Admixtures are substances which when added to concrete in small doses can overcome some of the drawbacks viz., poor workability, high shrinkage cracks, poor performance against chemicals, high permeability, inadequate protection of steel reinforcement from corrosion, low tensile strength and lower fracture toughness.
Admixtures can be defined as materials other than water, cement and aggregate, added to concrete immediately before or during mixing.
Admixtures are used to accelerate or retard the setting time of concrete, to reduce water content and improve strength and to increase slump or reduce cement content and to improve the overall durability of concrete.
Admixtures can enhance the workability of concrete. It is generally agreed that the use of fly-ash, particularly as an admixture rather than as a replacement of cement, reduces segregation and bleeding.
Fly ash has proved to be less expensive and when mixed with lime (liberated during hydration process), it makes excellent binders; improves durability of concrete, particularly its resistance to sulphate attack and alkali-silica reactions.
Characteristics of Fly ash
Chemical Composition of Fly ash:
The major constituents of most of the fly ash are Silica (SiO2), alumina (Al2O3), ferric oxide (Fe2O3) and calcium oxide (CaO).
The other minor constituent of the fly ash are MgO, Na2O, K2O, SO3, MnO, TiO and unburnt carbon.
There is wide range of variation in the principal constituents –
- Silica (25-60%),
- Alumina (10-30%) and
- Ferric oxide (5-25%).
When the sum of these three principal constituents is 70% or more and reactive calcium oxide is less than 10% – technically the fly ash is considered as Siliceous fly ash or class F fly ash.
Such type of fly ash is produced by burning of anthracite or bituminous coal and possesses pozzolanic properties.
The active constituents of class F fly ash is siliceous or alumino-silicate glass.
If the sum of these three constituents is equal or more than 50% and reactive calcium oxide is not less than 10%, fly ash will be considered as Calcareous fly ash also called as class C fly ash.
This type of fly ash is commonly produced by burning of lignite or subbituminous coal and possesses both pozzolanic and hydraulic properties.
In Calcareous fly ash the active constituents are calcium alumino-silicate glass, free lime (CaO), anhydrate (CaSO4), tricalcium aluminate and rarely, calcium silicate.
The glassy materials of fly ash are reactive with the calcium and alkali hydroxides released from cement FLY ASH system and forms cementitious gel, which provide additional strength.
Physical Properties of fly ash:
The fly ash particles are generally glassy, solid or hollow and spherical in shape.
The fineness of individual fly ash particle ranges from 1 µm to 150 µm size and has a significant influence on its performance in cement concrete.
Greater the surface area, more will be the fineness of fly ash.
The fineness of fly ash as measured in specific surface by Blaine’s permeability method is in 320 m2/kg (min.)
Pozzolanic Properties of fly ash:
Fly ash is a pozzolanic material defined as siliceous or siliceous and aluminous material which possesses little or no cementitious value, chemically reacts with Calcium Hydroxide (lime) in presence of water at ordinary temperature and form soluble compound comprising of cementitious property similar to cement.
Role of Fly ash As an Admixture in Concrete
1. Reduced Heat of Hydration:
The large temperature rise of concrete mass exerts temperature stresses and can lead micro cracks. When fly ash is present in the concrete mass, it reacts with released lime and produces binder and renders additional strength to the concrete mass.
The un-reactive portion of fly ash acts as micro aggregates and fills up the matrix to render packing effect and results in increased strength.
2. Workability of Concrete:
Fly ash particles, generally spherical in shape, reduce the water requirement for a given slump.
The spherical shape helps to reduce friction between aggregates and between concrete and pump line and thus increases workability and improves flowability of concrete.
3. Permeability and Corrosion Protection:
Higher the water cement ratio, higher will be the porosity and thus higher will be the permeability.
The permeability makes the ingress of moisture and air easy and is the cause for corrosion of reinforcement.
Higher permeability facilitates ingress of chloride ions into concrete and is the main cause for initiation of chloride induced corrosion.
When concrete is hardened, part of the entrapped water in the concrete mass is consumed by cement mineralogy for hydration. Some part of entrapped water evaporates, thus leaving porous channel to the extent of volume occupied by the water.
Some part of this porous volume is filled by the hydrated products of the cement paste. The remaining part of the voids consists of capillary voids and gives way for ingress of water.
Similarly, the liberated lime by hydration of cement is water-soluble and is leached out from hardened concrete mass, leaving capillary voids for the ingress of water. Additional cementitious material resulting from reaction between liberated surplus lime and fly ash, blocks these capillary voids and also reduces the risk of leaching of surplus free lime and thereby reduces permeability of concrete.
4. Effect of fly ash on Carbonation of Concrete:
Carbonation phenomenon in concrete occurs when calcium hydroxides (lime) of the hydrated Portland cement react with carbon dioxide from atmospheres in the presence of moisture and form calcium carbonate.
Carbonation process in concrete results in two deleterious effects
(i) shrinkage may occur
(ii) concrete immediately adjacent to steel reinforcement may reduce its resistance to corrosion.
The rate of carbonation depends on permeability of concrete, quantity of surplus lime and environmental conditions such as moisture and temperature.
When fly ash is available in concrete; it reduces availability of surplus lime by way of pozzolanic reaction, reduces permeability and as a result improves resistance of concrete against carbonation phenomenon.
5. Sulphate Attack:
Sulphate attacks in concrete occur due to reaction between sulphate from external origins (or from atmosphere) and surplus lime leading to formation of etrringite, which causes expansion and results in volume destabilization of the concrete.
Increase in sulphate resistance of fly ash concrete is due to continuous reaction between fly ash and leached out lime, which continue to form additional C-S-H gel.
This C-S-H gel fills in capillary pores in the cement paste, reducing permeability and ingress of sulphate ions.
6. Corrosion of steel:
Corrosion of steel takes place mainly
(a) due to carbonation attack and
(b) due to chloride attack.
In the carbonation attack, alkaline environment in the concrete comes down due to carbonation of free lime, which disturbs the passive iron oxide film on the reinforcement.
When the concrete is permeable, the ingress of moisture and oxygen infuse to the surface of steel initiates the electrochemical process and as a result rust is formed.
The transformation of steel to rust increases its volume thus resulting in the concrete expansion, cracking, and distress to the structure.
In the chloride attack, chloride ions become available in the concrete either through the dissociation of chlorides-associated mineralogical hydration or infusion of chloride ion.
The sulphate attack in the concrete decomposes the chloride mineralogy thereby releasing chloride ion. In the presence of large amounts of chloride, the concrete exhibits the tendency to hold moisture.
In the presence of moisture and oxygen, the resistivity of the concrete weakens and becomes more permeable thereby inducing further distress.
The use of fly ash reduces availability of free limes and permeability thus results in corrosion prevention.
7. Reduced alkali- aggregate reaction:
Certain types of aggregates termed as reactive aggregates react with available alkalis and cause expansion and damage to concrete.
It has been established that use of adequate quantity of fly ash in concrete reduces the amount of alkali aggregate reaction and reduces/ eliminates harmful expansion of concrete.
The reaction between the siliceous glass in fly ash and the alkali hydroxide of Portland cement paste consumes alkalis thereby reduces their availability for expansive reaction with reactive silica aggregates.
8. Environmental benefits of fly ash use in concrete:
Use of fly ash in concrete imparts several environmental benefits and thus it is ecofriendly.
It saves the cement requirement for the same strength thus saving of raw materials such as limestone, coal etc. required for manufacture of cement.
In the manufacturing of one tonne of cement, about 1 tonne of CO2 is emitted and goes to atmosphere. Less requirement of cement means less emission of CO2 resulting in reduction in green house gas emission.
Advantages of using fly ash in Concrete
Fly ash has several industrial applications and is widely found in power plant chimneys and being utilized as a significant building material for construction purposes due to its beneficial features.
It is vital to use only high quality fly ash to prevent negative effects on the structure of the building. Fly ash is used in many countries because of its advantages. Below listed are a few of the advantages of fly ash concrete, which are described in brief.
Advantages
- Fly ash in the concrete mix efficiently replaces Portland cement that in turn can aid in making big savings in concrete material prices.
- It is an environmentally friendly solution, which meets the performance specifications.
- It improves the strength over time and thus, offers greater strength to the building.
- Increased density and the long-term strengthening action of FLY ASH that ties up with free lime and thus, results in lower bleed channels and also decreases the permeability. The reduced permeability of concrete by using fly ash, also aids to keep aggressive composites on the surface where the damaging action is reduced.
- It is highly resistant to attack by mild acid, water, and sulphate.
- It effectively combines with alkalis from cement, which thereby prevents destructive expansion.
- It is helpful in reducing the heat of hydration. The pozzolanic reaction in between lime and fly ash will significantly generate less heat and thus, prevents thermal cracking.
- It chemically and effectively binds salts and free lime, which can create efflorescence. The lower permeability of fly ash concrete can efficiently reduce the effects of efflorescence.
I have gone through the writes up. This is really a helpful writings to whom are users of Readymix concrete as well as the manufacturer and suppliers of Readymix concrete also. Please keep it up and continue…
Regards
COO
Multazim Group
Bangladesh