Cement

Pila of cement bags in Tunisia.

Cement is a binder formed from a mixture of calcined and subsequently ground limestone and clay, which has the property of hardening after coming into contact with water. The product resulting from the grinding of these rocks is called clinker and becomes cement when a small amount of plaster is added to prevent the contraction of the mixture when it sets when water is added and hardens later. Mixed with stone aggregates (gravel and sand) and water, it creates a uniform, malleable and plastic mixture that sets and hardens, acquiring a stone consistency, called concrete or concrete. Its use is widespread in construction and civil engineering.

Cement is usually sold in sacks or bags, which, depending on the regulations of each country, have a specific weight and volume. In general, a bag of cement has a weight of 42.5 kg and a volume of 1 cubic foot. This is also why, in construction, "bag" or "bag" as a unit volume of cement.

History

Since ancient times, pastes and mortars made with clay or clay, plaster and lime have been used to join masonry in buildings. Cement began to be used in Ancient Greece using volcanic tuffs extracted from the island of Santorini, the first natural cements.

In the 1st century B.C. C. began to be used in Ancient Rome, a natural cement, which has resisted immersion in seawater for millennia, while Portland cements do not last more than 60 years in these conditions; Volcanic ash obtained in Pozzuoli, near Vesuvius, was part of its composition. The vault of the Pantheon is an example of this.

In the 18th century, John Smeaton built the foundation for a lighthouse on the Eddystone cliff on the Cornish coast, using calcined lime mortar. In the 19th century, Joseph Aspdin and James Parker patented Portland cement in 1824, named for its dark greenish-gray color similar to Portland stone. Isaac Johnson, in 1845, obtains the prototype of modern cement, with a mixture of limestone and clay calcined at high temperature. In the 20th century, the rise and generalization of the cement industry arises, due to the experiments of the French chemists Vicat and Le Chatelier and the German Michaélis, who achieve homogeneous quality cement; the invention of the rotary kiln for calcination and the tube mill and the methods of transporting fresh concrete were devised by Juergen Heinrich Magens, who patented them between 1903 and 1907.

Types of cement

Cemento being mixed.

There are two basic types of cement:

1. of clay origin: obtained from clay and limestone in proportion 1 to 4 approximately.
2. Pozolanic origin: the puzolana of the cement may be of organic or volcanic origin.

From a chemical point of view, it is generally a mixture of calcium silicates and aluminates, obtained by firing calcareous, clay and sand. The material obtained, very finely ground, once it is mixed with water, progressively hydrates and solidifies. Since the chemical composition of cements is complex, specific terminologies are used to define the compositions.

Portland cement

It is the type of cement most used as a binder for the preparation of concrete, a product that is obtained by pulverizing Portland clinker with the addition of one or more forms of gypsum (calcium sulfate). The addition of other products is allowed as long as their inclusion does not affect the properties of the resulting cement. All additional products must be sprayed together with the clinker. When Portland cement is mixed with water, a product with plastic characteristics with adherent properties is obtained that solidifies in a few hours and progressively hardens over a period of several weeks until it acquires its characteristic resistance. The solidification process is due to a chemical process called mineral hydration.

With the addition of particular materials to the cement (calcareous or lime) plastic cement is obtained, which sets more quickly and is more easily workable. This material is used in particular for the external cladding of buildings.

Regulations

The quality of portland cement must be in accordance with the ASTM C 150 standard. In Europe it must be in accordance with the EN 197-1 standard. In Spain, cements are regulated by the Instruction for reception of cements RC-08, approved by Royal Decree 956/2008 of June 6.

Special portland cements

The special portland cements are the cements that are obtained in the same way as the portland, but that have different characteristics due to variations in the percentage of the components that form it.

Ferric Portland

Image at the microscope of the ferric portland cement.

Ferrous portland is characterized by a flux modulus of 0.64. This means that this cement is very rich in iron. In effect, it is obtained by introducing pyrite ashes or powdered iron minerals. This type of composition therefore involves, in addition to a greater presence of Fe₂O₃(ferric oxide), a lower presence of 3CaOAl₂ >O₃ whose hydration develops the most heat. For this reason these cements are particularly suitable for use in hot climates. The best ferrous cements are those with a low calcareous modulus, in fact they contain a smaller amount of 3CaOSiO₂, whose hydration produces the greatest amount of free lime (Ca(OH)₂). Since free lime is the component most attackable by aggressive water, these cements, containing a smaller amount, are more resistant to aggressive water than plastic.

White cements

Unlike ferrous cements, white cements have a very high flux modulus, approximately 10. They therefore contain a very low percentage of Fe₂O₃. The white color is due to the lack of iron that gives a grayish hue to normal Portland and a darker gray to iron cement. The reduction of Fe₂O₃ is compensated with the addition of fluorite (CaF₂) and cryolite (Na₃AlF₆), necessary in the manufacturing phase in the kiln to lower the quality of the type of cement that today there are 4: which are type I 52.5, type II 52.5, type II 42.5 and type II 32.5; Also called pavi) an extra amount of limestone called clinkerite is usually added to lower the type, since normally the clinker ground with gypsum would be type I

Mixed cements

The mixture cements are obtained by adding other components such as pozzolana to normal Portland cement. The addition of these components gives these cements new characteristics that differentiate it from normal Portland.

Pozzolanic cement

pozzolana is called a fine volcanic ash that extends mainly in the region of Lazio and Campania, its name derives from the town of Pozzuoli, near Naples, on the slopes of the Vesuvius. It has subsequently been generalized to volcanic ash elsewhere. Vitruvius already described four types of pozzolan: black, white, gray and red.

Mixed with lime (in a ratio of 2 to 1) it behaves like pozzolanic cement, and allows the preparation of a good mix capable of setting even under water;

This property allows for the innovative use of concrete, as the Romans had already understood: The old port of Cosa was built with pozzolana mixed with lime just before use and cast under water, probably using a tube, to deposit it in the bottom without being diluted in seawater. The three piers are still visible, with the submerged part in good condition after 2,100 years.

Pozzolan is an acidic stone, very reactive, being very porous and can be obtained at a low price. A pozzolanic cement contains approximately:

• 55-70 % clinker Portland
• 30-45 % puzolana
• 2-4 % plaster

Since pozzolan is combined with lime (Ca(OH)₂), you will have less of the latter. But precisely because lime is the component that is attacked by aggressive waters, the pozzolanic cement will be more resistant to their attack. On the other hand, since 3CaOAl₂O₃ is only present in the component made up of Portland clinker, the pozzolanic cement pour will develop a lower heat of reaction during setting.. This cement is therefore suitable for use in particularly hot climates or for large-sized castings.

It is mainly used in elements where high impermeability and durability are needed.

Steel and steel cement

Pozzolan has been replaced in many cases by coal ash from thermoelectric power plants, slag from foundries or waste obtained by heating quartz. These components are introduced between 35 to 80%. The percentage of these materials can be particularly high, since it originates from silicates, it is a potentially hydraulic material. This must however be activated in an alkaline environment, that is to say in the presence of OH^- ions. It is for this reason that at least 20% normal Portland cement must be present. For the same reasons as pozzolanic cement, steel cement has poor resistance to aggressive water and develops more heat during setting. Another characteristic of these cements is their high natural alkalinity, which makes them particularly resistant to atmospheric corrosion caused by sulfates.

It has high chemical, acid and sulfate resistance, and a high setting temperature.

Fast-setting cement

The rapid-setting cement, also known as "Roman cement or natural prompt", is characterized by starting to set a few minutes after its preparation with water. It is produced in a similar way to Portland cement, but with the kiln at a lower temperature (1,000 to 1,200 °C). make a good application. Although controlled setting can be initiated using natural retarders (E-330) such as citric acid, it still starts setting after approximately 15 minutes (at 20°C). The advantage is that after approximately 180 minutes after the start of setting, a very high resistance to compression is achieved (between 8 and 10 MPa), so that great performance is obtained for rapid and definitive intervention works. There are fast cements that after 10 years obtain a compressive strength higher than that of some reinforced concretes (greater than 60 MPa).

Aluminous cement

Aluminous cement is produced mainly from bauxite with impurities of iron oxide (Fe₂O₃), titanium (TiO₂) and silicon oxide (SiO₂). Additionally calcium oxide or calcium carbonate is added. Aluminous cement is also called "molten cement", since the temperature in the furnace reaches up to 1600 °C, thus fusion of the components is achieved. The molten cement is poured into molds to form ingots that will be cooled and finally ground to obtain the final product.

Aluminous cement has the following composition of oxides:

• 35-40 % calcium oxide
• 40-50 % aluminum oxide
• 5 % silicon oxide
• 5-10 % iron oxide
• 1 % titanium oxide

Its complete composition is:

• 60-70 % CaOAl₂O₃
• 10-15 % 2CaOSiO₂
• 4CaOAl₂O₃Fe₂O₃
• 2CaOAl₂O₃Yes₂

Regarding silicon oxide, its presence as an impurity must be less than 6%, because the component to which it gives rise, that is, (2CaOAl₂O ₃SiO₂) has poor hydrophilic properties (low water absorption).

Hydration reactions

CaOAl₂O₃+10H₂O → CaOAl₂O₃ 10H₂O (hexagonal crystals)
2(CaOAl₂O₃)+11H₂O → 2CaOAl₂O₃ 8H₂O + Al(OH)₃ (crystals + gel) 2(2CaOSiO₂)+ (x+1)H₂O → 3CaO2SiO₂xH₂ O + Ca(0H)₂ (crystals + gel)

While Portland cement is a cement of a basic nature, thanks to the presence of lime Ca(OH)₂, aluminous cement is substantially neutral in nature. The presence of aluminum hydroxide Al(OH)₃, which in this case behaves like an acid, causing the neutralization of the two components and resulting in a neutral cement.

Aluminous cement should be used in cold climates, with temperatures below 30°C. Indeed, if the temperature were higher, the second hydration reaction would change and the formation of 3CaOAl₂O₃6H₂O (cubic crystals) and increased production of Al(OH)₃, which would lead to an increase in volume and could cause cracking.

General properties of cement

• Good resistance to chemical attack.
• Resistance to high temperatures. Refractory.
• High initial resistance that decreases over time.
• The use of armor must be avoided. Over time increases porosity.
• Appropriate use for low temperatures because it is very ethermal.

The use of aluminous cement in prestressed concrete is prohibited. The useful life of reinforced concrete structures is shorter.

The conversion phenomenon (increase in porosity and drop in resistance) may take time to appear under conditions of low temperature and humidity.

The designer must consider as the design value not the maximum resistance but the residual value, after the conversion, and this will not be greater than 40 N/mm².

A/C ratios ≤ 0.4, high amount of cement and increased coatings (due to lower pH) are recommended.

Physical properties of calcium aluminate cement

• Fragment: Normal 2-3 hours. Similar to Portland cement.
• Enduring: very fast. In 6-7 hours you have 80% of the resistance.
• Volume stability: Non-expansive.
• Hydration heat: very exothermal. It quickly unfolds a lot of heat.
• Very resistant to sulfates and very good durability and resistant to acid compounds
• Good refractory properties, hold on 1500-1600 °C maintaining physical strengths and properties.
• Exposed to high temperature and high humidity conditions (e.g. a coastal zone) undergoes an alteration in its chemical composition:

 3CAH10= 2005C3AH6+2AH3+18H

It loses 18 water molecules and leaves pores when evaporating, consequently it loses all resistance (Goes from a hexagonal crystal to a cubic one)

• Healing must be very careful (for a day)

Applications

Calcium aluminate cement is highly suitable for:

• Solid concrete

• Quick emergency repairs.
• Temporary foundations and benches.

When its use is justifiable, it can be used in:

• Works and prefabricated, mass concrete or non-structural concrete elements.
• Certain cases of mass concrete foundations.
• Projected concrete.

It is not suitable for:

• Structural armed concrete.
• Massive or armed concrete of large volumes.(very exothermal)

It is prohibited for:

• Concrete pretensed in all cases.

Common Uses for Calcium Aluminate Cement

• Sewers.
• Industrial discharge zones.
• Debuggers.
• Sulfate land.
• Marine environments.
• As a mortar of union in refractory buildings.
• Roads.

Manufacturing process

Cementera de Secil, Outão, Portugal.

The cement manufacturing process comprises four main stages:

1. Extraction and grinding of raw materials
2. Homogenization of raw material
3. Production of clinker
4. Cement milling

The raw material for the manufacture of cement (limestone, clay, sand, iron ore and gypsum) is extracted from quarries or mines and, depending on the hardness and location of the material, certain exploitation systems and equipment are applied. Once the raw material is extracted, it is reduced to sizes that can be processed by crude oil mills.

The homogenization stage can be wet or dry, depending on whether air currents or water are used to mix the materials. In the wet process, the raw material mixture is pumped to homogenization ponds and from there to the kilns where the clinker is produced at temperatures above 1500 °C. In the dry process, the raw material is homogenized in raw material yards with the use of special machinery. In this process, chemical control is more efficient and energy consumption is lower, since by not having to eliminate the added water in order to mix the materials, the kilns are shorter and the clinker requires less time subjected to high temperatures. temperatures.

The clinker obtained, regardless of the process used in the homogenization stage, is then ground with small amounts of gypsum to finally obtain cement.

Reaction of cement particles with water

1. Initial period: particles with water are in a state of dissolution, there is an intense initial ethermal reaction. It lasts about ten minutes.
2. Sleeping period: a gelatinous film is produced in the particles, which inhibits the hydration of the material for about an hour.
3. Rigidity start: by continuing the hydration of cement particles, the gelatinous film begins to grow, generating contact points between particles, which together immobilize the cement mass. It's also called a frieze. Therefore, the fraguado would be the increase of the viscosity of a mixture of cement with water.
4. Resistance gain: by continuing the hydration of cement particles, and in the presence of CaOH crystals₂, the gelatinous film (which is saturated at this point) develops tubulent filaments called "fusiform needles", which by increasing in number generate a plot that increases the mechanical resistance between the cement grains already hydrated.
5. Fraguado and hardening: the principle of fraguado is the time of a hard moulded and high viscosity cement paste. Then the pasta hardens and becomes a resistant solid that cannot be deformed. The time in which this state is reached is called the "end of fraguado".

Storage

If it is cement in bags, it should be stored on wooden racks or on a plank floor; It will not be stacked in superimposed rows of more than 14 bags in height for storage of 30 days, nor more than 7 bags in height for storage of up to 2 months. To prevent the cement from aging unduly, after arriving at the work area, the contractor must use it in the same chronological sequence of its arrival. No bag of cement that has been stored for more than two months in the works area will be used, unless new tests show that it is in satisfactory condition.

Cement production in Spain

Cement production in Spain is a good example of the country's economic evolution. From 1973 to the present, the graphs indicate a constant evolution, ending in 2013 after a seven-year production crisis that began in 2007.

Production of cement in Spain between 1973 and 1986 (in thousands of tons)
Source:oficemen
Graphics produced by: Wikipedia

Production of cement in Spain between 1987 and 1999 (in thousands of tons)
Source:oficemen
Graphics produced by: Wikipedia

Production of cement in Spain between 2000 and 2013 (in thousands of tons)
Source:oficemen
Graphics produced by: Wikipedia