Clinker is the primary material of cement, and it is called as semi-finished product. Clinker is a granulated material made of raw meal powder obtained by grinding limestone and clay together by sintering the raw meal in a rotary furnace at 1400°C-1500°C degrees.
Produced clinker’s mineralogical structure effects clinker’s grinding energy. And it determines the behavior of cement by changing the parameters such as strength and setting time.
Our post deals with clinker mineral structure’s grindability effects.
Figure 1: Gray Cement Clinker
Cement Production and Energy Consumption
Cement production is an energy-intense process and approximately one third of the energy required to produce one ton of cement is consumed during grinding of clinker and additive materials.
Cement industry is one of the biggest industries using almost 3.5% of the world’s energy. In cement industries, 40% of the total energy consumed during the production process is used for grinding.
Grindability in cement production is important due to two factors. First, cement’s properties depend on the fineness and grain size distribution of the cement other than chemical and mineralogical compound of it. Second; 1/3 of energy expense in cement cost is spent for grinding. 80% more energy is spent in grinding hard clinker compared to soft clinker.
Chemical Compound and Grindability
Chemical and mineralogical compound play an important role in the grindability of clinker. Grindability is observed to decrease with the increase of Silicate module, Aluminum oxide and free Calcium oxide determined through the chemical analysis of clinkers.
Clinker Microstructure and Grindability
Microstructure also has an effect on grindability alongside chemical and mineralogical elements. Heating and cooling speeds and furnace type are also effective in the formation of microstructure. Fine crystalline structure, especially small calcium silicate crystals, improve grinding. Large crystals not only make breaking down difficult, but they also increase the number of breaking areas.
Formation of Clinker Melt in the Production Process
Raw meal powder enters into the furnace system, it transitions into the melt phase, and clinker is obtained once furnace and cooling reactions are completed.
The first melt phase is formed at 1260°C-1310°C. Clinker melt’s rate increases as the temperature rises and it reaches up to 20-30% in weight at 1450°C depending on the chemical compound.
These temperatures enable alite formation which is the main content of Portland cement. As soon as sintering starts, high amounts of free Calcium oxide along with belite come out. With the melt phase, Calcium oxide and belite enter into the solid solution.
The Effect of Clinker Cooling on the Microstructure in the Production Process
Reactions occur in the rotary furnace at 1400°C-1500°C. Clinker should be cooled off rapidly once the reactions are completed. This process preserves the presence of alite which is the main phase of clinker. Belite and tricalcium aluminate phase emerges as a result of clinker melt entering into reaction with a certain proportion of alite due to slow cooling process. In addition, alite is not stable under 1250°C and it tends to decompose into belite and free Calcium oxide. Therefore, clinker requires a cooling process fast enough not to allow these reactions.
High cooling speed has the following effects: Grinding improves due to stress cracks in clinker. No alite dissolution occurs and this phase has a high amount. The fine crystalline aluminate and ferrite phases formed by shock cooling of clinker slow down cement hardening.
Clinker Phases
Clinker phases emerge as a result of reactions which occur during sintering of raw meal powder in the rotary furnace. These phases affect the behavior of clinker, thus the behavior of cement. Main phases of typical Portland cement are as follows: Alite (Tricalcium Silicate-C3S), Belite (Dicalcium Silicate-C2S), C3A (Tricalcium aluminate), C4AF (Tetracalcium Alumina Ferrite) Grinding becomes difficult when Alite/Belite rate and (Alite+Belite)/(C3A+C4AF) decrease in clinker phases.
Figure 2: Clinker View Under Microscope
Clinker view under microscope: Crystals with brown corners are alite crystals. Blue circular crystals are belite crystals. Frays seen around the circles are called fingertype and they indicate that alite crystals transformed into belites by decaying as a result of slow cooling.In addition, the sizes of crystals are also assessed. Crystal size is effective in grinding energy.
Clinker Phase – Alite (C3S)
Alite may range between 40-70% of the clinker mass. It reacts with water, and it is the phase which controls cement strength and hydration temperature. If the cooling speed is not too slow under 1250°C, it can preserve its stability down to normal temperatures. At very slow cooling speeds, however, some parts of alite dissolve and belite forms. This is the component with high hydration capacity as colorless crystals provide the first high strength in clinker. The quality of cement is measured by its alite concentration. Depending on cooling degrees, clinker’s alite rate reaches 59.8% in slow cooling, 65.2% in fast cooling, and 70% in express cooling.
Clinker Phase – Belite (C2S)
Belite may range between 15-45% of the clinker mass. It has a circular crystalline form. Its crystal size is between 5-40 μm. Belite is less reactive compared to alite and contributes to strength at later ages. It is a hydraulic binder which hardens slowly.
Clinker Phase – Tricalcium Aluminate (C3A)
It may range between 1-15% of the clinker mass. It is in cubic or orthorhombic form. Its crystal size varies between 1-60 μm. Its reaction with water is very fast and amorphous in structure. Hydration temperature is very high. It hardens immediately with water.
Clinker Phase – Tetracalcium Alumina Ferrite (C4AF)
It may range between 0-18% of the clinker mass. Its crystalline structure is dendritic and prismatic. It presents a thin and long view in the form of a sword in the crystalline section.
Clinker Phases and Crystalline Structure
Clinker silicate crystals’ grain size may be put forth by calculating equivalent diameters. Equivalent diameter is determined by calculating the arithmetic mean of the longest and shortest lengths which pass through the center of gravity of the cross-sectional area in a crystal cross-section. Alite crystals with equivalent diameters of 15-20 μm have positive impacts on the ease of grinding of clinker as well as the early strength of the cement.
Belite crystals with equivalent diameters of 25-60 μm may be a result of insufficient grinding of quartz grains in the raw mixture or of heating-cooling regime.
In summary, examining grain size distributions of silicate crystals through polarizing optic microscope and image processing software would guide the manufacturer for the raw material choice and optimization of heating-cooling regime.
In Rapidly Cooled Clinkers: There are cracks on the alite crystals in hexagonal crystal structure which are formed due to thermal pressing during express cooling process. These cracks increase hydraulic activity and grindability capacity. Crystal ends are sharp in a clinker which was properly cooled.
The size and shapes of pores (porosity) provide information about sintering conditions in microscopic examinations of the clinker. High porosity and large, long, and interconnected pores may indicate that the clinker was not adequately sintered. Small and round pores, on the other hand, are a sign of a good sintering.
Alite Phase Amount and Grindability: Job index value (kwh/ton;) decreases with the increase of alite amount in the clinker. This shows us that the energy amount required for grinding decreased.
Figure 3: The Relationship Between the Amount % of Alite Phase (C3S) in Clinker and Grindability of Clinker
Belite Phase Amount and Grindability: When belite amount is below 20% in the clinker, the energy demand required for grindability decreases with the increase in belite %. However, in case there is a certain amount of belite (around 22%), the grindability energy demand increases once again.
Figure 4: The Relationship Between the Amount % of Belite Phase (C2S) in Clinker and Grindability of Clinker
Intermediate Phase (C3A and C4AF) Amount and Grindability: With an increase in the intermediate phase of more than 6 – 6.5 %, energy requirement (kwh/ton) for grindability is observed to decrease.
Figure 5: The Relationship Between the Amount % of Intermediate Phase (C3A+ C4AF) in Clinker and Grindability of Clinker
Porosity and Grindability: In contrast to literature, the required energy demand for grindability was observed to increase in parallel with the increase in porosity amount in the clinker. The reason for this is seen when porosity’s fineness module is examined.
Figure 6: The Relationship Between the Amount % of Porosity in Clinker and Grindability of Clinker
Table-1: Comparison of Silicate Phases and Porosity’s Fineness Modules in Clinker Samples with Job Indexes
Grindability becomes easier with high alite content in the clinker, and it becomes more difficult with high belite content. High amount of liquid phase makes grindability of clinker easier.
Cement behavior may be altered by conducting studies on clinker chemistry and mineralogical phases during clinker production process. Therefore, clinker grinding energy varies as well depending on the phase structure. Energy recovery may be obtained in grinding by bringing the phase structures to the desired levels.
References:
