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Chemical Admixtures

Source: | Updated: Mar 02, 2016

A chemical admixture is any chemical additive to the concrete mixture that enhances the properties of concrete in the fresh or hardened state. It does not typically include paints and protective coatings (for steel or concrete). ACI 116R defines the term admixture as “a material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of concrete or mortar, and added to the batch immediately before or during its mixing.”

Classification

Water Reducers

Water reducers can be used in three ways:

  1. For a given workability, they can reduce the water demand, thus resulting in higher strength and durability.

  2. For a given w/c and strength, they can increase the workability.

  3. For a given w/c, strength and workability, the quantity of cement can be reduced.

Water reducers belong to a group of chemicals known as ‘dispersants’. The action of the dispersant is to prevent the flocculation of fine particles of cement. These dispersants are basically surface-active chemicals consisting of long-chain organic molecules, having a polar hydrophilic group (water-attracting, such as –COO-, -SO3-, -NH3+) attached to a non-polar hydrophobic organic chain (water-repelling) with some polar groups (-OH). As shown in Figure 1, the polar groups in the chain get adsorbed on the surface of the cement grains, and the hydrophobic end with the polar hydrophilic groups at the tip point outwards from the cement grain. The hydrophilic tip is able to reduce the surface tension of water, and the adsorbed polymer keeps the cement particles apart be electrostatic repulsion. With the progress of hydration, the electrostatic charge diminishes and flocculation of the hydrating product occurs.

Common normal water reducers:

  • Lignosulphonate salts (sodium salts of sulphonated lignin)

  • Hydroxycarboxylic acids – Citric, gluconic acid

  • Carbohydrates – Corn syrup, dextrin

The dosage of normal WRs is 0.3 – 0.5% by weight of cement. At higher dosages, there is danger of excessive retardation and bleeding. Also, returns diminish, and excessive air entrainment can occur.

Figure 1. Dispersion of cement by a water-reducing admixture

Figure 2. Schematic showing the working of a water reducer

Common high-range water reducers (or Superplasticizers):

1st generation: Lignosulphonates at high dosages
2nd generation:
Polysulphonates

- Sulphonated melamine formaldehyde (SMF)
- Sulphonated naphthalene formaldehyde (SNF)

3rd generation:

- Polycarboxylates
- Polyacrylates
- Monovinyl alcohols

Typical dosage: 0.7 – 1.0% by weight of cement.

The 1st generation HRWRs need a slump of around 75 mm for action (~0.45 w/c). The slump is increased up to 150 – 200 mm. The 2nd generation admixtures can work at reasonably low slumps (25 – 50 mm, corresponding to w/c of 0.35 – 0.40) to increase the slump to ~ 250 mm. The 3rd generation HRWRs, on the other hand, can even be used with no slump concrete (0.29 – 0.31 w/c), and the slump is increased to more than 250 mm. Concrete possessing slump above 225 – 250 mm is called ‘rheoplastic’.
Superplasticizing admixtures are prone to slump retention problems. The efficient dispersion of cement and reduced surface tension of water leads to hydration of cement, which in turn causes the diminishing of the electrostatic charge, and flocculation occurs. Moreover, w/c of superplasticized concrete is typically low.
New developments: Some new polycarboxylates have been developed that have a dual mode of action. In addition to providing dispersion by electrostatic repulsion, these chemicals provide added dispersion due to steric hindrance. This occurs because of the bulkiness of the polymer side chains. Thus, the slump can be retained for a longer time.

Mid-range water reducers

Usually, HRWRs give erratic performance when low slumps are desired (< 200 mm). Conventional water reducers cannot provide slumps higher than 150 mm. The intermediate gap can be covered by the application of Mid-range water reducers. These admixtures provide better finishing and pumping characteristics, in addition to controlled setting properties. Mid-range water reducers are typically low dosage or low active ingredient products based on superplasticizers.

Sequence of addition of water reducers

Generally, water reducers are added along with the mix water to the concrete mixture. In the case of superplasticizers, since the slump loss is rapid, it may be advantageous to add it to the mix in two, or even three, operations.

Set controlling admixtures

Set controlling chemicals are used to lengthen or shorten the setting time of concrete in order to suit the concrete to a specific casting condition. For example, when early strengths are desired, or when the temperature is low, accelerators may be added to the concrete. Conversely, when casting is done in hot weather, and there is a chance of increased transportation time of the concrete, retarders may be used.

While water-reducing admixtures have a mostly physical effect on the cement-water system, set-controlling chemicals cause a delay or increase of the setting time by chemical alterations to the hydration. Forsen has classified set-controlling chemicals in five categories, and the effects of these chemicals on the setting time of concrete is shown in Figure 3. The same chemical can sometimes act either as a retarder or an accelerator based on its concentration.

Type I: Gypsum
Type II: Calcium chloride, calcium nitrate
Type III: Potassium and sodium carbonate, sodium silicate
Type IV: Gluconates, Lignosulphonates and sugars, sodium salts of carboxylic acids, Zn and Pb salts
Type V: Salts of formic acid and triethanol amine (TEA)

Figure 3. Action of set-controlling admixtures

Typically, set controllers affect cement hydration during the early stages, namely, during the processes of dissolution cement compounds and nucleation of hydration products (see Cement Chemistry). According to Joisel, only the dissolution is affected by these admixtures. If we consider the hydrating PC to be a mixture of cations (Ca) and anions (silicate and aluminate), then the following scenarios can occur.

  • An accelerator should promote the dissolution of both cations and anions. Since several anions are present, the accelerator should promote the dissolution of that anion which has the lowest dissolving rate, i.e., silicate.

  • A retarder impedes the dissolution of Ca ions and aluminates.

  • . The presence of monovalent cations – K+ and Na+ - reduces the solubility of Ca, but promotes the dissolution of silicates and aluminates. At small concentrations, the former effect is predominant, and at high concentrations, the latter effect is predominant.

  • . Monovalent anions – Cl-, NO3-, etc. – reduce the solubility of silicates and aluminates, and promote the dissolution of Ca. At small concentrations, the former effect is predominant, and at high concentrations, the latter effect is predominant.

  • . In the case of salts of weak bases and strong acids (e.g. CaCl2) or strong bases and weak acids (e.g. K2CO3), at low concentration, the dominant effect is the retardation of Ca and aluminate dissolution; at high concentration, acceleration of the reaction occurs. Calcium chloride (at 1 – 3% by weight of cement) is the most effective accelerator.

Accelerators

Chloride accelerators: CaCl2 (most popular), NaCl
Non-chloride accelerators
Inorganic: Nitrates and nitrites of Ca and Na, thiocyanates, thiosulphates, and carbonates of Ca and Na.
Organic: Amines (triethanol amine – TEA, diethanol amine – DEA), carboxylic acids (Ca salts of formic and acetic acid), formaldehyde.

Retarders

Organic retarders: Lignosulphonates, hydroxycarboxylic acids (citric, gluconic), carbohydrates (corn syrup, dextrin). These are the same chemicals as normal water reducers.

Inorganic retarders: Borates, phosphates, Zn and Cu compounds. These are not generally used because of their high costs and low solubility.

Extended set admixtures: Phosphonates and other phosphorus containing organic acids and salts, gluconic acid, etc. These admixtures are used for the following purposes:

- Stabilization of washwater for concrete
- Stabilization of returned plastic concrete
- Use of fresh concrete for long haul (large travel times) applications

Sequence of addition of set-controlling admixtures:

Accelerators and retarders are added to the concrete either separately or with the mix water, soon after the cement and water come in contact. It must be noted that it is absolutely essential to pay particular attention to dosage – the same chemical may behave as accelerator or retarder depending on concentration (as described in the earlier discussion).

Air entrainers

Air entraining agents are used in concrete to generate air bubbles within the concrete, which help protect against damage due to freezing and thawing (see Durability). They also help in reducing bleeding and segregation, and improve the workability of concrete, since the air bubbles act in a manner similar to ball bearings.

Air-entraining agents are also surface-active chemicals. Unlike the water-reducing surfactants, the hydrocarbon chain does not have any polar groups, and is entirely hydrophobic. The hydrophilic polar groups are similar to water reducers. The mode of action of these chemicals is depicted in Figure 4. The polar group sticks outward, lowers the surface tension of water and promotes bubble formation. However, the polar ions get adsorbed on the cement surface with the hydrophobic chain sticking out.

Air bubbles are generated during the agitation and mixing of the concrete. The air-entraining agents simply helps to stabilize these bubbles by the above action. Some common chemicals used as air entrainers are neutralized vinsol resin, derivatized pine rosin, and fatty acids (detergents). Air entrainers are added to the concrete mixture either early in the process – with the sand and coarse aggregate – or after the cement has been added along with some of the mix water. Air entraining chemicals should never be mixed with any other chemical additives.

Figure 4. Mode of action of an air-entraining agent

Just providing an adequate air content inside concrete is not sufficient. Small and stable air bubbles are required for efficient protection against freezing and thawing. The air void parameters that need to be determined for the concrete are – total entrained air (found in fresh concrete during casting), and distance between voids (not more than 200 micron), which is determined from a petrographic analysis of the hardened concrete.

Entrapped air is different from entrained air. Entrapped air consists of irregular voids that are remnants of the compaction process. Some amount of entrapped air is always present in concrete, and the mix design codes generally stipulate the assumption of the amount of entrapped air based on the coarse aggregate size in concrete. In general, about 1 – 2% of entrapped air is present inside concrete. Entrained air, on the other hand, is generated using the admixture, and consists of small and spherical voids. A clear distinction can be seen in Figure 5.


Figure 5. Optical microscope image of an air entrained concrete

Specialty admixtures

Viscosity Modifying Agents (VMA)

These chemicals are added to the concrete for the following purposes:

  • To provide stability to extremely flowable concrete (which maybe prone to segregation)

  • To prevent the wash-out of concrete in underwater applications – In this case the VMA is also called ‘Anti-washout admixture’

VMAs are long-chain water soluble polysaccharides (Cellulose ether derivatives and microbial source polysaccharides, such as Welan gum) that enhance the water retention capacity of the paste. According to Khayat, these chemicals can act in the following ways:

  • Adsorption: Long-chain polymer molecules adhere to the periphery of water molecules, thus adsorbing and fixing part of the mix water and thereby expanding; this causes an increase in the viscosity.

  • Association: Molecules in adjacent polymer chains develop attractive forces, thus further blocking the motion of water by forming a viscous gel.

  • Intertwining: At low shear rates, polymer chains intertwine and entangle, causing an increase in the viscosity; shear thinning occurs at high shear rates when the chains disentangle and align in the direction of flow.


Shrinkage reducing admixtures

Drying of water from concrete capillary pores (primarily pores between 2.5 – 50 nm containing adsorbed water) causes the formation of menisci that results in an inward pull being exerted on the pore walls. The menisci form due to surface tension of water. Shrinkage reducing admixtures contain chemicals such as polyoxyalkylene alkyl ether that reduce the surface tension of water in the capillaries, thus reducing the tensile stresses on drying. These admixtures are typically used at a high dosage – about 2 – 4% by weight of cement.

Corrosion inhibitors

Corrosion of steel in reinforced concrete is initiated when the layer of passivating film on the surface of the steel (composed of FeO) breaks down at low pH levels. With the availability of moisture and oxygen, the corrosion reaction proceeds and results in the formation of various rust products. Corrosion inhibitors added to concrete can affect this process in various ways, such as:

  • Oxidizing or non-oxidizing passivators of steel

  • Oxygen scavengers

  • Film forming compounds (adsorption)

  • Cathodic effects: paste can be made hydrophobic

Some typical corrosion inhibitors are:

Inorganic: Calcium nitrite
Organic: Amines, esters, alkanolamines

These compounds are usually added at high dosages, ~ 2% by weight of cement.
Some commercially available admixtures are two-part products, such as those containing amines and esters. The amines coat the steel and provide a film on the steel surface, while the esters make the paste hydrophobic and reduce the availability of water for the cathodic reaction.

Other admixtures

Antifreeze compounds: These lower the freezing point of water. For example, NH4OH, calcium and sodium nitrates and nitrites, CaCl2, K2CO3, glycols, etc.

Waterproofing admixtures: These are organic compounds that adsorb on the pore walls and make them hydrophobic. Thus, once the concrete becomes dry, it is difficult to re-wet it. For example, oleic acid, emulsions of waxy materials, Ca and Al stearate.

Alkali-silica reaction mitigating admixtures: Compounds of active alkalis such as Lithium (e.g. LiOH, Li2NO3, etc.), which bind the reactive silica to form non-expansive compounds. The cost of these admixtures, however, is prohibitive.

Styrene-butadiene latexes: These are combinations of styrene and butadiene, as an emulsion in water. The solids content of typical slurries is 40 – 50%. A polymer film forms along with the hydrating cement, resulting in a monolithic matrix. This causes improved paste-aggregate bond, resistance to crack propagation, and watertightness of concrete.


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