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. Table (1) shows the approximate oxide composition limits of ordinary Portland cement.
Table (1) Approximate Oxide Composition Limits of Ordinary Portland Cement
Oxide Per cent content
CaO 60–67
SiO2 17–25
Al2O3 3.0–8.0
Fe2O3 0.5–6.0
MgO 0.1–4.0
Alkalies ( K2O, Na2O) 0.4–1.3
SO3 1.3–3.0
The identification of the major compounds of cement is largely based on Bogue’s equations and hence it is called “Bogue’s Compounds”. The four compounds usually regarded as major compounds are listed in table (2).
Table (2) Major compounds of cement
Name of Compound Formula Abbreviated Formula
Tricalcium silicate 3 CaO.SiO2 C3S
Dicalcium silicate 2 CaO.SiO2 C2S
Tricalcium aluminate 3 CaO.Al2O3 C3A
Tetracalcium aluminoferrite 4 CaO.Al2O3.Fe2O3 C4AF
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.
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