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Characteristics Of Lime Mortar Components Biology Essay

In this chapter, the characteristic of lime mortar components such as lime, mortar, aggregates, water and pozzolan will be discussed. In order to produce a good quality of lime mortar, the selection of good quality components should be made.

2.1 History of Lime

In the past, lime was an active ingredient in building and broadly used in construction industry. Previously lime was act as extender in the cement mixtures to produce a stronger and workable material which apply in walls, floors, ceilings and decorations. But the using of this vital ingredient had virtually ceased in the thirty years ago.

The role play by cement and mortar gradually substitute the lime in construction industry. Cement is easy to handle and relatively trouble free. Meanwhile, mortar can react with buildings at faster rates compared to lime. The industrial revolution also increases the number of urban buildings. Undoubtedly, the demand for lime in market gradually decline.

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In fact, there are problems existed on using these ingredients and techniques in ancient buildings and it usually take many years for the disadvantage to become apparent. Commonly, when these modern materials on ancient buildings, it will create an impermeable finishes which cause the dampness hard to evaporate thus speed up the decay process.

By the year 1970, Professor Baker at Wells Cathedral on the West Front creates a new generation for lime. By that time, the Society for the Protection of Ancient Buildings highly promotes the using of lime for conservation work on old buildings. Undoubtedly, application of lime on old buildings able the walls to absorb a certain amount of moisture and this moisture also evaporate easily and thus achieve moisture balance on the walls. By the way, cement impossible to achieve this result. Microscopically cracked problems will exist when the wall respond to the atmospheric changes. The will cause the moisture that sucked into the cracked unable to evaporate thus create a more damp wall (Schofield et al 1995)

2.2 Physical and chemical properties of lime

Lime is produce from limestone, chalk, coral, sea shells and other calcium-rich resources by heating them to a high temperature in a special type of kiln. Lime exists in white or grayish white colour finely crystalline substances. Due to iron impurities exist in lime, the lime sometimes has a yellow or brown tint colour. The table below had shown the physical properties of lime (Hornbostel 1991).

Type of properties

Specific Value

Specific gravity


Melting point


Boiling point


Table 2.1: Physical properties of lime

Source: (Hornbostel 1991)

Lime also called quicklime which consist chemical component of calcium oxide (CaO). Lime reacts vigorously with water to produce calcium hydroxide [Ca (OH) ²], known as slaked lime. The setting of lime is largely due to drying out and to recarbonation of calcium hydrate. When lime combines with the carbon dioxide of the air with some pozzolanic effect with the silica in the sand, it will from calcium carbonate. This reaction will cause the lime harden (Hornbostel 1991).

2.3 Classification of lime

In the past, lime was used principally in construction and in agriculture. As the time past, many research and development had been done on lime. Today, lime as a construction material, lime obtained from the calcinations of limestone is broadly classified into quicklime, hydrated lime, hydraulic lime, fat lime, and poor lime.

2.3.1 Quicklime

In order to produce quicklime, crushing, grinding and grading of limestone before heating. Graded limestones are heated in a horizontal kiln or in a vertical kiln at a temperature of 1093°C. The carbonates, thus heated, decompose into carbon dioxide and calcium oxide (Hornbostel 1991). Quicklime has high melting points. Besides, the quicklime is very volatile before it has been slaked and care has to be taken to avoid it reacts with carbon dioxide and the moisture in the air.

2.3.2 Hydrated lime

Hydrated lime is produced by burning the calcareous materials. In this burning process, quicklime is produced by given off the carbon dioxide. Then, it is reacted with water to produce hydrated lime. In the lime cycle, hydrated lime can further react with carbon dioxide to produce calcium carbonate. Due to this property, hydrated lime used in mortar can benefit to mortar of self-heating or autogenous healing (Newman et al 2003). Hydrated lime absorbs moisture and carbon dioxide from the air, and should therefore be store in a cool, draught-free building and used whilst still fresh.

2.3.3 Hydraulic lime

Hydraulic lime is produced by burning argillaceous limestone, which is subsequently reduced to powder by slaking. Hydraulic lime also known as "water lime" because of its ability to gain strength by hydration and set under water (Scottish Lime Centre 2003).

The general term "hydraulic lime" represents both natural hydraulic lime (NHL) and hydraulic lime (HL). Hydraulic lime is produced by limestone, chalk or other calcareous material containing high proportions of CaC03 at 1000 - 1250°C using vertical or shaft kilns, either as a batch or continuous process. This process is known as calcination to decompose CaCCh to CaO at the temperature just below the melting point of the product (Scottish Lime Centre 2003).

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To produce limes with hydraulic properties, argillaceous limestones that contain 10 types of clay in various proportions or processed limestones with silica inclusions are used (Smith et al 2004). An additional procedure is required to produce natural hydraulic lime which is called slaking. Certain amount of water is added to the hydrate and it converts the lumps of lime into a powder. Grinding is sometimes required to further reduce the particle size and obtain a uniform grading. Other than the uniform grading, natural hydraulic lime contains no additive as opposed to hydraulic lime (Smith et al 2004). The chemical composition of natural hydraulic lime is presented in Table 2.4.


Percentages related to original dry lime (%)



















Table 2.2: Chemical composition of natural hydraulic lime

Source: (Smith et al 2004)

2.3.4 Fat lime

This kind of lime also called as white lime, rich lime, or high calcium lime. Calcinations of pure limestone which should not contain impurities not more than 5% are made to produce this kind of lime. In the calcinations process, its volume is increased to about 2 and 2.5 times. This kind of lime is physically pure white in colour and set slowly in the presence of air. Usually, fat lime is not suitable used for the preparation of mortar but mostly used for plastering and white washing (Becha 2005).

2.3.5 Poor lime

Poor lime also known as lean lime or impure lime. This lime contains more than 30% of clay. Poor lime physically exists in muddy white colour. This kind of lime has very poor workability and poor binding properties. Due to this, poor lime is used when good lime is not available and make poor mortar which can be used for interior type of work (Becha, 2005).

2.4 Manufacture of lime

The original sources of lime are from quarried limestone. Quarried limestone is calcium carbonate. The limestone is the main source of material to produce lime putty or hydrated lime. The mineral is quarried, crushed, ground, washed, and screened to the required size range. The limestone is burn in a kiln either in a horizontal rotary kilns or vertical shaft kilns at a temperature around 900ËšC. in this burning process, carbon dioxide are given off to become quicklime (calcium oxide) (Lyons 2007).

CaCO3 (calcium carbonate) + heat (900°C) ® CaO (calcium oxide) + CO2 (carbon dioxide)

2.4.1 Slaking of lime

Subsequently, water is added to this quicklime, there will a vigorous chemical reaction take place. In this process, the water added is absorbed. As the result, a thick white slurry lime putty or calcium hydroxide (Ca (OH)2 are produced (Lyons 2007). The reactions which lead to the formation of this product are shown in below:

CaO (calcium oxide) + H2O (water) ® Ca (OH)2 (calcium hydroxide)

2.4.2 Carbonation

When these materials are used in lime mortar, the hardening process of lime mortar will occur. This is because when these materials exposed to the air the chemical composition of calcium hydroxide will react with the carbon dioxide that present in the air. In this reaction, water is given off and calcium carbonates are produced. The whole process is call carbonation process (Velosa et al 2002).

Ca (OH)2(calcium hydroxide) + CO2 (carbon dioxide) ® CaCO3 (calcium carbonate) + H2O (water)

Figure2.1: The lime circle

Sources: (Schofield et al 1995)

2.5 Mortar

Mortar is one kind of building materials which can classify as one of the oldest building materials. The main function of mortar is used to combine the masonry elements such as stones and brick. In order to meet the current construction demands, many study and research had been made to produce different technological characteristic in term of different type and size of aggregate, different compaction degree and different binder combination ratio. (Maravelaki-Kalaitzaki et al 2003). Mortars nowadays are widely used in bricklaying, plastering and rendering work. Due to few factors such as simplicity of the preparation, raw material availability, and also to the important aesthetic and structural functions, mortars play a strong character in the architecture from the prehistory.

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Previously, application of mortar is to protect masonry which exposed to environment and pollution. During the 19th century, development of modern construction materials in construction field such as modern cements and concretes have made the mortar re-evaluated as artistic elements.

There are two main kinds of binders: aerial and hydraulic which normally depend on the mechanism of hardening. Binders play an important role in the structural properties of mortar. This is because mortars are composite materials which are formed by the mixture of silica or carbonate sand (aggregate) with a binder fraction. The composition of calcium and magnesium hydroxides produced in the process of limestone calcinations and high percentages of clay act as a binder which contribute to workability and elasticity of mortars. The sand or aggregate contain inside the mortar act as filler and to prevent problems arising from shrinkage. (Moropoulou et al 2000)

2.6 Composition of mortar

Usually, mortars are composed of a binder, aggregates and additives. Particularly, the binder in the mortar is an important component to the durability and performance of the mortar. The common binder usually contain inside the mortar including clay, lime, gypsum or artificial cement.

2.7 Properties of mortar

Properties of mortar largely influence by the sand and fines particles contain in the mortar. Sand and fines particles can influence the workability, water retentivity, strength, and durability of mortar.

2.7.1 Workability

Workability of mortar is important, because it enable motor to fills the voids between masonry units. All the essential surfaces of masonry units are covered and thus create a strong bonding between the masonry units. Once the mortar are cured or hardened, the workability of hardened mortar such as cohesive strength, adhesive bond strength, modulus of elasticity, compression strength and degree of expansion and solubility can be tested (McKee 1973).

2.7.2 Water retentivity

This type of properties refers to the mortar's ability to retain the mix water and still remain workable when brought into contact with the masonry units. Different characteristic of fine particles mixed in the mortar affect the water retentivity of mortar. By using finely ground particles or air-entraining agents, the water retentivity can be increased (Smith et al 2001).

2.7.3 Strength

Many factors affect the strength of mortar such as type of sands, the particle shape, existence of the chemical impurities and the sand grain size. The mortar strength generally related to brickwork strength. The adhesion reaction between bricks and mortar generally depend upon by the flow of moisture from the mortar to the brick mortar interface (Smith et al 2001).

2.7.4 Durability

The durability of mortar refers to the mortar's ability to against surrounding deteriorating agents and remains serviceable for a long time period. The grading of sand is important as its can encourage cement paste to fill the voids thus increase mortar's frost resistance (Smith et al 2001).

2.8 Classification of lime mortars

Lime mortars can be classified as non-hydraulic lime mortars and hydraulic lime mortar. The significant differences between this two type of lime mortars is the existence of water during their production. Non-hydraulic lime mortars are produced by mixing the slaked lime with aggregates and harden by the process of evaporation and carbonation of lime due to carbon dioxide in the air.

Meanwhile, there are two common processes to produce hydraulic lime mortar. Firstly, hydraulic lime mortars are produced by mixing lime with pozzolans which containing amorphous active silicates and aluminates. Secondly, hydraulic lime mortars are produced by developing hydraulic phases through the calcinations of silica rich limestone directly quarried or synthetically mixed. After the mixing process, the hydraulic lime mortars are harden by evaporation, carbonation of lime and the reaction between the the lime and pozzolans or the hydraulic phases in the presence of water. Calcium silicate hydrates and calcium aluminate hydrates are produced during this process. (Lea FM 1970)

2.9 Aggregates

Aggregate is granular material such as sand, gravel, crushed stone, blast-furnace slag, and lightweight aggregates. The aggregate normally used in concrete are natural deposit of sand and gravel. In some places, large rocks are available and must be crushed to form the aggregates. In order to produce crushed aggregate, the crushing process requires high cost. The crushed aggregate also require more intention in order to prevent any improper dispersion of the sizes through the finished concrete (Intermediate Structural engineering guide book, n a).

2.10 Types of aggregate

Aggregate are divided into two types. Firstly is the coarse aggregate and secondly is fine aggregate.

2.10.1 Coarse aggregate

Coarse aggregate is that the particle which is larger than about 5mm. Generally, coarse aggregate consist of gravel, crushed stone, or blast furnace slag. Coarse aggregates are suitable to be used in concrete work. The coarse aggregate used in concrete work must be clean of organic impurities because it can affect the final strength of the hardened concrete (Nawy 2001). The common type of coarse aggregate and its details are listed in the table below.

Type of coarse aggregate


Natural crushed stone

Produced by crushing natural rock from quarries. This type of crushed rock is less workable when compare to other type of aggregate

Natural gravel

Result of the weathering action of running water on the beds and banks of streams. It is more workable if compare to crushed rock.

Artificial coarse aggregate

These are the by-product of other manufacturing processes such as blast furnace slag and mostly used to produce lightweight concrete.

Table 2.3: Types of aggregate and details

Source: (Nawy 2001)

2.10.2 Fine aggregate

Fine aggregate which comprises particles from 5mm down to 0.075mm. Fine aggregate contains not more than 5 per cent coarser material. Filler, binder and dust are particles which are finer than 0.0075mm. The main function of fine aggregate is to fill the voids in the coarse aggregate and to act as a workability agent (Bechar 2005). Fine aggregate may be classified as follow:

Natural sand: it is resulting from natural disintegration of rock or that which has been deposited by stream and glacial agencies.

Crushed stone sand: the fine aggregate produced by crushing natural stone

Crushed gravel sand: the fine aggregate produced by crushing natural gravel.

2.11 Characteristic of aggregate

Usually, aggregate accommodate almost 60-70% of the total volume of concrete mix and also the mortar, so aggregate characteristic determine the properties of hardened concrete and mortar. Further understanding about the aggregate features regard its size, shape, surface texture, strength, stiffness, and its overall soundness and durability are very important. (McNally 1998)

2.11.1 Strength of aggregate

Strength of aggregate related to the mechanical properties of aggregate which including toughness and hardness. The toughness of aggregate is measured as the resistance to failure by impact. The aggregate used in concrete work must have impact value which is not more than 45 per cent by weight for aggregate used for concrete. Meanwhile, the hardness of the aggregate is the measured of its resistance to wear obtained in the terms of aggregate abrasion value (Gambhir 2004).

In concrete work, strength of aggregate also measured on their ability on resistance to freezing and thawing. This ability largely depends on the aggregate's porosity and pore structure. Besides, modulus of elasticity of aggregate also must take into consideration in concrete work as it can bring major effect to the cement paste (Gambhir 2004).

Generally there are three type of test to determine the strength of aggregate such as aggregate crushing value, aggregate impact value and ten per cent fines value (Gambhir 2004).

2.11.2 Shape

Commonly, there are four type of classification for aggregate's shape which is rounded, irregular, angular and flaky (Gambhir 2004). Rounded aggregate frequently can be founded in the area of coastlines or in riverbeds. The aggregate founded in this area usually is smooth in their texture and rounded (Jeff C. et al. 2007) .Meanwhile, irregular aggregate are partly shaped by attrition and have rounded edges. The aggregates which grind by the machine usually have rough surface texture and commonly in angular shape (Jeff C. et al. 2007). Flaky shape aggregate is one type of the angular aggregates which obtain from laminated rocks having thickness lesser than the width and/or length (Gambhir 2004).

2.11.3 Surface texture

Surface texture of aggregates related to the characteristic appearance of aggregates surface having tactile quality. There are several factors affect the aggregate surface structure which including the amount of moisture and dust adhering to the aggregate surface, particle mineralogy, and surface roughness. Usually the aggregate chosen and used in the cement work must correct because aggregate can bring major effect to the chemical reaction, polishing resistance and cement adhesion to masonry units. The roughnesses of the aggregates are important to determine the quality of cement work. Commonly, there are two type of classification to the aggregates surface texture which is rough and smooth. Nowadays, most of the construction works prefer rough surface aggregate in construction work; this is because aggregates with rough surface texture can increase the bonding of cement mixture with the masonry units. (Jeff C. et al. 2007)

2.11.4 Moisture content of aggregate

The moisture content of aggregate is defined as the percentage of the weight of the saturated surface dry aggregate. In concrete work, the determination of moisture content of the aggregates is necessary and important because it will influence the net water-cement ratio for a batch of concrete. The concrete will become weak if the moisture content of aggregate is high because it increase the effective water-cement ratio. There are two types of methods to determine the moisture content which are displacement method and drying method (Gambhir 2004).

2.12 Sand

Nowadays, sand plays an important role building construction. Usually natural particles with a grain diameter size between 1/16mm and 2mm are called sand. Besides that, sand also can be defined as a loose aggregate of sandsized particles (Frazier 1996). The sand type has an important influence on the properties of mortar.

Sand is commonly extracted from borrow pits or dredged from coastal rivers. The sand is generally supplied as extracted, leaving the bricklayer to screen out oversize particles and vegetative matter. Some material is blended to improve mortar workability. Clearly, sands for masonry work have very wide-ranging characteristics (Walker et al 2003).

Sand is the common ingredient for masonry mortars even though varieties of cementitious materials are used for mortars. Sand constitutes bulk of the mortar volume. Composition of sand and its grading can influence the characteristics of mortars in fresh as well as in hardened state. Also, it could influence brick-mortar adhesion and other masonry characteristics (Reddy et al 2008).

Building mortar sands should be hard, durable, clean, sharp and free from contaminants (clay, iron pyrites, salts, organic matter) likely to impair hardening, strength and durability. In previous studies, mortar cube strength was reduced and shrinkage increased as grading of the sand became finer (Harrison 1986)

2.13 Sand production

There are two fundamental destabilization mechanisms occur in the sand production process. Firstly, during the production of hydrocarbons, the mechanical instabilities lead to the localized plastification and failure of the rock. Secondly, the invariables resistances cause the water power unstable such as interior and sheet erosion, which is manifested by the particle release and the mobilization. The internal corrosion may be co-related to micromechanical impacts which impose on the solid skeleton which produce mobile solid or flowing solid motion in the pore network (Vardoulakis et al 1996).

Apart from this, sand production processes consume a lot of work and time. The sand production process can be classified to several group which including extraction, preparation, sizing, and beneficiation (McNally 1998).

2.13.1 Extraction

This is the first step for the sand production process. Digging, loading and transporting works are involved in this step. Before begin of an extraction work, determination of alluvial pits whether it is wet or dry condition must be made. If exaction in dry pit condition, conventional moving plant such as bulldozers are used. Face collapse is main geotechnical problems in dry pits. On the order hand, in wet pits, the sand is mined by suction cutter dredging which using a barge mounted pump. In wet pit, the pond floor is simply agigated so that the sand can go into suspension (McNally 1998). Some geotechnical information is require in order to decide whether a material economically to be excavate or not which includes the following:

particles size distribution

stiffness of the matrix

existence of underground obstacles

aquifer characteristic of the alluvium

2.13.2 Washing and scrubbing

After extraction work, the next process comes to the washing process. The purpose of washing process is to remove any silt which is adhering to graved sized particles. Besides, the aggregate's screen capacity can be increased. Meanwhile, scrubbing is used when less clayey deposits were unavailable. The main different between the washing and scrubbing is the total energy involved (McNally 1998).

2.13.3 Coarse aggregate screening

In the screening process, the materials are turned into saleable products. There are three main purposes for screening process which are shown in the following:

Scalping oversize

Waste removal


The common type of screen used in the aggregate processing is the vibrating multi deck set.

Figure2.2: Vibrating multi deck set

Sources: McNally (1998)

The vibratory motion of the screen causes the crushed stones to separates and stratifies so that the finer particles will fall to the bottom and gradually migrating down the screen.

2.13.4 Fine aggregate classification

In this process, the screening is replaced by size separation through particles settling velocity or classification. In this process, the dirty product is removed by gentle elutriating currents and thus allow clean sand sink to base of the settling column. The separation efficiencies of sizing by classification are lower than screening coarse aggregates. Two type of high-value product are produced which are clean, coarse and for concrete mixes and fine sand for mortar (McNally 1998).

2.13.5 Dewatering

Hydrocyclones are used in the dewatering process. Hydrocyclones are used because it is cheap to build up and simple to operate. The waste water from classifier is pour into the chamber under pressure. The heavier grain contained in the waste water are forced outwards and downwards towards the apex of the cone, while the the suspended fines are drawn upwards and out of the chamber. Dewatering can improve the sell ability of the washed sand (McNally 1998).

Figure2.3: Dewatering hydrocyclone

Source: McNally, (1998)

2.13.6 Desliming and beneficiation

In the desliming process, the muddy water is discharged into tailings dams. Large area of level land and dry climate are required in order to encourage the muddy water to evaporate. When the surface water has been drained off, the speed of drying process can be increase by scooping up and turning over the tailings. If the climate is too wet for evaporation, the fine clay suspensions have to be chemically flocculated in thickening tanks. The main purpose of beneficiation process is to upgrade any low-grade sand by removing deleterious matter using gravity separation process (McNally 1998).

2.14 Water

Water is necessary for lubrication and hydration concrete mixture and lime mortar mixture (Walker et al 2003). There is a close relation between the void ratio of sand and the water content in mortar mixture. With the increase in void ratio (the percentage of voids in loose conditions) of sand, the water requirement for mortar increases for standard consistency. As the fineness modulus of sand decreases the requirement of water for a particular mix proportion increases. Specific surface of sand has no relation with the water requirement of mortar mix and it does not influence the mortar strength (Currie et al. 1981). Water used in lime mortar should be fresh and must not contain salt, sulphur or other substances that can break down the mixture.

2.15 Pozzolanas

Pozzolanas are materials containing reactive silica and/or alumina which on their own have little or no binding property but, when mixed with lime in the presence of water, will set and harden like cement.

2.16 Pozzolanic materials

Pozzolanic materials can be classified into two groups which are natural pozzolanas and artificial pozzolanas.

2.16.1 Natural pozzolanas

Natural pozzolanas are mainly volcanic in origin. According to ASTM-C618, natural pozzolana is defined as

"raw or calcined natural pozzolans that comply with the applicable requirements for the class given herein, such as some diatomaceous earths; opaline cherts or shaled; tuffs or volcanic ashes or pumicites, any of which may or may not be processed by calcinations; and various materials requiring calcinations to induce satisfactory properties such as clays and shales."

Silicates and high specific surface area is the main common feature of the natural pozzolanas. Most of the silicates are glassy because the process of rapid cooling of molten droplets of lava (Gani 1997).

2.16.2 Artificial pozzolanas

Artificial pozzolana can be organic or inorganic material. Examples of organic material include ash from rice husks and coffee hulls. Meanwhile, inorganic pozzolana including calcined clays and shales, furnace slag, fly ash, and brick powder. Generally, artificial pozzolanas produce the same hydraulic reaction as the the reaction produced by natural pozzolana. Artificial pozzolanas are fine grained and it will produce a gel like structure when mixed with wet lime. Artificial pozzolanas mix with wet lime can create a permanent bonding agent (Rapp 2009).

2.17 Rice husk ash (RHA)

Rice husk is a by-product of rice milling industry. By burning the husks of the rice paddy, rice husk ash is produced. In the high incineration temperature which around 500ËšC and 800ËšC would like to produce non-crystalline amorphous RHA (Mehta. et al 1993). RHA is in the colour blanch or the gray. In cellular structure, the RHA exist in condition which have a very high surface fineness. The RHA produced from the controlled incineration process contain 90% to 95% amorphous silica. Because of the high siliceous clay content, RHA has the outstanding pozzolanic activity (Mehta 1992).

2.18 Physical properties of RHA

The burning process of rice husk in the initial step largely affects the physical properties of RHA produced. The period and temperature of combustion influence the microstructure and characteristic of RHA (Nagataki 1994). Under controlled incineration of rice husks between 500ËšC and 800ËšC would like to produce white or grey RHA or non-crystalline or amorphous silica, which contain high and outstanding pozzolanic (Mehta. et al 1993). On the contrary, under improper or partial burning of rice husks will produce black colour RHA. Besides, under uncontrolled burning at high temperature, poor pozzolanic property of RHA will produce. RHA obtained from the complete burning process possess relative density of 2.05 to 2.11. This range of relative density value can be affect by the burning condition of rice husks (Ismail et al 1996).

Compared to silica fume, the RHA particles are porous and have a honeycomb microstructure (Zhang. et al 1996). Therefore, the specific are of RHA is very high which can be achieve the range of 50 to 100m²/g. Meanwhile, the size range of RHA particles is from 4μm to 75μm. This particles size range encourage the particles to pass the 45μm (No 325) sieve. On the other hand, the median particles diameter of RHA can be achieved in the range from 6 to 38μm (Mehta 1992).

2.19 Properties of rice husk ash

The typical chemical composition and physical properties of RHA are given in the Table (Mehta 1992; Zhang et al 1996)

Chemical properties


Mehta (1992)

















Loss on ignition


Physical properties

Specific gravity (g/cm3)


Mean particle size (μm)


Fineness: passing 45μm (%)


Table 2.4: Chemical and physical properties of RHA (Wt. %)

RHA produced from the burning process of rice husk contain high amount of amorphous silica. This type of chemical components is a naturally occurring or synthetically produced oxide of silicon characterized by the absence of pronounced crystalline. Under a controlled burning condition which has temperature around 500ËšC to 800ËšC, in a moderately oxidizing environment and for a suitable burning time ranging from 15 minutes to 1 hour can produces large amount of amorphous silica (Nagataki 1994).

2.20 Combustion of rice husk

The combustion process of rice husk largely depends on the temperature and the duration because these factors can determine the form of silica produced. In order to produce essentially amorphous silica, the combustion temperature should maintain below 500ËšC under oxidizing conditions for prolonged period or up to 680ËšC with hold time less than one minute (Mehta 1979).

There is another research show that the amorphous form of RHA can be maintained at combustion temperature up to 900ËšC if the combustion time is less than one hour, while in combustion temperature of 1000ËšC with time more than five minutes can produce crystalline silica (Yeoh et al 1979). X-ray diffraction method was used to observe the physical structure of RHA. It was observed that different burning temperature and the chemical composition of rice husk will create different type of reaction and produce different type of chemical composition. At burning temperature of 400ËšC, polysaccharides begin to depolymerise, while at temperature above 400ËšC, dehydration of sugar units occurs. Meanwhile, at temperature of 700ËšC, the sugar units decompose. At temperatures higher than 700ËšC, unsaturated products react together and form a highly reactive carbonic residue. In conclusion, the higher the burning temperature, the higher the component of silica in the ash, while K, S, Ca, Mg and other components were found to be volatile (Hwang et al 1989).

2.21 Grinding process of RHA

The second step in processing is grinding the RHA to a fine powder, and ball or hammer mills are usually used for this purpose. Crystalline ash is harder and will require more grinding in order to achieve the desired fineness. Fineness similar to or slightly greater than that of OPC is usually recommended for pozzolanas although some have been ground considerably finer. The minimum fineness recommended by the Indian Standards for pozzolana (1344) is 320 and 250m2/kg for grade 1 and 2 pozzolanas respectively, measured by the Blaine air permeability test. Although this standard is for calcined clay, the fineness requirements are also suitable for RHA.

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