Cement clinker microscopy

Belite clusters and crystal sizes 



Belite crystals begin to form in a kiln feed at quite an early stage of burning at contact points between silica and lime particles.  Very fine, poorly formed crystals can be present at temperatures below 800ᵒC, before the feed has entered the rotary kiln in a preheater system. The mineral belite is normally present in the cement kiln as one or more of three polymorphs known with increasing formation temperature as β, α’ (alpha prime) and α. There is also a lower temperature form γ which is very rare in clinker and also the α’ is subdivided into low and high temperature forms. 

In the cement kiln as the temperature rises above 1100ᵒC  belite is present as α’ crystals. These are very fine, relatively pure crystals which grow only slowly until, at about 1400ᵒC, they undergo a transformation to larger crystals of α belite. The α form absorbs impurities as it rapidly grows. In the burning zone of the kiln most of the belite crystals are converted to alite, but a proportion generally remains. This is because many belite crystals are present in clusters which make it difficult for the lime required to form alite to gain access. Also most clinkers are designed with a lime saturation factor below 100% which means there is insufficient lime present to convert all of the belite to alite.

On cooling, belite goes through the reverse transformation to α’ and then to β, and through these transformations structural changes produce the characteristic laminations. 

Belite crystals. Image is 300 microns wide

Any further growth with steadily falling temperature is frequently outwards along the planes of the laminae. This means that for the belite crystals remaining in the clinker which have been cooled rapidly enough to avoid the outward growths, the size of the crystals depends largely on the length of time spent above 1400ᵒC, effectively, for a given kiln speed, the length of the burning zone.

Belite clusters
Belite clusters occur for a number of reasons in clinker. All are essentially the result of non-homogeneity of the raw materials forming the kiln feed which may arise from the mixing or the milling of raw materials, possibly segregation of the feed components in the kiln or the nature of any ash from the fuels being poorly homogenised in the clinker.

Mixing

Mixing of raw materials under optimal operating conditions on a modern factory where there are no bottlenecks should not be an issue leading to serious inhomogeneity of the kiln feed. However, it is not unknown for a factory to seek to improve output of the kiln without adequately improving blending capacity or silo storage for the kiln feed. Clinker nodules within the same sample may contain either mostly belite or mostly alite and free lime which will not permit adequate combination.

Milling

Clinker raw materials need to be very finely milled so that the necessary chemical reactions can take place in the kiln in the short time available. In the case of belite crystals there is also a relationship between the size of belite crystals and the size of the silica grains around which the crystals were formed.

Coarse belite cluster after coarse silica grain. Image is 200 microns wide

The sequence above shows in a simplified diagrammatic form the development of coarse belite crystals around a coarse silica grain. As temperature rises, at the contact of silica with calcined lime a melt forms. Neither lime nor silica alone would melt until they were very much hotter, but when together a phase known as a eutectic melt can form at lower temperatures.

Lime loses a great deal of CO₂ during calcination but when silica is first in contact with calcined lime it has not yet collapsed into dense free lime crystals. In fact it is full of pores from where the gas has escaped. Capillary action draws the low limed melt into the porous lime and belite is formed. The fine belite crystals form a dense ring around the location of the silica grain and as the melt moves out a pore remains at the original site of the silica. This dense shell of belite contains no interstitial liquid so transport of further lime in order to convert the belite to alite is difficult. Dense rims of coarse belite surrounding a pore form and these are commonly observed in clinker as a result of the presence of coarse silica grains.

If the mix design intended that these belite crystals should become alite, then harder burning will be required to reduce the free lime and achieve the intended clinker compound composition.

Belite cluster after coarse shale particle. Image is 1mm wide

While silica is the most difficult of the main clinker raw materials to mill, the results of the presence of coarse grains of limestone and shale are also frequently encountered during microscopic examination. This figure shows a typical result of the shale producing an angular cluster of belite with interstitial liquid phase from the presence of aluminium and iron in the shale.




Segregation
Even in a well mixed kiln feed lime-rich zones and lime poor zones are found when clinker is produced in a modern preheater or precalciner kiln. Examination of the hot meal can reveal pellets of lime of a size which was not present in the kiln feed. Corresponding pellets of lime-poor material also form and in the final clinker these can be seen as rounded clusters of belite. 

Round belite cluster from segregated feed. Image is 200 microns wide

The segregation of lime is probably due to the arrival of grains of limestone in a calcined state and therefore with a low density at the kiln inlet which are carried up by the kiln gases for another trip through the preheater.

Ash Homogenisation

Poor coal milling and high ash coals can lead to deposition of low limed alumino-silicate material within clinker from the coal shale which forms the ash. Belite forms in these regions but because these are not mixed well with the kiln feed they are not easily converted to alite. In general modern coal milling is able to avoid coarse fuel problems like this although with high ash coal the fine ash projected through the kiln after ignition can merge into coarser particles which fall onto the clinker bed.

However, with the increasing use of alternative fuels the assimilation of ash into the clinker has returned as an issue. Solid recovered fuels fired at the front end through the main burner are orders of magnitude coarser than milled coal which presents two challenges. First the difficulty of projecting the fuel to the back of the kiln so that the ash can be assimilated before the burning zone and secondly the size of the fuel particles makes homogenisation of the ash into the kiln charge very difficult. The figure below shows the size of belite clusters which are common in clinker for which SRF has been a fuel.

Belite clusters from fuel ash deposition. Image is 1mm wide

The clusters appear to be very much too large to have formed from any kiln feed particles. Another indication that they derived from the front of the kiln rather than the kiln feed is that frequently the crystal size is finer than those crystals which have passed through the whole of the burning zone.

A measure of how significant the effect of this will be on the process and quality can be gained by looking at the size distribution of the belite crystal sizes. A large shoulder below the main peak as in the histogram below would probably indicate an issue with combinability due to poor projection of fuel through the kiln from the main burner.

Contact:

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