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Make better concrete with admixtures

The use of admixtures can enhance both the quality and workability of a cementitious mixture.

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Working with cement can be tough. Whether for small features, such as breeze blocks, or large ones, like the base of a skyscraper, plenty can go wrong in the usage and production. To reduce hassle – and possibly avert disaster – your mix ought to account for the specifics of both the project and the environment.

The use of admixtures has the advantage of radically increasing the quality of your cementitious mixture. Admixtures can have various benefits, including better strength, durability, cost reduction, adjusting of setting time, maintaining or improving workability and accelerating the rate of strength development during the early stages.

While admixtures can also compensate for the lack of quality materials in the mix, according to the Concrete Institute: “Admixtures should never be regarded as a substitute for good mix design, good workmanship, or use of good materials.”

The Concrete Institute lists six types of admixtures: plasticisers (water-reducing agents), superplasticisers (high-range water reducers), air entrainers, accelerators, retarders and others.

 

  • Plasticisers

Main types: lignosulphuric acids and their salts, hydroxylated carboxylic acids and their salts, and modifications of both.

The use of plasticisers leads to increased workability. Binder particles absorb the plasticiser and repel one another, breaking the clumps up into fine particles.

The addition of a plasticiser lowers the water requirement of a mix, while maintaining its workability, by about 10%. For a given strength, it is also possible for less cement to be employed.

 

  • Superplasticisers

Main types: chemical compounds such as sulphonated melamine formaldehyde, sulphonated naphthalene formaldehyde, modified lignosulphonates and Polycarboxylate-based materials.

Superplasticisers produce an effect like that of plasticisers, but more marked. Superplasticisers assist in areas of congested reinforcement, and decrease the water content of the cement mixture by 15-25%.

When used to produce flowing concrete, however, they should be added just prior to placing, as workability tends to decrease. Depending on the admixture, the effect may last between 30 minutes and six hours. Superplasticisers are advantageous in cases where a self-levelling consistency is desirable.

 

  • Air Entrainers

Main types: salts of wood resins, animal or vegetable fats and oils and sulphonated hydrocarbons.

The use of an air entrainer introduces small air bubbles that are distributed throughout the cement paste. Air entrainers stop particles that make up the mixture from separating into layers or ‘bleeding’. Benefits also include better workability and resistance to damage through freezing or thawing in the hardened concrete.

Typically, 100kg of cementitious mixture requires 50-150ml of air entrainers. An air volume of 1% can lower the concrete’s strength by around 5%, however, so avoid adding too much air entrainer in the mixing process. A change in ambient temperature may also impact the air quantity, as might the length of the mixing process.

 

  • Accelerators

Main types: chloride-based, non-chloride-based and shotcrete (sprayed concrete) accelerators.

As the name suggests, these lower the time cement takes to harden in the setting process. Rapid setting and high early strengths are among the main benefits, meaning mould and/or formwork may be produced rapidly. Moreover, should concreting take place in cold conditions, accelerators are found to be beneficial. With that said, accelerators work more effectively at lower ambient temperatures, according to the Concrete Institute.

Take care when choosing your accelerator to prevent unforeseen (and unwanted!) consequences. All chloride-based accelerators are corrosive and thus not suited to reinforced concrete, water-retaining structures, prestressed concrete and steam-cured concrete. Instant setting may result from too much accelerator, causing damage to tools and/or equipment.

 

  • Retarders

Main types: hydroxylated carboxylic acids, lignins, sugar and some phosphates.

The use of retarders slows down the setting of a cement mix as well as its initial strength gain. Points of weakness (‘cold joints’) that result from an interruption or delay in concreting operations are reduced due to the slower setting time. They are ideal when placing happens in hot weather or when cement is transported.

An overdose of a retarder may cause setting to last for an extended period. When using a retarder, be sure to not exceed the dose recommended by the supplier/manufacturer.

 

  • Others

Various admixtures let users tailor their mixes to their needs and specifications. For example, certain admixtures improve the pumping, while others add pigments, or enhance the expansion process.

 

Safety

Always be careful when handling chemicals. While most admixtures aren’t hazardous, some are caustic and others may be flammable in nature. Wash and remove clothes that become contaminated, and avoid eye, mouth and skin contact. Seek medical attention in cases of ingestion, eye contact or excessive inhalation.

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