Admixtures are those ingredients in concrete other than portland cement, water, and aggregates that are added to the mixture immediately before or during mixing. Construction and building material sciences is a highly matured technology, world over
Over decades, attempts have been made to obtain concrete with certain desired characteristics such as high compressive strength, high workability, and high performance and durability parameters to meet the requirement of complexity of modern structures.
The properties commonly modified are the heat of hydration, accelerate or retard setting time, workability, water reduction, dispersion and air-entrainment, impermeability and durability factors.
Types of admixturesChemical admixtures: Accelerators, retarders, water-reducing agents, super plasticizers, air entraining agents etc.
Mineral admixtures: Fly-ash blast-furnace slag, silica fume and rice husk ash GGBFS etc.
The major reasons for using admixtures are: • Reduce the cost of concrete construction • Achieve certain properties in concrete more effectively than by other means • Maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions • Overcome certain emergencies during concreting operations.
Concrete does not always behave as we would like, some of the undesirable behaviour can be seen as disintegration, spalling, cracking, leakage, deflection and settlement
Durable concrete structures can be achieved only through careful attention to many details.
Advantages of various types of admixtures:• Shortens the setting time of cement and therefore increases the rate of gain of strength. • Improves workability, cohesion and extends setting time, provides protection against delays and stoppages and facilitates keeping workable concrete for extended period • In the large construction, good workability of the concrete throughout the placing period and prevention of cold joints is ensured by adding retarders in the concrete. • Enables earlier release from precast moulds thus speeding guidelines on use of admixtures in concrete production.• Reduces segregation and increase density and compressive strength.• Early use of concrete floors by accelerating the setting of concrete. • Reduces water requirements, bleeding, shrinkage and time required for initial set.
Admixtures play an important role in today’s concreting to get durable concrete structures.
Types of STPs chemical admixtures • Accelerating admixtures (ShaliPlast FS)• Retarding admixtures (ShaliPlast R)• Water reducing admixtures (ShaliPlast SP/PCE)• Air entraining admixtures (ShaliPlast AE)• Shotcrete admixture ( ShaliPlast Shortcrete P/L)• Integral Waterproofing admixtures (ShaliPlast LW+ /LW++).
Dosages ranging from 0.06 to 3 per cent by weight of cement, different admixtures are recommended and a water reduction from 10 to 35 per cent or more is achieved. Admixtures help to achieve a much more watertight concrete.
General concrete deteriorationMost concrete deterioration can be attributed to water penetration. Since concrete absorbs moisture until it becomes saturated. Moisture hastens deterioration not only because it promotes chemical reactions but also because it carries dissolved chemicals that can react with the steel, lime and other components in the concrete. It also plays a major role in concrete deterioration. As a result of water entering in to concrete results in spalling of concrete.
Corrosion inhibitors are used in concrete for parking structures, marine structures, and bridges where chloride salts are present. The chlorides can cause corrosion of steel reinforcement in concrete. Ferrous oxide and ferric oxide form on the surface of reinforcing steel in concrete. Ferrous oxide, though stable in concrete’s alkaline environment, reacts with chlorides to form complexes that move away from the steel to form rust. The chloride ions continue to attack the steel until the passivating oxide layer is destroyed. Corrosion-inhibiting admixtures chemically arrest the corrosion reaction.
ConclusionIt should be borne in mind that no admixture of any type or amount can be considered a substitute for good concreting practice. Construction chemical manufacturers like STP Ltd are using durable technologies suitable to our national economy, affordability and environmentally friendly. 
Authored by__
Hasan Rizvi,Vice President, STP Ltd.