The primary raw material for cement clinker manufacture is limestone which provides the kiln feed with calcium carbonate.  Calcium carbonate is calcined to remove CO₂, leaving free CaO at high temperature and in a highly reactive state to combine with silica and form calcium silicates, which provide the strength potential in portland cement.  As well as calcium silicates the CaO combines with aluminium oxide and iron oxide to form a melt which facilitates the combination of CaO and SiO₂ in the kiln.  The liquid then cools as it passes the hottest part of the kiln to crystallise out as calcium aluminates and calcium alumino-ferrites.

Limestone therefore requires the presence of a further raw material to provide aluminium and iron oxides as well as silica which are required for the process.  In some cases these oxides can all be found in the appropriate proportions in a single second raw material and cement clinker can be produced from a simple two component mix.  Because of the inherent variability of naturally won raw materials however, the ability to control all of the chemical requirements with two components is limited.  With two components the lime saturation factor of the mix can be controlled and made consistent by making regular analyses of the mix and varying the proportions of the two materials.  However, making these changes to accommodate the natural variability of one or other or both of the raw materials will inevitably create wider variations in the other chemical parameters used in clinker making, the silica ratio and the alumina to iron ratio.

Generally the main source of silica, alumina and iron oxides has been clay or shale with control of the silica and alumina to iron ratio being accomplished by the addition of small quantities of sand and oxide wastes from steel manufacture. 

In many cases there is little realistic choice of second raw material.  The economics of transporting bulky materials over large distances usually means that the closest suitable material is the one which is used.   Frequently, therefore, the ideal raw material is not economically viable and a compromise is made with the choice of clay, shale, marl or other alumino-silicate deposit.  In some cases the aluminium oxide can be supplied from a by-product source and this will be discussed later, but in nearly every case there will be a need for a material extracted from a quarry.

Clays and shales share essentially the same genesis, having been formed by chemical weathering of minerals formed initially as igneous rocks, that is crystallised from a reservoir of molten rock either deep within the earth and subsequently exposed by wearing down of the surface or as outpourings of lava from volcanoes.  The components of these igneous rocks vary depending on their own origin but are variations on a mix of quartz, feldspars (predominantly alumino-silicate crystals but with alkali ) micas and other minerals containing iron, magnesium and alkalis.   In general igneous rocks contain very little sulphur, although clays and shales frequently have appreciable quantities of these which can sometimes cause problems when used as cement raw material. 

The chemical weathering of the igneous rocks (or more commonly rock fragments produced by mechanical weathering of the landscape by water and ice movement and by stresses caused by thermal expansion and contraction) breaks down the feldspars which are reformed as clay minerals.  These are sheet structures held together by layers of water.  Some clay minerals possess the property to absorb large amounts of water, swelling in the process.   This may present significant problems with handling the materials when introduced to a cement plant. 

Mineralogically the simplest form of clay raw material is one derived from weathered granite rocks and recovered in situ by removal from faces of granite with high pressure water jets.  The clay is removed from the parent rock along with sand, unweathered feldspars, micas and other minor components and is washed down slopes of low gradient so that successively finer material settles out with distance from the working face.  Eventually all that is left in suspension is the very fine clay fraction, in this case kaolinite clay, which is recovered from settling ponds.  Historically the primary use for this very pure clay is for fine china production and it is known as china clay.  It also has a use however in the production of white cement clinker because of it’s purity and absence of colouring elements such as iron.

Pressure on the clay minerals due to the laying down of very large volumes of the deposit over long periods of time compresses the clay minerals to the extent that the rock structure may become massive rather than plastic and may break along joint planes.  These materials are generally known as mudstones. 

When the weathering of the igneous rocks is less complete and fine silt sized particles of quartz and micas remain in the clay, compression can create a fissile rock known as shale.  In cement manufacturing there is a tendency to call all the materials, “Clays” whether fully weathered and compressed or not.

The nature of the raw material has an effect on the burnability of the feed as well as on the chemistry of the recirculating load in the kiln and the final clinker.  The presence of unweathered igneous minerals in the shale can include a proportion of very finely divided silica, especially if the igneous rock was one characterised as “acidic”, that is containing a high proportion of quartz.  This gives the shale a high silica ratio relative to a completely weathered clay.  The presence of very fine silica reduces the requirement to add sand at the raw mill to maintain the silica ratio of the raw feed and this, in turn reduces the milling required to achieve combination in the cement kiln.

 While high silica ratio clays are useful in reducing the sand input to the feed, there is a general relationship between the silica ratio of the clay and the alkali content as shown in Fig 2 for some typical raw materials.  As more unweathered fine sand is present in the clay there may also be a higher proportion of unweathered alkali feldspar.  The weathered products of feldspars hold bases weakly and slightly acidic rainwater can remove alkalis from the clays by leaching, whereas the unweathered alkali feldspars bring them along to the kiln feed.  Alkalis can provide problems of two kinds in cement manufacture.  They add to the recirculating load in the kiln and can be complicit in causing blockages along with chlorides and sulphate.  In addition there is a limit of alkali permitted in cement due to the potential for  a disruptive reaction in concrete between alkalis and some types of siliceous aggregate.

Many clay raw materials also contain significant proportions of sulphur.  The igneous rocks from which the clays derive do not usually contain more than trace amounts of sulphur and the presence in clays of both sulphates and sulphides is largely due to changes to the clay after it has been made from the parent rock.  Clays which are deposited in the sea may assimilate sulphur from the sea water and those which are formed at depth in the oceans with little oxygen available contain sulphur concentrated from the decay of organisms.

As well as sulphur these deposits may contain significant amounts of hydrocarbons and there are examples of clay raw materials in many parts of the world which contain an element of fuel when used in the cement kiln.  Use of these materials can, however, lead to problems with emissions of sulphur oxides and, if present as sulphides, of carbon monoxide as the sulphur combines with available oxygen in the preheater tower.

Alternatives to natural clays are available in various forms.  The most commonly used is probably fly ash recovered form the combustion of fuel in coal fired power stations.  Compositions of fly ash depend on the nature of the shales associated with the coal used, but in general they are close to the clay raw materials which they replace. 

An exception to this is the silica ratio of the fly ashes, which tends to be lower than natural shales as can be seen by comparison of the compositions in Table 1 with those in Fig.2.  The fly ash silica ratios are significantly lower than the coal shale example in the Figure.  This is perhaps because the relatively coarse silica particles in the shale are not carried to the same extent in the gas stream through the boiler and end up on the floor of the furnace in the bottom ash and not in the fly ash which was molten in the boiler itself.

With regard to kiln feed processing and clinker production a valued property of clays and shales is the fine fundamental particle size and the relative ease of reducing the rock produced from the quarry to the particle size required for cement clinker burning.  In general fly ash used for clinker production is also of a very fine particle size.  However, the finest portions of the ash produced from power stations also have a value as cement replacement materials if the carbon content is low enough to allow this.  The economics of using fly ashes as cement raw material therefore can depend on the ability to use those fractions which would not be suitable for cement replacement and which would therefore otherwise be landfilled at a cost to the producer.  The coarseness of the ash will have an effect on the ability to achieve combination with minimum use of energy.  Fig 3 demonstrates the difference in the ease of producing clinker of a given composition with ashes of different fineness.  The ash described as “Part 1” contains no more that 12% residue on a 45µ sieve  and no more than 7% LOI (essentially unburned carbon).  The residue is the remaining material left after the Part 1 has been mechanical separated from the whole ash and is therefore predominantly coarser than 45µ.

The shapes of the combinability curves are of interest, this difference has been observed in other such comparisons.  At lower temperatures the finer ash is able to reduce free limes more easily than the coarser ash, which would be a predictable response.  At the higher temperatures, however, the coarser ash is as well combined as the finer ash.  This demonstrates an important difference with the use of fly ash as opposed to natural clays or shales.

Clays are sheet silicate crystal structures with water holding the sheets together.  As the temperature in the cement kiln increases to about 700°C the first effect on the clay minerals is that they are dehydrated and begin at that stage to decompose.  This leaves them very susceptible to reaction with the surrounding materials and the first intermediate products of cement clinker are formed very early.  With fly ash this is not the case.  The ash is in the form of glassy spheres with little crystal structure to be disrupted and no inherent water to be lost.  Reactivity with the other components of the feed is very limited until the glass begins to melt at temperatures in excess of about 1000°C.  The chain of events to produce the intermediate products is therefore different and, it would seem from the combinability studies, slower.   The larger fly ash particles evidently take longer to melt and to become homogenised within the clinker, but when this is achieved combination can proceed as with the finer ashes.

Various other waste products from other industries can be used to augment Limestone and clay as cement raw materials.  Blastfurnace slag is used, but the quantity of magnesium in the slag frequently places a limit on the degree to which this is possible.  Steel slag is also valuable but the iron content will always place a limit on this.  The advantages of materials which have already been through a pyroprocessing stage are apparent in energy conservation.  It is always essential, however, to carry out thorough trials in the laboratory and on plant to ensure that the materials are compatible with the other raw materials and the particular process.

A Harrisson 2011