binders

Binding systems from history

The simplest, and possibly the earliest, binding material used was wet mud, and there are records of its use in ancient Egypt. Another example of a binder from the distant past is the use of naturally occurring bitumen by the Babylonians and Assyrians in their brick and alabaster (gypsum plaster) constructions.

Lime was known to the Greeks and was widely used by the Romans. The Roman architect and engineer Vitruvius published the first specification for the use of lime in building in his celebrated work De Architectura. The Romans also knew how to make a lime-pozzolana cement by adding materials such as volcanic ash or powdered bricks, tiles and pottery to lime.

That lime is an appropriate and durable binding material, especially when mixed with pozzolana, is well proven. The Pont du Gard at Nimes in France, a Roman aqueduct built in AD 18 with hydraulic lime-based mortar, is still waterproof; the excellence of the mortar is attributed to the selection of the materials and to the time spent tamping the mix into place during construction.

The rebuilding of the Eddystone lighthouse in the English Channel by John Smeaton in 1756 is a more recent development in 'lime technology'. It was achieved through Smeaton recognizing the hydraulic properties of lime that result from the burning of a clayey limestone. To make the highly water-resistant mortar needed for bonding the courses of stone, he thoroughly mixed this already hydraulic lime with an equal proportion of imported Italian pozzolana  to the mortar.

Why continue to use alternative cements

Major advantages of alternatives to Portland cement are that they are usually cheaper to produce, needing much lower or even negligible capital inputs to get started, and requiring far less imported technology and equipment. They can also be produced on a small scale to supply a local market resulting in greatly reduced transportation costs and a much greater degree of local accountability in the supply of building materials.

From an environmental angle lime-pozzolana cements can be produced with lower energy input than either lime by itself or Portland cement - giving a half to one third consumption in use compared with Portland cement and about one fifth compared with lime by itself. Low energy consumption is particularly prevalent with naturally occurring pozzolanas, or those from waste materials, which might need little additional processing other than drying. The use of clay as a binder, of course, results in negligible energy consumption in production.

Lower production costs mean lower prices for the consumer, enabling those who could not afford Portland cement to purchase and use a quality binding material.

Pozzolanic cements additionally have numerous other technical advantages to the user:

• Improved workability
• Improved water retention/reduced bleeding
• Improved sulphate resistance
• Improved resistance to alkali - aggregate reaction
• Lower heat of hydration

• The technical and economic advantages of alternative cements are not lost on architects and engineers from developed countries. Increasingly, architects, are becoming aware of the brittleness associated with Portland cement mortars, for example, and are now specifying blended lime/Portland cement mortars instead. As well as re-discovering the 'lost arts' of using alternative binders, recent research has enabled the properties of alternative binders to be thoroughly investigated and catalogued. A body of experience has built up on the appropriate application of traditional binders such as clay, lime and pozzolanas, not only in the repair and conservation of historic buildings, monuments and structures but also in adventurous and innovative new build applications.
  In some developing countries traditional binders are still slighted, probably because they might be associated with poverty or considered to be low status materials. Their performance and technical specifications might, completely unjustifiably, also be considered inferior to Portland cement, they might not be widely produced or available, or the skills to produce and use them might well have disappeared.