lexical root

The word cement is derived from a Latin word called cement , and it is a material that has the property of sticking materials to each other, and in fact, it is a bonding medium .

Cement in the construction industry

In the construction industry, cement is a substance that is used to bond different materials together, such as stone and sand, sand, brick, etc., and the main ingredients of this cement are lime materials . Lime cements are usually composed of silicate compounds and lime aluminates, which are both found naturally and can be produced in cement factories .

History

Although different peoples and nations have been profiting in some way by using cement in the construction of buildings, but for the first time in 1824, Portland cement was registered in the name of " Joseph Aspedin " , who was an English architect. In terms of the appearance and quality of the concrete produced from the primary cements to the stones of the Portland area in Dorset, England, the cement became known as Portland cement, and until today, it is used for cements made from mixing and heating lime and clay materials and materials containing silica and alumina. and iron oxide and clinker production and finally clinker grinding is obtained and used .

Cement structure

Basically, cement is obtained by grinding raw materials such as stone, lime , alumina and silica, which exist in the form of clay or clay stones, and mixing them in certain proportions and by heating them in rotary kilns up to about 1400 degrees Celsius. At this stage, the materials in the furnace turn into almost black pellets called clinker .
After cooling, the clinker is mixed and ground with some gypsum to adjust its grip, and a gray powder is obtained, which is the same as Portland cement. According to the type and quality of raw materials, cement is produced by two major methods and dry, while there are other methods as well. Of course, nowadays dry method is generally used in the production of cement, except in cases where the raw materials require more methods, because in dry method, less energy is required for production .

Chemical composition of cement

The raw materials used in the production of cement react with each other during baking and create other compounds. Usually, four major compounds are considered as the main components of cement, which are :
three calcium silicates (3 O ₂ = C ₃ S)

• Two calcium silicates (2 CaOSiO ₂ = C ₂ S)

• Three calcium aluminates (3 CaOAl ₂ O ₃ = C ₃ A)

• Four calcium aluminoferrite (4CaOAl ₂ O ₃ Fe ₂ O ₃ )

In short, CaO oxides are denoted by C , SiO ₂ by S , Al ₂ O ₃ by A , and Fe ₂ O ₃ by F. C ₃ S and C ₂ S silicates are the most important cement compounds in creating the strength of hydrated cement paste. In fact, silicates in cement are not completely pure compounds, but also have partial oxides in the form of solid solution . These oxides have significant effects on the arrangement of atoms , crystal form and hydraulic properties of silicates .
 

There are other compounds in cement that are not significant in terms of weight, but have significant effects on the properties of cement, which are mainly : MgO , TiO ₂ , Mn ₂ O ₃ , K ₂ O , NaO ₂ , which are sodium oxides and Potassium is known as alkaline oxides. Experiments have shown that these alkalis have reacted with some aggregates and the result of this reaction has caused the destruction of concrete . Of course, alkalis also have an effect on the strength of concrete .
The presence of three calcium aluminates (C ₃ A) in cement does not play a major role in the resistance of cement, except in the early stages, and it also causes problems against the attacks of sulfates, which lead to calcium sulfoaluminate, but its presence in the production stages, composition Facilitates lime and silica. The amount of C ₄ AF in cement is less compared to the other three compounds and they do not have a great effect on the behavior of cement, but in reaction with gypsum, it forms calcium sulfofrite and its presence accelerates the hydration of silicates .
The actual amount and size of oxides in cement compositions are different. Of course, the insoluble residue, which is mainly obtained from gypsum impurities, is also measured, up to about 1.5% of the weight in cement is allowed. Heat loss, which shows the range of carbonation and hydration of free lime and free magnesium in the vicinity of air, is measured up to about 3 to 4 percent of cement weight .

Cement hydration

The material we are looking for is mortar or cement paste, which becomes an adhesive material by mixing water and cement powder. In fact, cement silicates and aluminates form a hydration product in the vicinity of water, which gradually forms a hard body with the passage of time .

The two main silicate compounds of cement, C ₃ S and C ₂ S, are the main factors of cement hardening, and the hydration action on C ₃ S is faster than C ₂ S.

Heat of hydration

Like any chemical reaction, the hydration of cement compounds is also heat-generating, and the amount of heat that is produced in each gram of hydrated cement as a result of hydration at a certain temperature is called the heat of hydration and can be measured in different ways. The temperature and the temperature at which the hydrogenation process is performed have a significant effect on the rate of heat produced .

For ordinary Portland cements, about half of the total heat is released in three days, about 3.4% of heat is released in about 7 days, and almost 90 percent of the heat is released in 6 months. In fact, the heat of hydration depends on the chemical composition of cement and it is almost equal to the sum of the heats created by each of the pure compounds of cement, if it is hydrated individually .

Each gram of cement releases approximately 120 calories of heat. Because the thermal conductivity of concrete is low, heat can act as a thermal insulator. On the other hand, the heat produced by cement hydration can prevent the freezing of water in newly poured concrete capillary tubes . Therefore, knowing the heat-generating properties of cement can be useful in choosing the right type of cement for a specific purpose .

As mentioned, C ₃ S and C ₂ S play the main role in cement strength, and C ₃ S in the early 4 weeks of age and C ₂ S after that create cement strength. The role of these two compounds in cement strength is almost equal after one year .

Cement tests

In terms of the importance of cement quality in making concrete, manufacturers usually conduct numerous and standardized tests to control the quality of cement, and sometimes consumers request the properties of the produced cement from the factories to be sure, and sometimes they also conduct tests. The physical properties of cement are mainly cement softness, cement setting, cement health and cement strength .

Softness of cement

Since the hydration starts from the surface of the cement particles, the area of ​​all the cement surface is involved in the hydration. Therefore, the rate of hydration depends on the fineness of the cement, and for example, it is softer or finer to obtain faster resistance. But it should be noted that a soft cement is not always economically and technically affordable, because the cost of grinding and the effects of cement being too soft on its other properties, such as the need for more plaster to set the setting, the efficiency of fresh concrete, and other things. should also be considered .
Softness is one of the main properties of cement, which is usually determined in standards with specific surface area (m ² /kg). Common and different methods are used to determine the softness of cement in the world. Iranian National Standard No. 390 determines the softness of cement .

cement

The word "set" is used for the hardening of the cement paste, that is, the change of state from liquid to solid state. Setting occurs due to the hydration of C ₃ S and C ₂ A with increasing temperature of cement paste. The initial seizure is related to the rapid increase in temperature and the final seizure is related to the final temperature. The setting time of cement decreases with increasing temperature, but the experiment has shown that at a temperature of about 30 degrees Celsius, the opposite effect can be observed. At low temperatures, the setting of cement slows down .

Types of manufactured cement

Application of all types of cement

  (PC-type I) Portland cement type 1 (ordinary Portland cement)
is used in cases where no special properties like other types of cement are desired
(PC-type II) Portland cement type 2
for general and special use In cases where moderate heat of hydration is desired
(PC-type III), Portland cement type 3
for use when high resistance is desired in the short term
(PC-type V) Portland cement type 5
when high resistance against If sulfates are desired, white cement
is
used for use on the surface of buildings and when the use of colorless cements with high resistance is required, this cement is used in the production of colored cements
(SR). slag cement) anti-sulfate slag cement
is used when moderate resistance against sulfates or moderate heat of hydration is desired
(PP Cement) Portland-pozzolanic cement is used
in ordinary concrete buildings and more in cases where moderate resistance in Against sulfates and medium hydration heat, it is used
(PKZ Cement) Portland cement - lime , this type of cement can be used in the preparation of mortar and concrete in all cases where Portland cement type 1 is used. It improves the durability of concrete against freezing, melting and freezing solvents and chemical agents (Masonry Cement) Masonry Cement for use when masonry mortar with lower resistances than Portland cement type 1 is needed (Rf Cement 450). Refractory cement 450 with hydroxyl bonding and calcium aluminate phases is used as a refractory material in Al 2 O 3 containing more than 40% of thermal industries (Rf Cement 500) Refractory cement 500 is used as a refractory material with a percentage of CA 2 , CA is used with hydroxyl bonding and Al 2 O 3 phases containing more than 70%. High purity CO.H 2 is used in thermal industries and atmospheres (Rf Cement 550). Refractory cement 550 with hydroxyl bonding and calcium aluminate as the main composition. It has non-flammability and properties of Al 2 O 3 containing more than 80% high thermo-mechanical properties and special fireproof applications such as hydrogen recovery atmospheres. Oil well cements. These cements are used to seal oil wells. Most of these types of cements are slow and resistant to high temperatures and pressures. This cement may be used in digging water and sewage wells. Portland waterproof cements are produced in white and gray colors .














This type of cement reduces the capillary transfer of water under low or no pressure, but does not prevent the transfer of water
vapor
. The production times range from a few minutes to an hour
 

The colored cements
of these cements are mostly decorative and decorative and are used in cement facades and the production of profiled concrete.

" Characteristics of Portland cement "

1- Purpose and scope of application
The purpose of compiling this standard is to determine the physical and chemical characteristics as well as the method of sampling types of Portland cement, which is marked with the symbol "P"

2- Definitions
2-1 Portland cement clinker is a product A compound that is mainly composed of calcium silicates and aluminates and is obtained from the chemical thermal reaction of lime and clay materials in a cement kiln up to a certain
temperature . The appropriate amount of gypsum stone is obtained in the mill. 2-3 Portland cement with the characteristics determined in this standard creates heat in the vicinity of water and due to hydration reactions, and the resulting paste becomes hard and hardens with time . If this cement is measured and mixed with water and suitable aggregates, it will create a mortar or concrete with good performance and flow, which will gain certain resistances and its volumetric stability with the passage of time. It also preserves for long periods of time and has sufficient durability in facing environmental conditions. 2-4 The hardening process of cement in the vicinity of water. Cement in the vicinity of water hardens by performing the hydration reaction. The product of the reaction is mainly hydrated calcium silicates. Compositions Other chemicals such as aluminates are also effective in this process. 2-5 Additives - These are substances that regulate the properties of cement and its setting. Raw crystallized calcium sulfate is a common and virtual additive that is added to the cement production process when clinker is milled. It should be reminded - if special additive materials are used for special conditions, it must be ensured that it does not reduce the desired quality of cement and concrete mortar made from that cement, and does not increase the corrosion of the steel bars used inside that concrete. 3- Types Portland cement : Portland cement consists of five types as described below 3-1 Portland cement type one with the symbol P-1 is used as ordinary Portland cement for general purposes in making mortar or concrete 3-2 Portland cement type two with the symbol P-2 as a modified portland cement, its special use is in the construction of concretes for which medium hydration heat is necessary and the attack of sulfates is moderate . High initial strength is used in situations where high initial strength is desired. 3-4 Portland cement type four P-4
















It is used as Portland cement with low temperature in conditions where low hydration temperature of concrete is desired . 3-5 Portland cement type 5P-5 is used as anti-sulfate Portland cement in conditions where high resistance of concrete against sulfates is desired . 4- Characteristics The chemical and physical characteristics of the five types of Portland cement must comply with the contents of this standard . It is also determined in Table 2






 

The amount of oxides should be reported with an approximation of 0.01%
if the ratio of aluminum oxide to iron oxide is 0.64 or more, the percentage of three calcium silicates, two calcium silicates, three calcium aluminates and four calcium aluminoferrites using the amounts of oxides obtained from the analysis The chemistry of cement and the following expressions are calculated:
three calcium silicate (C3S) = (4.071 * %CaO) - (600 * 7.7% SiO2)
(6.718 * %Al2O3) - (1.430 * %Fe2O3) -
(2/ 852 * %SO3)
two calcium silicate (C2S)=(2.867 * SiO2)-(0.7544 *%C3S)
three calcium aluminate (C3A)=(2.650 *%Al2O3)-(1.692 * %Fe2O3)
four calcium aluminoferrite (C4AF) = 3.043 *%Fe2O3
If the percentage ratio of aluminum oxide to iron oxide is less than 0.64, a solid solution of calcium and aluminoferrite is made, whose formula is (C4AF + C2F) and In such cements, three calcium aluminates will not be formed. To determine three calcium silicates and (C4AF + C2F), the following formula is used, but two calcium silicates are determined according to the previous formula
(C4AF + C2F) = (10.2 * %Al2O3). ) + (1/702 * %Fe2O3)
(C3S) = (4/071 * %CaO) - (7/600 * %SiO2) -
(4/479 * %Al2O3) - (2/859 * %Fe2O3) -
(2.852% SO3)
C3A are used to calculate the values
​​obtained by approximating one hundredth percent, Al2O3 and Fe2O3 values , but in the calculation of other compounds , oxide values ​​obtained by approximating 0.1% are used. All the values ​​calculated on this basis should be reported with an approximation of one percent. 2-4 Physical properties The mandatory physical properties of the five types of Portland cement must be in accordance with the contents of table (3 ) and their optional properties are also determined in the table.

 

5- Sampling Sampling
of Portland cement must be done by one of the following methods
5-1 Sampling from the location of the conveyor belt or cement transfer pipe to the silo
In this sampling, the weight of the sample for every 40 tons of cement (or part of It is being transferred to a 5 kg silo. This sample can be taken continuously or
discontinuously
. For every one hundred tons of cement inside the silo, 5 kilograms of cement is taken as
a sample
. It can be prepared with a special sampling tool
5-4 Sampling from the cement bag warehouse
In this sampling, one cement bag is selected for every five tons or a part of it, and the necessary amount for the sample is prepared by a special sampling tool.
5-5 Sampling of truck cargo and other matters not mentioned
in this sampling, the cargo is taken from three different points, and if it is in several trucks, provided that the cargo is loaded from a specific silo and in one day, the samples are taken
5-6 each sample prepared by one of the above methods should be divided into three parts, one part should be sent
to the laboratory of the standard institute and one part should be sent to the factory laboratory for testing. Third, as a witness sample, it should be varnished and sealed in a tight and moisture-resistant packaging and kept in a trusted place.

6- Packaging and marking
6-1 The packaging
of Portland cement should be in suitable, resistant and easy-to-use bags. should be loaded flexibly, so that moisture and foreign materials do not penetrate into it and it does not tear during transportation. If paper bags are used, the specifications of the envelopes should be in accordance with the national standard of Iran, No. 4543.
2-6 Marking
on each The contents of the bag of portland cement should have the following items written clearly and in black color
6-2-1 The mark "P-1" for portland cement type one should be written on each bag with a thick line above the marking area
( at the same time, the resistance The cement should also be listed, for example (525-1 ) (425-1) (325-1
6-2-2) "P-2" sign for Portland cement type two should be written in a thick line above the marking on each bag.
6-2-3 mark "P-3 " for portland cement type three should be written with a thick line above the mark on each bag.
6-2-4 mark "P-4" for portland cement type four should be written with a thick line And above the marking place on each bag, write
6-2-5The sign "P-5" for Portland cement type five must be written in bold and above the marking on
each bag
. It
6-2-8 Net weight of Portland cement inside the bag in kilograms
6-2-9 The production date of Portland cement must be clearly written on each bag
6-3 All bags must be completely healthy during inspection and delivery to the consumer
6 - 4 - In cases where portland cement is delivered in bulk, the above information must be reflected in the bill of lading along with the date of delivery, order number and also the details of the recipient of the goods and sent along with the shipment.
7 - Quality acceptance conditions
7-1 If the characteristics If the tested samples of each shipment of Portland cement do not comply with any of the criteria of this standard, that shipment will be rejected.
7-2 If any shipment of Portland cement has remained in the silo of the factory for more than six months before being transported in bulk, or after If the package (bag) has more than three months left in the possession of the seller, it must be tested and conform to the standard before consumption.
7-3 The nominal weight of each bag of Portland cement is 50 kilograms (1). (2) If the weight of each bag is less than 49 kilograms, it cannot be accepted by the buyer, and if the average weight of each bag out of 50 randomly selected and weighed from a shipment is less than 50 kilograms, the entire shipment is rejected and It is not acceptable
8- Storage
8-1 The storage of portland cement in bags or in bulk in silos must be in accordance with Iran's National Standard No. 2761 so that the cargo can be distinguished from each other, inspection and sampling from each one as easily as possible. Acceptable
8-2 The storage place of Portland cement should be completely dry and moisture should not penetrate inside it

9- Confirmation of quality
according to the request of the buyer, if it is included in the purchase contract of Portland cement, the seller must provide a confirmation of the complete compliance of each shipment.

References _ _

Iran cement site:

www.irancement.com

Roshdh Encyclopedia website:

www.daneshnameh.roshd.ir