A material other than water, aggregates, or cement that is used as an ingredient of concrete or mortar to control setting and early hardening, workability, or to provide additional cementing properties.
Why is admixture used?
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 Admixtures
Chemical 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 etc
Types of Chemical Admixtures
1. Water-reducing admixture / Plasticizers:
These admixtures are used for following purposes:
- To achieve a higher strength by decreasing the water cement ratio at the same workability as an admixture free mix.
- To achieve the same workability by decreasing the cement content so as to reduce the heat of hydration in mass concrete.
- To increase the workability so as to ease placing in accessible locations
- Water reduction more than 5% but less than 12%
- The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts.
- Plasticizers are usually based on lignosulphonate, which is a natural polymer, derived from wood processing in the paper industry.
Surface active agents alter the physic chemical forces at the interface. They are adsorbed on the cement particles, giving them a negative charge which leads to repulsion between the particles. Electrostatic forces are developed causing disintegration and the free water become available for workability.
As these agents are organic by nature, thus they lubricate the mix reducing the friction and increasing the workability.
A thin layer is formed over the cement particles protecting them from hydration and increasing the setting time. Most normal plasticizers give some retardation, 30–90 minutes
An admixture which, when added to concrete, mortar, or grout, increases the rate of hydration of hydraulic cement, shortens the time of set in concrete, or increases the rate of hardening or strength development.
Accelerating admixtures can be divided into groups based on their performance and application:
1. Set Accelerating Admixtures,
Reduce the time for the mix to change from the plastic to the hardened state.
Set accelerators have relatively limited use, mainly to produce an early set.
2. Hardening Accelerators,
Which increase the strength at 24 hours by at least 120% at 20ºC and at 5ºC by at least 130% at 48 hours. Hardening accelerators find use where early stripping of shuttering or very early access to pavements is required. They are often used in combination with a high range water reducer, especially in cold conditions.
Calcium chloride is the most effective accelerator and gives both set and hardening characteristics. However, is limited due to acceleration of corrosion of steel reinforcement and decrease resistance of cement paste in a sulfate environment. For this reason, it should not be used in concrete where any steel will be embedded but may be used in plain unreinforced concrete.
Chloride-free accelerators are typically based on salts of nitrate, nitrite, formate and thiocyanate. Hardening accelerators are often based on high range water reducers, sometimes blended with one of these salts.
Accelerating admixtures have a relatively limited effect and are usually only cost effective in specific cases where very early strength is needed for, say, access reasons. They find most use at low temperatures where concrete strength gain may be very slow so that the relative benefit of the admixture becomes more apparent.
In summary, a hardening accelerator may be appropriate for strength gain up to 24 hours at low temperature and up to 12 hours at ambient temperatures. Beyond these times, a high range water reducer alone will usually be more cost-effective.
3. Set Retarders:
The function of retarder is to delay or extend the setting time of cement paste in concrete. These are helpful for concrete that has to be transported to long distance, and helpful in placing the concrete at high temperatures.
When water is first added to cement there is a rapid initial hydration reaction, after which there is little formation of further hydrates for typically 2–3 hours. The exact time depends mainly on the cement type and the temperature. This is called the dormant period when the concrete is plastic and can be placed. At the end of the dormant period, the hydration rate increases and a lot of calcium silicate hydrate and
calcium hydroxide is formed relatively quickly. This corresponds to the setting time of the concrete.
Retarding admixtures delay the end of the dormant period and the start of setting and hardening. This is useful when used with plasticizers to give workability retention. Used on their own, retarders allow later vibration of the concrete to prevent the formation of cold joints between layers of concrete placed with a significant delay between them.
The mechanism of set retards is based on absorption. The large admixture anions and molecules are absorbed on the surface of cement particles, which hinders further reactions between cement and water i.e. retards setting. The commonly known retards are Calcium Ligno-sulphonates and Carbohydrates derivatives used in fraction of percent by weight of cement.
4. Air Entrained Admixtures:
An addition for hydraulic cement or an admixture for concrete or mortar which causes air, usually in small quantity, to be incorporated in the form of minute bubbles in the concrete or mortar during mixing, usually to increase its workability and frost resistance.
Air-entraining admixtures are surfactants that change the surface tension of the water. Traditionally, they were based on fatty acid salts or vinsol resin but these have largely been replaced by synthetic surfactants or blends of surfactants to give improved stability and void characteristics to the entrained air.
Air entrainment is used to produce a number of effects in both the plastic and the hardened concrete.
• Resistance to freeze–thaw action in the hardened concrete.
• Increased cohesion, reducing the tendency to bleed and segregation in the plastic concrete.
• Compaction of low workability mixes including semi-dry concrete.
• Stability of extruded concrete.
• Cohesion and handling properties in bedding mortars.
See Also: Formwork Materials