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INGREDIENTS OF CEMENT

The main ingredients of cement are lime, silica, alumina and iron oxide. Besides these, most of 

the cement contain a small quantity of magnesia, sulphur trioxide, alkalies and gypsum.

Table: Percentage content of ingredients in cementNo. Ingredient Nomenclature

Percentage

1. Lime CaO 60-67

2. Silica SiO 17-25

3. Alumina AI2O3 3-8

4. Iron Oxide Fe203 0.5-6

5. Magnesia MgO 0.1-4

6. Sulphur Trioxide SO3 1-2.75

7. Alkalies K2O & Na20 0.4-1.3

8. Gypsum CaSO4 3-5

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Major compounds of cement

During clinkering, chemical reactions takes place and the various compounds as shown in table

are formed.

Table: Major compounds of cement

No. Compound Chemical Abbreviated formula Percentage formula

1.Tricalcium Silicate 3CaO.SiO2 C3S 54.1

2.Dicalcium Silicate 2CaO.SiO2 C2S 16.6

3.Tricalcium Aluminate 3CaO.AI2O2 C3A 10.8

4.Tetra calcium Aluminoferrite 4CaO.AI203Fe2O3 C4AF 9.1

Apart from the four major compounds listed in table 6.2, many minor compounds are also

present in cement. Two of the minor compounds, namely K2Oand N2O,referred to as alkalies

influence the properties of cement to a certain extent.

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SETTING OF CEMENT

When water is added to the constituent compounds of cement in the anhydrous state, the

setting and hardening starts taking place. This hydration may be visualised in two ways namely,

'through solution' and 'solid state mechanism'. 'The 'through solution' mechanism

predominates in the early stages of hydration resulting into the formation of paste.

The solid state mechanism continues for a long time converting the compounds into hydrated

products of cement, starting from the surface to the interior part. Since hydration generates lot

of heat, we observe in mass concrete works the interior portion having temperature as high as

50°C above the original temperature of concrete mix.

Different compounds hydrate at different rates as shown in Fig. It may be observed that C3A

and C4AFhydrate almost in one day, and hence they do not contribute to the strength of the

cement. The early heat of hydration and strength are contributed by C3S the next compound to

hydrate, C2S is the slowest compound to hydrate and the process continues for several days. It

contributes to the progressive increase in strength of cement, products. During hydration the

following compounds are formed.

Time (Log Scale) – Days

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C3A+ H2O –> C3A.6H2O = C3AH6

C4AF+ H2O –> CaO – Fe2O3 - H2O = C3FH6

C3S + H2O –> C3S2H3 + 3Ca(OH)2

C2S + H2O –> C3S2H3 + Ca(OH)2

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Compressive strength of cement compounds with age.

Figure shows the compressive strength gained by cement compounds with age.