IMPORTANCE OF ADMIXTURES (Chemical & Mineral ) IN

CONCRETE

Vinod Kumar SinghCo-Founder, Happho.com

www.happho.com is a One Stop Solution for Bungalow Construction, Interiors and Renovation and an Online Marketplace for Construction Materials

CONCRETEBasic Definition:

Concrete is a composite material that essentially consists of a binding medium embedded with Fine-Aggregate (typically sand) and Coarse Aggregate (typically gravel), water with or without chemical and mineral admixture and filler.

• Concrete is a highly heterogeneous material on a macroscopic scale, as it can be made up from an infinite combination of suitable materials and its final properties are dependent on the treatment it gets while production, placing, during curing and exposure conditions during its service life.

• The effectiveness of production( selection of combination of suitable materials, mixing etc.) , placing (consolidation) and curing procedures are critical for attaining full properties of a concrete mixture.

CONCRETE ADMIXTURES • Admixtures are material other than cement, aggregates & water that are added to

concrete either before or during mixing to alter its properties & performance in fresh (workability, setting time etc) and hardened state (strength, durability etc.)

Concrete admixtures are generally divided into 2 types i.e.,1. Chemical admixture2. Mineral admixture

Mineral and Chemical admixture are incorporated to modify the properties of concrete such as heat of hydration, accelerate or retard the setting time,improve workability,help in waterreduction ,disperse/deflocculates the cement particles,aid in entrain air and finally improve the impermeability and durability characteristics.

CONCRETE ADMIXTURES Typically incorporated to improve the following concrete properties

• Protect Against Freeze Thaw Cycles –Improve Durability • Mid- Range water reducers • High-Range water reducers – superplasticizers • High Strength Concrete • Corrosion Protection • Set Acceleration • Strength Enhancement • Set Retardation • Crack Control (shrinkage reduction) • Flowability • Self levelling• Finish Enhancement

CHEMICAL ADMIXTURES • Chemical admixtures are used to improve the quality of concrete during mixing,

transporting, placement and curing. They reduce the cost of construction, modify properties of hardened concrete, ensure quality of concrete during mixing/transporting/placing/curing, and overcome certain emergencies during concrete operations.

• They are basically chemical compounds. Dosage ranges from 0.2% to 2% by weight of cement.

CHEMICAL ADMIXTURES

Chemical admixtures fall into the following categories:

• Air entrainers • Water reducers • Set retarders • Set accelerators • Superplasticizers • Specialty admixtures: which include corrosion inhibitors, shrinkage control, alkali-silica

reactivity inhibitors, and coloring.

CHEMICAL ADMIXTURES • Air entraining agent: Air-entraining agents entrain small air bubbles in the concrete. The major benefit of this

is enhanced durability in freeze-thaw cycles, especially relevant in cold climates.

• Retarding Admixtures: Retarding admixtures slow down the hydration of cement, lengthening set time. Retarders are beneficially used in hot weather conditions in order to overcome

accelerating effects of higher temperatures and large masses of concrete on concrete setting time

• Accelerating admixture : Accelerators shorten the set time of concrete, allowing a cold-weather pour, early

removal of forms, early surface finishing, and in some cases, early load application.

CHEMICAL ADMIXTURES • Water Reducing Admixtures :

Water reducing admixtures require less water to make a concrete of equal slump, or increase the slump of concrete at the same water content. Typical water reduction is the range of 10 – 15%.

• Superplasticizers :

High range water reducers are admixtures that allow large water reduction or greater flowability (as defined by the manufacturers, concrete suppliers and industry standards) without substantially slowing set time or increasing air entrainment.

• Specialty admixtures:

Include corrosion inhibitors, shrinkage control, alkali-silica reactivity inhibitors, and coloring.

WATER REDUCING ADMIXTURES (Normal/High Range)

Reasons for Using a Water Reducer: Option 1

• To get a stronger and more durable concrete from a given amount of cement by allowing reduction of water-cement ratio

• Most regular water reducers will permit 5-15% reduction water

→ Example• Original mix: 330 kg cement, 181.5 kg water, w/c = 0.55→strength at 28 days = 30 MPa

• Mix with water reducer 330 kg cement, 165 kg water, w/c = 0.5→strength at 28 days = 36 MPa

WATER REDUCING ADMIXTURES(Normal & High Range)

Reasons for Using a Water Reducer: Option 2

• For a given w/c a mix may turn out to be unworkable

• Add water reducer to increase slump without making adjustment to mix proportions.

• Results in better compactibility of the mix and strength increase slightly

WATER REDUCING ADMIXTURES(Normal & High Range)

Reasons for Using a Water Reducers: Options 3

• Economy: a concrete with the same w/c and same strength can be produced with less cement.

• Normally the extra cost for the water reducer is more than offset by savings in cement.

• Example→Original Mix

330 kg cement, 165 kg water, w/c =0.5

→Mix with WR (10% water reduction) 150 kg water, For w/c = 0.5 cement required = 300 kg. ÎSave 30 kg cement/m 3)

CHEMICAL ADMIXTURES Terms associated with the interaction of chemical admixture and cementitious system.

• Saturation point :Saturation dosage of a given HRWRA (High Range water reducing admixture) exists for each cementitious system. Beyond this dosage any additional increase does not generate any increase in fluidity of the cement paste and the increase slump of concrete.• Incompatibility : A rapid loss is sometime observed in superplasticized concrete, although

the saturation dosage has been used, in this case the cement and HRWRA are said to be incompatible.• Compatibility : When a high slump can be maintained for a longer period of time, without

any significant segregation and bleeding, the HRWRA are said to be compatible.• Robustness: When a little variation in the composition of one of the ingredient of the mix

does not lead to a big variation of the properties of the concrete.

CHEMICAL ADMIXTURES Issues related to incorporation of chemical admixtures in concrete.

Compatibility and Robustness : Compatibility and robustness are the two main issues governing the choice of

chemical admixture in cementitious systems, to achieve the intended concrete properties.

• Simple tests on grouts, such as Marsh Cone test or Mini- slump cone test can give a fair idea of compatibility between chemical admixture and cementitious system.

• Fewer trials concrete mixture may be needed to obtain an optimized concrete mixture, once above test on grouts are performed.

CHEMICAL ADMIXTURES Key Factors governing chemical admixture and cementitious system compatibility.

• Chemical & phase compositions of cement especially C3A & alikali.

• Cement fineness.

• Amount and type of Gypsum

• Chemical & Average molecular weight of admixture.

• Dosage of Admixture & addition method.

• Degree of sulphonation, purity.

• Type and quantity of mineral admixture present in the concrete.

MARSH CONE

MARSH CONE TEST

MINI SLUMP CONE

MINI SLUMP CONE TEST

MINERAL ADMIXTURES• Mineral admixtures affect the nature of the hardened concrete through hydraulic or

pozzolanic activity. Pozzolans are cementitious materials and include natural pozzolans (such as the volcanic ash used in Roman concrete), fly ash, Silica Fume, Metakaolin, Rice Husk Ash, GGBFS.

• Mineral admixtures make mixtures more economical (enables reduction in total cement content), reduce permeability, increase strength, and influence other concrete properties.

• They can be used with Portland cement, or blended cement either individually or in combinations.

DIFFERENT TYPES OF MINERAL ADMIXTURES Fly Ash (By product of Thermal Power Plant) :

• Derived from burning coal, fly ash is a valuable additive that makes concrete stronger, more durable and easier to work with.

• Fly ash aids the formation of cementitious compounds to enhance the strength, impermeability and durability of concrete.

Two main classes of fly ash are used in concrete, Class F, and Class C.

Class FReduces bleeding and segregation in plastic concrete. In hardened concrete, increases ultimate strength, reduces drying shrinkage and permeability, lowers heat of hydration and reduces creep.

Class CProvides unique self-hardening characteristics and improves permeability. Especially useful in pre-stressed concrete and other applications where high early strengths are required. Also useful in soil stabilization.

DIFFERENT TYPES OF MINERAL ADMIXTURES Fly Ash (By product of Thermal Power Plant) :

• In addition to economic and ecological benefits, the use of fly ash in concrete improves its workability, reduces segregation, bleeding, heat evolution and permeability, inhibits alkali-aggregate reaction, and enhances sulfate resistance.

• Even though the use of fly ash in concrete has increased in the last 20 years, less than 20% of the fly ash collected was used in the cement and concrete industries

• One of the most important fields of application for fly ash is PCC pavement, where a large quantity of concrete is used and economy is an important factor in concrete pavement construction.

DIFFERENT TYPES OF MINERAL ADMIXTURES Silica Fume (By product of Silicon and Ferro-Silicon Alloy) :• When measured by nitrogen absorption techniques,its particles are approximately 100

times smaller than the average cement particle. Because of its extreme fineness and high silica content, Silica Fume is a highly effective pozzolanic material

• Silica fume can make a significant contribution to early-age strength of concrete. One kilogram of silica fume produces about the same amount of heat as a kilogram of portland cement, and yields about three to five times as much compressive strength.

Silica fume improves concrete in two ways – the basic pozzolanic reaction, and a microfiller effect. Addition of silica fume improves bonding within the concrete and helps reduce permeability, it also combines with the calcium hydroxide produced in the hydration of portland cement to improve concrete durability.

DIFFERENT TYPES OF MINERAL ADMIXTURES Silica Fume (By product of Silicon and Ferro-Silicon Alloy) :

• As a microfiller, the extreme fineness of the silica fume allows it to fill the microscopic voids between cement particles. This greatly reduces permeability and improves the paste-to-aggregate bond of the resulting concrete compared to conventional concrete.

• Silica Fume is used in concrete to improve its properties. It has been found that Silica Fume improves compressive strength, bond strength, and abrasion resistance; reduces permeability; and therefore helps in protecting reinforcing steel from corrosion.

Caution:The higher percentage of silica fume used, the higher the amount of super plasticizer needed - but mix can become "sticky".

DIFFERENT TYPES OF MINERAL ADMIXTURESGround Granulated Blast Furnace Slag (By product of Iron Industries) • Ground granulated blast-furnace slag is the granular material formed when molten iron

blast furnace slag is rapidly chilled (quenched) by immersion in water. It is a granular product with very limited crystal formation, is highly cementitious in nature and, ground to cement fineness, hydrates like Portland cement.

• Concrete containing GGBFS as a partial cement replacement has longer-lasting workability and low slump loss during hot weather construction.

• Concrete containing GGBFS exhibits a lower heat of hydration than conventional Portland cement concrete.

• The use of GGBFS as a partial replacement for Portland cement can reduce available alkalies and can reduce the reaction between certain siliceous components of concrete aggregates and the alkalies in the concrete

• Use of GGBFS as a partial cement replacement gives concrete moderate resistance to sulfate attack

DIFFERENT TYPES OF MINERAL ADMIXTURESMetakaolin:• Calcination of Kaolin (a fine, white clay mineral), results a highly pozzolanic material called

matakaolin.

Rice Husk Ash :• Rice milling generates a by product know as husk . This surrounds the paddy grain. During

milling of paddy about 78 % of weight is received as rice , broken rice and bran .Rest 22 % of the weight of paddy is received as husk . This husk is used as fuel in the rice mills to generate steam for the parboiling process .

• This husk contains about 75 % organic volatile matter and the balance 25 % of the weight of this husk is converted into ash during the firing process , is known as rice husk ash ( RHA ). This RHA in turn contains around 85 % - 90 % amorphous silica.

CONCLUDING REMARKS

• Mineral and Chemical Admixture play a very important role in concrete, its judicious selection improves its fresh and hardened concrete properties & enhances its long terms performance.

• New age concrete like Self Compacting Concrete, High Volume Fly Ash Concrete, High Performance Concrete etc, would not have been realized without the availability / incorporation of mineral and chemical admixture.

• Mandatory use of mineral and chemical admixture should be encouraged in order to attain true sustainable development.

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