Cement Energy and Environment

Table I Broad chemical specifications of cement-grade run-of-mine limestone Constituent Acceptable range Limiting values for manufacture taking into of ordinary consideration portland cement, other types of % cement, scope of beneficiation by blending, % CaO 44-52 Min. 40 MgO (Max.) 3.5 5 process involves use ofsome dopants in the raw mix to change the crystal structure and morphology of the dicalcium ~i l icate phase in the clinker coupled with alterations in burning and cooling regimes. In ordinary portland cement, the tricalcium silicate phase accounts for the early strength (up to 7 - I 0 days), and dicalcium silicate accounts for the late strength (beyond a week or I 0 days). High early-strength cements and also higher grade cements contain higher amounts (above 40- 45%) of tricalcium silicate. Also, dicalcium silicate is formed in the burning process at a lower temperature range - I 000° to 1200°Cwhereas tricalcium silicate is formed at the final stage ofclinkerisation through combination ofdicalcium silicate with lime at temperature ranges 1300° to 1450° following the reaction : Si0 2 +Aip 3 +Fep 3 To satisfy the LSF and Ms of raw mix Mnp 3 < 0.5 < 3.0 Rp(Kp + Nap) < 0.6 < 1.0 2Ca0. Si0 2 + CaO 1300° to 1450° 3Ca0. SiOr Total S and S0 3 < 0.6 < 0.8 PP s < 0.6 < I Cl < 0.05 < 0.1 Formation oftricalcium silicate thus needs additional input of CaO and heat. This implies that, if one can impart the hydraulic properties of tricalcium silicate to dicalcium silicate, cl inker can be manufactured at lower temperature and with lesserCaO content in the raw mi x. Table 2 Marginal-grade limestones potentially su itable for reactive belite cement * Success of this technology will open up a series of down-the– line benefits in cement manufacturing with several possib le and maj or spin off advantages for our cement industry. Table I gives the broad chemical specificationsofcement grade limestone. Table 2 shows the quality range of limestones amenable for manufacture of reactive belite cement. Table 3 shows the chemical and physical properties of clinkers synthesised from marginal grade limestone. It is seen from Tab le 3 that this cement satisfies the requirements ofdifferent grades of cement when designed for different levels of c)s and cls and ground to proper fineness. Constituent CaO LOJ Total Carbonate Raw mix: Table 3 Development of reactive belite cement with lower lime saturation facto r * Properties/ Composition Oxide/ Compound, % I Chemical Si0 2 22.5 Alp 3 11.9 Fep 3 3.5 2.2 CaO 57.9 61.0 LSF 0.73 Phase Composition (Bogue) C 3 S (-) 22.7 C 2 S 81.7 Physical Specific surface, 3606 cm 2 /gCompressive Strength, (MPa) 3 days 12.0 7 days 22.5 28 days 45.5 * Courtesy : ACC-RCD Range,% 15.6 - 16.0 2.5 - 2.6 · 0.8 - 1.1 42.8-43.6 34.5 - 35.0 76.5 - 77.8 97% Lst + 3% Laterite + Dopants Cements 2 3 24 .0 23.7 6.6 4.1 3.5 64.1 0.80 00.87 14.3 48 58.0 31 3680 4570 27.0 34.1 39.5 47.0 60.0 75.6 7