36. Use of RHA in Nornal Concrete - Oyelade - 061005

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THE USE OF RICE HUSK ASH AS A PARTIAL

REPLACEMENT OF CEMENT IN NORMAL CONCRETE

BY

Oyelade Akintoye Olumide

Matriculation Number: 010402104

A PROJECT SUBMITTED TO THE DEPARTMENT OF CIVIL

AND ENVIRONMENTAL ENGINEERING, FACULTY OF

ENGINEERING, UNIVERSITY OF LAGOS.

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR

THE AWARD OF BACHELOR OF SCIENCE B.Sc (Hons.) IN

CIVIL AND ENVIRONMENTAL ENGINEERING

AUGUST, 2005



II

Table of Contents

Title Page vCertification vi

Dedication vii

Acknowledgment viii

Abstract x

Chapter

1. Introduction 1

1.1 Objectives of the Study 3

2. Literature Review 4

2.1. Concrete 4

2.2. Composition and Structure 5

2.2.1. Cement 5

2.2.2. Water 7

2.2.3. Aggregates 7

2.2.4. 2.2.3.1 Light Weight Aggregates 8

2.2.3.2 Normal Aggregates 8

2.2.3.3 Heavy Weight Aggregates 8

2.2.5. Admixtures 9

2.3. Rice Husk Ash 9

2.3.1. Classification of rice husk ash 11

2.3.2. The Effect of Burning Temperature 13

2.3.3 The Effect of Burning Time and Furnace

Environment 17



III

2.4. Analysis of the Quality of RHA 20

2.4.1 Different Sources 20

2.4.2 Different processes 21

2.4.2.1 Open Field Burning 21

2.4.2.2 Fluidized Bed Furnace Burning 21

2.4.2.3 Industrial Furnace 23

2.4.3 The Effect of Burning Time and Temperature

On the surface area and its reactivity 23

2.5 Hydration Mechanism of paste with RHA 24

2.5.1 Paste/RHA heat Evolution Curve 24

2.5.2 Hydration Mechanisms of Paste with RHA 27

2.6 Early Characteristics of Concrete with RHA 28

2.6.1 The Workability of Fresh Concrete with RHA 28

2.6.2 The Setting Time of Concrete with RHA 29

2.6.3 The Compressive Strength and Impermeabilityof Concrete with RHA 35

3.0 Research Methodology 38

3.1 Collection and Burning of the rice husk 38

3.2 Laboratory test of the ash 38

3.3 Concrete Mix Design 38

3.4 Raw materials 39

3.5 Measuring and Mixing of Concrete 39

3.6 Concrete Test 40

3.6.1 Compression Test 40



IV

4.0 Results and Analysis 42

4.1 Results 42

4.2 Discussions 59

5.0 Conclusions 61

References 62



V

The Use of Rice Husk Ash as a partial replacement of

cement in normal concrete

Oyelade Akintoye Olumide

Matriculation Number: 010402104

In partial fulfillment of the requirement for the award of

BSc (Civil and Environmental Engineering) degree by

the Department of Civil and Environmental Engineering,

University of Lagos, Lagos

AUGUST, 2005



VI

Certification

This is to certify that the project entitled “the use of rice husk ash as a

partial replacement of cement in normal concrete” was carried out by

Oyelade Akintoye Olumide with Matriculation Number 010402104 in

the department of Civil and Environmental Engineering, Faculty of

Engineering, under the supervision of Dr.G.L.Oyekan

Dr. G.L. Oyekan Date

Supervisor

Prof.M.A. Salau Date

Head of Department



VII

Dedication

This project is dedicated to Him who saw me through the period of my

stay in University of Lagos-Jesus Christ



VIII

Acknowledgment

The Lord is thanked for His guidance, help and protection throughout my stay in

University of Lagos. He said to me ‘for brass I will bring gold, and for iron I will

bring silver, and for wood brass, and for stones iron: I will also make thy officers

peace, and thine exactors righteousness (Isaiah 60:17), which he did. Thank

you.

I heartily appreciate relentless assistance of my supervisor and timely pieces of

advice of my supervisor, Dr. G.L. Oyekan throughout the period of this project.

Pastor and Deaconess Yinka Sanni, you have contributed in a great measure in

making the programme a success, the spiritual covering you provided allowed

His grace to be sufficient for me. To all the members of Redeemed Christian

Church of God International Harvest Center, I say thank you. The Oyelowo’s

family accommodated me during my first two years in Lagos, thank you very

much.

I cannot but acknowledge the staff of Likusasa Nigeria Limited, you made

available the part of the cash for the project.

To all my senior colleagues in Unilag: Ayomide Oladiran, Clement, and Kola,

your advice assisted me when I started the degree course. My course mates I

say thanks to all of you especially, Olukoko, Ogbor, Rabiu, Lookman, Biola, Miss

Williams, Dare, Yinka (info).My project mate work tirelessly with me during the course of this project-Nyembo

Sangwa Smith. Your effort are appreciated

At the inception of the course, the encouragement received from my cousins

Kayode Oyelade and Akinola Akinbola kept me focus during the programme. To



IX

my cousin and friend Niyi Oyelade, you supplied my needs during your service

year. Thank you very much.

When I was without a shelter, Major Benjamin Etuk took me in, and protected me

from the scourge of the sun. I salute sir.

My able friends whom God used for me during the course of the programme;

Lekan Fatumbi, Bunmi Esho, Godwin Akpong, Rebecca Ajejevwe, Kemi Susu,

Pastor Tunde Ogunlana, Lanre Dina,

Also to my brothers and sisters, your love towards me during the seasons knows

no bounds. May God bless you.

Finally, to my dad and mum, for your love and care over me through these years.

I say a big thank you.



X

Abstract

Rice Husk is a natural by product obtained from the processing of paddy rice.

Rice husk contains a large percentage of silica (SiO2). Due to the increasing

amount of solid waste all over the globe, engineers and researchers are looking

for means to make use of such agricultural waste in concrete as an eco-friendly

measure to control pollution.

The present study deals with the use of rice husk ash (RHA) as a partial

replacement of cement in normal concrete. Experimental study was made on the

property of RHA in concrete by using it in concrete mixes.

A mix proportion of cement + RHA, sand and gravel of 1:2:4 and a constant

water cement ratio of 0.6 were used. The RHA content was varied from zero to

30 percent and at intervals of 5%. The effect of replacing cement partially with

RHA was investigated by assessing the compressive strength of the concrete

cube specimens.

The 86 cubes specimens were tested at 3, 7, 14 and 28 days. The results

showed a compressive strength of 25.13N/mm2 as control while an optimal

compressive strength of 29.35N/mm2

was obtained for 5% replacement ofcement with RHA at 28 days. This represented a differential increment of about

17% over the cube strength with zero RHA content in the mix.



1

1.0 INTRODUCTION

Increased agricultural production and the development of agro-based industries in

many countries of the world have brought about the production of large quantities

of agricultural wastes, most of which are not adequately managed and utilized.

Agricultural wastes have been used for animal feed, fertilizer and fuel for energy

production, but little work has been carried out to develop utilization of these

wastes in the production of building materials. The need to conserve the traditional

building materials which are facing depletion, have obliged engineers to look for

alternative materials. The rising cost of transportation encourages the use of

materials which are readily available within the region surrounding the construction

activity.

Low-cost building materials can be produced using inexpensive indigenous raw

materials. Agricultural wastes which are renewable and are found in abundance in

many countries, present an interesting alternative to the traditional and sometimes

imported building materials, particularly for low-cost construction. The types of

agricultural wastes available for use in each country need to be identified.

Utilization of these materials and the development of a domestic building materials

industry based on the available materials should be encouraged.

In Nigeria, the quantity of cement available is inadequate to meet the demand of

the construction industry. This has necessitated the need to import similar products

from almost all corners of the world. Usually, the locally manufactured cement

appears fresh when being batched at the jobsites. The imported cement wouldnormally have complied with the relevant specification in its country of origin but

due to long period of shipment and storage before arriving on site, they are usually

in an uncertain condition. Some would appear fresh and perform excellently well in

the mix while others would contain hardened nodules or lumps. When these

cement nodules or lumps are allowed in a concrete mix, they tend to behave like

aggregate thereby reducing the actual quantity of cement available in the mix for

hardening process. Since the nodules or lumps cannot withstand the stress which

real coarse aggregate would normally withstand they may disintegrate under



2

pressure thereby contributing to poor strength. Apart from aforementioned problem

associated with the imported cement, the high cost of cement arising from the

adverse economic conditions has made the material out of reach of a common

man.



3

1.1 OBJECTIVE OF THE STUDY BS 206-1, 6.1 states that the specifier of the concrete shall ensure that all the relevant

requirements for concrete properties are included in the specification given to the

producer. The specifier shall also specify any requirements for concrete properties that

are needed for transportation after delivery, placing, compaction, curing or further

treatment. The specification shall, if necessary, include any special requirements (e.g. for

obtaining an architectural finish).

The specifier shall take account of:

— the application of the fresh and hardened concrete;

— the curing conditions;

— the dimensions of the structure (the heat development);

— the environmental actions to which the structure is to be exposed;

— any requirements for exposed aggregate or tooled concrete finishes;

— any requirements related to the cover to reinforcement or minimum section width, e.g.

maximum nominal upper aggregate size;

— any restrictions on the use of constituent materials with established suitability, e.g.

resulting from exposure classes.

The objective of this research is to check the suitability of rice husk ash as a replacement

of cement in normal concrete.



4

2.0 LITERATURE REVIEW

2.1 CONCRETE.

The term concrete comes from the Latin word CONCRETUS which means something

grown together or compounded. Concrete is a composite material, consisting of a

hydraulic cementing substance, coarse aggregates which form the bulk of the mix, a fine

aggregate filling the voids between and water to bind the whole together. It begins as a

plastic mixture and gradually hardens into a stone-like mass.

The sand, or fine aggregate, and the cement may together be regarded as a mortar inwhich the coarse aggregate is set. The properties of concrete depend primarily on the

quality and amount of this interstitial mortar and only secondary on the coarse aggregate.

The latter must be hard so as not to break under the pressure to which it is subjected

when the concrete is stressed, and sufficiently impermeable not to act as a channel by

which water may pass into the concrete.

Concrete is the most widely used material in Civil Engineering, and its production and use

are extremely important aspects of construction. Despite been widely used, it is suffice to

say that certain limitations to the use of concrete as a building material are recognized.

These relate mainly to its low tensile strength, low energy absorption, susceptibility to

crack with changes in temperature and moisture, tendency to deteriorate as a result of its

absorption capacity and its low resistance to chemical attack under adverse

environmental conditions.

In order to understand the techniques involved in producing and using concrete, it is

necessary to have a general appreciation of its composition and properties.



5

2.2 COMPOSITION AND STRUCTURE

Concrete consists basically of cement, water, aggregates and at times an admixture.

2.2.1 CEMENT

This is one of the components of concrete. It is a material with adhesive and cohesive

properties, which enables it to serve as a binding medium to mineral fragments.

Different cements used for making concrete are finely ground powders and all have

important properties that when mixed with water, a chemical reaction (Hydration) takesplace, which produces a binding medium for aggregate particles. During this chemical

reaction, three important physical processes also occur; these are setting, otherwise

known as STIFFENING, generation of heat and hardening, which is, gaining strength.

These all take place over a period of time, at rates which vary over that period.

Cement from which concrete is made fall into groups: Portland cement and High Alumina

cement. The grouping arises as a result of their origin, chemical composition and their

physical characteristics.

The most commonly used type of cement is Ordinary Portland cement whose properties

conformed to the requirement of BS12 (Portland cement- ordinary and rapid hardening)

Part 2 British Standard Institution (1971). It is an air and water hardening hydraulic binder

manufactured by pulverizing together clinker and gypsum. Clinker is formed from a raw

mix (clay and limestone) of preset composition ensuring a predominant content of calcium

Silicates at a very high temperature. It is suitable for use in general concrete construction

when there is no exposure to sulphates in the soil or ground water. Rapid hardening

Portland cement is used where there is a need for rapid strength development e.g. early

removal of formwork for reuse; sulphate- resisting cement is used for structures exposed

to acidic soils and saline water. High Alumina cement was developed to resist sulphate

attack.



6

Cement, chemically consist of:

-Lime CaO 64-67%

-Silica SiO2 19-24%

-Alumina Al203 4-7%

-Iron oxide Fe2O3 2-6%

-Magnesia MgO 5%

-Sulphur trioxide SO3 3%

The main chemical compounds of Portland cement are: Tri-Calcium Silicate

(3CaO.SiO2), Di-Calcium Silicate (2CaO.SiO2), Tri-Calcium Aluminates (3CaO.Al203) and

Tetra-Calcium-Alumino-Ferrite (4CaO.Al2O3.Fe2O3).

Tri-Calcium Silicate contributes to the development of early strength particularly during

the 1st

- 14 days. It generates considerable amount of heat.

Di-Calcium Silicate hydrates slowly and is mainly responsible for the development of

strength after 7 days. It remains active for a considerable period.

Tri-Calcium Aluminates produces little increase in strength after 24hours.

Tetra-Calcium-Alumino-Ferrite is of less importance to the strength development.

The two Silicates are the most stable of the compounds and together they form between

70% and 80% 0f the constituent in cement.



7

2.2.2 WATER

Water used in concrete, in addition to reacting with cement, causing it to set and harden,

thus producing the binding qualities of cement, also facilitates mixing, placing and

compacting of the fresh concrete (workability).

The quantity as well as the quality of water in a concrete mix is of vital influence on the

properties of the resulting concrete. Both could affect the strength and durability of

concrete. Water containing undesirable organic substances or inorganic constituents in

excessive proportions could interfere with the setting of the cement, cause surface

dampness, staining of the concrete surface, efflorescence and excessive corrosion of

steel reinforcement. It is expressed in kilogammes of water per cubic meter of concrete.

2.2.3 AGGREGATES

Aggregates form bulk of the materials for the manufacturing of concrete. It usually

occupies about 70 to 75% of the total volume of a mass of concrete. They are obtained

by crushing quarried rock to the required size. The aggregates must be sufficiently strong

and well graded.

The aggregates performs the followings:

- provides a mass of particles which are suitable for resisting the action of applied

loads, abrasion, the percolation of moisture, and the action of weather;

- provides a relatively cheap filler for the cementing material;

- reduces the volume changes (Shrinkage) resulting from the setting and

hardening process and from moisture changes in the water-cement paste.

The classification of aggregates is as a result of its source, mineralogical composition,

mode of preparation and size.

Aggregates may be classified as fine aggregate and coarse aggregate with reference to

its size. Fine aggregate or sand are particles that pass through 5mm sieve, that is, grains



8

not larger than 5mm while coarse aggregate comprise particles larger than 5mm in size,

that is, they do not pass through 5mm sieve but are retained.

2.2.3.1 LIGHT WEIGHT AGGREGATE

They are material used to obtain concrete weight usually below the weight of the normal

concrete (2200 to 2600kg/m3). Their bulk density normally ranges between 350 to

850kg/m3 for coarse aggregate and from 750 to 1100kg/m3 for fine aggregate. Concrete

made from lightweight aggregate usually have as higher drying shrinkage and moisture

movement than heavy concrete and are more prone to shrinkage cracking. They give

better thermal insulation and fire resistance, allows for construction on ground with low

bearing-capacity.

Lightweight aggregates can be divided into two main groups: natural and artificial. Pumice

and periwinkle shells belong to the natural while the artificial can be further sub-divided

into two classes, the first consist of waste products, furnace clinker. The second class

consists of material made by processing artificial or natural products and these are much

more uniform in quality. For example, foamed or expanded slag, shales and slates,

expanded palite etc.

2.2.3.2 NORMAL AGGREGATES

They may either be natural or artificial in origin and they are used to produce concrete

with density in the range of 2200 to 2600kg/m3. The natural ones are crushed rock

(granite, sand and gravels). The rock aggregates are obtained by crushing quarried rock

to the required particle size or sizes while sand and gravel are obtained as deposits at theriver, alluvial and glacial. The artificial normal aggregates are obtained from brick and air-

cooled blast furnace. Aggregates, in particular sands and gravels, should be washed to

remove impurities such as clay and silt.

2.2.3.3 HEAVY WEIGHT AGGREGATES

They are produced from scrap iron and lead shot. Its density ranges from 4000 to

8500kg/m3

and it depends on the aggregate type, the dimensions and the degree of



9

compaction. Heavyweight aggregate provide an effective and economical use of concrete

for radiation shielding, and for coating of submerged pipelines. It is frequently difficult to

obtain a mix which is both workable and not prone to segregation with heavyweight

aggregates.

2.2.4 ADMIXTURES

This in the most general sense are materials that are added when mixing mortar or

concrete in order to modify the properties of the product either in its fresh or hardened

state. It is added to the concrete mix in quantities not larger than 5% by mass of cement

during mixing. They may be inorganic or organic in composition. Some types are used to

improve workability, to reduce the amount of water needed in mixing, to retard or

accelerate setting and to improve the frost resistance. Others are used to reduce the

permeability or to improve specific mechanical properties of the concrete such as

abrasion resistance. Others are used to improve chemical resistance as in POZZOLANIC

CEMENTS, to reduce alkali-aggregate expansion etc.

Examples of admixtures includes; air-entraining agents, accelerating agents, Retarders,

Water Reducers or Plasticizers, Super plasticizers, Binding admixtures, Water repelling

agents, Pigments, pore fillers and Pozzolanas.

2.3 RICE HUSK ASH

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)

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 & 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 is 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.



10

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

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

Tons).

COUNTRY RICE PADDY RICE HUSK

Bangladesh 27 5.4

Brazil 9 1.8

Burma 13 2.6

China 180 36.0

India 110 22.0

Indonesia 45 9.0

Japan 13 2.6

Korea 9 1.8

Philippines 9 1.8

Taiwan 14 2.8

Thailand 20 4.0

US 7 1.4

Vietnam 18 3.6

Others 26 5.2

Total 500 100

Rice Husk Ash is approximately 20% of the rice husk, i.e. the total world production

of RHA will be 20 million metric tons.

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.



11

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.

2.3.1 CLASSIFICATION OF RICE HUSK ASH

The chemical composition of rice husk is similar to that of many common organic fibers

and contains:

a) cellulose (C5H10O5), a polymer of glucose, bonded with B-1.4,

b) lignin (C7H10O3), a polymer of phenol,

c) hemicellulose, a polymer of xylose bonded with B-l.4 whose composition is like xylem

(C5H8O4,), and

d) SO2 the primary component of ash. (3)

The holocellulose (cellulose combined with hemicellulose) content in rice husk is about

54%, 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) (4-7)

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. 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). Mehta’s fluidized bed furnace

was designed in 1974 to produce energy and highly pozzolanic ash from incineration of

rice husks. Modern electric furnace has been used in order to maintain proper control of

the burning process and to produce better quality ash.



12

Table 2. Chemical Composition of Rice Husks.

Extractives

Chemical Composition (%)

Rice Husk

Alcohol

benzene

1%

NaOH

Hot

Water

Holo-

cellulose

Ash Lignin Ref.

Japonica 1.8 32.3 5.4 53.9 13.6 24.8 [3]

Indica 2.1 30.6 5.1 54.3 11.7 25.8 [3]

Anhydrous

Rice Husk

- - 8-15 40-50 15-20 25-30 [1]

Table 3. Ultimate Analysis of rice Husk, Hwang and Wu

Chemical Composition (Wt %)

C H O N S Cl Ash Ref

38.3

39.4

39.5

5.7

5.5

5.5

39.8

36.1

37.7

0.5

0.5

0.8

0.0

0.2

0.0

0.0

0.2

0.0

15.5

18.2

16.5

[5]

[6]

[7]



13

2.3.2 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)

Table 4

Temperature ºC

<300 400 600 700 1000

Element

%

Si

K

Ca

Na

Mg

S

Ti

Fe

81.90

9.58

4.08

0.9

1.25

1.81

0.00

0.43

80.43

11.86

3.19

0.92

1.20

1.32

0.00

1.81

81.25

11.80

2.75

1.33

0.88

1.30

0.00

0.68

86.71

7.56

2.62

1.21

0.5

1.34

0.00

0.00

92.73

2.57

1.97

0.91

0.66

0.16

0.45

0.68

Oxide

%

SiO2

MgO

SO3 CaO

K2O

Na2O

Fe2O3

88.01

1.17

1.122.56

5.26

0.79

0.29

88.05

1.13

0.832.02

6.48

0.76

0.74

88.67

0.84

0.811.73

6.41

1.09

0.46

92.15

0.51

0.791.60

3.94

0.99

0.00

95.48

0.59

0.091.16

1.28

0.73

0.43

a) At 400ºC, due to transglycosylation, polysaccharides begins to depolymerize,

producing levoglucosan, monosaccharide derivatives, and oligosaccharides.

b) Dehydration of the sugar units occurs above 400ºC producing 3-deoxyglucosene,

levoglucosenone, funicular and furan derivatives.

c) At 700ºC, sugar unit decomposes, producing some cabby compounds such as

acetaldehyde, glyoxal and acrolein.

d) At temperatures above 700ºC, these unsaturated products react and through free

radical reaction, form a highly reactive carbonic residue. (5,8-11)

Differential thermal analysis (DTA), thermogravimetric analysis (TGA), and TMA showed

the first peak at 95°C was caused by dehydration. Between 150°C and 250” C, a low



14

intensity endothermic reaction was followed by one extreme at about 300°C which may

have been due to oxidation reactions. The amount of char formed was about 60% of the

initial weight (Figure 1).



17

Table 5.

Pore Analysis of RHA under Different Burning Temperatures, after Hwang and Wu

{3}

Burning

Temperature (“C)

Mercury Penetration

Volume (cm3/gm)

Mean Pore (A) Median

Pore(A)

Char

400

600

700

800900

1000

1100

0.3567

0.3379

0.2912

0.0795

0.06430.0393

0.0323

0.0171

5257

5954

6069

5867

5446 5150

4972

3900

4585

6847

6847

5718

52504483

3867

9803

2.3.3 THE EFFECT OF BURNING TIME & FURNACE ENVIRONMENT

Above 8OO’C, an increase in the burning temperature, time, and environment tend tocause a sintering effect (coalescing of fine particles) as was mentioned earlier, and, is

indicated by a dramatic reduction in the specific surface (Table 6). Combustion

environment also plays an important role. It should be noted that a change in the rate of

oxidation from moderately oxidizing conditions

(CO2 environment) to highly oxidizing conditions (oxygen 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 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 800°C longer than one hour to

obtain suitable pozzolanic reactivity.

Also, it is believed that rapid cooling may increase the reactivity of RHA. (15)



20

Table 6.

Effect of Burning Conditions on the Crystal Structure and Surface Area of Rice

Husk Ash. Adapted from Ankra.

Properties of ASHBurningTemperature

Hold Time Environment

Crystalline SurfaceArea,m2

122500-600°C

1 min30 min 97

15min-lhr

moderatelyoxidizing

76700-800°C

>l hr

noncrystalline

100partially

crystalline

6-10>8OO”C >l hrHighlyoxidizing

crystalline <5

2.4 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.

2.4.1 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 (16):



21

Cellulose 40-50%

Lignin 25-30%

Ash 15-20%

Moisture 8- 15%

After burning at a suitable temperature and time (- 600- 7OO”C, 2 hours), in an industrial

furnace, the rice husk is composed of 90-95% SiO2 l-3% K2O and < 5% unburnt carbon,

as shown in Table 7 {1,3 17-21}. 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 ashwith 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%.

2.4.2 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. (7-12)

2.4.2.1 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.

2.4.2.2 FLUIDIZED-BED FURNACE BURNING

Fluidized-bed furnace is designed to control burning of rice husk {17}. 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 700°C for a relatively long period to remove

most of the carbon , or at temperatures around 700-800°C for less than one minute.



23

The chemical analysis of the ash samples produced by a fluidized-bed furnace showed

80-95% SiO2 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.

2.4.2.3 INDUSTRIAL FURNACE

A modern industrial furnace has been recently used due to environmental and economic

reasons (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.

2.4.3 THE EFFECT OF BURNING TIME AND TEMPERATURE ON THESURFACE AREA AND ITS REACTIVITY

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 3. Table 6 clearly

indicates that not only the burning temperature, the burning time is equally important in

removing carbon while keeping the silica in an amorphous and highly cellular form. The

surface area of RHA burned at 500-600°C 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, which consequently causes a

reduction in surface area . 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 3 indicates the ideal time/temperature path to obtain optimum quality rice

husk ash with a microporous and cellular structure which is highly reactive.



24

2.5 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. The heat evolution curve,

microstructure development, and ultrasonic velocity are important for studying hydration

mechanisms in paste made with RHA.

2.5.1 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. Figure 4

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 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 SiO2 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 secondpeak. 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 5).



27

Table 8.

The pH Values of RHA Dissolved in Water. Adapted from Hwang and Wu [3].

pH ValueRHAIW T W/RHA

1 day 3days 7days

0.05

0.1

0.15

0.2

0.25

0.3

20

10

6.67

5

4

3.33

9.91

9.98

10.00

10.12

10.14

3.33

9.59

9.82

9.91

9.91

9.98

9.98

9.55

9.75

9.80

9.86

9.91

9.91

• Ash burned at 700°C

2.5.2 HYDRATION MECHANISMS OF PASTE WITH RHA

Figure 6 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.According to the hydration mechanism of ordinary Portland cement without RHA a

transition zone exists in the ultrasonic pulse 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,

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 6b and 6c (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 7, 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



28

resistance may be primarily due to the formation of CH crystal. From the pulse velocity

curve as shown in Figure 10, the pulse changes 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 andreduces the permeability, which may be beneficial to the durability.

2.6 EARLY CHARACTERISTICS OF CONCRETE WITH RHA

The early characteristic 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.

2.6.1 THE WORKABILITY OF FRESH CONCRETE WITH RHAAt 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. This is mainly due to the large surface area

of rice husk ash which is in the range of 50 to 60m2 /g.

Large additions would produce dry or unworkable mixtures unless water-reducing

admixtures or superplastizers are used, as shown in Figure 8. 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 8. 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 mm in size, would supplement the cement grains in further



29

reducing the volume of voids in the concrete mixture. Consequently, it will require less

water to produce a concrete of a given consistency.

The workability of fresh concrete with RHA can be improved by densifying the mixture.

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 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.

2.6.2 THE SETTING TIME OF CONCRETE WITH RHAUnlike other pozzolanic materials, rice husk ash tends to shorten the setting time, as

shown in Figure 5. 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 6,

reflects that the rigid silica cellular skeleton 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 reduction in

early strength development.



35

2.6.3 THE COMPRESSIVE STRENGTH AND IMPERMEABILITY OFCONCRETE WITH RHA

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. 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

9. 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 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 10that 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

(Figure 11). 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 12. The

early strength of concrete is a function of water-to-binder ratio. As long as the water-to-

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.



36

The pore refining effect of rice husk ash has shown a surprising result in permeability of

concrete as shown in the last column of Table 9. 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.



38

3.0 RESEARCH METHODOLOGY

84 cubes test were tested in the laboratory. In preparing these samples, few importantsteps were taken into account, among others:

a) Collection and Burning of the Rice Husk

b) Laboratory test of the Ash

c) Concrete mix design

d) Raw materials

e) Measuring and mixing of concrete

3.1 COLLECTION AND BURNING OF THE RICE HUSK

The rice husk for this research was collected from Ofada a part in Ogun State. Four bags

collected for burning. The method of burning is important in this research. Although past

researchers in the department have used open burning. In this respect, we used furnaceburning in an industrial furnace at Clay Industry at Oregun. The husk was burnt at 900 oC

for 30mins in the furnace.

3.2 LABORATORY TEST OF THE ASH

The chemical constituent of the ash was done in the laboratories. The elements tested

are Silica (SiO2), Carbon, Barium Oxide (BaO), Aluminum Oxide (Al2O3), Sulphite (SO32-),

Na2O,CaO, MgO, and Fe2O3.

3.3 CONCRETE MIX DESIGN

The components of the mixes were by volume. The ratio of the mixes was 1:2:4:0.6. The

ratio of cement and husk ash varied as shown in table 9



39

Table 9

3.4 RAW MATERIALS

i. Aggregate- The coarse aggregate was from crushed granite of igneous origin. The

particle size range used is 10 – 20mm. River sand with less than 5mm size was

used

ii. In preparation of concrete mix, tap water from the lab was used.

iii. The rice husk ash was obtained from burning the husk at Clay Industry Oregun

iv. Cement – Portland Cement type whose properties confirm to the requirement of

BS 18 was used.

3.5 MEASURING AND MIXING OF CONCRETE

The prescribed mixes of cement + RHA, sand and gravel (1:2:4) for general reinforced

concrete work with water cement ratio of 0.6 were considered. Concrete was mixed using

concrete mixer. The raw materials were weighed and placed into the machine. The

machine was rotated until the materials were completely mixed.

Test Cement % Rice Husk Ash %

1 100 0

2 95 5

3 90 10

4 85 15

5 80 20

6 75 25

7 70 30



40

3.6 CONCRETE TEST

The 84 cubes were tested at 3, 7, 14 and 28 days. Three specimens were tested at each

age and the value of the crushing load were averaged and used to calculate the mean

strength for each group.

3.6.1 COMPRESSION TEST

The concrete mix was placed into the moulds, which has been applied with mould oil to

ease opening. Concrete was compacted into three layers using steel rods 25mm in size

with 35 blows per layer and the surface was leveled using trovel.

After 24 hours, the mould was opened and the cubes were treated in a pond. All tests

shall comply with the requirements of BS: 1881, Part 4, and MS 26: 1971.

Prior to the test, the cubes/specimens were weighed to obtain weight and the specimen

placed onto the lower steel platen plate with both smooth surfaces facing the top and

bottom platen plates (Fig 3.1).



41

Figure 13 – Cube Test



42

4.0 RESULTS AND ANALYSIS

4.1 RESULTS

The chemical analysis of the rice husk ash from Najco Laboratories Limited is shown inTable 10

Table 10

Test Performed Result

Silica (SiO2) 79.92 %Carbon 4.02%Barium Oxide (BaO) 0.01%Aluminum Oxide (Al2O3) 8.25%Sulphite (SO3

2-) 9.27%

The other result conducted at Federal Institute of Industrial Research, Oshodi (FIIRO)gives the following result for the ash.(Table 11)

Table 11Parameters Results

Na2O 0.26%PbO Not detectedCaO 0.112%MgO 0.17%Fe2O3 0.45%

Table 12 to Table 18 shows the result of varying the percentage of ash with cement from0% to 30%.



43

0% Ash Cast on 18/05/05

Table 12Weight Strength(Ton

ForceStrength (kN) Compressive

Strength kN/mm2

Average kN/mm2

9.2 22.6 226 10.049.4 28.8 288 12.809.2 41.2 412 18.31 13.72

3 Days

21/05/059.7 32.8 328 14.589.8 23.6 236 10.499.2 33.6 336 14.93 13.33

7 Days

25/05/059.7 24.4 244 10.849.4 49.8 498 22.13

9.3 31.6 316 14.13 15.70

14 Days

01/06/0510.3 51.8 518 23.028.8 63.4 634 28.188.7 54.4 544 24.18 25.13

28 Days

15/06/05

5% Ash Cast on 18/05/05



Strength kN/mm2

Average kN/mm2

9.5 43.4 434 19.2910.2 48.8 488 21.69

10.8 35.8 358 15.91 18.96

3 Days

21/05/059.1 32.2 322 14.319.1 25.2 252 11.209.2 32.2 322 14.31 13.27

7 Days

25/05/05

8.8 68.6 386 30.4910.4 69.8 698 31.028.9 60.6 606 26.93 29.48

14 Days

01/06/059.6 60.9 609 27.078.4 70.5 705 31.338.6 66.7 667 29.64 29.35

28 Days

15/06/05



44

10% Ash Cast on 18/05/05



Strength kN/mm2

Average kN/mm2

10.4 32.4 324 14.409.1 16.2 162 7.208.7 23.2 232 10.31 10.64

3 Days

21/05/058.8 17.2 172 7.649.2 23.8 238 10.588.9 25.8 258 11.47 9.90

7 Days

25/05/058.3 35.8 358 15.918.7 36.6 366 16.27

9.0 45.2 452 20.09 17.42

14 Days

01/06/058.5 15.6 156 6.939.9 48.6 486 21.608.8 50.2 502 22.31 16.95

28 Days

15/06/05

15% Ash Cast on 20/05/05



Strength kN/mm2

AveragekN/mm

2

8.4 28.2 282 12.538.8 17.8 178 7.918.4 18.2 182 8.09 9.51

3 Days

24/05/058.3 24.4 244 10.848.7 46.6 466 20.718.0 37.8 378 16.80 16.12

7 Days

27/05/058.5 39.6 396 17.608.8 41.8 418 18.588.8 43.6 436 19.38 18.52

14 Days

03/06/059.5 40.2 402 17.878.7 45.2 452 20.0910.1 42.6 426 18.93 18.96

28 Days

17/06/05



45

20% Ash Cast on 24/05/05



Strength kN/mm2

AveragekN/mm

2

8.8 23.4 234 10.408.3 32.6 326 14.498.7 39.8 398 17.69

14.193 Days

27/05/058.9 28.6 286 12.719.1 40.8 408 18.138.7 33.4 334 14.84 15.23

7 Days

31/05/058.3 48.6 486 21.609.4 35.8 358 15.91

8.7 36.2 362 16.09 17.87

14 Days

07/06/058.4 39.4 394 17.518.6 32.9 329 14.629.1 49 490 21.78 17.97

28 Days

21/06/05

25% Ash Cast on 24/05/05



Strength kN/mm2

AveragekN/mm

2

8.2 21.7 217 9.648.0 30.5 305 13.568.5 35.4 354 15.73 12.98

3 Days

27/05/058.8 25.0 250 11.118.9 35.0 350 15.568.1 34.5 345 15.33 14.00

7 Days

31/05/059.9 45.5 455 20.229.2 40.2 402 17.879.1 37.5 375 16.67 18.25

14 Days

07/06/0510.3 41.2 412 18.319.5 43.0 430 19.118.9 42.5 425 18.89 18.77

28 Days

21/06/05



46

30% Ash Cast on 24/05/05



Strength kN/mm2

AveragekN/mm

2

9.0 18.6 186 8.278.7 28.4 284 12.628.4 27.7 277 12.31 11.07

3 Days

27/05/059.5 24.7 247 10.988.3 32.8 328 14.588.7 30.2 302 13.42 12.99

7 Days

31/05/05

8.9 37.9 379 16.848.8 30.2 302 13.429.1 35.5 355 15.78 15.35

14 Days

07/06/059.2 35.0 350 15.568.9 37.4 374 16.6210.1 38.5 385 17.11 16.43

28 Days

21/06/05



13.7213.33

15.7

0

5

10

15

20

25

30

3 days 7 days 14 days

age (days)

c o m p

r e s s i v e s t r e n g t h ( N / m m 2 )

Figure 14: Relationship Between Compressive Strength (N/mm2) and Age (Days) for 0% RHA Content in



18.96

13.27

29.48

0

5

10

15

20

25

30

35


Age (days)

c o m p r e s s i v e s t r e n g t h ( N / m m 2 )

Figure 15: Relationship Between Compressive Strength (N/mm2) and Age (Days) for 5% RHA Content in the M



10.64

7.05

17.42

0

2

4

6

8

10

12

14

16

18

20


Age (days)

C o m p r e s s i v e s t r e n g t h ( N / m m 2 )

Figure 16: Relationship Between Compressive Strength (N/mm2) and Age (Days) for 10% RHA Con



9.51

16.12

18.52

0

2

4

6

8

10

12

14

16

18

20


Age (days)

c o m p r

e s s i v e s t r e n g t h ( N / m m 2 )

Fi ure 17: Relationshi Between Com ressive Stren th N/mm2 and A e Da s for 15% RHA Cont



14.19

15.23

17.87

0

2

4

6

8

10

12

14

16

18

20


Age (days)

c o m p r e s s i v

e s t r e n g t h ( N / m m 2 )

Figure 18: Relationship Between Compressive Strength (N/mm2) and Age (Days) for 20% RHA Co



14.19

15.23

17.87

0

2

4

6

8

10

12

14

16

18

20


Age (days)

c o m p


Figure 19: Relationship Between Compressive Strength (N/mm2) and Age (Days) for 25% RHA



11.07

12.99

15.35

0

2

4

6

8

10

12

14

16

18


Age (days)

c o m p


Figure 20: Relationship Between Compressive Strength (N/mm2) and Age (Days) for 30% RHA C



13.72

18.96

10.64

9.51

14.19

0

2

4

6

8

10

12

14

16

18

20

0% 5% 10% 15% 20% 2

Various Percentages of RHA Contents in the Mix at 3rd Day

C o m p r e s s i v

e S t r e n g t h ( N / m m 2 )

Figure 21: Relationship Between Compressive Strength (N/mm2) and Various Percentages of RHA conte



13.33 13.27

9.90

16.12

15.23

0

2

4

6

8

10

12

14

16

18

0% 5% 10% 15% 20%

Various Percentages of RHA Contents in the Mix at 7th Day

C o m p

r e s s i v e S t r e n g t h ( N / m m 2 )

Figure 22: Relationship Between Compressive Strength (N/mm2) and Various Percentages of RHA content in th



15.70

29.48

17.42

18.5217.87

0

5

10

15

20

25

30

35

0% 5% 10% 15% 20%


C o m p

r e s s i v e S t r e n g t h ( N / m m 2 )

Figure 23: Relationship Between Compressive Strength (N/mm2) and Various Percentages of RHA content in th



25.13

29.35

16.95

18.96

17.97

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

0% 5% 10% 15% 20%


C o m p

r e s s i v e S t r e n g t h 9 N / m m 2 )

Figure 24: Relationship Between Compressive Strength (N/mm2) and Various Percentages of RH



0

5

10

15

20

25

30

35

3 days 7 days 14 daysAge(days)

C o m p r e s s i v e S t r e n g t h ( N / m m 2 )

Figure 25: Relationship Between Compressive Strength (N/mm2) and Age for Various Percentage RHA



- 59 -

4.2 Discussions

The relationship between compressive strength and ages of various percentages of RHA

contents in the mix are shown in Figures 14 – 20.

In the control, there is a gradual increase of strength from 13.72N/mm2 at 3 days to

25.13N/mm2 at 28 days – 83% increment from the initial strength.

Unlike Di-calcium Silicate which hydrate slowly and is responsible for the development of

the strength after 7 days. The effect of the RHA in the cement was noticed in the first 3

days. A difference of 5.24N/mm2 in strength was observed in the control and the 5%

replacement of the cement with RHA. (Figure 25).

Figures 21, 23, and 24; reveal that 5 percentage replacement of cement with RHA gives

maximum values of 18.96N/mm2,29.48N/mm2, and 29.35N/mm2 at 3, 14 and 28 days

respectively. But at 14 days a maximum value of 16.12N/mm2 was obtained at 20%

replacement of cement by RHA. (Figure 22)

Chong (25) indicates that high silica content if present in an amorphous chemically

reactive form, enable pozzolan to exhibit pozzolanic properties. Pozzolanic materialschemically react with calcium hydroxide (CH). In the presence of moisture lime and Silica

(Sio2) react to produce tricalcium Silicate (C3S) and dicalcium Silicate (C2S). The

hydration products of the two compounds are tobermorite gel and calcium hydroxide. The

tobermorite particles are responsible for the cementing properties as well as other

important engineering properties such as strength and shrinkage while the calcium

hydroxide provides the alkaline medium which is beneficial for the protection against

corrosion of reinforcing steel.

At 5% replacement of cement with RHA, RHA exhibits its pozzolanic property by

increasing the compressive strength. However, further increase in the percentage of RHA

content in the cement leads to a reduction in the compressive strength of the concrete.

This could be attributed to the reduction in the quantity of cement available for hydration

process thus a reduction in the formation of stable strength producing cementitous

compound.



- 60 -

A reduction of compressive strength was noticed at 7 days testing for 5% and 10%

replacement of cement with RHA. This may be due to the hand compaction used in the

preparation of the cubes. For other percentages replacement of cement with RHA (15% -

30%) there is a gradual increase from 3 days to 28 days (Figure 17 -20).

There is no increase in the strength of 5% replacement of cement with RHA at 14 days

and 28 days. At 14 days the compressive strength was 29.48N/mm2 and at 28days

29.35N/mm2. So, the optimum compressive strength of the 5% replacement was attained

at 14 days.



- 61 -

5 CONCLUSIONS

Based on the discussion above, the following conclusions can be made

o The maximum compressive strength of 29.35N/mm2

was obtained at 5% RHAcontent.

o The compressive strength decreases as the content of the RHA in the mix

matrixes increases above 5%

o Siliceous agriculture wastes such as rice husk ash should therefore be carefully

utilized as important source of durable concrete. Obviously, the rice crop residues

have a great role in social economic development of areas where rice is being

produced.



- 62 -

REFERENCES

1. Mehta, P. K., Rice Husk Ash-A Unique Supplementary Cement Material, Advances in

Concrete Technology, Ed.by Malhotra, CANMET, Ottawa, Canada (1992).

2. Mehta, P. K., and Pitt, N., Energy and Industrial Materials from Crop Residues, Journal

Resource Recovery and Conservation, No. 2, pp. 23-38, Elsevier Scientific Publishing

Company (1976).

3. Hwang, C. L., and Wu, D. S., Properties of Cement Paste Containing Rice Husk Ash,

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36. Use of RHA in Nornal Concrete - Oyelade - 061005

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