54

CHAPTER 2

RICE HUSK ASH AND BAGASSE ASH CONCRETES

2.1 GENERAL

The waste materials locally available are efficiently deployed to

improve the functioning of the rural-based industries. Civil Engineers are

always in search of waste materials that can be used as a blending component

in cements to improve its quality and to reduce the cost. Several

investigations have been carried out to utilize waste material in construction.

Apart from getting rid of these materials, their use in construction protects the

environment from contamination. Industrial and agricultural wastes are used

for the production of low cost building materials. Agro-industrial wastes such

as tobacco waste, non-edible oil cake and hyacinth have been used

successfully for installing bio-gas plants. Agricultural residues such as

bagasse, rice husk and rice straw are utilized in the production of light weight

clay bricks.

Industrial wastes such as blast furnace slag, fertilizer wastes, fly

ash, silica fumes and incinerators ash are silica-based materials having

pozzolanic properties. These are used for the development of novel low

temperature cements and added to portland cement as supplementary

cementitious materials with improved properties compared to portland

cement. In addition to the above industrial wastes, the agro wastes such as

rice husk ash , sugarcane bagasse ash , rice straw ash, wheat straw ash, hazel

nut shell ash are having pozzolanic properties and used as cement

55

replacement materials. As a cement replacement material, many unique

properties of these silica-based materials have been reported by several

investigators.

2.2 AGRICULTURAL RESIDUES

Agricultural residues are composed of organic constitutes such as

cellulose, lignin, fibre and small amount of crude protein and fat. In addition

they contain a range of minerals which include silica, alumina and iron oxide.

During growth, some plants absorb various minerals and silicates from the

soil and accumulate it into their structures. Inorganic materials, especially

silicates are found in higher proportions in annually grown plants than long

lived trees. Such plants with a high concentration of silica are therefore rice

plant, wheat plant, sunflower, tobacco and sugarcane. The agricultural residue

itself cannot be used as replacement and it is the ash obtained from controlled

processing the residue which is of interest. Two factors require for the

consideration in the controlled processing are the ash content and the

chemical constituents of the ash. The ash content is important as it indicates

the amount of residues which needs to be burnt. Silica is generally the major

chemical constituent of the ash which is important for cement replacement.

These ashes will also contain as much as 10% alkalis, traces of iron,

aluminum, calcium and magnesium oxides plus oxides of trace elements

introduces into the soil through fertilization practices. Table 2.1 shows the

ashes and silica content of some plants as well as those of rice husk, rice straw

and bagasse (Cook 1986).

56

Table 2.1 Ash and Silica Content of Plants

Plant Part of plant Ash(%) Silica(%)

Sorghum Leaf sheath epidermis 12.55 88.70

Wheat Leaf sheath 10.48 90.56

Corn Leaf blade 12.15 64.32

Bamboo Nodes(inner portions ) 1.49 57.40

Bagasse -- 14.71 73.00

Lantana Leaf and stem 11.24 23.28

Sunflower Leaf and stem 11.53 25.32

Rice husk -- 22.15 93.00

Rice straw -- 14.65 82.00

Bread fruit tree Stem 8.64 81.80

From Table 2.1, it may be observed that rice husk has the greatest

yield (ash content 22.15%) and silica content (93%). Bagasse has also good

yield (ash content 14.71 %) and silica content (73%). Rice husk is a residue

waste product, generated from the accumulation of the outer covering of rice

grains during the milling process, produced in significant quantities on a

global basis. It constitutes about 20% of 500 million tons of rice produced

annually in the world. Bagasse is the fibrous residue left after sugarcane is

crushed to extract its juice. Though less benefits of bagasse have been reaped,

they have enough potential as fuel for boiler in sugar mills and co-generation

plants. These residues cannot be considered as wastes for they were

traditionally used as a fuel sources, feed for animals and raw materials for

various manufacturing processes. Every 1000kg of rice husk burnt, 200kg of

ash are produced while 1000kg of bagasse burnt only 150kg of ash are

produced. Rice husk ash (RHA) and bagasse ash (BA) available in plenty,

57

dumped in open air are particularly important resources to supplement the

portland cement. Attempts to utilize rice husk ash and bagasse ash are

advantages as their high ash content consists essentially of silica in

amorphous form.

2.3 AGRICULTURAL - WASTE RESOURCES IN INDIA

According to National Statistical data in 2002, India had 45.0

million hectares of land area for rice and 44.0 million hectares for sugarcane

production. The disposal of such a huge amount of rice straw, rice husk,

sugarcane bagasse, the by-product of rice and sugar production is a serious

problem. India being the second largest producer of rice and sugarcane in the

world produces 35 million tons of rice husk annually and it is estimated that

approximately 25 million tons are readily available for disposal from rice

mills (Bhanumathidas and Mehta 2004). It is estimated that about 1/3 of the

available husk can be collected and converted to ash for use as a Portland

cement replacement. Hence in India, about 5.0 million tons of rice husk ash is

potentially available as an excellent cement replacement material. Similarly

sugar production in India is about 20 million tons and sugarcane production is

300 million tons. About 2.0 millions tons of bagasse ash is available from

sugar mills and co-generation plants.

The most recent assessment available (TIFS 1993) indicated that

there are over 1,30,000 rice mills in India-reputedly, the largest number in any

single country. About 70 % of these are small operations with capacities

below 500Kg paddy/hour. Approximately one-half of the paddy produced in

India is milled in rice mills that are designated as modern. There are over

30,000 of these modern mills with an average capacity of 2 tons of

paddy/hour scattered throughout India. The rice milling industry generates

between 30 and 35 million tons of rice husks annually. Most of this waste

58

produced is generated in relatively small quantities at thousands of small mills

that do not operate on 12 month yearly cycles.

The average area under cultivation of sugarcane in Tamilnadu is

2.75 lakh hectares which is 2% of the total area under cultivation in the state.

The average production of sugarcane is about 270 lakh tons per year, of which

50-55% of the sugarcane is drawn and crushed by sugar mills in Tamil Nadu

and the balance quantity of sugarcane is utilized for jaggery manufacture,

seed and chewing purposes.

In addition to the wide dispersal and sporadic production of rice

husks and bagasse, there are two other principal problems related to their

availability as a raw material. These are their low bulk density (0.10 ton/m3)

and their uses as fuel both in the mills itself and in a wide range of other local

industrial activities. The extent of rice husk utilization as an industrial fuel is

reportedly higher in India than in any other rice growing developing country.

Nearly half the rice husk and bagasse produced in India are used as boiler fuel

in mills. The rest is much sought after by industry for use as fuel in fuel-short

India.

Even though the above-mentioned problems of raw materials

availability and supply are existed, the RHA and BA commercial venture in

India could be made viable with the continuous supply of rice husks from

‘modern’ rice mills and bagasse from the sugar mills with year round

operation. Production activities would therefore need to be located in those

regions of the country where there was a concentration of such mills.

‘Puduvayal’ town in ‘Karaikudi ‘ taluk of Sivaganga district of Tamil Nadu

state is such a place for the commercial production of RHA cement, where

about 100 modern mills are situated with year round production. There are 36

sugar mills in this State, of which 16 are in Cooperative sector, 3 are in Public

59

sector and 17 are in Private sector. Madurai city in south India is such a place

for commercial production of BA cement, where about 5 sugar mills are

situated in and around.

Utilization of RHA and BA to produce a Portland cement

replacement has become most attractive in India. It reduces the demand and it

should theoretically reduce the cost of cement and to provide cement in a

rural area typically of those traditionally deprived of cement.

2.4 RHA AND BA AS CEMENT REPLACEMENT MATERIALS

2.4.1 Historical Development

In modern rice mills, rice husk disposal is usually a serious problem

due to very low bulk density and require large space for storage and hauling.

Due to their negligible protein content, they are not suitable for animal fodder.

A convenient method of getting rid of rice husk is burning them in open fields

or as fuel in steam generators. The burning operation produces large

quantities of ash and the ash consists of essentially of silica which is in a

relatively inert form and this is not useful either for agricultural or industrial

purpose.

Prior to 1970, RHA was usually produced by uncontrolled

combustion and the ash produced was generally crystalline and had poor

pozzolanic properties. The primary work on RHA was started during 1974 at

the Asian Institute of Technology. In 1975, Mehta in the United States

published papers dealing with rice husk utilization. A study on the efficiency

and effects of burning methods on the quality and their influence on the ash

reactivity of RHA were carried out.

60

The concept of cement based on rice husk ash provided the field for

research, particularly in the rice growing countries of South-East Asia in

1979, and the first workshop on RHA cement was held in Peshawar, Pakistan.

In the later part of 1979 a further meeting on RHA cement was held in Alor

Selar, Malaysia. The main outcome of the meeting was the adoption of the

Indian Standard for masonry cement, (IS 4098). It was then considered that

cement based on RHA would be used for mortar and plaster applications.

Another meeting was held in New Delhi in 1981 and the meeting

concluded by indicating that the available technology on RHA cement was

sufficiently enough and that the information should become more widely

available.

In 1984, the United Nations Industrial Development Organization

(UNIDO) published a comprehensive state-of-the-art development of cements

based on RHA in recent years. The report was intended as a guide for the

production and utilization of rice husk ash cement.

.

Sugarcane bagasse is an industrial waste which is used worldwide

as fuel in the same sugarcane industry. Very limited development on bagasse

ash has been published. A study of the properties of bagasse ash for using in

concrete production was first presented in 1998 and very few papers have

been reported so far.

2.4.2 Thermal Treatment of Agricultural Residues

RHA is an important source of silica. By controlled thermal

decomposition of rice husk, it is possible to produce an ash which contains

reactive silica. In the conversion of rice husks to ash, the combustion process

61

removes the organic matter and results in structural transformation of husk

that influence both the pozzolanic activity of the ash and its grindability.

When the rice husks are first heated, weight loss occurs up to 100°C due to

evaporation of absorped water. At 350°C, the volatiles ignite, causing further

weight loss and husks commence to burn. From 400 to 500°C, residual carbon

oxidies and the majority of the weight loss occurs in this period.

The thermal decomposition process of rice husks exposed to higher

than 300°C are shown in Figure 2.1. The process can be classified into two

stages, carbonization and decomposition (Meada et al 2004). Carbonization of

rice husks occurs when rice husks are treated at a temperature higher than

300°C and releases combustible gas and tar. On the other hand, decarbonation

is considered as the combustion of fixed carbon in the rice husk char.

Decarbonation occurs when rice husk char is treated at a higher temperature

in the presence of sufficient oxygen. Silica phase in RHA is obviously

influenced by incinerating temperature of rice husk and it is an important

factor in the chemical reactivity of silica in RHA. If the incinerating

temperature of rice husk is lower than 700°C, silica in RHA remains in

amorphous state. If the incinerating temperature of rice husk is higher than

800°C, silica in RHA crystallizes as cristobalite or tridymite which have a

lower chemical reactivity compared to amorphous state of silica.

Moreover, chemical reactivity of silica is directly proportional to

the specific surface area of RHA. The heating rate is also another factor in

incinerating rice husk. Rice husk contains a small amount of potassium,

which comes from fertilizer. If the heating rate is high, potassium in rice husk

does not volatile and reacts with silica, burning into potassium polysilicate

combined with carbon. The rapid burning of rice husks cause high residual

carbon in RHA. Thus incinerating process should be designed with

62

considering the above-mentioned thermal decomposition properties of rice

husks.

Heat O2

Combustible heat

gas tar CO2

Carbonization Decarbonization

[Decomposition of [Carbonization of

volatile matters] fixed carbon]

Figure 2.1 Thermal Decomposition of Rice Husk

2.5 CHARACTERISTICS OF RHA AND BA

The characteristics of RHA and BA depend on the geological and

geographic factors related to the types of the rice and sugarcane plants, soil

types, climatic conditions, burning temperature, duration of burning, grinding

methods and collecting devices. According to the ACI committee

recommendations, the following are the primary factors, which will influence

the effective use of agro ashes in concrete. These are as follows: Chemical

and phase composition of the ashes and alkali- hydroxide concentration of the

reacting system, the morphology of the RHA and BA particles, the

development of heat during the early phase of the hydration process and the

changes in the mixing water requirement when using RHA and BA. For an

63

effective utilization, it is necessary to know about the characteristics of RHA

and BA and their effects on the properties of blended concrete.

2.5.1 Physical Properties

The physical properties of agro–ashes like density, colour, fineness,

mean diameter and shape are mainly depend upon the type of burning the rice

husk and sugarcane bagasse and grinding time of the above ashes. Little

information could be found in the literature on the physical properties of the

ashes, but it was apparent that the properties were influenced by the condition

of pyroprocessing.

2.5.1.1 Density

The density of RHA and BA depends on the constituents (iron,

silica, aluminium and calcium) and higher carbon content tents to lower the

density. The compacted unit mass of RHA ranges from 200 to 600 Kg / m3

while the values of concrete incorporating RHA ranging from 2000 to

2300 Kg / m3. The compacted unit mass of the bagasse ash ranges from 200 to

500 Kg / m3 and in concrete it ranges from 2000 to 2200 Kg / m3. The specific

gravity of RHA and BA varies from 2.02 to 2.08 and 2.00 to 2.06

respectively.

2.5.1.2 Colour

The colour of the RHA and BA may vary from white, light tan to

grey and almost black depending on the type and temperature of burning and

duration of burning. If the pyroprocessing occurs between 450˚C and 550˚C,

carbon will remain in the ash which is black. As the temperature of processing

becomes higher, the ash becomes progressively whiter. However, ash

64

recovered from the interior of large masses of burnt husks where air access is

restricted, such as in heap burning, is a lilac pink colour.

2.5.1.3 Fineness

The fineness of RHA and BA will have an influence on the

pozzolanic reactivity and workability of concrete. The use of RHA and BA

increases the water demand due to fineness of these ashes.

Fineness of these ashes is normally measured by BET’s nitrogen

adsorption method or Blaine’s air permeability method in m2 /Kg. The RHA

has specific surface area (Blaine’s) varies from 300 m2 / Kg to 2000 m2 / Kg.

Little information on fineness of BA is available and still research is in

progress. It is reported that the specific surface area (Blaine’s) of BA may

vary from 450 m2 / Kg to 1000 m2/kg. The fineness of RHA and BA increases

with increase in grinding time for all burning temperatures. In general for a

given grinding time, there was a considerable reduction in the specific surface

area of RHA as the burning temperature increased. The influence of fineness,

as determined by the Blaine’s air permeability was studied on compressive

strength of mortars and showed that the compressive strength increases as the

fineness increases.

2.5.1.4 Shape and size

The shape and size of ash particles mainly depend upon the

mineralogical phases and pyroprocessing. Normally the shape of RHA

particles is irregular and cellular texure. The analysis of RHA particles using

SEM showed that the ungrounded RHA particles are in tubular form split

longitudinally with the presence of small bristles distributed over undulated

pores. The well ground ash particles showed the cellular structures. The RHA

65

has mean particle diameter varies from 1 µm to 50 µm. The information on

shape and size of BA particles are not available and to be determined in future

studies.

2.5.1.5 Amorphous SiO2

The reactivity of RHA or BA as pozzolanic material depends on the

crystalline / amorphous ratio. Therefore, for characterization of RHA or BA,

the evaluation of the amount of amorphous silica become very important. For

this purpose, there are some specific methods in the literature. Silica phase in

RHA and BA is obviously influenced by incinerating temperature of rice husk

and it is an important factor in the chemical reactivity of silica in RHA. When

the BA was burnt under controlled condition, it also gave ash having

amorphous silica which has improved pozzolanic properties.

2.5.2 Chemical Characteristic

RHA and BA are fine particulate materials with the main chemical

constituents being SiO2, Al2O3, Fe2O3 and CaO which are responsible for its

pozzolanic activity. They also contain MgO, K2O, N2O, SO3 and unburnt

carbon. There is a possibility of variation in composition from mill to mill,

place to place and type of fertilizer used. The general variation in three

principle constituents in RHA and BA will be as follows:

RHA - SiO2 (80-98%), Al2O3 (0.10-0.6%), Fe2O3 (0.15-0.60 %)

BA - SiO2 (60-80%), Al2O3 (5.0-10.0%), Fe2O3 (3.0-6.0 %)

There are some differences in the standard requirements in the case

of SO3 and loss on ignition. RHA and BA are having some minor amount of

crystalline constituents like quartz, crystobalite and tridymite and free calcium

66

oxide (up to 10 %). In the following sections the significance of each of the

chemical constituents of RHA and BA on the behaviour of concrete is

discussed.

2.5.3 Effect of SiO2

The latest ASTM C618 specifications suggest that the minimum for

the sum of the oxides (reactive glasses) is 70% for class F fly ash and 50% for

class C fly ash. The chemical requirement of OPC and other cements shall

comply with the class 4 of IS 8112 – 1989, according to this class,

CaO

the ratio lime saturation factor =

2.8 SiO2 +1.2 Al2O3+0.65 Fe2O3 (2.1)

shall be within 1.02 to 0.66. Ratio of percentage of alumina to that of iron

oxide shall not more than 1.5%.

The intension to specify the limit is to ensure that sufficient reactive

glassy constituents are present. Many researchers have found that there was

little effect of these three components on the performance of concrete.

The chemistry of rice husk ash and bagasse ash cements involves

the chemical reaction of the amorphous silica in the ash with lime to form

calcium silicate hydrates, which is primarily responsible for strength. In the

case of mixture of OPC – RHA or OPC-BA, the silica reacts with extra lime

in the OPC which in some cases can be as high as 30 % (Boateng and Skeete

1990). The silicate formed are of the kind, CSH (I) and CSH (II) (Jose James

and Subba Rao 1986 – 1).

67

It has been established that silica in the RHA undergoes structural

transformations under various temperature conditions (Mehta 1979)

Incineration of rice husk in the temperature range of 550-700˚C generally

found to produce amorphous silica in the ash while temperature in excess of

900˚C produced unwanted crystalline forms. Smith and Kamwanja (1986)

however, have reported that the temperature of about 800˚C maintained for

12 hours showed small proportions of crystalline silica.

Singh et al (2000) reported that OPC blended cement clearly

indicated higher amount of calcium silicate hydrates formation in the presence

of BA than OPC cement. The structural changes during hydration affect the

grindability and reactivity of these ashes since the larger the surface area of

the silica the greater the extent of the chemical reaction with the lime.

2.6 LITERATURE REVIEW

2.6.1 Earlier Studies on RHA Blended Cement

Columna (1974) had started the primary work on rice husk ash at

the Asian Institute of Technology. Rice husks were converted into ash at

temperature less than 300˚C. Little has been reported about the characteristics

of cement based on uncontrolled combustion of the husks. However, since the

ash is largely crystalline, the lime-silica reaction was weak and reported that

the strength of resulting cement was low.

Mehta (1975) has reported that when the rice hull (husk) was burnt

in the open air in conventional rice mill plants, rice hull produced crystalline

silica ash. A process for developing cement from rice hull has been developed

and patented by Mehta at the University of California at Berkely. When burnt

under proper conditions, especially in a controlled temperature furnace, the

residue was a highly reactive black ash, which when mixed with lime, became

68

rich black cement. Blend of rice hull ash with portland cement produced a

cementing material which was capable of moulding mortars and concretes

with 28 day compressive strength of over 8000 psi (27 MPa). Laboratory

results had shown that rice hull cement containing only from 15- 20 % lime

was acid resisting. Reinforcing of rubbers with rice hull ash was another

application of rice hull ash.

Mehta (1977) has shown that when rice husks were burnt under

controlled condition, the ash was in the form of amorphous silica with a

cellular microstructure, making it a super pozzolan of high reactivity. When

RHA was added to Portland cement, RHA reacted faster than fly ash with the

lime released from Portland cement during hydration and that helped to

improve the early-age strength of concrete, but formed a calcium silicate

hydrate (CSH gel) around the cement particles with dense and less porous. In

this study properties of both lime- RHA and OPC-RHA cements were

investigated. Due to relatively high water demand, the lime-RHA cement

developed lower compressive strength. However the strength characteristics

were considered adequate for general masonry works. OPC-RHA cement

containing up to 50% ash showed compressive strengths which were

considerably higher than that control portland cement even at the early age of

3 and 7 days. It is also reported that the cement containing RHA had excellent

resistance to dilute organic and mineral acids.

Mehta and Pirtz (1978) have reported that a highly reactive silica

ash produced by incineration of rice hulls could be successfully used in mass

concrete applications requiring high strength without excessive rise in

adiabatic temperature. Due to exceptionally high surface area of the ash, the

concrete containing RHA showed only 13mm slump as compared to 95mm

slump for control concrete. To provide the same slump as that of control

concrete, additional water and corresponding cementitious material would

69

have been needed, hence the strength. Under this condition the difference in

heat evaluation could have been substantially reduced. In a concrete mixture

when 30% RHA by weight of total cementing material was present, the 7 and

28 days compressive strength were higher and the adiabatic temperature rise

was 18° F (10°C) lower than that of control concrete. It was also shown that

mortars made with rice husk ash cement had superior resistance to acidic

environments compared even to OPC and other pozzolans. When the lime

RHA mixes were stored in 1% acetic acid solution, it remained in excellent

condition for more than five years, whereas Portland cement showed surface

softening and substantial weight loss within one year.

.

Mehta (1978) proposed the silica activity index as the measure of

the degree of amorphousness of silica in the ash. The index was determined

by calculating the percentage of available silica that dissolved in an excess of

boiling 0.5N sodium hydroxide in a three minutes extraction in a stainless

steel beaker on a minus 325 mesh sample. Results reported by Shah (1979)

indicated that the amount of silica dissolved decreased as the temperature of

incineration of the ash increased. Shah (1979) further proposed that the silica

activity index was somewhat tedious and that a simpler method is to titrate a

solution containing one gram of ash in 25ml of distilled water against

methylene blue. The amount of methylene blue required to produce a colour

change could be used as an index of the state of the silica in the ash. Shah

(1979) results are shown in Table 2.2.

70

Table 2.2 Reactivity Measurement of RHA

Burning temperature

for 1 hour

Silica activity

index(%)

Volume of

methylene blue

500°C 70.4 15.0

600°C 70.4 14.5

700°C 70.4 13.0

800°C 66.7 9.5

900°C 55.1 6.0

1000°C 54.1 5.0

Mehta (1979) stated that totally amorphous silica could be

produced by maintaining the combustion temperature below 700°C under

oxidising conditions for prolonged periods. Yeoh et al (1979) on the other

hand, showed that if the duration time was less than one hour, a combustion

temperature of 900°C could be used with the ash still remaining amorphous.

At 1000°C, a duration time of greater than 5 minute was sufficient to produce

crystalline silica forms. From the above studies, it was observed that the

incineration temperature must be between 450°C to 700°C and duration of

incineration must be sufficient long enough (2 to 3 hours) to get high quality

of RHA.

Manmohan and Mehta (1981) showed that adding RHA to OPC

caused a pore refinement or a transformation of large permeable pores to

small impermeable pores. Compared to a type F flyash at 28 days hydration,

the volume of pore greater than 50nm was significantly lower in equivalent

mixtures containing RHA. It was also reported that even though the total

71

porosity was greater for mixture containing RHA, the permeability was

considerably reduced. This finding was corroborated by Cook (1986).

Cook and Suwanvitaya (1981) examined the influence of

combinations of time of pregrinding amorphous ash and intergrinding with

lime on the compressive strength of mortars. They showed that pregrinding

the ash prior to intergrinding produced higher 7 day strengths but thereafter

the effect of pregrinding was negligible. Intergrinding for period in excess of

4 hours only marginally increased compressive strength and was not

warranted from an energy consumption point of view. The best balance

between time of grinding and strength was an intergrinding time of two hours

with no pregrinding. They have also indicated that the morphology depended

on the lime-ash ratio. For high lime mixes (lime / ash = 1: 0.25), the structure

consisted of poorly defined crystalline forms; the well-recognizable

hexagonal plates of calcium hydroxide were not observed. For lime / ash

ratios of 1: 1.5 and 1:0.67 crystalline forms were observed, but the crystals

were only about 2µm across. In the low lime mix (lime / ash = 1: 4) no

crystalline forms were observed.

Chopra et al (1981) studied the influence of fineness, as determined

by the Blaine’s air permeability method on compressive strength of RHA

blended mortar and showed that the compressive strength increased as the

fineness increased. The presence of up to 20% carbon (by weight) in the ash

did not significantly influence compressive strength. Strength reduction only

became noticeable at carbon content of 25% and higher, mainly due to a

corresponding reduction in silica content.

Mehta (1983) examined the properties of lime-RHA cement as per

ASTM C109. Satisfactory hydraulic cement were made from the reactive rice

husk ash by simply blending from the previously ground ash or by

72

intergrinding the ash with a calcareous material such as quick lime or

hydrated lime. It was concluded that the strength of both the interground and

the blended cements containing either CaO or Ca(OH)2 were adequate for

general masonry work as per ASTM C91. The 0.5 water cement ratio mortars

made with interground cements containing 20% CaO or 25% Ca(OH)2 by

weight was satisfactory for many structural applications because the 2, 7 and

28 day strengths are same as many commercial ASTM type cements. Also the

lime RHA cements were found to be quick setting.

Cook (1984) has reported that in the OPC–RHA mixtures, the

surface area of the RHA particles became coated with a calcium silicate

hydrate (CSH) product as the portland cement hydration commenced.

Ettringite, calcium mono sulphoaluminate, lime and various forms of CSH

were also observed in the OPC –RHA mixtures. As the hydration proceeded,

the hydrate shell around the RHA particles was observed to break away from

the RHA particles. At ages beyond 28 days, the RHA particles were observed

to be etched away and at later ages, around one year, these particles could not

be detected in the matrix which had assumed the relatively featureless

characteristic of OPC hydrated for the same period of time.

Al-khalaf and Yousif (1984) studied the effect of grinding the husk

for various burning temperatures. Grinding was effected by using a modified

Los Angels machine .Samples were burnt at 450, 500, 550, 600 and 700°C for

2 hours. When the husk was converted to ash by uncontrolled burning below

500°C, the ignition was not completed and considerable amount of unburnt

carbon was found in the resulting ashes. The result showed that the most

convenient and economical burning condition required to convert rice husk

into homogenous and well burnt ash was 500°C for 2 hours. The relationship

between grinding time & fineness of RHA burned at various temperatures

suggested that for a given grinding time, there was a considerable reduction in

73

the specific surface area of RHA as the burning temperature increased. A

highly active RHA could be obtained when specific surface was about

21000 cm2/gm, by grinding the ash for 12 hours. The finer the particle (more

specific surface) more the reactive with lime, liberated during the hydration of

cement.

Cook (1986) pointed out that the setting time and strength of lime

rice husk ash mortars could be accelerated by the addition of small amount of

OPC. Up to 10% had been added as an accelerator to lime- RHA mixes. Other

accelerators such as sodium aluminate, calcium chloride, sodium carbonate

and ferric chloride have also been used but the cost of these chemicals was

relatively high. They reported that as the percentage of addition of sodium

aluminate additions increases, the initial and final setting time reduced in the

lime- RHA mixes.

Jose James and Subba Rao (1986-1) examined the reaction product

of lime and silica from RHA. The setting process for lime-excess and lime-

deficient mixtures has been investigated. The product has been shown to be a

calcium silicate hydrate, C-S-H (I) gel by a combination of thermal, XRD and

electron microscopy. The formation of C-S-H (I) gel during the setting of a

paste of lime and silica from RHA accounted for strength or its enhancement.

Jose James and Subba Rao (1986-2) have studied the effect of lime-

silica ratio on the kinetics of the reaction of silica in RHA. It was shown that

below lime/silica ratio = 0.65 the reaction did not proceed to completion and

even in the presence of a large amount of silica, only 90% lime was

consumed. A parameter, lime reactivity index, has been defined to quantify

the reactive silica present in the RHA. The product of the reaction between

RHA and saturated lime was reported as Calcium silicate hydrate, C-S-H(I)

gel. The fibrilar structure and the hollow tubular morbology of the fibres of

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C-S-H(I) gel had been explained by a growth mechanism, where the driving

force was osmotic pressure.

Rahman (1987) observed that when the RHA was prepared in a

furnace by burning the husks at 500ºC for 2 hours, the RHA was whitish grey

in colour. RHA passing through 0.075mm sieve was used. A geochemical

break down showed that it was composed of mainly silica and loss on ignition

of rice husk ash was 3.6%. Bricks were made from clay-sand mixes with

different percentage of RHA and burnt in a furnace for different firing times.

It was shown that light weight bricks could be made with RHA without any

deterioration in the quality of bricks. Further the compressive strength of

bricks was higher with RHA contents and clay-sand-RHA mixes could be

used for load bearing walls.

Boateng and Skeete (1990) conducted a survey of the availability of

rice husks in Guyana and found that the quantities were sufficient to support a

rural housing industry. A prototype incinerator was designed with special

features to burn the rice husk and capable of maintaining bed temperature in

the range of 800-900 ºC. It was reported that the ash produced was amorphous

and highly reactive when mixed with lime and water. The compressive

strength of the mortar produced to the extend of cement of 1:1 blend and

cured for 3 to 28 days ranged between 11.25 and 20.42 MPa. Those values

were in excess of 8 to 31 % over control OPC mix.

Zhang and Malhotra (1996) used the RHA which was a processed

waste material incinerated at a temperature not exceeding 800°C for few

minutes. The RHA contained a carbon content of 5.91% and therefore was

slightly black in colour. Chemical analysis indicated that the material was

principally composed of SiO2 (87.2%) and relatively high K2O content of

3.68% that originated mainly from the soil due to the use of fertilizer. RHA

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has a specific gravity of 2.06 and median particle size of the RHA was

approximately 7µm while that cement has approximately 13µm. The RHA

had extremely high specific surface of 38.9 m2 /g by nitrogen adsorption.

They also reported that the compressive strength of concrete containing 15%

of RHA was higher than the control concrete. The RHA concrete had higher

compressive strength at ages up to 180 days. The strength of the concrete

increased with decreasing water-cementitious material ratio w/(C+RHA). The

RHA had slightly longer setting time, negligible bleeding and required a high

dosage of super plasticizer (more than 10Kg/m3) to obtain the proper slump.

The 28 and 91 days test results for the resistance to penetration of chloride

ions into concrete for durability aspects of control concrete, RHA concrete

and silica fume concrete, measured in terms of the electric charge passed

through the specimens in coulombs (ASTM C 1202) were comparable. The

RHA had excellent resistance to chloride ion penetration and charged passed

in coulombs was below 1000 both at 28 and 91 days which was well below

that of the control concrete but higher than that of silica fume concrete. The

RHA concrete also showed excellent performance in freezing and thawing

test.

Ciasse et al (1998) studied the mechanical and dimensional

properties of concrete containing rice husk ash as filler. The results obtained

indicated that the performance of such concrete were very close to those of

concrete with sand filler and that the performance was better when the ash

was pulverized. It was concluded that the use of RHA in sand concrete

allowed the RHA as an elaborate economical building material which would

solve the pollution problems caused by the waste.

Gjorv and Mehta (1998) conducted research program on long-term

durability of concrete structures in marine environment and the ability of the

RHA to improve the resistance of concrete against chloride penetration was

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investigated. The results showed that both the capillary suction and the

accelerated chloride diffusivity were distinctly improved by the addition of

RHA. The result also indicated that RHA has a similar effect on both chloride

diffusivity and electrical resistivity as that of condensed silica fume.

Qijun Yu et al (1999) have studied and confirmed that at a

temperature around 40°C in the presence of water, the amorphous silica

contained in RHA could react with Ca (OH) 2 to form one kind of C-S-H gel

(Ca1.5SiO 3.5 H2 O). The C-S-H gels with CaO and SiO2 molar ratio of about

1.43 filled in pores and played a bridging role between hydrates and

unhydrated cement particles. In comparison to the concrete without RHA,

there would be more C-S-H gel and less portlandite in the concrete with RHA

addition. This contributed the improvement in the strength of the concrete and

its resistance to acid attack, carbonization and chloride penetration.

Cissa and Laquerba (2000) have examined two types of ashes,

ground and ungrounded in the study of characterization of filler sandcrete

with RHA. The use of ungrounded RHA has enabled the productivity of light

weight insulating concrete. Chemical analysis revealed that the RHA was

amorphous structure of silica. The analysis using SEM showed that the

ungrounded ash particles were in a tubular form spilt longitudinally with the

presence of small bristles distribution over an undulated structure and

confirmed the presence of hydroscopic pores whereas the ground ash was in

cellular structure. The report revealed that the mechanical strength increased

throughout the curing period, more rapidly for the samples cured in water.

The mechanical strength of the sandcrete with ground ash was on average

twice that of sandcrete with ungrounded ash (28 days strength 18.59 MPa as

against 9.56 MPa) due to pozzolanic nature of ground ash. It is also reported

that the sandcretes made with rice husk ash demonstrated a superior

mechanical strength (18.59 MPa) when compared to sandcretes made with

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filled lime stone (9.10 MPa) or chet (7.47 MPa) and had the same or higher

cement content. Sandcrete blocks with RHA addition has a lower cost

compared with those using other types of additions.

Nicole et al (2000) undertook an experimental program to study the

effect of different level of replacement by silica fume and RHA on the

expansion of mortar bars containing two reactive aggregates. The morphology

and composition of alkali silica reaction gel were determined using SEM and

X-ray. They reported that there was significant reduction in mortar expansion

for both the silica fume and RHA blended mortars. The replacement levels of

15% of either SF or RHA produced the least expansion and replacement level

of 12% reduced expansion below the threshold of 0.10% at 16 days.

Paya et al (2001) proposed a rapid analytical method for evaluating

amorphous silica in RHA to assess its potential pozzolanic activity. This

method was based on bringing the siliceous non-crystalline fraction of the

pozzolan into solution as glycerosilicate by treating the test material with

glycerol. The titration of the resulting solution was performed with an

aqueous glycerol solution of barium hydroxide. A good concordance results

were obtained when compared to these obtained using a standard method.

Singh and Sarita Rai (2001) studied the effect of 3.0 wt% polyvinyl

alcohol (PVA) on the hydration of OPC (Ordinary Portland Cement) in the

presence and absence of 10% RHA. The results indicated that the PVA with

RHA increased the strength and decreased the porosity due to the formation

of new compound that filled the pores or improved the bond between the

cement particles.

Singh et al (2002) carried out studies on the effect of lignosulfonate

(LS), calcium chloride and their mixture on the hydration of RHA-blended

portland cement and reported that lower amount of water was required for

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consistency of the paste. The result showed that the compressive strength of

10% RHA-blended Portland cement was maximum in the presence of a

mixture of 2 wt% CaCl2 and 1% LS. However the results on corrosion

studies have indicated that RHA-blended cement without any admixture

(CaCl2, LS or both) was more resistant to the corrosion atmosphere.

Bhanumathidas and Mehta (2004) carried out a preliminary

research study on the role of RHA in enhancing the properties of high volume

fly ash concrete. They described that the use of RHA in ternary blends offered

a good solution to enhance the early-age engineering properties of concrete

with high volume of fly ash blended cements. Through cement conservation,

enhancement of durability and environmental protection, such ternary blends

obviously contributed to sustainable development.

Meada et al (2004) developed a new furnace for the production of

high specific surface area of RHA and investigated the effect of incinerating

conditions on the specific surface area and ignition loss on RHA. They

showed that RHA having specific surface area larger than 150 m2/g with

ignition loss less than 3% could be produced by incinerating the husk at a

temperature below 400ºC. Irrespective of RHA content, water-cement ratio

and aging, the compressive strength of mortars incorporating the high surface

area RHA was higher than that of mortars incorporating the low surface area

RHA and silica fume.

Qingge et al (2004) used three methods to study the pozzolanic

properties of RHA by hydrochloric acid pretreatment. The heat evaluation,

heat of hydration of cement, the Ca(OH)2 content in the mortar and pore size

distribution were determined. A significant increase in strength of the RHA

(pretreated) specimen was observed. The results of heat evaluation indicated

that the RHA by pretreatment increased the hydration of cement. The cement

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added with RHA by pretreatment had lower Ca(OH)2 content after 7 days and

the pore size distribution of the mortar with RHA with pretreatment showed a

tendency to shift towards the smaller pore size.

Deepa et al (2006) investigated the properties of RHA samples

produced from different types of field ovens to compare the performance of

the ovens and to identify the most feasible method to produce a reactive

pozzolan as alternative to cement for building application requiring lower

strength. The RHA samples from the annular oven consisting of two co-axial

hollow cylinders made with welded meshes, produced better results in all mix

composition compared with RHA samples from other field ovens. The

annular type of oven was suggested as an affordable and simple option for the

small-scale production of RHA in rural areas.

2.6.2 Earlier Studies on BA Blended Cement

Hernandez et al (1998) presented a study that was aimed at

monitoring the reaction between lime and the waste of the sugar industry

having pozzolanic properties by evaluating the content of calcium hydroxide

dependence on lime, development of pore structure dependence on time,

study on the reaction product at different stages and mechanical properties of

hardened pastes. The results of the study showed that sugar cane bagasse ash

did not act like a reactive pozzolan, mainly due to the presence of unburned

materials and carbon, where as sugar cane straw ash showed good pozzolanic

activity comparable to that of RHA.

Singh et al (2000) evaluated the hydration of the bagasse ash-

blended portland cement by employing a number of experimental techniques.

It was found that in the presence of bagasse ash, setting times were increased

and free lime content was decreased. The overall results indicated that BA act

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as a pozzolanic material, and in its presence the extent of hydration was lower

as compared to that of control. However, in the presence of 10 % BA the

compressive strength values were found to be higher than that of control at all

ages of hydration. The chemical deterioration of blended cement mould in the

presence of N/60 H2SO4 was lower compared to that of control. It was due to

the pozzolanic reaction of BA and the reduction in permeability in its

presence.

Hernandez at el (2000, 2001) have also reported the pozzolanic

properties of well burnt residues of sugar industries.

Paya et al (2002) studied the properties of sugar cane bagasse ash to

reuse in concrete production. The combustion yield ashes contained high

amount of unburned matters, silicon and aluminuim oxides as main

components. The sugarcane bagasse ashes have been chemically, physically

and mineralogically characterized in order to evaluate the possibility of their

use as a cement replacement material in concrete industry. It is reported that

due to the presence of amorphous silica in its chemical composition, the sugar

cane bagasse ash could be used as mineral admixture in concrete.

Cardeiro (2004) carried out study to enhance the pozzolanic activity

of the sugar cane bagasse residual ash by increasing the fineness of the

material. In their study mechanical grinding has been used to improve the

pozzolanic activity of the sugar cane bagasse ash. The influence of grinding

time on the sugar cane bagasse residual ash reactivity was presented. The

bond grindability test of the residual ash was performed and the work index

was determined at a test size of 325 mesh (45µm). The results demonstrated

that the pozzolanic activity of the residual sugar cane bagasse ash

significantly increased due to mechanical grinding in a vibratory mill.

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From the literature survey, it is clear that sufficient research have

been done on the strength and durability behaviour of fresh and hardened

concrete incorporating fly ash, silica fume and blast furnace slag.

From the literature survey carried out on the RHA or BA blended

concretes, the following observations can be made

1. Use of RHA in concrete as supplementary cementing

material provided economic, energy saving, ecology benefit

and improvement in properties of materials.

2 In many of the studies carried out on the utilization of RHA

in concrete, the rice husks were ignited under controlled

temperatures. The incinerating temperature was in the range

of 500°C to 700°C. The duration of incineration was

between 1 hour to 2 hour and ground for 1 hour.

3 Amount of water required for standard consistency of

cement RHA pastes was always higher than that required to

balance against the ash reactivity and produce a more

workable concrete.

4. Curing process was one of the decisive factors in obtaining

higher compressive strength of RHA concrete when higher

dosages of RHA (> 10% by weight of cement) were added.

5. The higher the percentage of RHA, the higher were the

volume change characteristic compared to those of the

corresponding plain mixes. However the percentage increase

in drying shrinkage of mortar bars were within the limit

specified.

6. One of the main reasons for the improvement in properties

of concrete such as strength, durability, resistance to acid

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attack, carbonation and penetration by adding RHA to

concrete possibly may be attributed by the formation of C-S-

H gel and less portlandite.

7. RHA concrete had slightly longer setting time than those of

control concrete and the bleeding of concrete incorporating

RHA was negligible. The concrete incorporating RHA

requires higher dosage of superplasticizer and air- entraining

admixtures than control concrete.

8. RHA concrete had higher compressive strength at the end of

180 days compared to control concrete. The flexural and

splitting tensile strength, modulus of elasticity, drying

shrinkage of control concrete and concrete incorporating

RHA were comparable.

9. The RHA concrete showed excellent resistance to chloride

ion penetration and the charged passed in coulombs was

below 1000 both at 28 days and 91 days which were below

those for the control concrete.

10. RHA may be used supplementary cementing material for

manufacturing of other cement based product like, concrete

block, light weight insulating concrete block, clay-sand-

RHA bricks, filler sandcrete, etc.

11. The pozzolanic activity, pore refinement, strengthening of

micro structure and crack resistance at interfacial zone of

RHA concrete were similar to silica fume concrete. The

problem of more water demand and shrinkage cracking can

be eliminated by using suitable dosage of superplasticizer

and method of curing.

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12. Well burnt bagasse residue may be used as pozzolanic

materials.

13. Bagasse ash blended cements increased setting times and

reduced the free lime in OPC.

14. Due to the presence of reactive silica in BA, they may be

used as mineral admixtures for blended cement.

15. In the presence of 10% BA, compressive strength was higher

than that of OPC.

16. Due to mechanical grinding, the pozzolanic activity of the

sugar cane bagasse ash significantly increased.

Very limited information is available on the water and chloride

permeability characteristics of concrete containing RHA. In general there is a

specific need to study the performance of RHA prepared from uncontrolled

boiler burnt rice husk residue available in rice mills as reactive pozzolan for

strength, chloride permeability and corrosion resistance properties in blended

cement concretes.

Only a few studies have been reported on the use of bagasse ash as

pozzolanic material with respect to cement pastes. Thus a systematic and

detailed study on the utilization of BA prepared from uncontrolled boiler

burnt bagasse residue of the sugar mills for strength and durability properties

of concrete is limited.

Literature survey indicates that very little information is available

on the corrosion resistant properties of concrete blended with RHA and BA.

There is a need to study the corrosion resistance of RHA concrete and BA

concrete using various electrochemical monitoring techniques.

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2.7 NEED FOR RHA AND BA BLENDED CONCRETES

Fly ash, ground granulate blast furnace slag and condense silica

fume are well-established mineral admixtures. The improvements in the

properties of fresh and hardened concrete resulting from the pozzalanic

behavior of these admixtures are well understood. Among the mineral

admixtures, only fly ash is available readily from thermal power plants. BFS

and silica fumes require excessive processing before they are used as a

mineral admixture. These materials are not available in many countries. Rice

husk ash and bagasse ash have special relevance to many countries. Some 500

million tons of paddy and 1500 million tons of sugar cane are produced

worldwide annually, from which some 20 million tons of rice husk ash and 10

million tons of bagasse ash as pozzolanic materials could be extracted.

Cement production is associated with the utilization of large

amount of energy. Replacement of cement with RHA and BA results in

energy saving as RHA and BA do not need addition of energy input before

use and they are local waste materials. The salient features in the production

of cement based on RHA and BA are the conversion of the husk or bagasse to

ash and the grinding them to a suitable particle size. However the conversion

of husk or bagasse may be undertaken in different ways.

Rice husk and bagasse represent valuable sources and are used as

fuel in the mills. When used as boiler fuel or for any commercial application,

the pyroprocessing is not controlled and the ash so produced contains

crystalline components (hard burnt ash). The alternative process in the

conversion of hard burnt rice husk residue and bagasse residue obtained from

the mills into reactive amorphous ash is controlled burning using the fuel

value of these residues and pulverizing them to required fineness with less

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energy compared to that of grinding in ball mills for several hours. The

amount of cement replacement with RHA or BA, favour energy saving.

Thermodynamic computation indicates that 1 ton of cement replacement with

RHA and BA saves approximately 5000 MJ of energy which is equivalent to

a barrel of oil or 0.25 ton of coal. Utilization of RHA and BA as cement

replacement material in construction, they will provide economical benefit by

way of energy saving.

The Portland cement production is also associated with carbon

dioxide emission, which is a major source of global warming, and the use of

Portland cement with cement replacement materials carbon dioxide emission

is controlled. A typically efficient cement plant will release about 0.65 ton of

carbon dioxide for each ton of cement produced. If carbon fuel is used to burn

the clinker, an addition of 0.35 ton of carbon dioxide is released. Each cement

plant releases at an average of 1 ton of carbon dioxide to the atmosphere to

produce 1 ton of cement. RHA typically replaces 30 % of cement and BA

typically replaces 20 % of cement and thus, at an average a 25 % replacement

of 1 ton of cement would result in a reduction of 0.25 ton of carbon dioxide.

The use of RHA and BA in the blended concretes is expected to

enhance the strength and durability properties of concrete for technical

benefits :

1. Reduced bleeding and segregation

2. Reduced heat of hydration

3. Improved early strength development

4. Lower water permeability

5. Less shrinkage

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6. Increased resistance to cracking

7. Higher compressive strength

8. Lighter colour

9. Reduced chloride ion permeability and diffusion

10. Increased resistance against corrosion of steel in concrete.

A number of tests and processing of rice husk and bagasse ash for

cement replacement have to be carried out before they can be suitably blended

with cement. Hence in this research work experimental investigations have

been planned for the evaluation of RHA and BA prepared from the mill

residue as cement replacement materials for strength and durability properties

of concrete and to find an optimal level of replacement on the blended cement

concrete system for the strength and resistance against chloride penetration

and corrosion of steel.

It is felt that a more comprehensive and systematic evaluation on

RHA blended cement concrete and BA blended cement concrete carried out in

this report will lead to its widespread application in the corrosion prone

reinforced concrete structures. Apart from reducing demand for Portland

cement, its cost, ecological, economical and technical benefits, rich husk ash

and bagasse ash will provide substitute for cement to rural areas which are

traditionally deprived of cement.