Admixtures & Plasticizers of Concrete

4. 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.

5. 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 Traditionally, they were based on fatty acid salts or vinsol resin but these have largely been replaced by synthetic surfactants or blends of surfactants to give improved stability and void characteristics to the entrained air.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. These include:

• 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.

Types of Mineral Admixtures

1.     Cementitious

These have cementing properties themselves. For example:

§  Ground granulated blast furnace slag (GGBFS)

2.    Pozzolanic

A pozzolan is a material which, when combined with calcium hydroxide (lime), exhibits cementitious properties. Pozzolans are commonly used as an addition (the technical term is "cement extender") to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete and in some cases reduce the material cost of concrete. Examples are

§  Fly ash

§  Silica Fume

§  Rice Husk Ash

§  Metakaolin

Pozzolanic Action:

The additive act in three ways

1. Filler
2. Nucleating
3. Pozzolanic

1. Filler:

These additives/admixtures are finer than cement, so when added to concrete they occupy the small pores previously left vacant.

2. Nucleating:

These fine particles accelerate the rate of hydration and precipitation starts.

3. Pozzolanic:

When cementing material reacts with water the following reaction take place:

C2S + H CSH + CH

C3S + H CSH + CH

CSH is responsible for strength while CH is a soluble material reacts and dissolves in water leaving behind pores.

So when admixture is added

SiO3 or Al2O3+CH CSH

Thus it reduces the amount of CH & increase CSH

Conditions to Declare a Material Pozzolan:

§  Having silica + Alumina oxide+ ferrous oxide more than 70%.

§  Surface area on normal admixture is more than 300m²/kg.

§  Surface area should be more than cement used.

3. Ground Granulated Blast Furnace Slag (GGBFS)

Ground granulated blast-furnace slag is the granular material formed when molten iron blast furnace slag (a by-product of iron and steel making) is rapidly chilled (quenched) by immersion in water. It is a granular product, highly cementitious in nature and, ground to cement fineness, hydrates like Portland cement.

(Blast-Furnace Slag: A by-product of steel manufacture which is sometimes used as a substitute for Portland cement. In steel industry when iron ore is molted, then in the molted state all the impurities come at its surface which are removed called slag. It consists mainly of the silicates and aluminosilicates of calcium, which are formed in the blast furnace in molten form simultaneously with the metallic iron. Blast furnace slag is blended with Portland cement clinker to form PORTLAND BLASTFURNACE SLAG CEMENT).

GGBFS is used to make durable concrete structures in combination with ordinary Portland cement and/or other pozzolanic materials. GGBFS has been widely used in Europe, and increasingly in the United States and in Asia (particularly in Japan and Singapore) for its superiority in concrete durability, extending the lifespan of buildings from fifty years to a hundred years.

Concrete made with GGBFS cement sets more slowly than concrete made with ordinary Portland cement, depending on the amount of GGBFS in the cementitious material, but also continues to gain strength over a longer period in production conditions. This results inlower heat of hydration and lower temperature rises, and makes avoiding cold joints easier, but may also affect construction schedules where quick setting is required.

Use of GGBFS significantly reduces the risk of damages caused by alkali-silica reaction (ASR), provides higher resistance to chloride ingress, reducing the risk of reinforcement corrosion, and provides higher resistance to attacks by sulfate and other chemicals.

Benefits:

1.    Durability

2.    GGBFS cement is routinely specified in concrete to provide protection against both sulphate attack and chloride attack

3.    GGBFS is also routinely used to limit the temperature rise in large concrete pours. The more gradual hydration of GGBFS cement generates both lower peak and less total overall heat than Portland cement.

4.    Appearance

5.    In contrast to the stony grey of concrete made with Portland cement, the near-white color of GGBFS cement permits architects to achieve a lighter colour for exposed fair-faced concrete finishes, at no extra cost.

6.    Strength

7.    Concrete containing GGBFS cement has a higher ultimate strength than concrete made with Portland cement. It has a higher proportion of the strength-enhancing calcium silicate hydrates (CSH) than concrete made with Portland cement only, and a reduced content of free lime, which does not contribute to concrete strength. Concrete made with GGBFS continues to gain strength over time, and has been shown to double its 28 day strength over periods of 10 to 12 years.

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