other binders

Lime

There are two forms of lime: quicklime and hydrated lime.

Quicklime is produced by heating rock or stone containing calcium carbonate (limestone, marble, chalk, shells, etc.) to a temperature of around 1000°C for several hours in a process known as 'calcining' or sometimes simply 'burning'. It is an unstable and slightly hazardous product and therefore is normally 'hydrated' or 'slaked', by adding water, becoming not only more stable but also easier and safer to handle.

To produce dry powdered hydrated lime just sufficient water is added for the quicklime lumps to break down to a fine powder. This material would have a 'shelf life' of only a number of weeks, depending on storage conditions. 'Old' hydrated lime would have partially carbonated and become a less effective binder.

However, if quicklime is hydrated with a large excess of water and well agitated, it forms a milky suspension known as milk of lime. Allowing the solids to settle, and drawing off the excess water, forms a paste-like residue, termed lime putty, which is the form of lime which can be used in building applications to best effect. This will keep almost indefinitely and, in fact, improves with age. In most countries, though, lime is most widely available as a powder, due to its widespread utilisation in process and treatment industries rather than in construction. Lime putty, which needs a stiff bag or container for transportation, is more rarely produced.

Limes with high calcium content, often called 'fat' or 'white' limes are desirable for most industries, although the construction industry can use limes containing impurities. For instance, limestones containing a proportion of clay are often seen as an advantage in building as they produce hydraulic limes which will set under water and will produce stronger mortars.

In the construction industry, lime, in its hydrated or putty form, is mixed with aggregate and water to produce concrete or mortar in the usual manner. Lime putties generally produce mortars and renders of excellent quality and consistency.

Plain lime-sand mortars are quite weak; any early adhesive strength results from drying out, and longer term hardening occurs through the action of the air's carbon dioxide on the lime.

Traditionally lime renders and plasters were often mixed with animal hair to improve cohesion. Today the addition of gypsum or Portland cement and/or pozzolanas to increase durability and give faster setting times is usual.

Pozzolanas

Pozzolanas are materials which, although not cementitious in themselves, will combine chemically with lime in the presence of water to form a strong cementing material. They include:

• Volcanic ash
• Power station fly ash (usually known as pfa)
• Burnt clays
• Ash from some burnt plant materials
• Silicious earths (such as diatomite)

Materials not already in a fine powdered form must be ground, and some require calcining at around 600 -750 °C to optimize their pozzolanic properties.

Pozzolanas can be mixed with lime and/or Portland cement and can improve quality and reduce costs of concretes made from both materials.

In some countries (e.g. India and Kenya), pozzolanas are mixed with Portland cement and sold as blended cement, which in many respects is similar to Portland cement. In other countries (e.g. Cuba) lime/pozzolana/Portland cement blends are sold as an alternative to Portland cement. Lime-pozzolana cement by itself can make an excellent cementing material for low-rise construction or mass concrete and in some countries (e.g. Indonesia) is still produced extensively.

Gypsum plaster

Gypsum is a not an uncommon mineral, and needs only a low temperature, of around 150°C, to convert it into a very useful binding material, known as hemi-hydrate or plaster of Paris.

On its own, plaster of Paris sets very rapidly when mixed with water. To give time for it to be applied, around 5% of lime and 0.8% of a retarding material (such as the keratin glue-like extracts from boiling fish bones or animal hoof and horn) are added to the plaster.

Retarded plaster of Paris can be used on its own or mixed with up to three parts of clean, sharp sand. Hydrated lime can be added to increase its strength and water resistance. Gypsum plasters can be reinforced with various fibrous materials from reeds to chopped glass fibres.

Gypsum plaster is not wholly resistant to moist conditions and so is normally used internally, except in the drier Mediterranean and Middle Eastern countries where it has traditionally been used as an external render.