Chemical Composition of Cement

Chemical Composition of Cement
The raw materials used for the manufacture of cement consist mainly of lime, silica, alumina
and iron oxide. These oxides interact with one another in the kiln at high temperature to form
more complex compounds. The relative proportions of these oxide compositions are responsible
for influencing the various properties of cement; in addition to rate of cooling and fineness of
grinding
The identification of the major compounds of cement is largely based on Bogue’s equations
and hence it is called “Bogue’s Compounds”
It is to be noted that for simplicity’s sake abbreviated notations are used. C stands for CaO, S
stands for SiO2, A for Al2O3, F for Fe2O3 and H for H2O. The equations suggested by Bogue for
calculating the percentages of major compounds are given below.
C3S = 4.07 (CaO) – 7.60 (SiO2) – 6.72 (Al2O3) – 1.43 (Fe2O3) – 2.85 (SO3)
C2S = 2.87 (SiO2) – 0.754 (3CaO.SiO2)
C3A = 2.65 (Al2O3) – 1.69 (Fe2O3)
C4AF= 3.04 (Fe2O3)
In addition to the four major compounds, there are many minor compounds formed in the kiln.
Two of the minor oxides namely K2O and Na2O referred to as alkalis in cement are of some
importance and Expressed in terms of Na2O. These alkalis basically react with active silica in
aggregate and produce what is called alkali-silica gel of unlimited swelling type under
favourable conditions of moisture and temperature in voids and cracks and further it causes
disruption and pattern cracking.
Tricalcium silicate and dicalcium silicate are the most important compounds responsible for
strength. Together they constitute 70 to 80 per cent of cement. The average C3S content in
modern cement is about 45 per cent and that of C2S is about 25 per cent.
The calculated quantity of the compounds in cement varies greatly even for a relatively small
change in the oxide composition of the raw materials. To manufacture a cement of stipulated
compound composition, it becomes absolutely necessary to closely control the oxide
composition of the raw materials.
SO3 also appear in cement analysis which comes from adding gypsum (4-6) % during clinker
gridding. The Iraqi and British specification for normal high rapid Portland cement pointed that
SO3 content must be between ( 3-2.5 )% according to type of cement and C3A content.
The percentage of MgO in cement which is come from Magnesia compounds in raw material.
is about ( 4-1)% and 5% as maximum range to control expansion from hydration of this oxide
in hard concrete.
An increase in lime CaO content beyond a certain value makes it difficult to combine with
other compounds and free lime will exist in the clinker which causes unsoundness in cement.
Insoluble residue is that part of the Cement non-soluble in hydrochloric acid HCl and arise
mainly from non active silica to form cement compounds dissolved in this acid therefore it
express the completeness of the chemical reactions inside the rotary kiln.