Cement clinker microscopy

Clinker cooling

The cooling of clinker covers the period from when it passes the hottest part of the burning zone until it reaches ambient temperature in storage. The microstructure of the clinker and the properties of the cement which is produced from it can be strongly affected by this stage of production depending on factors such as the distance from the burning zone from the kiln outlet, the range of temperatures through which the clinker minerals pass, the presence of rings or snowmen, whether coating is loose or the cooler type and the rate of cooling after leaving the rotary kiln.

Slow cooling is generally a negative influence on quality for reasons which can be observed by microscopic examination of clinker as discussed below.

Matrix crystal size

The liquid phase of cement clinker comprises the aluminate phase and the ferrite phase as well as a few others such as the alkali sulphates. When a liquid cools below its crystallisation temperature the size of the crystals which form depend largely on the rate of cooling, with fast cooling producing very fine crystals and slow cooling producing coarser ones.

Figure 1. Fast cooled clinker nodule

Figure 1 shows a clinker section in which the cooling was fast, The individual crystals of the matrix are difficult to distinguish and in some cases measuring the size is difficult at the magnifications usually used for quantitative examinations. The grey aluminate crystals in Figure 1 are approximately 2 microns in size which represents fast cooling. In more extreme cases the crystals may hardly be distinguishable. 

Figure 2. Slowly cooled clinker nodule

The blue angular alite crystals in Figure 1 have clearly differentiated edges which is usually the case with fast cooling.

By contrast Figure 2 shows a clinker nodule which has been very slowly cooled. The matrix can be seen to contain well crystallised grey and white aluminate phase and ferrite phase respectively. 

Within the grey aluminate and to a lesser extent within the ferrite, pinpricks of very fine belite crystals have formed. The alite crystals, which are brownish in this image, instead of having clear edges in contact with the liquid phase are corroded and surrounded by fine crystals of belite.

Belite pinpricks

Because the cooling of clinker is not under conditions which would lead to equilibrium crystallisation, some of the silica which may eventually form alite or belite by combination with lime remains within the liquid phase in the burning zone and when rapidly cooled exists as substituent material within the aluminate or ferrite crystals. On slow cooling however, the lime and silica are still able to combine within the liquid phase and to nucleate new crystals of belite during the cooling stage. Alite will not normally be formed at this stage because the temperature has fallen below that at which alite will easily form.

Coatings of belite around alite

As well as not forming below the high temperatures in the burning zone, alite is not normally stable at lower temperatures. However rapid cooling and the presence of some impurities usually preserves it in a metastable state.  In Figure 2 the slow cooling rate has left the alite in contact with the liquid phase while it was still liquid and some corrosion of the crystal has occurred. The alite (essentially C₃S) had broken down to belite (C₂S) and free lime (C). The free lime re-dissolved into the liquid and the belite remained as a coating around the alite crystals.

Secondary growth of belite

Figure 3. Fast cooled belite crystals

During cooling from about 1450°C belite crystals pass through a number of phase transitions. The high temperature form is known as ɑ belite, the next lower temperature form is ɑ' (alpha prime) and the lowest which is found in most Portland cement clinkers is β belite. The lowering of symmetry due to these changes produces sets of parallel striations within the crystals as are seen in Figure 3. This is known as Type I belite.

Figure 4. Slowly cooled belite crystals showing outgrowths

 If the temperature is held below the alite formation temperature but with liquid still present the belite crystals can continue to grow. This growth tends to follow the lines of the striations leading to the formation of finger like extensions from the belite crystals. Figure 4 shows an example. 

Figure 5. Very slow cooled belite beginning to dismember.

If the cooling is exceptionally slow the outgrowths can become the dominant feature and effective dismemberment of the crystals occurs, obscuring the original form of the crystals as in Figure 5. In some crystals the core of primary belite remains but it has begun to recrystallise to a very fine form.

When belite forms during the cooling stage at temperatures just below 1400°C it forms as ɑ' and the structural change is just that due to the phase transition from ɑ' to β. This results in a single set of striations which are polysynthetic twins as shown in Figure 6. This is most commonly seen in coating fragments which have been included in the clinker. This is Type II belite.

Figure 6. Type II belite

Effects on quality

The size of the aluminate crystals has an effect on the grindability of the clinker because coarser crystals associated with slow cooling increase the overall difficulty of grinding and also affect the surface area of the phase in a cement. The reactivity of aluminate has a very significant influence on the water demand of a cement in concrete and therefore on the strength of concrete in practice and if the aluminate phase provides varying surfaces for the setting to be controlled the quality of the cement will be compromised.

The effect of alite decomposition in the event of slow cooling has been implicated in loss of strength due to the lower reactivity of the alite crystals. The loss of the quantity of alite expected from the chemistry can also be significant.

Contact:
For any further information on clinker examinations or to request a quotation please contact arthur.harrisson@gmail.com