Cements and limes

Cement is a collective name for mineral binders in powder form, which set to become solid when mixed with water. Pure lime binders are not usually considered cements. The main difference is that lime solidifies when it reacts chemically with air, while cement reacts with water in a hydrating process. It is a hydraulic binder that can also be used under water.

The cement most usually used in building today is Portland cement, but there are many other cements that have been used throughout the ages. The high energy consumption and emissions of carbon dioxide associated with Portland and similar cements based on lime have recently led to experiments with alternatives. In geopolymeric cement, the Portland cement is completely replaced by sodium oxide Na2O.

Cements in construction have three basic functions: as plaster, mortar and concrete. The consistency depends on the number and size of the constituents, cement, sand, gravel and stone, the proportion of water, and quite often various additives; for example, antifreeze liquids and plasticizers.

HISTORY

The use of calcined lime and gypsum-based materials goes back to approximately 10 000 BCin Mesolithic Palestine (Wright, 2005). Excavation of Neolithic dwellings in Jericho dated back to 8000 BC has revealed an extensive use of concrete as a floor material. This concrete is almost completely made of lime, used as both cast material and filler. The technical quality can be compared with modern concrete as regards its absorption of water and compressive strength, and it is so widespread that there must have been a relatively well-developed production technique using high-temperature kilns (Malinowski efa/.,1987).

In Egypt there are solid structures that are 5000 years old and have gypsum as the main constituent in the mortar, whilst Greece used lime mortar. In Mychae on the Greek mainland, exposed lime mortar 3000 years old is still intact.The mortar was made the 'modern'way by mixing burnt and slaked lime with sand in the proportions 1:1 or 1:2.

The Romans mixed finely ground volcanic stone with their lime mortar 2000 years ago.They thereby produced a hydraulic mortar which could withstand both saltwater and freshwater.The volcanic stone was fetched from Pozzuoli, and named pozzo/ana. The Romans laterdiscovered other mineral substances that could be used as ingredients, suchas groundbricks andpottery.

The introduction of different pozzolanas revolutionized the building of walls and stronger arches and vaults.The Pantheon in Rome has a cassette vault cast in pozzo-lana cement. These cements were also used to make baths, water pipes and aqueducts watertight, and as a jointing material between roof tiles.

During the 'Dark Ages' after the fall of the Roman Empire, pozzolana techniques seem to have been forgotten. With very few exceptions, such as the Sophiysky Cathedral in Kiev (1000-1100), builders returned to slaked lime. Certain regions managed with clay; for example, the stone churches of Greenland (1100-1400), but this was rather disappointing for future archaeologists: when the roofs had disintegrated, the rain washed the clay away, leaving only a pile of stones.

During this period, additivestothe lime such as oxblood and casein provided water-tightness and more elastic mortars.The pozzolana mixture turned upagain in England during the sixteenth century. Around 1800 James Parker from Northfleet made 'Roman cement' - a somewhat misguiding nomenclature - by firing crushed argillaceous limestone, which contains small amounts of fossil meal and is found along the banks of theThames.

In 1824, an Englishman, Aspedin, patented what he called Portland cement, because it resembled rock quarried on the Portland peninsula in the south of England. Within 20 years it had developed into the mixture still in use today. Many more cements similar to Portland cement have been developed sincethen, in which Portland cement is often an important ingredient. These cements have different expanding, elastic or quick-drying properties.

In northern Europe there are approximately 35 different types of cement on the market. In the industrial countries its use is of the order of 1.7 m3/year/per person; in countries with low and middle industrialization it is approximately 0.3 m3.

Apart from problems associated with centralized industry, such as vulnerability to market forces and distance from the user, the cement industry also has high transport costs because of its weight, and extra care is required because of cement's sensitivity to moisture.

An alternative is a cement industry based on medium or small-sized units. Establishing such units takes little time, and investment is small enough to be covered locally. This has been successfully demonstrated in countries like India. These smaller plants can be placed where the cement is to be used and the raw materials extracted. The technology is relatively straightforward and can be adequately served by local workshops and services.

Hydraulic binders

Hydraulic binders include lime pozzolana cements, hydraulic lime, Portland cement, Portland pozzolana cements and mixtures of lime and Portland cement.

A hydraulic binder can harden with humidity even under water, but it must contain an acid substance. The most suitable are silicon dioxide and aluminium silicates, which are plentiful in clay. Argillaceous ingredients such as pulverized brick as well as silicium rich substances such as fossil meal and volcanic earth can be used, as well as waste ash from silica plants (see page 178). The hardening reaction is:

2(2CaO x SiO2) + 4H2O = 3CaO x 2SiO2 x 3H2O + Ca(OH)2 (3)

One might think that quartz sand, which is almost pure silicon dioxide SiO2, would be usable. However, quartz sand cannot, in principle, form silicic acid under normal pressure and temperature conditions. It can in a damp, warm atmosphere and under pressure - a method used in the manufacture of lime sandstone. In many of the European castles of the Middle Ages a mixture of lime and quartz sand was used as a cold mix; we must assume that the silicic acid has been released from the sand over time, thus forming a durable binder, as these buildings are still solid today.

Pozzolana cements are low energy materials, because pozzolana production requires only moderate heat. For the same reason there is very little gaseous pollution during production. Heavy metals such as nickel and thallium need a much higher temperature before vaporizing. Pozzolana cements can also be produced more economically than Portland cement, but they are often weaker. A ton of Portland cement is equivalent to 1.7 tons of lime pozzolana cement.

The following hydraulic binders are of current interest.

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