How To Observe Clinker Quality

On-site observation of clinker can identify appearance, colour, internal denseness, and the presence or absence of sandwich structure, yellow core, or undercooking. It is the most direct, simple, reliable, and convenient method for judging the calcination and quality of clinker.

Ideal clinker appearance characteristics are as follows:

(1)Grain: Particles are uniform, with more than 90% in the size range of 5 mm to 30 mm. Some are egg-sized, while particles smaller than 1 mm (referred to as "flying sand material") are rarely seen.

Reasons for the production of fine powder material include:

① High KH and SM in the clinker result in fewer fluxing minerals and a lower liquid phase, producing finer grains and more flying sand material.

② Improper operation, such as excessively high kiln temperature or over-preburning of the material, leads to full decomposition, reducing surface activity and lattice activity. This hinders the formation of alite. The reduced liquid phase cannot bond the material into particles, resulting in serious overburning and the generation of large amounts of fine powder (flying sand).

③ High alumina and alkali content in raw materials can easily lead to flying sand formation. Sticky bulk material is characterized by a sticky firing zone, where the material slides in flakes and rolls infrequently. This prevents clinker granulation and leads to large quantities of flying sand. High levels of harmful components in raw materials, or excessively high clinker sulfate saturation, reduce liquid phase viscosity and surface tension, resulting in poorly granulated clinker and increased flying sand.

④ A cooling rate that is too slow can lead to the crystalline transformation of belite.

(2) Texture: The texture should be dense and not overburned, with a grey-black core and a uniform fracture surface. In contrast, the texture of kiln coating and underfired material is loose, with an uneven fracture surface.

(3) Colour: Colour reflects not only the thermal conditions of calcination but is also influenced by variations in raw material composition. Ordinary silicate cement clinker is grey-green in colour, with uniform, rounded, and smooth particles. When broken open, it is dense, contains shiny crystals, and shows no serious signs of melting. This indicates normal calcination, proper composition, and good clinker quality.

If light green highlights are observed, it suggests that magnesium oxide and ferric oxide have formed a solid solution phase. Trace elements such as chromium, manganese, and others can cause the clinker to appear green, blue, or yellow.

Clinker that is dark green in colour, with a rough surface and small pockmarks, may contain shiny crystals (more than in normal clinker). It has a slight frictional texture, and small particles may fall off. It is lightweight and, when broken open, shows numerous voids. This type of clinker is characterised by low weight, low free calcium oxide, low early strength, and difficulty in forming kiln coating. It also causes refractory lining damage and leads to shorter operating cycles. The main cause is a high silica ratio and low content of fluxing minerals, combined with improper kiln operation.

Grey clinker, with a smooth, non-porous surface and a dense internal structure when broken open, shows a strong fusion appearance. This is indicative of clinker calcined under high humidity. It is heavy, has high strength, and is of good quality. However, it often results from high coal usage and high temperatures during calcination, which can cause damage to the kiln lining and shorten its service life, thereby affecting long-term kiln stability.

Clinker that is brownish-yellow or consists of overburned lumps of various colours results from reduction of the raw material. This type of clinker forms when kiln ventilation is poor and coal burns in a low-oxygen, strongly reducing atmosphere. Reduced clinker has poor grindability and low strength development.

The colour of clinker produced in a rotary kiln can vary due to changes in temperature, the batching ratio, the effectiveness of material preheating, and the completeness of coal combustion. These variations result in clinker of different colours, each reflecting different issues related to the calcination process.

Correctly analysing the colour of clinker and identifying its underlying causes is an essential part of kiln firing technology. Below is an introduction to eleven common clinker colour conditions and their corresponding causes.

1. Clinker is grey-green

Clinker particles are uniform, round, and smooth. When broken open, the interior is dense with a few small pores and shiny crystals. There are no significant signs of fusion. This indicates normal calcination, appropriate raw material composition, and good clinker quality-making it the most desirable type.

2. Clinker is dark green

The surface of the clinker is rough with small pockmarks and shiny crystals (more numerous than in normal clinker). It has a slight frictional texture, and small particles may fall off easily. The clinker is lightweight, and when broken open, it shows a porous structure. Calcination produces a large amount of "flying sand" and is commonly referred to as sticky or viscous bulk material.

This type of clinker is characterised by low weight, low free calcium oxide, and low early strength. It is difficult to form stable kiln coating, leading to refractory lining loss and a short operating cycle. The main causes include a high silica ratio, a low content of fluxing minerals, and improper kiln operation.

Sticky bulk clinker generally occurs when the raw material composition is unstable or when the mixing proportions are incorrect.

Solution:It is necessary to strengthen control over raw material composition, ensure the batching ratio stays within the required range, maintain a reasonable silica ratio, optimize the fluxing (melt-forming) mineral content, and adjust combustion parameters to match actual kiln conditions.

3. Clinker is grey

The clinker surface is smooth and non-porous. When broken open, the structure is dense with a strong sense of fusion. This indicates clinker calcined under high humidity conditions. It is heavy, has high strength, and is of good quality.

However, this type of clinker is typically produced using high amounts of coal and at high temperatures. These conditions can easily damage the kiln coating, shorten the service life of the refractory lining, and affect the long-term safe operation of the kiln.

Solution: Avoid excessively high flame temperatures. Control the apparent density of the clinker within the prescribed range.

4. Clinker has a grey-green surface and brown interior

At first glance, the surface of the clinker appears no different from normal clinker. However, when broken open, it shows a brown interior wrapped by a grey-green outer layer with a dense structure. This is caused by the incomplete combustion of pulverised coal, which produces a reducing flame. As a result, the higher valence iron (Fe₂O₃) is reduced to lower valence iron (FeO), leading to the formation of "yellow core" material.

Incomplete combustion typically occurs due to poor coal quality, improper kiln operation, excessive coal feed, or when the coal injection pipe is positioned too low. It can also result from an unstable thermal system in the kiln or frequent kiln start-ups and shutdowns.This clinker has little effect on quality, but it is not easy to crush, has high grinding power consumption, and seriously affects the colour of the cement, which is particularly unfavourable to the ex-factory cement.

Solution:Ensure the quality of coal, strengthen the wind, coal with the control to ensure complete combustion of pulverised coal, reduce the number of kiln starts and stops, and stabilise the thermal system.

5. Clinker is grey-green on the outside and white on the inside

The external appearance of the clinker is similar to normal clinker. However, when broken open, the interior is white, enclosed by a grey-green outer layer. This phenomenon is typically caused by wet production processes, damaged or leaking arc sieves, or the presence of residual "small stones" in the slurry. When slurry contains unground particles, these can become encapsulated during firing, leading to a white core. The white material inside the clinker is free calcium oxide (f-CaO).

Solution: Repair or replace the arc sieve and ensure that raw material fineness remains within the specified range.

6. Clinker is white on the outside and grey-green on the inside

This type of clinker is coated with a layer of raw meal on the surface. Although the apparent density is often not low, the outer coating can be worn off after prolonged friction or washed away by water. When broken open, the interior grey-green layer appears similar to normal clinker.

The external raw meal coating results in high free CaO content, poor stability, low strength, and poor clinker quality. This occurs when raw meal is accidentally introduced into the cooling zone, causing previously burned clinker to become coated with unburned raw materials. Such "raw meal coating" usually results from an unstable thermal system, incorrect operating parameters, excessively thick kiln coating or rings, or delays in handling operational issues. Short flames and improper combustion can also contribute to this problem.

Solution: Maintain a proper air-to-coal ratio, avoid over-firing or under-firing, and control the flame length to ensure complete combustion of coal powder. Prevent the formation of thick kiln coating, and strengthen fire-watching and kiln operation management.

7. Clinker is grey-green on the outside and powdery inside

The clinker has a normal colour on the outside, but the interior shows serious chalking. In severe cases, the chalking spreads from inside to the surface until the entire clinker particle crumbles into powder. The chalking material can be grey, brown, or brownish-yellow and has almost no strength.

This phenomenon most often occurs in newly commissioned kilns. The chalking rate can exceed 50%. Sometimes, clinker observed from the firing zone to the cooling zone appears normal, but after entering the cooler, it shows extensive chalking. By the time it leaves the cooler, almost all clinker has turned to powder. The higher the firing temperature, the more severe the pulverization.The cause is generally a low KH ratio in the raw mix, a high content of fluxing minerals, slow kiln speed, overfiring, and poor cooling.

Solution: Adjust the raw mix to increase the KH ratio, reduce the content of fluxing minerals, moderately increase the kiln speed, and control clinker particle size to avoid excessive fines.

8. Clinker is black with no crystals

This clinker is produced at a very high firing temperature and has a high apparent density. The clinker particles have smooth surfaces but lack crystals or contain only very fine crystals when broken open. There are serious signs of melting, low free calcium oxide content, and low strength.This phenomenon is caused by a low KH ratio in the raw mix and insufficient C₃S mineral content.

Solution: Adjust the raw mix to increase the KH ratio and keep it within the proper range.

9. Clinker is rough grey-green

Clinker particles have a rough surface and a loose structure with poor roundness. When broken open, the interior shows many pores and lacks density. This indicates insufficient calcination temperature and underfiring, resulting in poor-quality clinker.

Solution: Strengthen calcination control and increase the firing temperature appropriately.

10. Clinker is brownish-red

The clinker is loose, porous, and lightweight, with many particles containing flake-like, slag-shaped materials. This occurs due to low-temperature calcination combined with excessive coal feed, which creates a strongly reducing flame. The resulting clinker is of very poor quality.

Solution: Strengthen kiln monitoring and fire-watching operations to ensure proper calcination temperature. Use coal in appropriate amounts and ensure complete combustion of pulverized coal.

11. Clinker is yellow and white powder

This type of material results from unburned raw meal-essentially a waste product. The cause of this phenomenon is usually delayed or improper response by the kiln operator when problems arise.

Solution: Kiln operators should monitor the flame carefully and frequently, and continuously improve their operational judgment and responsiveness.