Composition Of Cement Clinker

Composition Of Cement Clinker

The various constituents combine in burning and form cement clinker. The compounds formed in the burning process have the properties of setting and hardening in the presence of water. They are known as Bogue compounds after the name of Bogue who identified them. Le-Chatelier and Tornebohm have referred these compounds as Alite (C₃S), Belite (C₂S), Celite (C₃A) and Felite (C₄AF). The following Bogue compounds are formed during clinkering process.

The properties of Portland cement varies markedly with the proportions of the above four compounds, reflecting substantial difference between their individual behaviour.

Tricalcium Silicate is supposed to be the best cementing material and is well burnt cement. It is about 25-50% (normally about 40 per cent) of cement. It renders the clinker easier to grind, increases resistance to freezing and thawing, hydrates rapidly generating high heat and develops an early hardness and strength. However, raising of C₃S content beyond the specified limits increases the heat of hydration and solubility of cement in water. The hydrolysis of C₃S is mainly responsible for 7 day strength and hardness. The rate of hydrolysis of C₃S and the character of gel developed are the main causes of the hardness and early strength of cement paste. The heat of hydration is 500 J/g.

Dicalcium Silicate is about 25-40% (normally about 32 per cent) of cement. It hydrates and hardens slowly and takes long time to add to the strength (after a year or more). It imparts resistance to chemical attack. Raising of C₂S content renders clinker harder to grind, reduces early strength, decreases resistance to freezing and thawing at early ages and decreases heat of hydration. The hydrolysis of C₂S proceeds slowly. At early ages, less than a month, C₂S has little influence on strength and hardness. While after one year, its contribution to the strength and hardness is proportionately almost equal to C₃S. The heat of hydration is 260 J/g.

Tricalcium Aluminate is about 5-11% (normally about 10.5 per cent) of cement. It rapidly reacts with water and is responsible for flash set of finely grounded clinker. The rapidity of action is regulated by the addition of 2-3% of gypsum at the time of grinding cement. Tricalcium aluminate is responsible for the initial set, high heat of hydration and has greater tendency to volume changes causing cracking. Raising the C₃A content reduces the setting time, weakens resistance to sulphate attack and lowers the ultimate strength, heat of hydration and contraction during air hardening. The heat of hydration of 865 J/g.

Tetracalcium Alumino Ferrite is about 8-14% (normally about 9 per cent) of cement. It is responsible for flash set but generates less heat. It has poorest cementing value. Raising the C₄AF content reduces the strength slightly. The heat of hydration is 420 J/g.

Calculation of Compound Composition of Portland Cement: Bogue developed a method for calculating the compound composition from the oxide analysis of a cement. This method is based upon cooling of the clinker at such rate that equilibrium is maintained. Although equilibrium does not usually obtain in commercial operations, valuable information can be derived from such calculations. The method is summarized in the following steps and in Table 5. An accurate chemical analysis is entered in the first column of the table as shown.

Table 5 Record of Significant Data for Computing Compound Composition

Since the ratio of the atomic weight of CaO : SO₃ = 56.07 : 80.065 = 0.70:1, each percentage of SO₃ combines with 0.70 per cent of Cao to form 1.70 per cent of CaSO₄. Hence, the percentage of lime required to satisfy SO₃ (= 0.7 × per cent SO ₃) is recorded as c₁ in column 4 of the table, opposite CaO; the percentage of SO₃ is also entered in column 4; and the CaSO₄ content is summed and entered at the bottom of column 4.

Similarly, since the atomic ratios Al₂O₃ : Fe₂O₃ = 101.92 : 159.68 = 0.64 : 1 and 4CaO : Fe₂O₃ = 224.28 : 159.68 = 1.40 : 1, it is evident that each percentage of Fe₂O₃ enters into combination with 0.64 per cent of Al₂O₃ and 1.40 per cent of CaO to form 3.04 per cent of 4CaO × Al ₂O₃ × Fe ₂O₃. Therefore 0.64 × percentage of Fe ₂O₃ is entered as a₁, opposite Al₂O₃ in column 5, and 1.4 × percentage of Fe ₂O₃ is entered as c₂, opposite CaO in column 5; the percentage of Fe₂O₃ is re-entered in the same column; and the percentage of C₄AF is summed at the bottom.

Since practically none of the magnesia is combined, it is entered as free at the bottom of column 2. The total alumina minus a₁, entered as a₂ in column 6, is available to combine with lime to form C₃A

in the ratio 3CaO : Al₂O₃ = 168.21 : 101.92 is 1.65 : 1. Hence, each percentage of this available alumina × 1.65 is the percentage of CaO required for C ₃A, and it is entered opposite CaO as c₃ in column 6. Summing quantities in column 6 gives percentage of C₃A.

The CaO available to combine with SiO₂ is total CaO minus (free CaO + c₁ + c₂ + c₃); call this difference c. Then the total silica (s) is calculated first to combine with CaO to form C₂S. Since the ratio 2CaO × SiO ₂ : SiO₂ = 172.20 : 60.06 = 2.87:1, each percentage of s × 2.87 is the percentage of C ₂S. This first approximation of C₂S is entered in column 8, opposite SiO₂. By subtracting this value of C₂S from the sum s + c, the amount of CaO (called c₄) available for combination with 2CaO × SiO ₂ to form 3CaO × SiO ₂ is determined. Since the ratio 3CaO × SiO ₂:CaO = 228.27:56.07 = 4.07:1, multiplying c₄ by 4.07 gives the amount of C₃S which is entered at the foot of column 7. By subtracting this value of C₃S from c + s, the true percentage of C₂S is found and entered in column 8.

Should the computed percentage of C₃S be greater than c + s, no C₂S is present. In that case the content of C₃S is found from the ratio 3CaO × SiO ₂ : SiO₂ = 228.27 : 60.06 = 3.8 : 1. Hence, the percentage of C₃S is obtained by multiplying the percentage of SiO₂ by 3.8. This latter value of C₃S, subtracted from c + s, gives the percentage of uncombined lime. This last condition can only be obtained when lime is in excess of the amount required for equilibrium and the free lime has not been deducted.

Since errors in chemical analysis of 0.2 per cent in determinations of lime, alumina, silica, or iron oxide will make errors up to 1.5 per cent in certain compounds, percentages for the compounds should be rounded off to whole numbers. If the ignition loss is high, the analysis should be reduced to a clinker basis prior to compound calculations.

As previously mentioned, Bogue's method of calculation is based on the assumption that the clinker is slowly cooled at such rate that equilibrium is maintained and the crystallization is complete. Lea and Parker have shown that values calculated by the bogue method may be considerably in error if the clinker liquid crystallizes independently of the solids formed, or if cooling is so sudden that no crystallization takes place and glass is formed. For the case of independent crystallization and a clinkering temperature of 1400 o C, they show that Bogue's method is correct for cements with Al₂O₃/Fe₂O₃ ratios between 0.9 and 1.7, but for ratios between 1.7 and 6.1 their corrections to be added are :

C₃S, + (1.8Al₂O₃ - 2.8Fe₂O₃)

C₂S, + (2.1Fe₂O₃ - 1.4Al₂O₃)

C₃A, + (2.5Fe₂O₃ - 1.6Al₂O₃) C₄AF, Nil

Thus for a cement with Al₂O₃ = 7 and Fe₂O₃ = 3 per cent, the correction to C₃S = 4.2, to C₂S = -3.5, and to C₃A = -3.7 per cent.

For very rapid cooling of the clinker, the liquid is formed into glass and they show that no C₃A or C₄AF appear but the amount of glass is + (2.95Al₂O₃ + 2.2Fe₂O₃). For this case their corrections to Bogue's values for C₃S and C₂S are: C₃S, + (1.8Al₂O₃ - 2.8Fe₂O₃); C₂S + (1.9Fe₂O₃

- 2.1Al₂O₃).