Definition of Cement: Chemical Composition
Definition of Cement: Chemical Composition
Definition of Cement: Chemical Composition
Cement, in general, adhesive substances of all kinds, but, in a narrower sense, the binding
materials used in building and civil engineering construction. Cements of this kind are finely ground
powders that, when mixed with water, set to a hard mass. Setting and hardening result from hydration,
which is a chemical combination of the cement compounds with water that yields submicroscopic
crystals or a gel-like material with a high surface area. Because of their hydrating properties,
constructional cements, which will even set and harden under water, are often called hydraulic cements.
The most important of these is portland cement.
This article surveys the historical development of cement, its manufacture from raw materials,
its composition and properties, and the testing of those properties. The focus is on portland cement, but
attention also is given to other types, such as slag-containing cement and high-alumina cement.
Construction cements share certain chemical constituents and processing techniques with ceramic
products such as brick and tile, abrasives, and refractories. For detailed description of one of the
principal applications of cement, see the article building construction.
Composition of Cement
Chemical Composition
Portland cement is made up of four main compounds: tricalcium silicate (3CaO SiO2), dicalcium
silicate (2CaO SiO2), tricalcium aluminate (3CaO Al2O3), and a tetra-calcium aluminoferrite (4CaO
Al2O3Fe2O3). In an abbreviated notation differing from the normal atomic symbols, these compounds
are designated as C3S, C2S, C3A, and C4AF, where C stands for calcium oxide (lime), S for silica, A for
alumina, and F for iron oxide. Small amounts of uncombined lime and magnesia also are present, along
with alkalies and minor amounts of other elements. The composition ranges of various kinds of portland
cement are shown in the table
.
Properties of Cement
HYDRATION
The most important hydraulic constituents are the calcium silicates, C2S and C3S. Upon mixing
with water, the calcium silicates react with water molecules to form calcium silicate hydrate (3CaO
2SiO2 3H2O) and calcium hydroxide (Ca[OH]2). These compounds are given the shorthand notations C
SH (represented by the average formula C3S2H3) and CH, and the hydration reaction can be crudely
represented by the following reactions:
2C2S + 4H = C3S2H3 + CH
During the initial stage of hydration, the parent compounds dissolve, and the dissolution of their
chemical bonds generates a significant amount of heat. Then, for reasons that are not fully understood,
hydration comes to a stop. This quiescent, or dormant, period is extremely important in the placement
of concrete. Without a dormant period there would be no cement trucks; pouring would have to be
done immediately upon mixing.
Following the dormant period (which can last several hours), the cement begins to harden, as CH
and CSH are produced. This is the cementitious material that binds cement and concrete together. As
hydration proceeds, water and cement are continuously consumed. Fortunately, the CSH and CH
products occupy almost the same volume as the original cement and water; volume is approximately
conserved, and shrinkage is manageable.
Although the formulas above treat CSH as a specific stoichiometry, with the formula C3S2H3,
it does not at all form an ordered structure of uniform composition. CSH is actually an amorphous gel
with a highly variable stoichiometry. The ratio of C to S, for example, can range from 1:1 to 2:1,
depending on mix design and curing conditions.
* Iron Oxide
* Magnesium Oxide
* Sulphate
* Alkalis
Calcium Oxide: It is the main ingredient of cement. 60% to 65% of the mass of cement is Calcium Oxide.
it is expressed by CaO. In general, we know the calcium Oxide as lime.
Function:
Silicon dioxide: It is also a major ingredient of cement. It is known as silica. It holds 20-25% of cement
mass. It is chemically expressed by SiO2.
Function:
It imparts strength to the cement due to formation of di-calcium silicate (2CaO SiO2 or C2S) and
tri-calcium silicate (3CaO SiO2 or C3S).
Silica in excess provides greater strength to the cement but at the same time it prolongs its
setting time.
Aluminum oxide: It is called alumina. The chemical name of it is AI2O3. Cement contains 4%-8% alumina
of its mass.
Function:
Iron Oxide: It is also called Ferric Oxide. Cement has 2%-4% iron oxide of its mass. Iron Oxides chemical
name is Fe2O3.
Function:
if the content of MgO exceeds 5%, it causes cracks after mortar or concrete hardness
It also provides color and hardness to cement..
Classification of Cement
Natural Cements
Those obtained by calcining and grinding to fine powder lime stone containing 20 to 40% clay. It
is brown in color and sets very quickly when water is added to it. These cements have variable
properties, because the clay content in lime stone in various batches cannot be ensured.
Artificial Cements
Those obtained by calcining and grinding to fine powder controlled quantities of lime and clay mixed
thoroughly in order to ensure a product of homogenous composition and of known properties. Various
types of artificial cements i.e. Ordinary Portland Cement (O.P.C) available are enumerated below:
STRUCTURAL PROPERTIES
The strength developed by portland cement depends on its composition and the fineness to
which it is ground. The C3S is mainly responsible for the strength developed in the first week of
hardening and the C2S for the subsequent increase in strength. The alumina and iron compounds that
are present only in lesser amounts make little direct contribution to strength.
Set cement and concrete can suffer deterioration from attack by some natural or artificial
chemical agents. The alumina compound is the most vulnerable to chemical attack in soils containing
sulfate salts or in seawater, while the iron compound and the two calcium silicates are more resistant.
Calcium hydroxide released during the hydration of the calcium silicates is also vulnerable to attack.
Because cement liberates heat when it hydrates, concrete placed in large masses, as in dams, can cause
the temperature inside the mass to rise as much as 40 C (70 F) above the outside temperature.
Subsequent cooling can be a cause of cracking. The highest heat of hydration is shown by C3A, followed
in descending order by C3S, C4AF, and C2S.
Slag cements
The granulated slag made by the rapid chilling of suitable molten slags from blast furnaces forms
the basis of another group of constructional cements. A mixture of portland cement and granulated slag,
containing up to 65 percent slag, is known in the English-speaking countries as portland blast-furnace
(slag) cement. The German Eisenportlandzement and Hochofenzement contain up to 40 and 85 percent
slag, respectively. Mixtures in other proportions are found in French-speaking countries under such
names as ciment portland de fer, ciment mtallurgique mixte, ciment de haut fourneau, and ciment de
liatier au clinker. Properties of these slag cements are broadly similar to those of portland cement, but
they have a lower lime content and a higher silica and alumina content. Those with the higher slag
content have an increased resistance to chemical attack.
High-alumina cement
High-alumina cement is a rapid-hardening cement made by fusing at 1,500 to 1,600 C (2,730 to
2,910 F) a mixture of bauxite and limestone in a reverberatory or electric furnace or in a rotary kiln. It
also can be made by sintering at about 1,250 C (2,280 F). Suitable bauxites contain 50 to 60 percent
alumina, up to 25 percent iron oxide, not more than 5 percent silica, and 10 to 30 percent water of
hydration. The limestone must contain only small amounts of silica and magnesia. The cement contains
35 to 40 percent lime, 40 to 50 percent alumina, up to 15 percent iron oxides, and preferably not more
than about 6 percent silica. The principal cementing compound is calcium aluminate (CaO Al2O3).
High-alumina cement gains a high proportion of its ultimate strength within 24 hours and has a
high resistance to chemical attack. It also can be used in refractory linings for furnaces. A white form of
the cement, containing minimal proportions of iron oxide and silica, has outstanding refractory
properties.
Gypsum plasters
Gypsum plasters are used for plastering, the manufacture of plaster boards and slabs, and in one
form of floor-surfacing material. These gypsum cements are mainly produced by heating natural gypsum
(calcium sulfate dihydrate, CaSO4 2H2O) and dehydrating it to give calcium sulfate hemihydrate (CaSO4
1/2H2O) or anhydrous (water-free) calcium sulfate. Gypsum and anhydrite obtained as by-products in
chemical manufacture also are used as raw materials.
The hemihydrate, known as plaster of Paris, sets within a few minutes on mixing with water; for
building purposes a retarding agent, normally keratin, a protein, is added. The anhydrous calcium sulfate
plasters are slower-setting, and often another sulfate salt is added in small amounts as an accelerator.
Flooring plaster, originally known by its German title of Estrich Gips, is of the anhydrous type.
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http://theconstructor.org/building/building-material/cement/types-of-cement-composition-uses/5974/
http://acivilengineer.com/2013/04/Cement-Ingredients.html
http://cescientist.com/chemical-composition-of-cement-and-functions-of-ingredients/
http://cescientist.com/classification-of-cement/