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Last edited 09 Dec 2020
Plasticisers (admixtures or dispersants) are substances added to materials such as cement mortar mixes (as well as to plaster and concrete) to decrease viscosity and plasticity and increase workability and performance. In the case of mortar, they make the mix more flexible and easier to maintain a more consistent mortar thickness. In turn, this can mean that bricklayers are able to lay more bricks than they otherwise might.
Numerous types of plasticisers are available commercially to enhance workability. They typically work by retarding the mortar from setting and providing air entrainment to assist workability. They can also help prevent mortar shrinkage, which if it occurs, does not only look unsightly in mortar joints but also can allow water ingress. Excessive use of plasticisers can have an adverse effect on the mortar.
Lime added to mortars is well known for giving them a soft, putty-like consistency which can make them easier to work and retard their setting. Air entrainment plasticisers when used in the correct proportions can have a similar effect and are sometimes used to replace lime within the mix.
 Washing-up liquid
Washing up liquid is commonly used as a plasticiser in cement mortar. However, it is thought to affect the long-term structural integrity of the mortar as it can add too much air, thereby creating bubbles. The surfactants in washing up liquid may also produce foam if used in excess. These bubbles may fill with moisture in winter and if freezing occurs could cause pore expansion, cracking and disintegration of the mortar. That said, it is a very common practice in the UK, and is used in training brick laying as it can prevent the mortar from setting so it can be re-worked or re-used.
The use of washing-up liquid is discouraged by many professionals, as well as The Brick Development Association which says: “Washing-up liquids and any additives containing calcium chloride should not be used as they can weaken the mortar and contribute to efflorescence staining.”
Concrete superplasticisers play a vital role in improving the quality and reducing the cost of construction. Superplasticisers are particularly suited to the manufacture high strength concrete for construction. They are chemical admixtures that have water-reduction capabilities that range from 5.0% to 40.0% in concrete mixes.
See also: Admixtures in concrete.
 Related articles on Designing Buildings Wiki
- Admixtures in concrete.
- Binding agent.
- Bullseye window.
- Cement mortar.
- Concrete superplasticisers.
- Defects in brickwork.
- Defects in stonework.
- Hot-mixed mortars: the new lime revival.
- Lime mortar.
- Mortar analysis for specifiers.
- Types of mortar.
- Ultra high performance fibre concrete.
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