ROLE OF IMPURITIES IN THE FORMATION OF DICALCIUM SILICATE PHASE

Abstract

Dicalcium silicate (C2S) is an important mineral Constituent of ordinary portland cement clinker. The phase occurs in clinker as belite, formed by the incorporation of large number of ions present in cement raw materials and has a general formula Ca8^AgAl2Fe (Na1/2 Kl/2^ (Al2Si43 °180^* C0S phase exists in several polymorphic forms such as a, a' , a» p and y which are formed under specific conditions of heating and cooling and stabilized in presence of certain stabilizing ions. Belite contains principally the p form of dicalcium silicate, which of all the polymorphs has superior hydraulic characteristics. Dicalcium silicate phase is formed at a temperature, of— 1450°C and as such is an energy intensive phase. Alarge number of anions and cations have been reported to significantly affect the formation temperature of dicalcium silicate, the reactivity of the reacting mix and stabilization of various polymorphs of C2S in the clinker. Present investigations have been aimed at studying the effect of cationic dopants such as Li, Na, K, Mg, Sr, Ba, Al, Fe, Cr, Ti and Mn in varying percentages in the forma tion of C2S from 2:1 molar ratio mixtures of CaCo3 and quartz powder. The reaction of CaCo^ and Si02 (2:1) to form C2S may be broken up into two distinct steps. (ii) (a) Decomposition of CaCOg (fa) Reaction of CaO with Si02 to form C2S (a) The decomposition reaction involves dissocia tion of CaC03 into CaO and C02# This is a major endothermic reaction and generally controls the kinetics of formation of C2S, Following investigations have been carried out to determine the energetics of the decomposition reactions; (i) The threshold (Ti), peak (Td) and termination (Tf) temperature of CaC03 decomposition have been recorded for CaC03-Si02 (2:1) system without dopants and in presence of vary ing concentration of the dopant. (ii) Activation energy (EJ, enthalpy (AH) and shape inde,x (S) of the decomposition peaks have been calculated. (b) The formation of C2S involves interaction of lime and silica and stabilization of the phases by sintering. The intermediate phases observed at the DTA peaks after the decomposition of CaC03 have been identified by XRD by studying the samples quenched in liquid nitrogen. The infrared spectra (IR) and scanning electron microscopy (SEM) of the final products obtained by heating the mix to final DTA peak temperature for ^ hr have also been recorded. The cationic dopants used in these investigations have been grouped as under :- (iii) (i) The alkali metal dopants (Li2C03, Na2C03 and K2C03 calculated as per cent oxide) (ii) The alkaline earth metal dopants (MgO, SrOCu and BaCQ calculated as per cent oxide) 3 (iii) Aluminium oxide (Al^-J and Iron oxide (FeJD3) (Iv) Transition metal oxides (Cr^^ Ti02 and Mn02) which are present as minor impurities in cement raw materials. The investigations carried out have been dealt with in six chapters. Chapter I presents a brief resume of the work reported in the literature on the decomposition of CaC03 formation of C2S and the effects of various dopants on these reactions and on the stabilization of various polymorphs of dicalcium silicate phase. Chapter II covers the methodology followed during the course of the investigations and outlines the materials used, method of preparation of the samples, thermal analysis technique, method of calculation of kinetic parameters such as activation energy (E ), Enthalpy of the reaction (4kH) 3 and shape index (S) from the thermal data. The details of XRD, infrared spectroscopy and scanning electron microscopy have also been given. The chapter also includes the proce dure for the estimation of free lime. Chapter III deals with the role of alkali metal carbonates on the kinetics of decomposition of CaCCU in (iv) CaC03-Si02 (2:1) system, formation of dicalcium silicate phase and stabilization of various polymorphs of C2S. All the three dopants lower the temperature of decomposition of CaC03 and of the formation of dicalcium silicate phase. Li2C03 dopant acts as an intensifier and lowers the decomposition temperature of CaC03. The activation energy (E ) and enthalpy (AH) of CaCOo decomposia J tion is minimum with 1.0 per cent dopant and then increases slightly with 5 per cent of the dopant. The reaction is complete at 1350°C with the formation of f5-C2S phase. Li2C03 (5.0 per cent) brings down the final reaction tempe rature to 1290°C with the formation of predominantly p-C2S and small quantity of C3S phases. Sodium and potassium carbonate dopants (0,1 - 0*5 per cent) lead to the forma tion of a mixture of 0 and y-C^S, whereas 1.0 per cent dopants promote the formation of the more desired p-C2S phase. 5,0 per cent dopants give a mixture of p and a'-C2S phases. In addition, small quantities of alkali silicate phases are also formed. Chapter IV deals with the effects of alkaline earth metal dopants (MgO, SrC03 and BaC03). MgO (0.1 - 1.0 per cent) leads to the formation of p and y-C2S phases at 1420-1430°C and 5.0 per cent MgO results in the formation of predominantly p-C2S phase at 1360°C. SrC03 in the con centration range studied, leads to the stabilization of P-C2S without lowering the temperature of its formation. (v) BaC03 (0.1 - 1.0 per cent) promotes the formation of P-C2S phase but 5.0 per cent BaC03 forms a mixture of p and oc'p^S. The free BaO and SrO present in the phases are extracted with hot ethylene glycol leading to a higher values of free lime using the solvent extraction and titra tion procedure. A method is proposed for the estimation of free lime in presence of MgO, SrO and BaO. Role of A1203 and Fe,^ on the reaction kinetics have been discussed in Chapter V, Addition of Al3*" and Fe3*" in CaC03-Si02 (2;1) mixture give the least value of activation energy (Ej and enthalpy (AH) of the reaction with 0.1 per cent concentration of the dopant and the values increase with the increasing percentage of the dopants. Use of Alr03 dopant with different concentrations leads to the formation of p-C2S phase along with other minor phases e.g. CAiCaAlgO^, CgAS (Gehlenite) and C3A(Ca3Al206) . However, the temperature of formation of C2S is not significantly affected by the concentration of the dopant. Fe203 with all the concentfations studied helps the formation of a mixture of 0 and y-C2S phases along with other Iron bearing phases like a-Fe^, Fe304, CaFe305 etc. Both Al3*" and Fe3*", are capable of substituting for Si yielding a compound of chemical composition Ca2 [MxSii..xJ04_x/2 (wnere Mis Al 3+ or Fe3*"). Role of minor constituents (Cr^g, Ti02 and Mn02) in the formation of dicalcium silicate phase have been discussed (vi) in Chapter VI. Cr^-j, Ti02 and Mn02 also play an important role in the early decomposition of CaC03 in CaC03-Si02 (2:1) system. Cr (0.1 and 0.5 per cent) gives varying amounts of p and y-C2S phases at the final DTA exotherms obtained after the decomposition of CaC03 but 1.0 per cent dopant stabilizes the 0-C2S phase. Small quantities of CaCr04 and Cr2Si04 and a'-C2S are also formed alongwith the major j3-C2S phase indicating that both Ca2+ and Si44" in the C2S lattice can be substituted by Cr3*", 4+ 4| Mn and Ti^ both act as mineralizers in lowering the decomposition of CaC03 in CaC03-Si02 (2:1) system. The activation energy (E ) decreases with dopant concentration 3 upto 1.0 per cent and then increases. The final phase formed at all concentrations in p-C2S phase alongwith minor manga nese and Titanium bearing phases. However, the temperature of final product formation is not significantly decreased except with 5.0 per cent concentration of the dopants.Mn02 does not give any free lime in the reaction, showing that it does not substitute calcium but makes phases like MnSi03 and CaMn^y. Free lime values go on decreasing with increasing concentration of Ti02 and ultimately disappears with 5.0 per cent Ti02 dopant. Major conclusions emerging out of these investigations are summarised at the end of Chapter VI. These investiga tions have led to the conclusion that use of cationic dopants in the CaC03-Si02 system (2:1) result in considerable (vii) lowering of the decomposition temperature activation energy (E ) and enthalpy (^H) of the CaC03 decomposition reaction, This has resulted in improving the kinetics of the decompo sition reaction, and lowering the energy required for the formation of C2S phase.