Using rice husk ash as a cement replacement material in concrete

Waste Materials in Construction G.R. Woolley,J.J.J.M. Goumans and P.J. Wainwright(Editors) 9 2000 Elsevier Science Ltd. All rights reserved.

U S I N G R I C E H U S K A S H AS A C E M E N T R E P L A C E M E N T IN C O N C R E T E

671

MATERIAL

M. Anwar a, T. Miyagawa b, and M. Gaweesh c aAssistant Professor, Construction Research Institute, NWRC, Delta-Barrage 13621, Egypt. bprofessor, Civil Engineering Department, Kyoto University, Kyoto, Japan. cprofessor, Director of Construction Research Institute, NWRC, Delta-Barrage 13621, Egypt.

Many countries have the problem of shortage of conventional cementing materials. Recently there are considerable efforts worldwide of utilizing indigenous and waste materials in concrete. One of such materials is the rice husk which under controlled burning, and if sufficiently ground, the ash that is produced can be used as a cement replacement material in concrete. This research has been undertaken to study the main characteristics of the Rice Husk Ash (RHA), properties of fresh concrete and development of the fundamental properties of hardened concrete. Also, the research involves developing a comprehensive engineering data-base on RHA concretes including durability aspects such as long term performance in artificial sea water (5% NaC1 solution). The chloride ions permeability and chloride diffusion coefficient were measured using potentiometric titration analysis. Moreover, the porosity and pore structure of concrete were performed using Mercury Intrusion Porosimetry (MIP). The obtained test results showed that using RHA improved the different studied properties of concrete. In this paper, experimental program, test results and analysis as well as conclusions are presented.

1. INTRODUCTION Recently housing construction with local raw materials has received research attention in developing countries. Pozzolanic materials of plant and artificial origins and natural occurring pozzolans have been of much concern in Africa and Asia [ 1]. During growth, some plants absorb silica from the soil and assimilate it into their structures; one such plant, with a high concentration of silica, is the rice plant [2]. Rice husk has been reported to constitute about 20% by weight of ash when incinerated; the resultant ash contains 90-96% silica [3]. Also, Mehta [4] has reported that its high ash and lignin contents (20-30% lignin) make it unsuitable as raw material for paper manufacturing. Apart from limited uses as a source of heat in some rice mills, the bulk is burned in open heaps in order to dispose it. The high ash and lignin present in the husks make them unsuitable economic material for cellulose production. A convenient method of getting rid of rice husks is by burning them in open fields or as fuel for steam generators. The burning operation produces large quantities of

672 ash, about 20% by weight of husks. This consists essentially of silica which is in a relatively inert form and is thus not useful either for agricultural or industrial purposes [5]. Rice husk ash concrete is very much like fly ash/slag concrete with regard to strength development. The important exception is that rice husk ash is a very active pozzolanic material, and the results of the pozzolanic reactions are evident at early ages rather than later as is the case with other replacement cementing materials [6]. Replacing by even small amount of RHA is beneficial to the strength development of concrete. This suits well with the present understanding in concrete technology, whereby most current national specification allows the addition of minor constituents to cement. Strength reductions are fairly obvious at higher replacement levels; this is somewhat expected as the water to cementitious material ratios of these mixes are rather high [7]. Chloride ions may enter easily into flesh concrete from the mix components, such as cement, aggregate, mixing water and chemical admixtures, or from chloride contaminates. The chloride ions may penetrate into hardened concrete from external sources, such as curing water, deicing salts, salt spray and sea water. Chloride ions may be present in concrete in several states [8] : (a) strongly bound by tricalcium aluminate hydrates (and to a lesser extent by tetracalcium alumino-ferrite hydrates) mainly in the form of calcium chloroaluminate, (b) loosely bound (immobilised by calcium silicate hydrates), and (c) free in solution (watersoluble) in the pore space. Migration of chloride ions occurs primarily through diffusion processes. The arbitrary limits of diffusion coefficient fall in the range; of 10 -7 and 10 -8 cm2/sec [9]. It is well known that the rate of chloride ion diffusion into concrete is related to the permeability and pore size distribution. Concretes made with blended cements generally have lower permeability and more discontinuous pore structure than plain portland cement concrete. Therefore, the diffusivity of chloride ions in blended cement concretes tends to be lower [10]. Therefore, the pore structure of concrete, perhaps more than any other characteristic of the materials, affects the behavior of the concrete [ 11 ].

2. MAIN CHARACTERISTICS OF RICE HUSK ASH The rice husk ash is abbreviated here as RHA10 and RHA60. 10 and 60 in the subscript indicate the grinding time in minutes. The colour of RHA is white. The specific gravity of ash (2.25) is about two-thirds the specific gravity value of ordinary portland cement (3.16). The fineness (specific surface area) of RHA varies slightly depending on the period of grinding. The relationship between fineness and particle size of RHA with the grinding time is indicated in Figure 1. The effect of increasing the grinding time on the particle size and shape of particle size ditribution profiles of RHA10 and RHA60 are shown in Figure 2. Determination of these properties was undertaken at the facilities of Osaka Cement Company according to the specifications provided by Japanese Industrial Standard (JIS). The chemical composition of the RHA indicates that the combined proportion of silicon dioxide (SiO2), aluminum oxide (A1203), and iron oxide (Fe203) in the RHA10 and RHA60 ash was 93.25%. This satisfies the ASTM C618-78 [12] requirement for chemical composition which stipulates a minimum combined proportion of 70%. The carbon content of the RHA10 and RHA60 ash determined as loss on ignition, was 3.3%. This also satisfies the ASTM requirement for loss on ignition which should not exceed 12%.

673 The pozzolanic activity of RHA was examined by the method based on variation in electric conductivity of RHA in a saturated Ca(OH)2 solution [13]. According to Sugita et al [14], the pozzolanic activity can be estimated using the variation in electric conductivity. The results of variation in electric conductivity of RHA10 and RHA60 are 0.7 mS/cm and 0.9 mS/cm, respectively and RHAlo and RHA60 fall within the range of variable pozzolanic. 3.4

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40 60 80 100 120 Grinding time (minutes) Figure 1. Particle size and fineness of RHA vs. grinding time.

ioo 80

2 60

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40

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20 0 0.i

i

i0 Diameter

i00

i000

(~m)

Figure 2. Panicle size distribution of RHAlo and RHA60. The initial and final setting times of Ordinary Portland Cement (OPC) with RHA pastes are shown in Table 1. It is concluded that the water-cement ratio increases with the increasing of the RHA content and water-cement ratios of RHA10 are larger than those of RHA60. The initial and final setting times of RHAlo pastes increase with increasing of the replacement percent but the initial setting time of RHA60 pastes decreases when the replacement percent increases.

674 Table 1 Initial and final setting time of OPC with RHA. Cement W/(OPC+RHA) RHA Settin~ time (hour-minute) OPC" RHA ratio type Initial Final 'i00" 0 0.280 ' 2-25 3-22 90 9 10 0.364 2-31 3-49 80 9 20 0.444 RHA 1o 3-03 4-28 70 930 0.525 3-02 5-17 60 940 0.608 3-10 5-40 100" 0 0.280 2-25 3-22 90" 10 0.308 2-09 3-18 80"20 0.353 RHA60 1-55 3-39 70"30 0.399 1-29 4-17 60"40 0.448 1-24 4-20 Table 2 Mix proportions details of studied Mix No. Replacement, percent OPC, kg/m 3 RHA 10, kg/m 3 Water, litre/m 3 Sand, kg/m 3 Gravel, kg/m 3 Sand/total aggregate, ratio Water/(OPC + RHA), ratio

RHAlo concrete mixtures. 1 2 3 0 10 20 350 315 280 0 35 70 210 210 210 844 870 865 854 881 875 0.50 0.50 0.50 0.60 0.60 0.60

On the other hand, the final setting time of RHA60 pastes increases with the increase in the replacement percent. The results indicated that the setting times were still within the recommended range for ordinary portland cement (OPC) paste [ 15]. Also, The results agree with the findings of Cook et al [ 16] who reported increases in setting times of RHA pastes over those of plain cement paste.

3. EXPERIMENTAL WORK Three concrete mixtures with RHA10 only were studied. The OPC was replaced with 0%, 10%, and 20% of RHA10. The mix proportions details of concrete mixtures are summarized in Table 2. The materials that have been involved in the experimental work were selected from local sources in Japan. The properties of used OPC comply with (JIS) specifications. The used sand has 2.58 specific gravity, 2.76 fineness modulus and 1.37 % water effective absorption. The coarse aggregate is crushed basalt has 2.61 specific gravity, 15 mm nominal maximum size, and 6.9 fineness modulus.

675 The measured parameters represent : (a) properties of fresh concrete such as slump, air content, and unit weight, (b) properties of hardened concrete such as compressive strength, tensile strength, flexural strength, static Young's modulus, horizontal and vertical dynamic Young's modulus, Poisson's ratio, pulse velocity, (c) chloride ion permeability, which includes total and soluble chloride contents as well as the diffusion coefficient, and (d) pore structure, which include cumulative pore volume, total porosity, pore size and its distribution as well as total pore surface area. The procedures, equipment, and apparatus used are described in details in [ 17].

4. FUNDAMENTAL PROPERTIES OF RHA CONCRETE The properties of fresh concrete are included in Table 3. Furthermore, Table 4 shows the properties of RHA concrete cured in water until testing. While Table 5 gives the properties of RHA concrete cured 7 days in water and put in curing room conditions with temperature degree of 20~ and 80% relative humidity until testing. Data of Table 3 show that : (a) the concrete slump decreases with increasing the replacement percent of RHA due to large specific surface area of RHA, (b) the air content of RHA concrete increases with increasing RHA content due to the difference between the finesses of OPC and RHA, and (c) the unit weight of concrete decreases with increasing the replacement percent of RHA due to the change in the specific gravity of both OPC and RHA. Tables 4 and 5 indicate that with increasing RHA content the compressive strength decreases up to 28 days. After that, RHA concrete shows the same or higher strength than those of control mix. Further increase in the RHA content retards the strength development largely due to the higher water demand for these mixes. Hilmi Bin Mahmud [7] has studied the development of RHA concrete strength with high replacement percents and his findings confirmed the results of present study. In the second type of curing, the RHA concrete strength shows about 85% in compressive of that cured in water. The development rate of tensile strength with time is slightly low for the first 7 days, after that, almost all mixes have the same rate of the tensile strength development. The difference between the tensile strength of concrete of the two types of curing is found to be not more than 5%. The increasing of RHA content does not affect the values of tensile strength especially after 28 days. Flexural strength results show approximately similar trend as that of the tensile strength. The results show that no change in the value of flexural strength takes place except at 180 days due to increasing the content of RHA from 10% to 20%. Also, from the results of Table 5, it can be concluded that the inclusion of RHA in concrete does not significantly affect the static modulus of elasticity. The obtained data of specimens cured in the second curing type confirmed with the work reported in Ref. [ 18]. The horizontal dynamic Young's modulus (Eh) decreases as the RHA content increases at the early age of concrete. After that, RHA concretes show slightly higher values of (Eh) than those of OPC mix. The long term results of (Eh) showed that the static and dynamic moduli of elasticity of RHA concrete did not affect. The second curing method shows lower values of Eh than those of water curing.

676 Table 3 Properties of fresh concrete of RHA. Mix No.

1

2

3

Unit weight (t/m 3)

2.324

2.292

2.244

Slump, (cm)

14.40

11.10

5.40

Air content

1.70

1.80

2.25

Table 4 Properties of RHA hardened concrete (water curing). Mix

Property Strength (kg/cm2)

Compressive Tensile Flexural

1-day 3-day 7-day 28-day 90-day 213 29

317 32

341 39

180-day

36 5.4

130 20.5

402 40

12

32

41

49

59

63

0.92

1.9

2.5

2.96

3.18

3.42

Young's

Static

Modulus

Dynamic (Eh) 1 . 3 8

2.45

3.25

3.47

q.49

3.93

(x 105 kg/cm2)

Dynamic (Ev) 1 . 3 9

2.54

3.28

3.54

3.82

4.02

Poisson's ratio

0.152

Pulse velocity (km/sec) Strength (kg/cm2)

Compressive

0.178 0.198

0.217

0.217

0.214

1.54

2.20

2.37

2.46

4.41

4.45

28

107

174

251

312

428

Tensile

4.4

16.6

23.1

28.4

38

39

Flexural

10.6

30.4

40.8

45

55

58

Young's

Static

0.44

1.87

2.29

3.22

3.29

3.53

Modulus

Dynamic (El0

0.81

2.34

2.89

3.53

3.43

3.76

(xl 05 kg/cm2)

Dynamic (Ev) 0.80

2.40

2.91

3.66

3.61

3.99

Poisson's ratio

0.16

0.173

0.180

0.22

0.188

0.212

Pulse velocity (km/sec)

2.30

3.64

2.31

4.29

4.37

4.44

Strength (kg/cm2)

Compressive

24

80

177

209

275

377

Tensile

2.7

13.5

22.5

35.0

37.9

39.42

Flexural

9.7

28.5

41

45

54.5

65.2

Young's

Static

0.62

1.63

1.96

2.95

2.48

3.40

Modulus

Dynamic (Eh)

0.85

2.15

2.74

3.62

3.87

3.94

(xl 05 kg/cm2)

Dynamic (Ev) 0.88

2.18

2.79

3.64

3.93

4.02

Poisson's ratio

0.149

0.156 0.180

0.211

0.192

0.215

Pulse velocity (km/sec)

2.46

4.37

4..46

4.50

3.6

3.94

677 Table 5 Properties of RHA hardened concrete (7 days water curing and 20~ + 80% RH till testing). Mix

Property

28-days

90-days

180-days

Strength

Compressive

312

326

368

(kg/cm2)

Tensile

33.3

35.7

38

Flexural

47.7

57.5

80

Young's

Static

2.86

2.88

2.70

Modulus

Dynamic (Eh)

3.45

3.48

3.56

(x 105 kg/cm2)

Dynamic (Ev)

3.50

3.65

3.80

Poisson's ratio

0.199

0.173

0.172

Pulse velocity (km/sec)

2.22

3.34

4.17

Strength (kg/cm2)

Compressive

239

261

351

Tensile

27.6

32.1

34.7

Flexural

39.1

46.2

61.4

Young's

Static

2.98

2.55

2.22

Modulus

Dynamic (Eh)

3.29

3.42

3.07

(xl 05 kg/cm2)

Dynamic (Ev)

3.32

3.15

3.42

Poisson's ratio

0.181

0.189

0.157

Pulse velocity (km/sec)

4.16

4..27

4.27

Strength (kg/cm2)

Compressive

216

247

320

Tensile

29.3

31.69

34.52

Flexural

31.1

45.4

59.1

Static

2.68 298 3.22

2.27 3.09 3.42

1.95 3.10 3.36

Poisson's ratio

0.185

0.166

0.194

Pulse velocity (km/sec)

4.08

4.15

4.17

Young's Modulus (xl 05 kg/cm2)

Dynamic (Eh) Dynamic (Ev)

The OPC mix gives higher Eh than the values of RHA concrete. After 7 days, with the second curing method there was no difference in the vertical dynamic Young's modulus (Ev) due to increasing the content of RHA from 10% to 20 %. The concretes cured in water show higher values of Ev than those of the second curing method. RHA concretes show slightly lower Poisson's ratio than those of the OPC mix at early ages in the case of water curing. At long term ages of water curing, all mixes give approximately the same Poisson's ratios and OPC mix shows higher Poisson's ratio than those of the RHA mixes. RHA concrete with the second curing method shows different values of Poisson's ratios with progress in time and all values are lower than those of water curing. Further, the values of pules velocity of concrete cured in water are higher than those of the second curing

678 method. The obtained results of pulse velocity conflict with the finding of Ikpong [19], in his work, the pulse velocity decreases with increasing RHA replacement percent but in tthe present research, the pulse velocity fairly increases with increasing RHA content until 20%. The RHA concrete give higher pulse velocity than those of the OPC mix.

5. PERMEABILITY AND CHLORIDE DIFFUSION IN RHA CONCRETE Chloride in concrete is an important factor contributing to concrete deterioration. The chlorides may be considered to be present either in a bound or a free conditi~on. It is the free chloride levels in structural concrete that promote corrosion of the reinforcement. However, most existing specifications restricting the chloride content of concrete are based on total chloride levels. Thus it is important to understand the relationship between free and total chloride content. Sangha [20] stated that chloride threshold limits should eventually be expressed in terms of free chloride content. Marusin [21] mentioned that the corrosion threshold limit for soluble chloride ion concentrations in normal weight reinfiarced concrete is about 0.03 percent by concrete weight. The total chloride content is generally obtained by removing the chloride ti'om a sample by titration analysis (using nitric acid). On the other hand, the water soluble chloride content is determined by immersion of the sample in hot water, also using titration analysis. Furthermore, from the obtained results listed in Table 6, the distribution of chloride concentrations at different depths of cover were plotted in Figure 3 and Figure 4 to obtain the chloride concentrations profiles. The ability of RHA mixtures to reduce the potential detrimental effects of chloride intrusion into concrete is made clear from the results obtained, where, concretes containing RHA outperform the specimen containing OPC alone. The levels of total and soluble chloride ions of Table 6 show large reductions as the depth of concrete zones surveyed increased as shown in Figure 3 and Figure 4. For concretes studied, the first 10 mm of concrete cover provides little barrier to chloride ion penetration and underscores the importance of concrete cover to the reinforcement. On the other hand, all the results of zone 20--30 mm show lower values of total chloride ions content than the limits of reinforcement corrosion threshold. From the data of Table 6, it is evident that there are significant reductions in chloride ions permeability due to replacing the OPC with RHA. As the replacement level of the RHA increases from 10% to 20% by weight the results were affected and low chloride ions contents were obtained. Consequently, concrete containing RHA may require less depth of cover to protect the reinforcing steel than those concretes using OPC alone. The obtained results of soluble chloride ions contents of zone (20,-,30 mm) for RHA concretes are smaller than the limits of threshold for corrosion of steel. Gaynor [22] reported that one-half or three-fourths of penetrated clalorides ions in hardened concrete are soluble in water and free to contribute to corrosion, but RHA concrete mixes show lower percent than that reported by Gaynor. In the present research, RHA concretes show lower ratio of soluble/total chloride ions content than those of OPC concretes.

679

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0~10 10-~20 20---30

Depth from surface (mm)

(a) Mix No. 1 .8

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,

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(a) Mix No. 1

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0~10 10,-,20 20~30

0~10 10~20 20--30

Depth from surface (mm)

Depth from surface (mm)

(b) Mix No. 2 0.8 I 0.7 I ~~, 0.6 f -~ 0.5 "~ 0.4 "0.3 ~o 0.2 0.1

(b) Mix No. 2

,

.

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2-m 1-m 3-m 4-m 5-m

0.35 /

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0.30 t o 0.25 9~ o 0.20 F

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0-~10 10,--20 20~30 Depth from surface (mm) (c) Mix No. 3 Figure 3. Total chloride content profiles.

.

.

.

.

.

0-~10 10,--20 20~30 Depth from surface (mm) (c) Mix No. 3 Figure 4. Soluble chloride content profiles.

680 Table 6 Results of total and solubl e chloride ions as percent by weight of concrete. Mix Chloride Depth Exposure pe,,riod n to 5% NaCI solution (month) 2 3 4 No. type (mm) 1 0.69219 0.7218 0~10 0.47998 0.4'9890 0.65234 0.24204 0.22993 0.29789 10~20 0.01212 0.12410 0.08625 0.01107 0.12708 20~30 0.00740 0.01107 0.75589' 0.53628 b-l'O 0.15910 0.5'0508 0.6499 0.15069 0.1178 0.11433 2 Total 10-20 0.00690 0.05515 t3,.00676 0.0073 0.00758 20~30 0.00670 0.00676 ().42095 0.54099 0.40973 0-10 ~28980 '0.35746 0.06254 0.02626 0.06789 10-20 0.00710 0.01905 0.00652 0.00710 0.00729 20-30 0.00660 0.00743 0.18957 0.18429 0.19448 0~10 0.15597 0.10870 0.07512 0.05682 0.07688 10-20 0.00650 0.02986 0.02011 0.00599 0.03237 20~30 0.00570 0.00750 0.34672 0.18230 0.16161 0-10 0\05090 0.14437 (t.05542 0.02950 0.03384 2 Soluble 10~20 0.00640 0.01293 0.00660 0.00630 0.00576 20~30 0.00610 0.00585 0.15599 0.19151 0.12313 0--10 0.09090 0.12999 0.02210 0.00799 0.02078 10~20 0.00630 0.00859 0.00588 0.00499 0.00614 20~30 0.00570 0.00614 ,..

,

,,,

,

..,

.

,..,

Table 7 Diffusion coefficient (Dc) of studied concretes ,~xl0-7 cm2/sec). Mix No. 1-month 2-month 3-month 4-month 1 1'34 0.965 "' 0.552 0.451 2 1.23 0.617 0.544 0.420 3 1.13 0.602 0.446 0.391

5-month 0.439 0.336 0.324

Figure 5. Diffusion coefficient of chloride ions for the studied mortar.

681 Table 8 Summary of pore structure data Measured property Total pore volume, cc/g Total porosity, % Total pore surface area, sq-rn/g Average pore diameter, grn Median diameter (volume), larn Median diameter (area), ~m

of mortar at 28 and 90 days. 90 - days 28 - dales Mix 2 Mix 1 Mix 1 Mix 2 Mix 3 0.0929 0.0956 0.1006 0.0869 0.1017 6.5126 6.6957 7.2698 6.1634 7.2698 13.094 15.1870 17.7980 14.6660 19.263 0.0248 0.0252 0.0226 0.0237 0.0211 0.0732 0.0435 0.0363 0.0524 0.0338 0.0103 0.0107 0.0099 0.0095 0.0093 m

..... Mix 3 0.1067 7.4697 21.812 0.0196 0.0260 0.0098

The diffusion coefficient results of chloride ions into the concrete studied support the fact that the diffusion coefficient is affected by replacing the OPC with RHA. From Table 7 and Figure 5, it is evident that diffusion coefficient values decrease with increasing the replacement percent of RHA and the diffusion coefficient values of RHA concretes are lower than those of OPC concrete at all the exposure periods.

6. PORE STRUCTURE OF RHA M O R T A R The resistance of cementitious materials to chemical attack or physical degradation is related to the mechanical properties of the material, but more importantly the chemical and microstructural characteristics and particularly the pore structure of the hardened material [23]. Therefore, three mixtures of OPC and RHA mortars were made with the same mix proportions of the concrete studied without coarse aggregate (see Table 2). The properties studied include the pore structure of mortar which involved, porosity, total pore volume, total pore surface area, and pore size distributions. The most important output data of Mercury Intrusion Porosimetry (MIP) are summarized in Table 8. The results obtained show that the pore volume increases with the increasing RHA content and this confirms the previous notes reported by Roy [24]. Also, the pore volume of OPC mortar gives lower values than those of RHA mortar. This is due to the same reasons mentioned by Roy, where, adding RHA as mineral admixture leads to decrease in pore size or increase in the fraction of porosity in the finer pore. The RHA mortars indicate higher values of pore surface area than those of the OPC and the measured pore surface area increased with the increased RHA content due to the existance of more fine pores. The mortar of OPC mix shows lower porosity than the range of OPC mortar reported by Raymond[25]. Further, porosity of RHA mortars shows higher values than those of the OPC mortar mix and total porosity increases with an increase in RHA content. Using RHA cement affects the properties of product mortar, therefore, the open channels are blocked by the effect of RHA and hydration products, leading to a change of pore structure such as the formation of finer and discontinuous pores. Moreover, the obtained results demonstrate the importance of curing on the properties of studied mortar. The profiles curves of the relationships between the applied pressure and measured pore diameter, cumulative pore volume, cumulative pore surface area, and cumulative porosity at 28 days and 90 days for the mortars studied are shown in Figure 6 and Figure 7, respectively.

682

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100 1000 10 4

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Pressure (psia)

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Mix 1 Mix 2

0.12 o 0.10 0.08 _ 0.06 0.04 0.02

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10 100 1000 104 Pressure (psia)

(b) Pressure vs. cumulative pore v o l u m e ~ N

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(a) Pressure vs. pore d i a m e t e r

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Pressure (psia)

(a) Pressure vs. pore diameter

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Figure 6. Profile curves at 28 days.

Figure 7. Profile curves at 90 days.

683 7. CONCLUSIONS 1. The properties of RHA varies depending on burning and grinding methods. Moreover, The initial and final setting times of OPC-RHA pastes are affected due to adding RHA. Furthermore, the chemical composition of RHA satisfies the ASTM C618-78 requirements for chemical composition. 2. The compressive strength of concrete decreases with increasing RHA content up to 28 days. After that, RHA concretes show the same or higher strength than those of an OPC mix. Therefore, concrete containing RHA exhibit higher long term strengths and better durability. 3. RHA has its special properties and good influence in concrete properties especially those controlling the durability such as low chloride ion permeability and low diffusion in concrete. Total chloride contents indicate that RHA concrete outperform the concrete of OPC alone and its levels show large reductions as the depth of concrete zones surveyed increased. Therefore, RHA concrete may require less depth of cover to protect the reinforcing steel than those concretes using OPC alone. 4. The addition of RHA as blending materials improves the different characteristics of product concrete due to both chemical reactions and physicochemical effects. The open channels are blocked by the effect of RHA and hydration products, leading to change of pore structure such as the formation of finer and discontinuous pores.

REFERENCES

1. Okot-Uma and W.O. Roger, A Survey of Local Building and Housing Construction - Phase I. Local Raw Material and Technology for Housing Construction Project (Africa), CSC Publication, 1987, p31. 2. R.G. Smith and G.A.Kamwanja, The Use of Rice Husk for Making a Cementitious Material, Proc. Joint Symposium on the Use of Vegetable Plants and their Fibers as Building Material, Baghdad, 1986. 3. B.O. Juliano, Rice: Chemistry and Technology, American Assoc. of Cereal Chemists, St. Paul, Minneapolis, 1985. 4. K. P. Mehta, The Chemistry and Technology of Cements Made from Rice Husk Ash, proc. UNIDO/ESCAP/RCTT Workshop on Rice Husk Ash Cement, Peshawar, Pakistan, 1979, pp. 113-122. 5. P. K. Mehta, Properties of Blended cements, Cements Made from Rice Husk Ash, J. Amer. Conc. Inst. V 74, 1977, pp. 440-442. 6. V. M. Molhotra, Fly Ash, Slag, Silica Fume, and Rice Husk Ash in Concrete : A review, Concrete International, April 1993, pp. 2-28. 7. Hilmi Bin Mahmud, Rice Husk Ash as a Cement Replacement Material in Concrete-The Malaysian Experience, The 4th JSPS-Vcc Seminar on Integrated Engineering, Kyoto, Japan, Oct. 1992, pp. 280-285. 8. R. S. Ravindrarajah and P. R. Moses, Effect of Binder Type on Chloride Penetration in Mortar", 4 th Int. Conf. on Structural Failure, Durability and Retrofitting, Singapore, July 1993, pp. 303-309.

684 9. H. Nagaro and T. Naito, Application of Diffusion Theory to Chloride Penetration into Concrete Located in Splasing Zones, Transactions of the Japan Concrete Institute, Vol. 7, 1985, pp. 157-164. 10. D. J. Cook, I. Hinczak, M. Jedy, and H. T. Cao, The behaviour of Slag Cement Concretes in Marine Environment-Chloride Ion Penetration, proc. of 3 rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, Trondheim, Norway, 1989, pp. 14671483. 11. W. Czermin, Cement Chemistry and Physics for Civil Engineers, 2 nd English Edition, Foreign Publications Inc., New York, 1980, P. 58. 12. ASTM, ASTM C618-78, Specifications for Pozzolanas, ASTM, Philadelphia, 1978. 13. M. P. Luxan, et al, Rapid Evaluation of Pozzolanic Activity of Natural Products by Conductivity Measurement, Cement Concrete Research, Vol. 19, pp. 63-68, 1989. 14. S. Sugita, M. Shoya, and H. Tokuda, Evaluation of Pozzolanic Activity of Rice Husk Ash, ACI, SP-132, May 1992, pp. 495-512. 15. A. A. Ikpong, The Relationship Between the Strength and Non-Destructive Parameters of Rice Husk Ash Concrete, Cement and Concrete Research, Vol. 23, 1993, pp. 387-398. 16. D. J. Cook, R. P. Pama, and B. K. Pual, Bldg. Envir. 12, 282, 1977. 17. M. Anwar, Use of Rice Husk Ash as Part of Cement Content in Concrete, Ph.D. thesis, Cairo University, 1996. 18. A. M. Neville, Properties of Concrete, 3rd ed., Pitman Publishing Ltd., London, 1981. 19. A. A. Ikpong, The Relationship Between the Strength and Non-Destructive Parameters of Rice Husk Ash Concrete, Cement and Concrete Research, Vol. 23, 1993, pp. 387-398. 20. Sangha, C. M., et al, "Effect of Cement Type on the Relationship between Free and Total Chlorides in Concrete", Proc. 4th Int. Conf. on Structural Failure, Durability and Retrofitting, Singapore, 14-15 July 1993, pp. 310-315. 21. S. L. Marusin, Influence of Length of Moist Curing Time on Weight Change Behaviour and Chloride Ion Permeability of Concrete Containing Silica Fume, 3 rd int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, trondheim, Norway, SP-114, 1989, pp. 929-944. 22. R. Gaynor, Understanding Chloride Percentages", steel Corrosion in Concrete : causes and restraints, ACI SP- 102, 1987, pp. 161-165. 23. D. M. Roy, The Effect of Blast-Furnace Slag and Related Materials on the Hydration and Durability of Concrete, ACI, SP- 131, 1992, pp. 195-208. 24. D. M. Roy and K. M. Parker, Proc. CANMET/ACI First Intl. Conf. on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-products in Concrete, Vol. I, Ed. Malhotra, V. M., ACI SP-79, ACI, Detroit, 1983, pp. 397-414. 25. A. Raymond, Cook, C. Kenneth and Hover, Mercury Porosimetry of" Cement-Based Materials and Associated Correction Factors, ACI Materials Journal, March-April 1993, pp. 152-161.