2. The Use of RHA in

THE USE OF RICE HUSK ASH IN CONCRETE Chao Lung Hwang andsatish Chandra

INTRODUCTION

Rice husks are by-products of rice paddy milling industries. For rice growing countries, rice husks have attracted more attention due to environmental pollution and an increasing interest in conservation of energy and resources [ 1,2]. There are several reasons that draw the attention of concrete researchers and are discussed below. Proper Disposal About 20% of a dried rice paddy is made up of the rice husks. The current world production of rice paddy is around 500 million tons and hence 100 million tons of rice husks are produced, as shown in Table 1 [ 1,3]. The rice husk has a large dry volume due to its low bulk density (90-150 kg/m3), and possesses rouge and abrasive surfaces that are highly resistant to natural degradation. Disposal has become a challenging problem. It is recognized that only the cement and concrete industries can consume such large quantities of solid pozzolanic wastes.

184

The Use of Rice Husk Ash in Concrete

Supplementary

Binder

For developing countries where rice production is abundant, the use of rice husk ash (RHA) to partially substitute for cement is attractive because of its high reactivity. As the production rate of rice husk ash is about 20% of the dried rice husk, the amount of RHA generated yearly is about 20 million tons worldwide. Also, properly treated ashes have been shown to be active within cement paste. Hence, the use of rice husk ash in concrete is important. In this chapter, the characteristics, quality, hydration mechanism and influence of rice husk ashes on the quality of concrete are discussed. CLASSIFICATION OF RICE HUSK ASH

The chemical composition of rice husk is similar to that of many common organic fibers and contains: a) cellulose (C,H,,O,), a polymer of glucose, bonded with B-1.4, b) lignin (C,H,,O,), a polymer of phenol, c) hemicellulose, a polymer of xylose bonded with B-l.4 whose composition is like xylem (C,H,O,), and d) SO,, the primary component of ash [3]. The holocellulose (cellulose combined with hemicellulose) content in rice husk is about 54% [4], but the composition of ash and lignin differ slightly depending on the species, as shown in Table 2. The critical composition of rice husks from different species also varies slightly (Table 3) [4-71. After burning, most evaporable components are slowly lost and the silicates are left. The characteristics of the ash are dependent on the components, temperature and time of burning [I]. In order to obtain an ash with high pozzolanic activity, the silica should be held in a non-crystalline state and in a highly microporous structure [ 1,3]. Hence, the burning process should be controlled to remove the cellulose and lignin portion while preserving the original cellular structure of rice husk. Traditional open-field burning can create air pollution that is suspected to cause lung and eye diseases within the human population, as well as damage to plant life [ 1,2]. Mehtas

185

Waste Materials Used in Concrete Manufactuting

Table 1. World Production Rate for Rice Paddy and Rice Husk (Million Metric Tons).

Thailand us Vietnam Others Total

20 7 18 26 500

4.0 1.4 3.6 5.2 100

Rice husk ash is approximately 20% of the rice husk, i.e., the total world production of RHA will be 20 million metric tons.

186


Table 2. Chemical Composition

of Rice Husks.

Table 3. Ultimate Analysis of Rice Husk, Hwang and Wu [3].

Chemical C 38.3 39.4 39.5 H 5.7 5.5 5.5 0 39.8 36.1 37.7

Composition N 0.5 0.5 0.8

vt S 0.0 0.2 0.0

4Cl 0.0 0.2 0.0 Ash 15.5 18.2 16.5 Ref. I51 [61

(71

fluidized bed furnace was designed in 1974 to produce energy and highly pozzolanic ash from incineration of rice husks [S]. Modern electric furnace has been used in order to maintain proper control of the burning process and to produce better quality ash [ 11. The Effect of Burning Temperature Reactions at different burning temperatures are summarized below, and the chemical compositions of RHA produced are shown in Table 4 [3].

187

Waste Materials Used in Concrete Manufacturing

Table 4. Chemical Compositions of RHA (Taiwan) under Different Burning Temperatures, Hwang and Wu [3].I

700 86.7 1 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 1000 92.73 2.57 1.97 0.91 0.66 0.16 0.45 0.68 95.48 0.59 0.09 1.16 1.28 0.73 0.43

Oxide (%I

~ K@Na,O Fe,O,

(4

At 4OOC,due to transglycosylation, polysaccharides begin to depolymerize, producing levoglucosan, monosaccharide derivatives, and oligosaccharides. Dehydration of the sugar units occurs above 400C producing 3-deoxyglucosenone, levoglucosenone, funicular and furan derivatives. At 7OOC,the sugar unit decomposes, producing some cabby compounds such as acetaldehyde, glyoxal and acrolein. At temperatures above 7OOC, these unsaturated products react together and through free radical reaction, form a highly reactive carbonic residue [5, 8-l I]. 188

(b)

cc>Cd)


Differential thermal analysis (DTA), thermogravimetric analysis (TGA), and TMA showed the first peak at 95C was caused Between 150C and 250 C, a low intensity by dehydration. endothermic reaction was followed by one extreme at about 300C which may have been due to oxidation reactions. The amount of char formed was about 60% of the initial weight (Figure 1). X-ray, chemical, and EDAX analyses revealed that the higher the burning temperature, the greater the Si content in the ash. The K, S, Ca, Mg as well as several other compounds were revealed to be volatile, as shown in Table 4 and Figure 2 [3]. The SEM micrographs in Figure 3a and 3b [3] illustrate how the surface fiber was burnt to ashes first, while the inner fiber had not been fully decomposed. Ground husk decomposed more completely at high temperatures (Figure 3~). It is clear that the heating rate and time affect the quality of ash produced. The amount of ash formed at around 600C was greater than that at 400C and it was in a dispersed state (Figures 3d to 3 I). At 700C, the char ash changed from a gray to a pink color. Furthermore, some spherical debris were observed (Figure 3j). The type of crystals formed at 700C and 800C were similar, however, some conversion to a coral-shaped configuration occurred (Figure 3 k). possibly due to different temperature effects. Between 900C and lOOOC,the formation of coral-shaped crystals increased (Figure 3m to 30). Although the crystals retained their coral shape at 1 lOOC, they were finer and melted considerably, as can be seen in Figure 3p or 3r. A phase diagram of the CaO-SiO, system can be used to determine the type of phases formed. The types of silica formed between 900C and 1000C may be tridymite, wollastonite, p-Ca,SiO, and P-quartz [12]. At 700C to 800C a-quartz and wollastonite phases coexist. Because the clinkering temperature of rice husk is under 15OOC,polymorphism of silica such as quartz, tridymite, and cristobalite with open structured SiO, in RHA is produced which can promote pozzolanic reaction. However, this reaction may vary with the solubility of SiO,, depending on both the soluble layer and the density of the silica [ 131. According to Table 5 [3], the mean

189


I

____

-

I

-----

__-_------

1

I

I-

300-C ASH_ ______-_-_ - --a-

!iah-_ -_--. -------___--_-____

\ TGS ---I I I

-\ \ \ \ 1 \ 1

DTA RICE HUSK I 0 , A 200

I 400

I 600

\

, 800

Temperature

(C )

Figure 1. The DTA and TGA spectra for the rice husk under different burning temperatures. Adapted from Hwang and Wu [3]. 190


60O"CASH

Figure 2. The x-ray diffraction pattern of RHA under different burning conditions. Adapted from Hwang and Wu [3]. 191


Figure 3a-f. The SEM micrographs of RHA burned at different temperatures, after Hwang and Wu [3]. 192


Figure 3g-1. The SEM micrographs of RHA burned at different temperatures, after Hwang and Wu [3].

193


Figure 3m-r. The SEM micrographs of RHA burned at different temperatures, after Hwang and Wu [3]. 194


diameter of pores of RHA between 600 and 700C is highest and therefore, the pozzolanic reaction of ash formed at this temperature should also be greatest. The lower the burning temperature, the lower the energy consumption and thus, temperatures of 600 to 700C for rice husk ash formation do not involve excessive energy. Table 5. Pore Analysis of RHA Under Temperatures, after Hwang and Wu [3].Mercury Penetration Volume (cm3/gm) 0.3567 0.3379 0.2912 0.0795 0.0643 0.0393 0.0323 0.0171

Different

Burning

Burning Temperature(C) Char 400 600 700 800 900 1000 1100

Mean Pore (A) 5257 5954 6069 5867 5446 5150 4972 3900

Median Pore (A) 4585 6847 6847 5718 5250 4483 3867 9803

The Effect of Burning Time & Furnace Environment Above SOOC,an increase in the burning temperature, time, and environment tend to cause a sintering effect (coalescing of fine particles) as was mentioned earlier, and, is indicated by a dramatic reduction in the specific surface (Table 6) [ 141. Combustion environment also plays an important role. It should be noted that a change in the rate of oxidation from moderately oxidizing conditions to highly oxidizing conditions (oxygen (CO, environment) environment) was responsible for the steep drop in the microporosity (Table 5) and surface area (Table 6). The diffusion process for obtaining a reactive cellular rice husk is shown in Figure 4 and is based on the data presented in

195


Table 6. Effect of Burning Conditions on the Crystal Structure and Surface Area of Rice Husk Ash. Adapted from Ankra [ 141.

Burning Temperature Hold Time Environment

Properties of ASH Crystalline Surface Area, m2/g 122 moderately oxidizing noncrystalline / 15min-lhr 97 76

1 min 500-600C 30 min 2 hrs

7004300c

>l hr

>8OOC

>I hr

Tables 6 and 7. It shows that optimum incineration condition is important to obtain reactive rice husk ash with microporous and cellular structure. But, it is not suggested to burn rice husk above 800C longer than one hour to obtain suitable pozzolanic reactivity. Also, it is believed that rapid cooling may increase the reactivity of RHA [15]. ANALYSIS OF THE QUALITY OF RHA

The quality of RHA actually depends on the method of ash incineration and the degree of grinding. It also depends upon the preservation of cellular structure and the extent of amorphous material within the structure.

196


1000 900 800 700 600 500 400 300 200 100 0 Time (hours) curve for obtaining cellular structure

Figure 4. The optimum incineration condition reactive cellular RI-IA.

197


Different Sources The production of rice husk in the whole world is about 100 million metric tons a majority of it coming from countries in Southeast Asia. Depending on growing conditions and the variety of rice species, the quantity of straw also varies, but as indicated in Table 1, the chemical composition of different sources is quite similar. Typical components of RHA, as shown in Table 2 are [ 161: Cellulose 40-50% Lignin 25-30% 15-20% Ash Moisture 8- 15% After burning at a suitable temperature and time (- 6007OOC, 2 hours), in an industrial furnace, the rice husk is composed of 90-95% SiO,, l-3% K,O and < 5% unburnt carbon, as shown in Table 7 [ 1,3, 17-211. The quality of ash from different sources after proper burning varies only slightly. For use, the RHA should be amorphous and highly porous. The pozzolanic reactivity is also dependent on the surface area of RHA. Rice husk ash with a surface area of 50-100m2/gm is now available. The summary of the research conducted from 1976 to 1991 is shown in Table 7. The pozzolanic activities for the RHA when mixed with < 70% (by weight) cement is greater than 100%. Different Processes Burning process affects the quality of rice husk ash produced. Research studies have shown the influence of processing on a variety of properties of the RHA, as shown in Table 7 [ l-3,7-12].

198

Table 7. The Incineration Conditions, Chemical Compositions, Ash. Adapted from Hwang and Wu [3] and Mehta [2].

Characteristics

and Strengths of Rice Husk

T&

i:*Ed -

ioz 88 3.9: 93 -

-

-

-

-

-

-

l I3-7I 8

amorphau* 17.57

92

4

41 -

*.,I

III I

-

5

1

,-! -

/


a. Open-Field Burning Open-field burning of rice husk not only produces poor quality of ash, but is banned in many countries due to pollution problems. Uncontrolled burning results in a structure of highly crystalline form that is of low reactivity. b. Fluidized-Bed Furnace Burning

Fluidized-bed furnace is designed to control burning of rice husk [ 171. In the process, the heat from the combustion of rice husk was utilized to produce steam or electricity. A close control of the time-temperature parameter in the burning operation is maintained. A highly pozzolanic ash is produced. Highly pozzolanic RHA is created by maintaining husk combustion temperatures between 500 and 700C for a relatively long period to remove most of the carbon [I], or at temperatures around 700-800C for less than one minute. The chemical analysis of the ash samples produced by a fluidized-bed furnace showed 80-95% SiO,, l-2% I&,0, and 3-18% unburned carbon. The ash was highly cellular, with 50-60 m2/g surface area measured by nitrogen adsorption. The fibrous texture of the silica in ash results presumably from its deposition between cellulose fibrils in the microvoids. c. Industrial Furnace A modern industrial furnace has been recently used due to environmental and economic reasons [l] (Table 7). Depending upon the efficiency of combustion, the silica content of RHA may be in the range of 90-95% with residual carbon as the main remaining ingredient. In addition to residual carbon, alkalis ranging from 1 to 3% form the other impurity. By controlled combustion in the industrial furnace, it is simple to produce RHA with silica in an amorphous and highly cellular form, with 50- 100 m2/g surface area. This type of rice husk ash is highly pozzolanic. 200


The Effect of Burning Time and Temperature Area and Its Reactivity

on the Surface

The RHA must be burned for the correct time and temperature to achieve the requisite pozzolanic activity, as indicated in Table 7 and illustrated in Figure 4. Table 6 clearly indicates that not only the burning temperature [ 141, the burning time is equally important in remvoving carbon while keeping the silica in an amorphous and highly cellular form. The surface area of RHA burned at 500-600C for 1 minute is as high as 122 m2/g without causing crystallization, as shown in Table 6. Longer burning times will cause collapse of the cellular form and also coalescence of the fine pores [3], which consequently causes a reduction in surface area [ 11. At higher temperatures with longer burning times, a crystalline structure is formed with a sharp reduction in surface area. This lowers the pozzolanic activity. Figure 4 indicates the ideal time/temperature path to obtain optimum quality rice husk ash with a microporous and cellular structure which is highly reactive.

HYDRATION

MECHANISMS

OF PASTE WITH RHA

Understanding the hydration of paste with RHA is important for using pozzolanic materials and controlling the properties of the paste [3]. The heat evolution curve, microstructure development, and ultrasonic velocity are important for studying hydration mechanisms in paste made with RHA. Paste/RHA Heat Evolution Curve The study of the rate of hydration can be used to predict strength development. However, if the interior temperature is too high, cracks may develop in the cement paste [3]. Figure 5 reveals that the heat evolution curve of cement paste with RHA is similar in shape to that without RHA. It also shows the effect of water to

201


6

-0

4

_._._._._.

15

___._._

1

10

100

Time (Hours) Figure 5. Heat evolution of cement paste with RHA. Adapted from Hwang and Wu [3].

202


cement ratio and the amount of replacement by RHA on heat evolution. The first peak is higher than the second peak. The larger the amount of RHA added, the lower the amount of heat evolved. At higher RHA contents, K and SiO, react with Ca2 to lower both the first and second peaks. High water to cement ratio may also lower the heat of hydration due to the diluting effect of water. At the second stage of hydration (dormant period), the concentration of Ca2 decreases, thereby increasing the saturation time of ion and thus delaying the second peak. However, the pH value increases primarily due to the potassium content in RHA dissolved in water, as shown in Table 8. This compensates for the alkali concentration consumed by Ca2 to nucleate earlier and to accelerate the second peak. The setting time of ordinary cement paste occurs before the second peak of the calorimetric curve; such a mechanism can be responsible for short setting time of cement paste containing RHA (Figure 6). Table 8. The pH Values of RHA Dissolved in Water. Adapted from Hwang and Wu [3].

RHAIW 0.05 0.1 0.15 0.2 0.25 0.3

T

pH Value 1 day 3 days 9.59 9.82 9.91 9.91 9.98 9.98 7 days 9.55 9.75 9.80 9.86 9.91 9.91

W/RHA

* Ash burned at 700C Microstructure of Hydrated Cement Paste with RHA C-S-H gel, CH, ettringite, and of capillary pores and unhydrated core on cement composition and curing additives may alter the crystalline 203

Cement hydrates to monosulfoaluminate. The type varies with age and depends conditions. However, different


_____._ _ __,.. s *t __ _..__3I

a

,

.c-_~,______~_________l.....__......._. __..-.-.-.!

I

_----i ..-. ----/ I

&.

-

---+_______~~_~_y..-.. z.E.*.* I

+._-~,,-

F if3 v)

2

0

,

!

I

/ (b) wbo.47 j

&

0

5

10

IS

30

RHA (W

Figure 6. Setting time of cement paste with RHA. Hwang and Wu [3].

Adapted from

nature of products, thereby resulting in the generation of more hydrates. From optical microscopy observations, a magnification exceeding 400X or an excessively high concentration of cement paste (low water to cement ratio), produces an unclear image. Only the development of CH crystals can be observed. A high water to cement ratio of approximately 0.6 is therefore used to obtain clear images. The symbol I--+ in Figure 7 indicates that a CH crystal appears between unhydrated cores and that C-S-H gel appears after 5 hours [3]. The crystal at 6.5 hours is 4 times the size of that after 5 hours; after 7.5 hours it is 8 times greater and after 12.5 hours, 26 times greater. The free water generates a significant amount of C-S-H gel, and consequently, RHA and unhydrated cores become large due to 204


(c) 12.5 hours

I

(fl 15 hotlrs

--I-

I__(i) 13 hours

I ,

Figure 7. Optical micrographs of cement paste with RHA. Adapted from Hwang and Wu [3]. 205


the build-up of hydrates. This may be related to the dense sponge matrix, as shown in the SEM micrograph of cement paste containing RHA. However, such a phenomenon does not occur in cement paste without RHA (Figure 8g-8h). The symbol --+Iin Figure 8a to8f indicates the occurrence of a hexagonal crystal after 7 hours. A cement paste formed with a water to cement ratio of 0.4 and containing 15% RHA was investigated using scanning electron microscopy and revealed a significant difference between pastes with and without RHA. After one day, the C-S-H gel was in the form of a dense sponge matrix (Figure 8a, b, c) [3]. Most of the CH may have reacted with RHA. Some hexagonal plates (about 0.1 pm) grow on the surface of the RHA similar to that of monosulfoaluminate (Figure 8d). After 3 days, the dense fibers bond with the matrix within large pores (Figure 8e). The fiber surface is no longer smooth and some of it is filled with C-S-H gel (Figure 8f). After 7 days, the crystal (1.5 urn) looks like a bundle of fibers, constrained in the middle, but flower-like at the ends (Figure 8g). After 28 days, the matrix has become more dense and the undecomposed CH crystal clogs the matrix (Figure 8h). After 60 days, the matrix bonds together and the presence of CH becomes unclear (Figure Si). A cement paste with water to cement ratio of 0.47 and containing 10% RHA was also investigated. The porosity of this system was found to be higher than that of cement paste with a water to cement ratio of 0.35 which may result in lower compressive strength. The pore structure of the cement paste containing 10% and 15% RHA replacement are nearly identical (Figure 9) [3]. Therefore, theoretically, the compressive strengths of these pastes should not differ much from each other. Hydration Mechanisms of Paste with RHA

Figure 10 reveals the hydration behavior which includes the calorimetric curve, the ultrasonic pulse velocity curve, the penetrative resistance curve, and the setting time of paste with 20% RHA (22). According to the hydration mechanism of ordinary portland cement without RHA (23), a transition zone exists in the ultrasonic pulse

206


207

Vaste Materials Used in Concrete Manufacturing

1 DAY

lb) w/s -

= 0.47 RHA=5% 10%

. . . . . . . . RHA=

0.30 t1 DAY 3 DAY 7 DAY 28 DAY

ICI w/s 0.30 -.-.-

=0.47 .RHA = 15% _=20%

Figure 9. Pore analysis of cement paste. Adapted from Hwang and wu [3].

208


300

250

200

150

100

50

0

400

5 >__ ? I !ji 300

8

,

.Lo

!--

200

$2 ._

4

100

%

0

400

300

200

100

0 0.1 I I

Time (Hours)

Figure 10. The relation of calorimetric curve, ultrasonic pulse curve, penetrative resistance strength curve and setting time (w/c=O.47), Hwang, Peng and Lin [22].

209


velocity curve which corresponds to the period between the dormant and deceleration periods in the calorimetric curve. The transition zone occurs roughly at the points of inflection in both the calorimetric and the penetrative resistance curves, with the setting time measured with a Vicat needle. Moreover, the time of appearance of the second peak in the calorimetric curve approximately coincides with the final setting time. Such a transition zone will be delayed with the increase in water to solid ratio. The addition of RHA will cloud the transition zone [22], but the hydration mechanism is similar to that of ordinary Portland cement. The final setting of paste with 5% and 20% RHA added begins at the second peak as shown in Figure lob and 1Oc [22,23]. This implies that the addition of RHA will strengthen the matrix. The hydration mechanism of paste with RHA can be hypothesized, may appear as shown schematically in Figure 11 [22], and may be described as follows: The penetration resistance coincides with the growth of CH (Calcium hydroxide) up to 8 hours, and this is similar to the behavior of ordinary portland cement paste. The early resistance may be primarily due to the formation of CH crystal. From the pulse velocity curve as shown in Figure 10, the pulse change coincide with the early detection of penetration resistance. The formation of CH at the surface of rice husk ash may be due to the adsorption by cellular structure of RHA. In such case the bleeding water will be significantly reduced. The adsorbed water enhances the pozzolanic reaction inside the inner cellular spaces and gain significant strength. The ultra-sonic velocity gain rapidly after the formation of CH and the contact of RHA solid through the silica skeleton. This behavior is different from that of pure Portland cement paste. After 40 hours, the pozzolanic reaction further binds Si in RHA with CH to form C-S-H gel and solid structures. This means that RHA fills the finer pores and reduces the permeability, which may be beneficial to the durability.

210


EARLY CHARACTERISTICS

OF CONCRETE

WITH RHA

The early characteristics of concrete with RHA depends on the water to cement ratio, the amount of paste used, the amount of RHA added, other admixtures used and mixture proportion. The Workability of Fresh Concrete with RHA

At a given water to cement ratio, small addition (less than 2 to 3 by weight of cement) of RHA may be helpful for improving the stability and workability of concrete by reducing the tendency towards bleeding and segregation [24]. This is mainly due to the large surface area of rice husk ash which is in the range of 50 to 60m2/g [ 11. Large additions would produce dry or unworkable mixtures unless water-reducing admixtures or superplastizers are used, as shown in Figure 12 [3]. Due to the adsorptive character of cellular rice husk ash particles, concrete containing RHA require more water for a given consistency. At high water-cement ratio, the workability tends to improve, as shown in Figure 12 [3]. The addition of sand will significantly reduce the flow table spread. For a given consistency, the reduction of water requirement can lead to an overall improvement in many engineering properties. Granulometric characteristics of the coarse aggregate, fine aggregate, and cement particles influence the volume of voids and water requirement of a concrete mixture. The addition of fine particles of a mineral admixture, typically in the order of 1 to 20 ym in size, would supplement the cement grains in further reducing the volume of voids in the concrete mixture. Consequently, it will require less water to produce a concrete of a given consistency [25]. The workability of fresh concrete with RHA can be improved by densifying the mixture [25,26]. The process uses cement and rice husk ash with water to fill the pores and voids within well-compacted aggregates. The density of concrete made by this process is higher

211


IaJ o-3

hOlh-s

b) 3-8 hour:

(cl 8-40 hour

.(d) after 40 hours

Figure 11. Schematic drawing of the hydration of cement paste with RHA. Adapted from Hwang, Peng and Lin [22]. 212

The Use of Rice Husk Ash in Concrete180 160

... ......... __. .... _. .. ........... I..................... _..._ _... .,Paste,[W/S=0.47 i .... ...... ___________.____._.~_ ................................. : .... : l *., .... .................... ............................. ........ j...... . ....... r.%.&_y......i..................._.................

L%&.~_._. ................................ ......... ........ ..ji

...................... .!a... &.._. 140 _. ............................... +___.

120 100 80 60 40 20 0 0

........

I.._.-.................. ........ .............. j _..._..__~:~~ .

5

10

15

20

RHA (/) Figure 12. Flow table spread of cement paste and mortar with RHA. Adapted from, Hwang and Wu [3].

than that of conventional mixtures with more cement. Slump can be controlled to 250 + 20mm range with excellent rheological properties and with small reduction in slump after 45 minutes [25]. The Setting Time of Concrete with RHA Unlike other pozzolanic materials, rice husk ash tends to shorten the setting time, as shown in Figure 6. This may be due to the water adsorption ability of the cellular form of rice husk ash and hence, the surrounding water-to-cement ratio is reduced. It is further substantiated by the early detection of the ultrasonic pulse velocity, as shown in Figure 10, reflects that the rigid silica cellular skeleton 213


also plays an important role in setting time. Higher water-to-cement ratio tends to increase the setting time because there is less contact between the open matrix and the silica cellular structure causes a reducuction in early strength development. The Compressive Strength and Impermeability RHA of Concrete with

In normal concrete, the transition zone is generally less dense than the bulk paste and contains a large amount of plate-like crystals of calcium hydroxide, with the c-axis perpendicular to the aggregate surface [27]. This is suspected to induce microcracks due to the tensile stresses induced by thermal and humidity change. The structure of the transition zone is the weakest phase in concrete and has a strong influence on the properties of the concretes. The addition of pozzolanic materials can affect both strength and permeability by strengthening the aggregate-cement paste interface and by blocking the large voids in the hydrated cement paste through pozzolanic reaction. This phenomenon is shown in Figure 13. It is known that the pozzolanic reaction modifies the pore-structure. Products formed due to the pozzolanic reactions occupy the empty space in the pore-structure which thus becomes densified. The porosity of cement paste is reduced, and subsequently, the pores are refined. Mehta [I] has shown significant reduction in the porosity of cement paste with RI-IA additions and refinement in the porestructure. Pozzolanic reaction is a slow process and proceeds with time. It is illustrated in Figure 14 [I] that the pore refinement is in progress even after 28 days. Rice husk ash adsorbs large amount of water due to its high specific surface area. This reduces bleeding water. The high absorption of the RHA has been shown by Hwang [3] (Figure 15). It improves the weakest zone under the aggregate. However, adding the correct amount of rice husk ash is important for achieving high strength. Large amounts of rice husk ash have an adverse effect and reduce strength as shown in Figure 16 [3]. The early strength of 214

III \\Cement Grain

CI

I I ICement Grain

Figure 13. Mechanism

of void filling and transition zone strengthening

effects of RHA.

0.25

Pore Diameter

(A)

Pore Diameter

(A)

Figure 14. Pore refinement of hydrated cement pastes by RHA. Adapted from Mehta and Monteiro [27].

The Use of Rice Husk Ash in Concrete concrete is a function of water-to-binder ratio. As long as the waterto-binder ratio is kept constant, the early strength of concrete will be similar, but the ultimate strength will be enhanced due to pozzolanic reactions. It is further seen that above 54 kg/m3 rice husk ash addition, there is no influence on the strength. However, it decreases the permeability of concrete. At a high water to cement ratio, the addition of RHA to cement paste will not only reveal a significant effect on strength at early ages, but the strength at later ages also tend to be higher than those with lower water to cement ratios. A higher water to cement ratio also contributes to a lower heat of hydration. The pore refining effect of rice husk ash has shown a surprising result in permeability of concrete as shown in the last

2

3

4

5

Time

(Hours)

Figure 15. Bleeding ratio of cement paste with RHA. Adapted from Hwang and Wu [3].

217


0

5

10 FWA

15 (%I

20

25

30

Figure 16. The effect of RHA on the compressive Adapted from Hwang and Wu [3].

strength of paste.

column of Table 9 [ 11. Each percent of rice husk ash can improve at least 0.6 times of permeability at 1.year. No admixtures or processing techniques in concrete technology are known to yield a concrete product with such low chloride permeability so far. The potential usefulness of rice husk ash, as a cement or concrete additive, for applications where the corrosion of reinforcing steel is a major concern is obvious. Hence, the use of rice husk ash is quite significant for those areas that need water resistance, and good durability like in the marine environment.

218

Table 9. Mix ProportionsMix NO. a 1 b a 2 b a 3 b 17 356 72 13 0 356 428 54 9 0 356 410 36 RHA (96) 0

and Properties of Concrete Containing RHA. Adapted from Mehta [ 11.Fresh Properties W W/B WK: 0.33 0.36 0.31 0.36 0.30 0.36 Air,% 1.0 1.5 1.0 1.5 1.5 1.5 Slump,mm 200 225 240 175 225 200 Compressive Strength, Mpa 3-d 45 42 47 45 47 46 l-d 56 56 60 60 62 65 28-d 65 77 66 80 70 80 1-yr 80 86 80 92 81 92 Permeability, Coulombs 28-d 3500 1260 3260 870 3000 390 I-yr 2200 420 2200 250

Mix Proportion, kg/m3 C 392 RHA C.A. 1062 1062 1044 1044 1026 1026 F.A. 786 786 786 786 786 786

128 0.33 128 0.33 128 0.31 128 0.31 128 0.30 128 0.30

1800 190

9 All mixtures contained a constant amount of a superplasticizer in order to obtain high consisrency.l

Coulombs passed in a 6 hours standard test (AASHTO T-277), based on FHWA Report No. RD-81/119, Aug. 1981.


The Modules of Elasticity, Creep and Shrinkage of Concrete with RHA Modulus of elasticity, creep and drying shrinkage characteristics of concrete are greatly influenced by strength of concrete and stiffness of aggregate. Since ultimate strength of concrete containing pozzolans will result in significant gain in the modulus of elastic and creep will be low after 28 days [24]. Since the addition of rice husk ash reduces bleeding, the constructor needs to carefully protect the concrete surface when conditions for plastic shrinkage cracking prevail [24]. The pozzolanic reaction of rice husk ash refines the pore structure as is indicated in Figure 14, hence at the same water-to-binder ratio the amount of drying shrinkage of concrete with the addition of rice husk ash is slightly higher than that of concrete without rice husk ash [3]. Figure 17 indicates such tendency. THE DURABILITY RHA PROPERTIES OF CONCRETE WITH

The addition of pozzolans can improve the properties of concrete by modifying the micro- and macro-structure of cement paste. The direct beneficial effect of fine and cellular RHA particles on the bleeding and segregation characteristics of concrete mixtures is due to large water adsorption ability, high internal surface area, as well as microporous and amorphous particles. The nature of rice husk ash can be viewed in Figure 3. A reduction in bleeding water results in a stronger transition zone between solid matter and cement pastes. This will lead to a more impermeable and durable concrete.

220


1.6 1.4 1.2 1 0.8 0.6 0.4 0.2

1.4 1.2 1 0.8 0.6 0.4 0.2 0 1 Time (days) 10 100

Figure 17. Drying shrinkage of cement Adapted from Hwang, Peng and Lin [22]. 221

paste containing

RHA.


The Reduction in the Permeability of Concrete The permeability of concrete plays an important role in durability because it controls the rate of entry of moisture that may contain aggressive chemicals and the movement of water during wetting or drying. The single parameter that has the largest influence on durability is water to cement or water to binder ratio [27]. As the water-to-binder or water-to-cement ratio is decreased, the porosity of the paste is decreased and hence, the concrete becomes more impermeable. The addition of microporous rice husk ash adsorb large amount of water surrounding the solid matter, producing a low water to binder ratio and refining the pore structure. This consequently reduces the permeability of concrete-to-chloride penetration as shown in Table 9 [ 11. It has been found that the permeability is significantly reduced after 28-day curing for the cement paste with 10, 20 or 30 percent rice husk ash [ 11. The permeability of the cement paste with rice husk ash is believed to be in the range of 1x10 cm/set. Resistance to Thermal Cracking The mineral admixtures which react slowly at normal temperatures can benefit from the heat of hydration of Portland cement without sacrificing ultimate strength. The use of rice husk ash to replace a portion of the cement directly reduces heat evolution. The possibility of thermal cracking for massive concretes or plastic shrinkage cracking for large flat surface construction will thus be reduced. Resistance to Alkali-Aggregate Reactions

Alkali-Aggregate reaction is a slow process between the alkali solution from a high alkali portland cement (greater than 0.6% NqO eq.) and active aggregate [24,27]. A partial replacement of high-alkali cement by rice husk ash is a direct way to cut down the alkali supply and has been effective in reducing expansion due to the

222


alkali-aggregate reaction. Mehta and Folliard [28] have reported a 95% reduction in expansion for blended cements containing 90% high alkali Portland cement (1 .O% as N%O) and 10% RHA (Figure 18). This is above the minimum limit of 75% proposed in ASTM C441 for assessment of pozzolans in reducing the alkali-silica expansion. Hence, the addition of rice husk ash plays several roles. First, RHA helps to reduce the supply of alkali from cement. The RHA competes with the slow reactive aggregate and consumes the alkali from cement due to its greater reactivity. Further, it adsorbs the surrounding water and refines the pore structure. This further hinders the diffusion of alkali ions to the surface of aggregate by microporous rice husk ash. Table 8 shows the effect of rice husk ash on the pH value of water.

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Documents

2. The Use of RHA in