Structure Of Transition Zone Biology Essay

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2.1 GENERAL: Fly ashes from Dadri TPP, UP and Panipat TPP, Haryana were used in the experiments of the present study. A brief review of literature about the physical and chemical properties; mineralogy and morphology behaviour of fly ashes is presented. Literatures regarding concrete applications of fly ashes have been used in construction are also discussed.

Fly ash is produced from burning of pulverized coal in thermal power plants. The pulverized coal is fed into the boilers and burnt with the supply of additional air. The temperature in the boiler exceeds 16000C and the most of the mineral matter present in the coal are fused and altered physically and chemically. The resulting residue is called coal combustion by-products namely bottom ash, economizer ash, air pre-heater ash, and electrostatic precipitator ash (fly ash). These ashes are handled and disposed off separately to their differing qualities by mechanical, hydraulic and pneumatic conveying systems. The quality of ash produced is depends on various factors like source coal and its degree of pulverization, design of furnace, changes in coal supply, changes in boiler load, and firing condition. Because of this inherent variability of the material, it is necessary to study the characteristics and engineering behavior of fly ash in detail before its use in an application.

Fly ash is a promising and economical alternative material to construction engineering applications. Review of literature shows that fly ash has been utilized in the construction of pavement construction, in high strength concrete, high performance concrete and in other applications.

Concrete Basics: Concrete consists of three basic ingredients viz Portland cement aggregates (both fine and coarse) and water. These are mixed together in a flow able mixture so that it can be placed and moulded in the desired shape. Various factors are affecting the durability of concrete or to make good quality concrete. The basic chemistry of concrete is summarized in given table-2.1.

Table -2.1 Concrete Basics


Crushed limestone+ clay shale


Key ingredient hardens concrete through chemical reaction called hydration.

The quality of water plays an important role towards the long-term durability of concrete.


Chemically inert, solid materials held together by cement. Selection of aggregate is determined based on desired characteristics of concrete.

When aggregates are reactive, problems of alkali-aggregate reaction / alkai-silica reaction can be encountered having deleterious effects on concrete.


Substances other than major ingredients, which are added during mixing process. Selection and use of admixture is depending on the need of the user.

1.Air entraining




5.Mineral admixtures


2.2 Classification of concrete:

Based on unit weight

Ultra-light concrete <1,200 kg/m3

Lightweight concrete 1200- 1,800 kg/m3

Normal-weight concrete ~ 2,400 kg/m3

Heavyweight concrete > 3,200 kg/m3

Based on strength (of cylindrical sample)

Low-strength concrete < 20 MPa compressive strength

Moderate-strength concrete 20 -50 MPa compressive strength

High-strength concrete 50 - 200 MPa compressive strength

Ultra high-strength concrete > 200 MPa compressive strength

Based on additives:

Normal concrete

Fiber reinforced concrete

Shrinkage-compensating concrete

Polymer concrete

Three-phase theory of Concrete: Concrete consists of three phases, aggregate (coarse and fine), hardened cement paste (hcp) and transition zone. Each phase can be further divided into multiple phases. For example, the aggregate contains various minerals, voids and micro cracks. Here we discussed the properties of transition zone only.

Transition zone

Structure of transition zone: The transition zone is defined as the region between large aggregate particles and the hcp (or mortar). It exists as a thin shell, typically 10-50 micron thick. Formation of transition zone can be attributed to poor packing and the formation of water films around large particles during mixing. Owing to higher w/c ratio, the transition zone is more porous than the bulk cement paste or mortar matrix.

Influence on concrete properties: The transition zone is generally weaker than both the aggregate and the hcp. Though, the transition zone occupies much less volume than the other two phases, its influence on concrete properties is very large. The existence of transition zone helps in explains why:

Cement paste or mortar will always be stronger than concrete provided at same w/c ratio and same age.

The permeability of concrete is much higher than cement paste.

Under the same loading, components of concrete (aggregate and hcp) can show linear behaviour while concrete itself shows a nonlinear behaviour.

The first questions are easily explained due to the existence of high porosity and micro-cracks in transition zone. So far as the third question is concerned it does not take a lot of energy for the propagation of pre-existing micro-cracks in transition zone. Even at 40% of the ultimate strength of concrete, nonlinear behaviour can also observe.

Scanning electron microscopy indicates that HCP and transition zone have the same constituent phases but at different fractions. The transition zone is more porous in which large CH crystals are present provide smooth planes for cracks to develop. This observation cast light on the 'engineering' of microstructure to improve concrete strength. By adding silica fume, a very small particle that can react with CH to form additional C-S-H, diminishing the CH crystals and making the interface much denser. Nowadays use of silica fume for high strength concrete is a common practice.

Fresh concrete: Fresh concrete is defined as concrete at the state when its components are fully mixed but its strength has not yet developed. This period corresponds to the cement hydration stages 1, 2, and 3. The properties of fresh concrete directly influence the handling, placing and consolidation, as well as the properties of hardened concrete.

2.3. Workability: Workability is defined as the amount of mechanical work required for full compaction of the concrete without segregation. The final strength of the concrete is largely influenced by the degree of compaction. A small increase in void content due to insufficient compaction could lead to a large decease in strength.

The characteristics of workability are consistency (or fluidity) and cohesiveness. Consistency is used to measure of flow of fresh concrete. Cohesiveness is used to describe the ability of fresh concrete to hold all ingredients together without segregation and excessive bleeding.

2.3.1. Factors affecting workability:

2.3.2. Water content: Except for the absorption by particle surfaces, water must fill the spaces among particles. Additional water "lubricates" the particles by separating them with a water film. Increasing the amount of water will increase the fluidity and make concrete easy to be compacted. Indeed, the total water content is the most important parameter governing consistency. But, too much water reduces cohesiveness, leading to segregation and bleeding. With increasing water content, concrete strength is also reduced.

2.3.3 .Aggregate mix proportion: For a fixed w/c ratio, an increase in the aggregate/cement ratio will decrease the fluidity. In general a higher fine aggregate/coarse aggregate ratio leads to a higher cohesiveness.

2.3.4. Maximum aggregate size: For a given w/c ratio, increase in maximum size of aggregate gives effect in increase of fluidity. This happens due to overall reduction in surface area of the aggregates.

2.3.5. Aggregate Properties: The shape and texture of aggregate particles can also affect the workability. A general rule the spherical and smoother particles, produce more workable concrete.

2.3.6. Cement: Increased fineness reduces fluidity at a given w/c ratio, and increase its cohesiveness. Under the same w/c ratio, higher the cement content, the better is the workability due to increase in total water content.

2.4. Admixtures used in concretes: Air entraining agent and super plasticizers are used to improve the workability. Admixtures are almost as old as concrete itself. It is known that the Romans used animal fat, milk, and bloods to improve the concrete properties and workability. Blood is a very effective air-entraining agent and has improved the durability of Roman concrete. In more recent times, calcium chloride has been used to accelerate hydration of cement. The study of admixtures began with the introduction of air-entraining agents in the 1930s when people accidentally found that cement ground with beef tallow (grinding aid) had more resistance to freezing and thawing than a cement ground without it. The concrete properties, both in fresh and hardened states, can be modified or improved by admixtures. In some countries, 70-80% of concrete (88% in Canada, 85% in Australia, and 71% in US) contains one or more admixtures. Therefore knowledge of admixtures for engineers has become essential. Admixtures can be roughly divided into the following groups:

i). Air-entraining agents (ASTM C260): This kind of admixture is used to improve the frost resistance of concrete (i.e., resistance to stresses arising from the freezing of water in concrete).

ii). Chemical admixtures (ASTM C494 and BS 5075): This kind of admixture is mainly used to control the setting and hardening properties for concrete, or to reduce its water requirements.

iii). Mineral admixtures: They are finely divided solids added to concrete to improve its workability, durability and strength. Slags and pozzolans are important categories of mineral admixtures.

IV). Miscellaneous admixtures: include all those materials that do not come under the above mentioned categories such as latexes, corrosion inhibitors, and expansive admixtures.

Temperature and time: As temperature increases, the workability decreases. Workability decreases with time also. These effects are related to the progression of chemical reaction. 2.5. Segregation (separation): Segregation means separation of the components of fresh concrete, resulting in a non-uniform mix. More specifically, this implies some separation of the coarse aggregate from mortar.

2.6. Bleeding (water concentration): Bleeding is a particular form of segregation in which some water from concrete comes out to the surface of the concrete. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete. Sometimes, along with this water, certain quantity of cement also comes to the surface. The locations of increased water concentration can be concrete surface, bottom of large aggregate and bottom of reinforcing steel. Bleed water trapped under aggregates or steel lead to the formation of weak and porous zones, within which micro-cracks can easily form and propagate.

2.7. Setting of concrete: Setting is defined as the onset of rigidity in fresh concrete. It is different from hardening, which describes the development of useful and measurable strength. Setting precedes hardening although both are controlled by the continuing hydration of the cement.

2.7.1.Abnormal setting: False setting: If concrete stiffens rapidly in a short time right after mixing but restores its fluidity by remixing, and then set normally, the phenomenon is called false setting. The main reason causing the false setting is crystallization of gypsum. In the process of cement production, gypsum is added into blinker through inter-grinding. During grinding temperature can rise up to 120oC, thus causing the following reaction:

CSH2 → CSH1/2

The CSH1/2 is called plaster. During mixing, when water is added, the plaster will re-hydrate to gypsum and form a crystalline matrix that provides estiffnessto the mix. However, due to presence of plaster small amount of plaster in the mix, very little strength actually develops. Fluidity can be easily restored by further mixing to break up the matrix structure.

2.7.2.Flash setting: Flash setting is caused by the formation of large quantities of monosulfoaluminate or other calcium aluminate hydrates due to quick reactivities of C3A. This is a rapid set with the development of strength and thus is more severe than false setting. Flash setting can be minimized and eliminated by addition of 3-5% gypsum into cement. Thixotropic set is another phenomenon which happens due to the presence of abnormally high surface charges on the cement particles. It can be taken care of by additional mixing.

As the hydration reaction progresses with time, the concrete becomes less flowable, and the slump values start decreasing accordingly. However, if the slump value decreases at an abnormally fast rate, the phenomenon is called "slump loss". It is often due to the use of abnormal setting cement, unusual long time taken in the mixing and placing operations, due to the high temperature of the mix (e.g., when concrete is placed under hot weather, or when ingredients have been stored under high temperature). In such case, ice chips are used to replace part of water to lower the temperature.

2.8. Properties of Fly Ash: Fly ash is a by-product of combustion of coal, a mixture of vegetation, clay and rocks, comprises a wide range of inorganic matters. Physically, fly ash occurs as very fine spherical particles, having diameter in the range from few μ to 100μ, low to medium bulk density, high surface area and sandy silt to silty loam texture. Chemically, fly ash is an amorphous ferro-alumino silicate mineral with major matrix elements like, Si, Al, and Fe with significant amount of Ca, Mg, K, P, and S. The concentration of total and available trace/ heavy metals and radio nuclides in fly ash samples is in traces and their availability/ leach ability is negligible. The range of physico-chemical properties including trace/ heavy metals and radionuclide in fly ash samples are given in Tables 2.2 to 2.6[1].

Table- 2.2Physical Properties of Fly ash


Fly ash



Specific gravity


Bulk density(g/cc)


Grain size distribution

Sandy silt to silty loam

Porosity (%)


Water holding capacity (%)


Electrical conductivity (dS/m)


Table -2.3Chemical Properties of fly ash



















Table -2.4Total and available micronutrient in fly ash



Available (ppm)



















Table -2.5Total and available trace/heavy metal



Available (ppm)

























Table -2.6Radioactivity levels in fly ash


Bq Kg-1







2.9. Mineralogy of Fly Ash: Fly ash consists of both crystalline and amorphous phases. The crystalline phases could be α-quartz, mullite, silimanite, crystallite, cristobalite, sulphates of iron, magnetite etc. The amorphous phases could be of silica and silicates predominantly of aluminium but containing calcium, magnesium, and iron in varying concentration with and without traces of sodium and potassium. The reactivity of fly ashes depends on the non-crystalline or glass content in it. The chemical composition of the glass in the high calcium fly ash is different from the low calcium fly ash and hence the reactivity of both the ashes is different. The high calcium fly ashes are more reactive than low calcium fly ashes pointed out that the composition of glass in low calcium fly ashes is different from high calcium fly ashes. Diamond [2] and Mehta [3] pointed out that the composition of glass in low calcium fly ashes is different from high calcium fly ash. Typically low calcium fly ashes show a diffused halo with maxima at 2 = 21-250 and high calcium fly ashes at 30-34o.Garg (1999) [4] conducted XRD studies of Indian fly ashes and the crystalline constituents identified were quartz (SiO2), mullite (3 Al2O3. 2SiO2), hematite (Fe2 O3) and magnetite (Fe3 O4). The quartz and mullite were the main crystalline constituents in British fly ashes and the American fly ashes contained magnetite and hematite in large proportions. The range of quantitative measurement in British fly ashes was quartz (1-6.5%); mullite (9-35%); magnetite and hematite (5% or less). For American fly ashes the proportions were quartz (0 - 4%); mullite (0-16 %) magnetite (0-30%); and hematite (1-8%). The glass proportions in these fly ashes were found to range from 50 to 90%.

2.10. Classification of Fly Ash: According to ASTM C-618 Fly ash is broadly classified into two major categories: Class F and Class C fly ash. The chief difference between these two classes is the presence of amount of Gold, calcium, silica, alumina, and iron content. Chemical properties of fly ash are largely influenced by the presence of chemical content in the coal burned (i.e., anthracite, bituminous, and lignite).

2.10.1. Class 'F' fly ash: Burning of old anthracite and bituminous coal typically produces Class F fly ash which contains less than 10% lime (CaO). In fly ash of class F the presence of glassy silica and alumina requires a cementing agent, such as Portland cement, quicklime, or hydrated lime, in presence of water in order to react and produce cementitious compounds. Alternatively the addition of a chemical activator such as sodium silicate (water glass) to Class F ash can lead to the formation of a geo polymer.

2.10.2. Class C Fly ash: Class C Fly ash produced from the burning of younger lignite or sub bituminous coal generally contains more than 20% lime (CaO). This type of ash does not require an activator & the contents of Alkali and sulphates (SO4) are generally higher as compare to the Class F Fly ash.

2.10.3. Source of Fly Ash in India: According to Central Electricity Authority of India, there are around 83 major coal fired thermal power plants and 305 hydro plants existing in India. As per the ministry of power statistics, the total installed generating capacity (Thermal + wind) during 2003-2004 was about 79838 MW and hydropower generation was 29500 MW. In addition to this, there are more than 1800 selected industrial units which had captive thermal power plants of >1MW.

2.10.4. Ash Collection: Ash can be collected in following categories

Dry Fly Ash: Dry ash is collected from different rows of electrostatic precipitators. It is available in two different grades of fineness in silos for use as resource material by different users.

Bottom Ash: Bottom ash is collected from the bottom of the boiler and transported to Hydro bins and then ash mound for use in road embankment

Conditioned Fly Ash: Conditioned fly ash is also available in ash mound for use in Landfills and ash building products.

2.10.5. Ash Content in Indian Coal: The quality of coal depends upon its rank and grade. The coal rank arranged in an ascending order of carbon contents is:

Lignite > sub bituminous coal > bituminous coal > anthracite

Indian coal is of mostly sub-bituminous rank, followed by bituminous and lignite (brown coal). The ash content in Indian coal ranges from 35% to 50%. The coal properties including calorific values differ depending upon the colliery. The calorific value of the Indian coal (~15 MJ/kg) is less than the normal range of 21 to 33 MJ/Kg (gross).Coal is used for approximately 62.3% of electric power generation in India, oil and gas accounts for 10.2%, hydro's share is 24.1%, nuclear, wind, and other contribute remaining 3.4%.Some of the innovative and commonly manufactured eco-friendly building material utilizing Fly Ash.

2.11. Fly Ash Utilization: Fly ash from thermal power plants can be considered either as a waste or as a resource yet to be fully utilized. Indian coals have very high ash content. The fly ash content of coal used by thermal power plants in India varies between 25 and 45%, with average fly ash content being 40%. The various uses of fly ash can be summarised as below:

2.11.1.Cellular Light Weight Concrete (CLC) Blocks: These are substitute to bricks and conventional concrete blocks in building with density varying from 800 kg/m3 to 1800 kg/m3. The normal constituents of this Foaming Agent based technology from Germany are cement, Fly Ash (to the extent 1/4th to 1/3rd of total materials constituent), sand, water and foam (generated from biodegradable foaming agent). Using CLC walling & roofing panels can also be manufacture. Foaming agent and the Foam generator, if used for production of CLC with over 25% fly ash content invites concession on import duty by Govt. of India [5].

2.11.2. Development of Fly Ash Based Polymer Composites as Wood Substitute: Fly ash based composites have been developed using fly ash as filler and jute cloth as reinforcement. After treatment, the jute cloth is passed into the matrix for lamination. The laminates are cured at specific temperature and pressure. Number of laminates is used for required thickness. The technology on fly ash Polymer Composite using Jute cloth as reinforcement for wood substitute material can be applied in many applications like door shutters, partition panels, flooring tiles, wall panelling, ceiling, etc. With regard to wood substitute products, it may be noted that the developed components / materials are stronger, more durable, resistant to corrosion and above all cost effective as compared to the conventional material i.e. wood. This technology has been developed by Regional Research Laboratory, Bhopal in collaboration with Building Materials & Technology Promotion Council (BMTPC) and TIFAC. One commercial plant has also been set up based on this technology near Chennai [6].

2.11.3. Portland Pozzolana Cement: 35% of suitable fly ash can directly be substituted for cement as blending material. Addition of fly ash significantly improves the quality & durability characteristics of concrete. In India, present cement production per annum is comparable to the production of Fly Ash. Hence even without enhancing the production capacity of cement; availability of cement (fly ash based PPC) can be significantly increased [7].

2.11.4. Ready mixed Fly Ash concrete: Though Ready Mix concrete is quite popular in developed countries but in India it consumes less than 5 percent of total cement consumption. Only recently its application has started growing at a fast rate. On an average 20% Fly ash (of cementitious material) in the country is being used which can easily be enhanced further. In ready mix concrete various ingredients and quality parameters are strictly maintained/controlled which is not possible in the concrete produced at site and hence it can accommodate still higher quantity of fly ash [8].

2.11.5. Fly Ash- Sand-Lime-(Gypsum /Cement) Bricks /Blocks: Fly Ash can be used in the range of 40-70% as a replaceable material. The other ingredients are lime, gypsum /cement, sand, stone dust/chips etc. Minimum compressive strength (28 days) of 70 kg/cm2 can easily be achieved and this can go up to 250 Kg/cm2 (in autoclaved type) [9].

2.11.6. Fly Ash in Road Construction: Fly Ash may be used in road construction for:

i) Stabilizing and constructing sub-base or base. ii) Upper layers of pavements. iii) Filling purposes. Concrete with Fly Ash (10-20% by wt) is cost effective and improves performance of rigid pavement. IV) Soil mixed with Fly Ash and lime increases California Bearing Ratio (CBR), increased (84.6%) on addition of only Fly Ash to soil. Addition of Fly Ash has not shown any adverse effects on the ground water quality in the vicinity of experimental plots. V)National Highway Authority of India (NHAI) is currently using 60 lakh m3 of Fly Ash and proposed to use another 67 lakh m3 in future projects [8].

2.11.7. Use of Fly Ash in Agriculture The large volume of fly ash occupies large area of land and possesses threat to environment. There is an urgent and imperative need to adapt technologies for gainful utilization and safe management of fly ashes on sustainable basis. Agriculture and waste land management have emerged as prime bulk utilization areas for fly ash in the country. The field demonstration experiments carried out under varied agro-climatic conditions and soil types across the country by various R & D Institutes / Universities on the cultivation of different field crops (cereals, pulses, oil seeds, sugar cane, vegetables, etc.) and forestry species with different doses of fly ash and pond ash as soil modifier / source of economical plant nutrients with and without organic manure bio-fertilizer and chemical fertilizers in respect to crop yield, soil health, quality of crop produce, uptake of nutrients and toxic heavy metals, ground water quality etc. have revealed the following:

1. It improves permeability status of soil.

2. Improves fertility status of soil (soil health) / crop yield.

3. Improves soil texture.

4. Reduces bulk density of soil.

5. Improves water holding capacity / porosity.

6. Optimizes pH value.

7. Improves soil aeration.

8. Reduces crust formation.

9. provides micro nutrients like Fe, Zn, Cu, Mo, B, Mn, etc.

10. Provides macro nutrients like K, P, Ca, Mg, S etc.

11. Works as a part substitute of gypsum for reclamation of saline alkali soil and lime

2.12. Fly Ash in Concrete / Mortar: Fly ash can be used in concrete/mortar both as an aggregate or replacement of cement. A brief discussion about the basics of concrete, chemistry of cement and the factors affecting the durability of concrete is given below.

2.13. Factors affecting durability of Concrete: Concrete deterioration generally takes place when it is exposed to weather, water or chemicals for a considerable period of time. There are both physical as well as chemical factors which affect the durability of concrete. The physical factors include cycles of freezing and thawing, abrasion due to winds, mechanical loads and temperature. As far as chemical factors are concerned, there are a number of chemicals which attack concrete and affect it adversely. Chemical factors mainly include exposure of sulphate, soft water, sea water, acid attack, reactions between alkali & aggregates and alkali & silica. Corrosion is another major factor responsible for deterioration. It can be caused by chloride attack and/or carbonation [10]. Apart from these, microbiological induced attack also deteriorates concrete. Causes of deterioration which need most attention are:

Causes of deterioration which need most attention are:

Corrosion of reinforcing steel.

Chemical effects on hydrated cement paste from external agents (viz. water containing carbon dioxide, sulphates or chlorides).

Physical-chemical effects from internal phenomenon, such as alkali-aggregate reaction and salt weathering.

Frost action in cold climates.

Table -2.7 Major factors that affect strength of concrete

Internal Factors

Faulty planning





plumbing and sanitation



constituent materials





Inferior workmanship

Lack of quality assurance / control

Improper use

Poor maintenance and upkeep

Environmental Factors

Physical effects

and weathering



Moisture movement


1. Rate of chemical reaction increases with increase in temp.

2. As temp. Increases, the moisture content of concrete reduces, making it drier & more permeable to additional fluids. When temperature falls it may sometimes cause sufficient shrinkage to open small cracks & allow greater penetration to liquid into concrete.

Chemical effects

Corrosion of reinforcing steel embedded in concrete

1) Chloride attack

2) Carbonation of concrete

Soft water leaching

Acid attack

Sulphate attack

Alkali-aggregate reactions

Alkali-silica reaction

Mechanical effects




Other effects

Microbiological induced attack

Effect of chelating chemicals




In practice, several of these deterioration mechanisms can act simultaneously with possible synergistic effects [12]

Table -2.8 Chemistry of cement

Cement Compound

Weight percentage

Chemical formula

Tricalcium silicate

50 %

Ca3SiO5 or 3CaO.SiO2

Dicalcium silicate

25 %

Ca2SiO4 or 2CaO.SiO2

Tricalcium aluminates

10 %

Ca3Al2O6 or 3CaO.Al2O3


10 %

Ca4Al2Fe2O10or 4CaO.Al2O3.Fe2O3


5 %


Tricalcium silicate + water→ Calcium silicate hydrate + calcium hydroxide + heat (173.6kJ) Initial reaction is very fast.

Dicalcium silicate+ water →Calcium silicate hydrate + calcium hydroxide + heat (58.6kJ) (Slow reaction).

When water is added to cement, each of the components undergo hydration and contributes to the final concrete product [13].

2.14. Sulphate Attack: Sulphates attacks concrete, deteriorate and disintegrate due to expansion of concrete. Sulphate attack takes place when water containing dissolved sulphate penetrates the concrete. In early stages of attack there is little visual evidence of any change in the concrete, though there may be some reduction in strength as the chemical action proceeds. This is accompanied by slight expansion, which may be apparent in the concrete itself, but may cause trouble at points of restraints. The deterioration can take place in any of the following forms:

Extensive cracking


Loss of bond between the cement paste and aggregate

Alteration of paste composition, with monosulfate phase converting to ettringite and, in later stages, gypsum formation. The necessary additional calcium is provided by the calcium hydroxide and calcium silicate hydrate in the cement paste

Sulphate attack can be 'external' or 'internal'. External sulphate attack is due to penetration of sulfates in solution into the concrete from outside whereas internal sulphate attack causes due to a soluble source of sulphate being incorporated into the concrete at the time of mixing such as gypsum in the aggregate [14].

Naturally occurring sulphates of Na, K, Ca or Mg that can attack hardened concrete are sometimes found in soil or dissolved in ground water adjacent to concrete structures. The rate of attack depends on a number of factors such as concentration of sulphate, pH and temperature of solution and also the physical state of structure. Higher the concentration of sulphate in solution and if the pH value is below 6.0, attack will be of more serious nature. The severity of attack is increased where a flow of sulphate bearing water brings a continuous supply of the salts in contact with the concrete. Also the activity of sulphate increases with the increase of temperature. Attack proceeds along the line of cracks, particularly when the movement of moisture along any crack is encouraged by one sided water pressure or evaporation from the free surface. In addition the aggressiveness of the conditions also depends on soil saturation, water movement, ambient temperature and humidity, concentration of sulphate and type of sulphate. Other sources of sulphate which can cause sulphate attack include:


Oxidation of sulphide minerals in clay adjacent to the concrete - this can produce sulphuric acid which reacts with the concrete

Bacterial action in sewers - anaerobic bacteria produce sulphur dioxide which dissolves in water and then oxidizes to form sulphuric acid

In masonry, sulfates present in bricks and can be gradually released over a long period of time, causing sulphate attack of mortar, especially where sulfates are concentrated due to moisture movement [15].

Mechanism: The reaction(s) of sulphate with concrete and its results are given below:

Table - 2.9 Sulphate Reactions and Results



1. Combination of sulphate with free calcium hydroxide (hydrated lime) liberated during the hydration of cement, to form calcium sulphate (gypsum).

Formation of gypsum can lead to softening and loss of concrete strength

2. Combination of gypsum and hydrated calcium aluminate to form calcium sulphoaluminate (ettringite).

The formation of ettringite (3CaO.Al2O3.CaSO4.31H2O) can result in an increase in solid volume, leading to expansion and cracking, because ettringent has a large amount of crystalline water and hence produces expansive disruptive force within the concrete.

2.15. Codes and Practices: The USBR has recommended a separate classification for sulphate aggression levels as given below.

Table -2.10USBR Classification of Sulphate Aggressivity

Relative degree of sulphate attack

Percent water soluble sulphate (as SO3) in soil sample

Sulphate (as SO4) in water sample,



0.00 - 0.10

0 to 150


0.10 - 0.20

150 to 1500

Severe (2)

0.20 - 2.00

1500 to 10000

Very severe (3)

2.00 or more

10000 or more

According to classification different types of use of cements has been suggested as mentioned below:

1. Use Type II Cement (containing maximum of 8% C3A).

2. Use Type V cement (containing maximum of 5% C3A) or approved combination of Portland cement and pozzolana which has been shown by tests to provide comparable sulphate resistance when used in concrete.

3. Use Type V cements plus approved pozzolana which has been determined by tests to improve sulphate resistance when used in concrete with type V cement.

The Indian standard 456‑2000 has given different requirement for concrete under various sulphate concentrations as shown below:

Table -2.11Requirements for concrete exposed to sulphate as per IS: 456-2000


Concentration of Sulphate

Expressed as SO3

Type of


Dense, fully compacted concrete made with 20 mm nominal MSA aggregates complying with IS:383

In Soil

In Ground water











SO3 %

SO3 in 2:1 Water








Less than



or PPC




0.2 to


1.0 to 1.9

0.3 to 1.2


or PPC

Super sulphated cement or sulphate resisting Portland cement (SRPC)






0.5 to


1.9 to


1.2 to 1.5

Super sulphated

cement or SRPC















Super sulphated

Cement or SRPC




More than 2.0

More than 5.0

More than 5.0

SRPC or Super sulphated cement with protective coatings




1. Cement content given in the table is irrespective of grades of cement

2. Use of super sulphated cement is generally restricted where the prevailing temperature is above 400 C.

3. Super sulphated cement gives an acceptable life provided that the concrete is dense and prepared with a water-cement ratio of 0.4 or less, in mineral acids, down to pH 3.5.

4. The cement contents given in col. 6 of this table are the minimum recommended. For SO3 contents near the upper limit of any class, cement contents above these minimums are advised.

5. For severe conditions, such as thin sections under hydrostatic pressure on one side only and sections partly immersed, considerations should be given to a further reduction of water-cement ratio.

6. Portland slag cement conforming to IS: 455 with slag content more than 50% exhibits better sulphate resisting properties.

7. Where chloride is encountered along with sulphate in soil or ground water, ordinary Portland cement with C3A content from 5 to 8 percent shall be desirable to be used in concrete, instead of sulphate resisting cement. Alternatively, Portland slag cement conforming to IS 455 having more than 50 percent slag or a blend of ordinary Portland cement and slag may be used provided sufficient information is available on performance of such blended cements in these conditions.