92 3.59 3.57 3.48 2.88 2.83 2.78 3.23 3.02 2.98 3.17

7000

Coulombs passed

6000

Control

7% RHA

10% RHA

15% RHA

5000 4000 3000 2000 1000 0 7 days

28 days

90 days

Figure 13.19 Resistance to chloride ion penetration of RHA concrete at various ages (Ramezanianpour et al., 2009).

Rice husk ash

451

13.7.2 Corrosion resistance and carbonation RHA plays an important role in mitigation of carbonation as well as in improving corrosion resistance of concrete. Being a pozzolanic material, it improves impermeability of concrete which in turn is conducive to reduction of carbonation. However, according to Sugita et al. (1997) and Yamamichi et al. (2003), at high cement replacement levels with RHA coupled with high w/c ratio, depth of carbonation increases. Saraswathi and Song (2007) examined the corrosion performance of RHA blended concrete using impressed voltage test, and their results are shown in Table 13.25. As this table shows, concrete up to 10% RHA replacement failed within 74 hours of exposure, while beyond 10% replacement level, no cracking was observed even after 144 hours of exposure, which is indicative of reduced corrosion. Chindaprasirt and Rukzon (2008) have also reported the positive effects of RHA on corrosion resistance of mortars and have concluded that when compared to fly ash, RHA is better in improving corrosion resistance. According to these authors, whereas the time of first crack of the specimens made with PC was 89 hours, this time for the mortars blended with 10%, 20%, and 40% RHA was 167, 168, and 166 hours, respectively. Cizer et al. (2006) investigated the effect of RHA inclusion on carbonation by testing several cement mortars blended with RHA and lime. The composition of the control mortar was 1:3 cementsand ratio (by weight) and in the blended mortars cement was replaced with 30% (RHA-C.3-7), 50% (RHA-C.5-5) and 70% (RHAC.7-3) of RHA. Two types of ternary blended mortars were also prepared with cement, RHA, and lime. The tests were carried out on standard prismatic specimens of size 40 3 40 3 160 mm, and carbonation depth was measured up to 120 days. Results of the tests show that carbonation depths in the cement mortar specimens containing RHA increased with increasing dosage of RHA. This study indicates that increasing cement replacement levels with RHA in the mortar specimens had a deleterious effect on carbonation behavior. More investigations, especially on concrete containing RHA, are required before the role of RHA in carbonation of concrete can be conclusively established.

Table 13.25 Results of impressed voltage tests on concrete containing RHA (Saraswathi and Song, 2007) S. no.

Replacement percentage

Time to cracking (h)

1 2 3 4 5 6 7

Ordinary Portland cement 5 10 15 20 25 30

42 72 74 No cracking even after 144 h of exposure No cracking even after 144 h of exposure No cracking even after 144 h of exposure No cracking even after 144 h of exposure

452

Waste and Supplementary Cementitious Materials in Concrete

13.7.3 Freezing and thawing resistance Mehta and Folliard (1995) conducted tests to check the frost resistance of highperformance concrete in which RHA and silica fume were separately used for partial replacement of 15% (by weight) of cement at each of the w/c ratios of 0.30 and 0.35. The reference or the control concrete was prepared with cement as the only binder. Prisms of size 76.2 3 101.6 3 406.4 mm were cast and cured for 14 days prior to being subjected to freezing and thawing cycles in accordance with the procedure mentioned in ASTM C666A. The freezethaw tests were conducted for a total of 300 cycles or till achievement of a durability factor below 60, whichever was earlier. Test results illustrated in Fig. 13.20 show that the concretes containing RHA had superior freezethaw performance and could endure 300 cycles without reaching the failure limit of durability factor being less than 60. Concrete containing silica fume in the binary blend showed the worst performance. The better performance of concrete containing RHA may be attributed to the fact that RHA has a microporous structure which provides space for water to expand during freezing, thus reducing internal stresses. The results of Zhang and Malhotra (1996) on the freezethaw resistance of concrete blended with 10% RHA and 10% silica fume are presented in Table 13.26. The authors have reported excellent freezethaw performance of all their control RHA and silica fume concrete with the RHA concrete in particular having a durability factor of 98.3 and showing negligible changes in length, mass, pulse velocity, and resonant frequency even after exposure to 300 freezethaw cycles.

13.7.4 Acid and sulfate resistance Concrete exposed to acidic environment is prone to the progressive neutralization of its alkaline nature associated with the dissolution of portlandite, C-S-H gel, and removal of alkalies. Exposure to HCl may lead to the formation of soluble CaCl2 as 100

Durability factor

80 60 40 20 0

REF30

SF30 RHA30 REF35 SF35 RHA35

Figure 13.20 Durability factors of concrete tested for freezethaw resistance as per ASTM C666 (Mehta and Folliard, 1995).

Rice husk ash

453

Freezethaw test results of concrete containing RHA (Zhang and Malhotra 1996)

Table 13.26

Mix no. Type of concrete

Change at the end of 300 freezingthawing cycles

Compressive strength (MPa)

CO-D Control 36.5 R10-D 10% RHA 45.5 SF10-D 10% SF 42.8

Durability factor (%)

Weight Length

Pulse Resonant velocity frequency

0.08 0.02 0.12

2 0.55 0.01 0.19

0.006 0.001 0.001

2 0.84 2 0.86 0.47

98.0 98.3 101.0

100 90 Percent of original mass

80 70 Failure limit (25% mass loss)

60 50 40 30

Concrete without RHA

20

Concrete with RHA

10 0

0

4

8

16 20 12 Weeks of exposure

2.4

2.8

Figure 13.21 Mass loss of concrete cylinders immersed in 1% HCl solution (Mehta and Folliard, 1995).

a result of which porosity and permeability of concrete increases. In the presence of H2SO4 solution, SO422 ions in the presence of calcium hydroxide form gypsum leading to expansion of concrete and cracking. Mehta and Folliard (1995) examined the influence of RHA in reducing chemical deterioration of concrete upon exposure to 1% HCL solution. Experiments were conducted on cylindrical specimens (76.2 mm 3 152.4 mm) of concrete with 0.33 w/c ratio and RHA content of 13% by mass substitution of PC. Results were compared with control mix of the same w/c ratio and are shown in Fig. 13.21. Concrete containing RHA was found to be much more resistant to the HCL solution than the concrete without RHA. The reason behind such behavior was attributed to the significant reduction in permeability and the relatively lower calcium hydroxide content of concrete containing RHA, resulting in the improvement of transition zone of concrete and reduced porosity.

454

Waste and Supplementary Cementitious Materials in Concrete

Saker (2006) studied the effect of RHA incorporation (15% by mass of cement) on sulfate resistance of concrete containing heavy weight (ilmenite, barite, and gravel) fine and coarse aggregates. For finding sulfate resistance, 100-mm cubes were immersed in a MgSO4 solution for 1, 3, and 6 months, and the loss of compressive strength after these immersion periods was taken as an indicator of the strength of the sulphate attack. It was concluded that: (1) reduction in compressive strength after immersion in 5% MgSO4 for 28 days of the gravel, baryte, and ilmenite control concretes was 8.5%, 7%, and 8%, respectively, while the reductions after 90 days of immersion were 16%, 14%, and 14%, respectively; (2) the reduction percentage of compressive strength of the gravel, baryte, and ilmenite concrete containing 15% RHA for the same immersion conditions were 5%, 6%, and 6%, respectively. Chindaprasirt et al. (2007) investigated the sulfate resistance of mortar made from ternary cement consisting of a blend of fly ash and ground RHA. Class F fly ash and RHA were used at replacement dosages of 20% and 40% by weight of cement and the mortars were made with a sand-to-binder ratio of 2.75, and the water contents were so adjusted such that a flow of 110% 1 / 2 5% was achieved in all the mortars. The testing for sulfate-induced expansion was done following the procedure recommended in ASTM C1012 using 5% sodium sulfate solution. It was noted by the authors that expansion of the control specimens were significantly larger than that of the specimens made with the blended cements and accelerated expansion in the control specimens was noted from 120 days onwards, whereas no such accelerated expansion was seen in the specimens made with the blended cements. Fig. 13.22 shows the pH levels of the sodium sulfate solution, and it may be seen in this figure that the highest pH of 12.5 was observed in all the mortars within the first 7 days of immersion. This observation is indicative of the fact that a

14 7 days

90 days

180 days

13 12

pH

11 10 9 8 7 6 OPC

FA20

FA40

RHA20

RHA40

Figure 13.22 pH values of sodium sulfate solution immersed with mortar bars containing RHA (Chindaprasirt et al., 2007).

Rice husk ash

455

Compressive strength of concretes after 2 months of sulfate exposure (Ramezanianpour et al., 2009)

Table 13.27

CTRL 7% 10% 15%

28-day curing

2 months in 5% Na2SO4

2 months in 5 % MgSO4

41.92 44.61 46.19 47.46

41.58 49.78 51.40 47.74

45.5 50.28 55.49 52.16

substantial amount of the calcium hydroxide had leached out during this period and thus had increased the pH of the solution. It may be noted that the pH of the fresh solution was 77.5. At 90 and 180 days, the pH levels of the sulfate solutions were significantly lower but they depended upon the binder type. The pH value of the solution with the PC mortar specimens was the highest at 11 followed by those containing fly ash (10.7 and 10.5) and the lowest pH values of 10.1 and 9.5 were noted for the specimens in which 20% and 40% of the cement had been replaced with the RHA. The expansion of the test specimens is sensitive to the pH of the solution and lower pH values are indicative of reduced susceptibility to sulfate attack. Ramezanianpour et al. (2009) have reported the results of their investigation of the effect of RHA on sulfate resistance of concrete, and the compressive strengths of specimens exposed to 5% Na2SO4 solution are presented in Table 13.27. This table shows that when subjected to continuous sulfate exposure, all specimens showed an increase in compressive strength up to 2 months, and this is attributed to the hydration of calcium silicates and to the pozzolanic reaction of the blended cements. However, subsequently compressive strengths of the control concretes began to decrease significantly, and this has been attributed to formation of ettringite and gypsum.

13.7.5 Alkalisilica reactivity Alkalisilica reaction takes place between alkalis (Na2O and K2O) from cement or other sources and certain reactive silica minerals present in the coarse or fine aggregates. The chemical reaction can result in abnormal expansion and cracking of concrete. To avoid this reaction in concrete, the use of low alkali cements is one option or otherwise the use of mineral admixtures is recommended. In this section, the role of RHA in mitigating the adverse effects of alkalisilica reaction in mortars and concrete is discussed. Finely ground RHA can react with the alkalis in cement paste to form alkali silicates. Such a development will reduce the available alkalis in the pore solution, and this will limit attack on reactive siliceous aggregates. Alkalisilica reactivity will also be suppressed in concretes containing RHA since such concretes are likely to be less permeable, and this will prevent the penetration of water necessary for the

456

Waste and Supplementary Cementitious Materials in Concrete

Table 13.28 Expansion of mortar bars containing RHA (Hasparyk et al., 2000) Replacement (%)

Expansion (%) Quartzite

0 4 8 12 15

Basalt

16 day

30 day

16 day

30 day

0.28 0.16 0.21 0.09 0.06

0.53 0.36 0.45 0.25 0.15

0.84 0.76 0.32 0.08 0.04

1.10 0.99 0.81 0.33 0.25

0.05 0.045

Expansion (%)

0.04 0.035 0.03 0.025 0.02 0.005 0.01 0.005 0 0

7 10 RHA replacement level (%)

15

Figure 13.23 Expansion of RHA concrete prisms at 175 days (Ramezanianpour et al., 2008).

alkalisilica reaction. Therefore, expansion due to alkalisilica reaction will be less in concretes containing RHA. However, the amount of RHA required for prevention of alkalisilica reaction varies from aggregate to aggregate. The expansion of mortar bars made with different levels of cement replacement with RHA was investigated by Hasparyk et al. (2000) by using two types of reactive aggregates, quartzite and basalt. Cement replacement with RHA were 0%, 4%, 8%, 12%, and 15%, and the tests were conducted according to recommendations of ASTM C1260. Measured expansions due to alkalisilica reactivity at 16 and 30 days are given in Table 13.28, which shows that inclusion of RHA was very effective in controlling the expansion of mortar bars at the age of 16 and 30 days. Ramezanianpour et al. (2008) investigated alkalisilica reactivity by studying the expansion of concrete prisms made with different levels of cement replacement with RHA. The expansion of the test prisms when measured as per ASTM C1293 is shown in Fig. 13.23. This figure shows that after 175 days, in the concretes containing 7%

Expansion (%)

Rice husk ash

457

0.050 0.045 0.040 0.035 0.030

0% 7% 10% 15%

0.025 0.020 0.015 0.010 0.005 0.000 0

50

100

150

200

Time (days)

Figure 13.24 Trends in expansion of concrete prisms containing different RHA replacement levels (Ramezanianpour et al., 2008).

and 10% RHA expansion due to alkalisilica reaction reduced by about 52% and 33%, respectively, relative to the prisms made with the control concrete. However, specimens containing 15 % RHA showed the highest expansion of about 0.045 %. Fig. 13.24 shows the expansion of the concrete prisms up to the hardening age of 175 days, and the trends in this figure indicate that the optimum dosage of RHA in the context of controlling alkalisilica reaction is in the range of 7%10%.

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