Last edited 29 Apr 2021

Binding agent

A binding agent, or binder, is a material used to form materials into a cohesive whole, as a means of providing structural stability. Binding agents harden chemically or mechanically, and in the process bond fibres, filler powder and other substances together.

Binding agents have been commonly used in construction for a very long time; for example, the use of straw and natural fibres to strengthen clay in wattle-and-daub construction. A very common binding agent used in contemporary construction is cement, which is used to make concrete. Other common examples include bitumen binder which is used for asphalt pavements, and clay for binding bricks.

Hydraulic binding agents are often used in underground and underwater engineering projects. On mixing with water they continue to preserve and increase their strength. Examples include; Portland cement, pozzolanic cement, blast-furnace cement, alumina and expanding cement, hydraulic lime, and so on.

Air-entrained binding agents harden and maintain their strength after mixing in air rather than water. Examples include; gypsum cement, magnesium cement, air-hardening lime, and so on.

Acid-resistant binding agents can maintain their strength after mixing in air while also in contact with acids, and form various kinds of acid-resistant cement, such as silicon fluoride cement, quartz cement, and so on.

Binding agents can also be 'organic', where they are of organic origin, such as asphalt, bitumen, pitch, polyvinylacetate, resins, and so on. Under the influence of physical or chemical processes they transition from a plastic state to a hard state.

Durability Guaranteed - Pulhamite Rockwork - Its conservation and repair, Published by Historic England in 2008, defines a binder as: ‘The medium or vehicle that binds together the particles of aggregate in a mortar, render or concrete, or the pigments and fillers in a coating (in a mortar or render the binder is usually lime or cement). The binder coats each particle and fills the voids between them, and it is the drying or curing of the binder that causes a mortar or coating to set. Its drying or curing properties and the proportion of binder to aggregate are very important in determining the durability of a mortar.’

NB: The use of 'mechanical' binders refers to bond stones in masonry, and tie beams in timber framing.

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