Binders are substances which are used to bind inorganic and organic particles and fibres to form strong, hard and/or flexible components. This is generally due to chemical reactions which take place when the binder is heated, mixed with water and/or other materials, or just exposed to air.
There are four main groups of binders:
• Mineral binders
• Bituminous binders
• Natural binders
• Synthetic binders.
These can be divided into three categories:
• Hydraulic binders, which require water to harden and develop strength.
• Non-hydraulic binders, which can only harden in the presence of air.
• Thermoplastic binders, which harden on cooling and become soft when heated again.
• The most common hydraulic binder is cement (see chapter on Cement).
• Hydraulic and semi-hydraulic limes (see chapter on Lime) are obtained from burning limestone, which contains a large or moderate amount of clay. This can be easily understood, since limestone and clay are the main raw materials for cement production.
• Pozzolanas (see chapter on Pozzolanas), when mixed with non-hydraulic lime, form a hydraulic cement.
• Hydraulic binders are usually available in the form of a fine powder: the finer they are ground (usually in a ball mill), the larger is the specific surface area (of the sum of the particles) per unit weight. And the larger the surface area, the more effective and complete is the chemical reaction with the water that it comes into contact with.
• On account of their affinity to water, hydraulic binders must be stored in absolutely dry conditions, to avoid premature setting and hardening. Even humid air can cause hydration.
• The most common non-hydraulic binder is clay, which is present in most soils, causing them to harden on drying and soften when wet. Its main uses are in earth constructions and in the manufacture of burnt clay products.
• Another common non-hydraulic binder is high calcium or magnesium lime (see chapter on Lime). Hardening depends on its combination with carbon dioxide from the air (carbonation), by which it again becomes calcium carbonate (limestone). But limes are rarely used as the only cementitious binder, and more usually react with clay or a pozzolana to form a hydraulic cement.
• Gypsum is a non-hydraulic binder which occurs naturally as a soft crystalline rock or sand. The chemical name is calcium sulphate all-hydrate (CaS04.2H20). By gentle heating up to about 160° C, calcium sulphate hemi-hydrate (CaSO4.1/2H2O) is produced, more commonly known as "Plaster of Paris", which when mixed with water sets in 8 to 10 minutes. Gypsum plaster has successfully been produced by means of solar energy. Further heating of gypsum, slightly beyond 200° C (not achieved by solar energy) produces anhydrite gypsum (CaSO4), which when mixed with water, sets very slowly.
• Gypsum is also abundantly available as an industrial by-product from the evaporation of seawater to produce common salt, or from the manufacture of fertilizer from phosphate rock. The latter is called phosphogypsum, which contains more water than natural gypsum, is more acidic and has more impurities, so that costly processing is required. It is also to some extent radioactive and therefore not recommended for use in building.
• Gypsum is used as a building material, mainly as a retarder to regulate the setting of various types of hydraulic cements, and in conjunction with a variety of other materials (eg lime, sand, sawdust, jute, sisal, linseed oil, paper) to produce renders, boards and masonry blocks.
• The main advantages of gypsum are the low energy input during burning to produce gypsum plaster; rapid drying and hardening, with negligible shrinkage (needing no form-work); good adhesion to fibrous and other materials; good fire resistance; good sound reflection (if made dense and hard); superior surface finish; resistance to insects and rodents.
• The principle drawback of gypsum plaster is its solubility in water (2 g gypsum per litre of water). Humid air can also soften gypsum plaster. Frost and sudden temperature changes can also cause damage.
• On account of this drawback, gypsum should not be used on external surfaces in humid climatic zones, unless it is well protected by wide overhanging roofs and a water resisting coating (eg hot linseed oil).
• Thermoplastic materials require heat in order to be processed, and harden on cooling. Their properties remain unchanged on reheating and cooling, so that they can be reclaimed and reprocessed numerous times.
• Probably the only thermoplastic mineral binder used for building is sulphur. For details, see chapter on Sulphur.
• Bitumens are mechanical mixtures of different hydrocarbons (compounds of carbon and hydrogen) and a few other substances, and is obtained as a residue in the distillation of crude oil, either in petroleum refineries or in nature (in pores of rocks or in the form of lakes, close to petroleum deposits). Bitumens are generally dark black, oily, fluorescent thermoplastic substances, which are highly viscous to almost solid at normal temperatures. Compounds consisting of at least 40 % of heavy hydrocarbons are called bitumens.
• Asphalts are defined as mixtures containing bitumen and a substantial proportion of inert mineral matter (sand, gravel, etc.). In the USA, bitumen is called asphalt, thus causing some confusion.
• Tar is the thick black substance produced by the destructive distillation (or carbonisation) of organic matter, such as wood or coal.
• Pitch is the residue after distilling tar from coal.
• Bitumen is not affected by either light, air or water individually, but in combination they can make it brittle, porous and susceptible to oxidation, forming blisters and cracks. It becomes soft at temperatures between 30° and 100° C (no sharp melting point),and therefore must be protected from exposure to heat. It is insoluble in water and fairly resistant to most acids. Although bitumen is combustible, composite products, such as mastic asphalt, are not readily ignited. Bitumen and coal tar products may be poisonous, hence contact with drinking water should be avoided.
• Bituminous products can be used as waterproofing materials (in soil stabilization, as paints, damp-proof membranes, roofing felt, joint fillers, etc.), as paving materials (roads and floors) and as adhesives (for wood block flooring, insulating linings and felts).
• When bitumen is used, it must be either heated; or mixed with solvents (eg gasoline, kerosene or naphta), which is called "bitumen cutback"; or dispersed in water, which is called "bitumen emulsion".
• A variety of binders are obtained from plants and animals, and can be used in their natural form or after processing.
• Examples of natural binders are plant juices (eg juice of banana leaves; latex of certain trees; sisal juice; coconut, cotton and linseed oils), animal excrete (eg cow dung; horse urine) and other animal products (eg bull's blood; animal glues from horn, bone, hooves and hide; casein or whey, made from milk).
• Natural birders have played an important role in traditional constructions since prehistoric times, but nowadays face low social acceptance. However, research today is giving such materials increasing importance, especially with a view to Cost effectiveness and environmental acceptability.
• These binders are generally produced by industrial processes and, therefore, often expensive. Some synthetic binders are toxic.
• They can either be used as admixtures, as adhesives or as surface coatings and are either applied hot, or as an emulsion, or with a solvent.
• Synthetic admixtures which bond loose particles together are mainly resins derived from plant materials or mineral oil. The variety of commercial products is very large and their use depends on the required performance (strength development, waterproofing, elasticity etc.).
• Adhesives are used to stick larger particles, components, membranes, sheets, boards, tiles, etc. on another surface. Some adhesives are designed specifically for one job, whilst others can be used for a number of applications. Adhesives can have one or two components. Some adhesives are thermoplastic and retain their properties when reheated and cooled.
• Surface coatings can be used as a protective film, as a decoration or even to achieve a surface bonding. Here again the variety of products is too large to be dealt with here.
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