The earliest builders tried to find compounds that would hold individual bricks or stones together in a structure so that the loading on them was suitably spread. Such a material is known as a mortar and usually consists of an inert substance such as sand, a binder (i.e., cement), and water, which renders the mass plastic. Cements currently used in construction are hydraulic, that is, they react with water to form a rigid structure, or set, and by continued chemical reaction, this structure hardens and develops strength even when used under water. If a large aggregate of crushed rock or gravel is incorporated, concrete is produced, and this concrete can completely replace stone in large structures. Since concrete is cast as a plastic mass, the opportunities for using it in design and construction are considerable at acceptable cost.
Early cements
The Egyptians used a mortar based on gypsum plaster—obtained by heating gypsum, a calcium sulfate (CaSO4·2H2O)—in building the pyramids, but plaster can be used only in dry conditions. Calcium hydroxide, Ca(OH)2, also known as slaked lime, was used by the Greeks and Romans as a mortar in many of their buildings and made by heating limestone, which is mostly calcium carbonate (CaCO3), to form lime (calcium oxide, CaO). The lime is then slaked, that is, reacted with water. Unfortunately, mortar made with slaked lime tends to crack and crumble when exposed to weather, but both the Greeks and the Romans developed a much more satisfactory hydraulic cement based on lime and a reactive volcanic ash, the latter found in particular near the Italian town of Pozzuoli. Ash, which slowly reacts with lime to set and harden when water is added, is described as pozzolanic. The Romans also used concrete, the Colosseum being an early example of its use. Similar cements, using trass, a material similar to pozzolana found in northern Europe, remained in use until the late 18th century, although quality deteriorated after the collapse of the Roman Empire because the importance of careful formulation and dense compaction were not appreciated. John Smeaton, an English civil engineer who was commissioned to rebuild the Eddystone lighthouse off the coast of Cornwall, Britain, experimented with hydraulic lime made by heating limestone and clay to eliminate water and carbon dioxide and found a product that was superior to pozzolanic cement for underwater use.
In 1824, Joseph Aspdin, an English stone-mason took out a patent in Britain on a process for making Portland cement, so-called because of the similarity in color between the set cement and Portland stone. Aspdin’s process used a higher temperature than had been used before and produced a cement that had a much improved strength over previous materials.
Production
As a manufactured material, cement is relatively inexpensive. The raw materials needed are chalk or limestone, the source of lime, and clay or shale, the source of silica (SiO2) and of aluminum and iron oxides (Al2O3 and Fe2O3). Silica is generally present both in the complex clay mineral structure and as quartz. Marl, which is a natural mix of chalk and clay, is often used, but adjustment of the composition by blending is necessary. Small amounts of "imported" raw materials such as sand and iron oxide are sometimes necessary to adjust the composition.
The composition of the input into the kiln is carefully controlled to ensure an end product with suitable properties. The raw materials are reduced in particle size by crushing and grinding and thoroughly mixed. When they have a high moisture content (often in excess of 20 percent in southeast Britain), they are blended as a slurry (30 to 35 percent water), and in the wet process, the slurry is fed directly into an inclined rotary kiln. Heat is usually supplied by burning powdered coal that is blown into the kiln through the firing pipe. Natural gas and oil are less frequently used owing to cost.
In the first section of the kiln, the water is evaporated by heat supplied by the combustion gases. In the middle section, the clay minerals and the calcium carbonate are decomposed as the material temperature rises to 1650°F (900°C). Finally, close to the flame at temperatures of 2550 to 2730°F (1400–1500°C), the lime combines with the other oxides as about a quarter of the materials melts to form the clinker (lumps of cement). A wet process kiln may be up to 600 ft. (180 m) long by 20 ft. (6 m) in diameter, these dimensions being determined by the throughput required and the amount of heat to be transferred for evaporation of water, calcining, and fusion.
Because the wet process involves the evaporation of a large quantity of water, fuel consumption is high—about 0.2 tons standard coal per ton (0.18 tonnes/tonne) of clinker—and the process is now largely obsolete. In the semiwet process, the slurry is filtered under pressure after removal of the flint if present, and the resulting cake is fed to a shorter kiln to make the clinker.
Where drier hard materials such as limestone and shale are employed, they are crushed, dried, placed into a mill, and ground together. After further blending, the powdered mix is transferred to a short kiln through a system of heat exchangers in which cyclones move the powder against the hot gas stream and capture dust blown back out of the kiln. At its best, fuel consumption approaches 0.1 ton standard coal per ton (0.09 tonnes/tonne) of clinker.
Cement clinker is most often in the form of spherical lumps (nodules) produced as a result of the consolidating action of the melt and rolling in the kiln. It is cooled by passing air through it to provide preheated combustion air. To produce Portland cement, clinker is ground with about 5 percent natural gypsum in a cylindrical ball mill. The energy consumed is considerable. A modern mill will draw more than 4,000 kW and total energy usage is of the order of 30 kWh per ton for ordinary Portland cement fineness.
As the manufacture of Portland cement involves much movement of fine powders, it is necessary to prevent dust emission at all stages. Dust carried out of the kiln system by fast-moving combustion gases (chiefly carbon dioxide and nitrogen) is usually removed from the gas stream by electrostatic precipitators. Particles of dust are charged as they pass through a negative corona discharge produced by an electric potential of 40 to 70 kilovolts (mean), and they are collected when they migrate to the grounded electrodes. Bag filters, which function like giant vacuum cleaners, are favored in the United States for cleaning stack gases, and smaller units are widely used to clean air wherever handling creates airborne dust.
