Effect of Kiln Temperature on Clinker Formation
Effect of Kiln Temperature on Clinker Formation
1. Introduction
Temperature is the single most critical process variable governing clinker formation in Portland cement production. The rotary kiln's thermal profile determines the extent and rate of the chemical reactions that transform raw meal into the four principal clinker minerals: alite (C₃S), belite (C₂S), tricalcium aluminate (C₃A), and tetracalcium aluminoferrite (C₄AF). While the nominal burning zone temperature target is approximately 1450°C , the actual influence of temperature extends far beyond this single setpoint. It governs reaction kinetics, liquid phase formation, mineral phase equilibria, crystal morphology, and ultimately the hydraulic reactivity and grindability of the finished clinker.
This investigation examines the relationship between kiln temperature and clinker formation through multiple analytical lenses: the thermodynamics of phase formation, the kinetics of lime assimilation, the role of the liquid phase, and the consequences of temperature deviations in either direction. The analysis draws upon laboratory sintering studies, industrial kiln observations, and mineralogical investigations.
2. Temperature Thresholds for Clinker Phase Formation
2.1 Onset of Alite Formation
The formation of alite, the most important strength-giving mineral in Portland cement, is strongly temperature-dependent. Laboratory studies using X-ray powder diffraction have established that alite begins to appear at temperatures as low as 1200°C when mineralizers such as fluorine ions are present in the raw mix . In the absence of mineralizers, the critical temperature range for abundant alite formation is between 1300°C and 1350°C. Below 1300°C, belite and free lime remain the dominant phases, indicating incomplete reaction .
A study investigating clinker produced with desulfurization slag (DS) additions demonstrated this relationship quantitatively. Clinkers prepared with 10% DS exhibited detectable alite at 1200°C, with most alite formation completed before 1250°C. In contrast, clinkers without DS showed the critical temperature range for alite formation to be between 1300°C and 1350°C, with alite content increasing gradually from 1350°C to 1450°C .
2.2 Free Lime Consumption as a Function of Temperature
The disappearance of free lime (f-CaO) serves as the most direct and reliable indicator of clinker formation completeness. Free lime represents unreacted calcium oxide that has not combined with silica, alumina, and iron oxide to form clinker minerals. High free lime content is associated with poor clinker quality, reduced cement strength, and potential unsoundness.
Experimental sintering studies consistently demonstrate that residual free lime decreases markedly with increasing firing temperature. In a study firing raw meal with mineralizers at 1200°C, 1300°C, and 1400°C for two hours, high amounts of free lime were present in samples fired at 1200°C. Free lime content decreased progressively with temperature, and at 1400°C, complete disappearance of free lime was observed, with insoluble residue below 1.40%, indicating completely formed clinker .
Similar results were reported for low-temperature clinker synthesis using raw materials from Western Mongolia. Increasing sintering temperature from 1100°C to 1300°C markedly reduced residual free CaO, with gypsum addition further promoting CaO assimilation. Nonlinear regression identified an optimum near 1.39 wt.% gypsum at 1300°C, corresponding to a predicted free-lime content of approximately 1.25 wt.% .
A comparative study of clinkers synthesized from ordinary raw mixture and fly ash raw mixture at 1250°C, 1300°C, 1350°C, and 1400°C with 60-minute retention concluded that 1250°C and 1300°C are insufficiently high for proper clinker formation. Both mixtures required sintering at 1350°C or 1400°C to achieve acceptable clinker quality .
3. The Role of Liquid Phase Formation
3.1 Liquid Phase as a Reaction Medium
At the burning zone temperature of approximately 1450°C, approximately 20–30% of the clinker material enters a liquid (molten) state . This liquid phase is not merely a byproduct of high temperature but serves an essential function: it acts as a solvent and transport medium that facilitates the reaction between calcium oxide and the silicate, aluminate, and ferrite components of the raw meal. Without the liquid phase, the solid-state reactions required to form alite would be impractically slow.
The formation of liquid phase is highly temperature-sensitive. The amount of liquid increases with temperature above the eutectic point of the system, and its viscosity decreases, enhancing ionic mobility and reaction rates. Minor components such as alkalis, sulfur, and magnesium influence the liquid phase properties, including its viscosity, surface tension, and temperature of formation.
Date: 2026-09-17 00:00:00.000000