The growth of Portland Cement manufacture, as patented by Joseph Aspdin, largely took place in the South and East of England during the 19th and early 20th Centuries.  The raw materials available were a soft, generally saturated limestone, known as Chalk, and a soft largely waterlogged clay.  It is debatable whether these circumstances enhanced the development of Portland Cement manufacturing technology or perhaps held it back by several decades.

The Chalk of Southern England is a soft, white, very fine grained  limestone composed largely of minute fossils known as coccoliths.  These creatures lived in ancient oceans in the Cretaceous Period of Geological history some 60 to 95 million years ago, roughly coincident with the demise of the dinosaurs.  Similar creatures to those which formed the ancient Chalk still dwell in modern oceans, but the Cretaceous was the height of their rock-building activities.  When the creatures died and fell to the ocean floor they were compressed over millions of years by the addition of further debris from above.  As well as the calcium carbonate which formed the bulk of the skeletal material there were some creatures which were built on a silica framework.  The silica also fell to the bottom of the ocean and under the pressure of the overlying sediments was redistributed to form nodules within the mainly limestone rock.  The nodules are known today as flint.  

                                                                Figure 1. A coccolith from chalk in the electron microscope.

Fig. 1 shows a typical fossil coccolith from the  chalk.  Much of the chalk is made up of many millions of tiny skeletons such as these.  The individual plates which form the fossil are of the order of one micron across and the fragility of the structure when washed in a washmill or a washdrum means that a large proportion of the fragments of chalk in the slurry are one micron or less in size with no further milling required.  The size distribution in Fig. 2 is typical of a chalk slurry and shows that over 90% of the chalk passes a 90µ sieve with no grinding other than being agitated in water and the autogenous action of any flint component.  The harder silica of the flint almost certainly makes up the bulk of the 90µ+ material.

Portland cement clinker production grew from the burning of limestone to produce quicklime, a process which has been taking place since ancient times.  The Roman architect, Marcus Vitruvius Pollio, wrote a series of books “de Architectura” in which he described the construction practices of his day.   He sets out preferences for which type of stone is best for which purposes.  

Fig. 2  Typical particle size distribution of chalk slurry.

It is clear that in his view the best stone to make lime for building was regarded as thick and hard (“spisso et duriore”).  These limestones could be more easily stacked and burned in lime kilns until calcination was complete at relatively low temperatures. 

The burning of impure limestones led to the use of hydraulic lime in construction, that is a lime with a proportion of calcium silicates and calcium aluminates as well as calcium oxide.  The silicates and aluminates were produced by the reaction of calcined lime with clay minerals at temperatures above about 900°C.  These limes were slaked in the same way as quicklime for use, but care needed to be taken to avoid hydration of the other phases in the material before it was used in practice.

Modern producers of cement clinker will be aware that the combination of calcium oxide with silica, alumina and iron oxide requires higher temperatures than lime burning but also, more crucially, it requires that the raw materials are extremely fine before being placed in the cement kiln so that the reactions can take place on a very small scale between grains of different composition. When firing an impure limestone the clay may be evenly distributed within the stone and already  able to react with the lime without the necessity for extreme size reduction.  There are some cement raw materials which contain the appropriate mixture of limestone and clay to allow burning with little or no extra effort or addition, however, in the vast majority of cases two (or more) components must be brought together from different locations and the required chemical reactions need to be facilitated by making the individual constituents as fine as possible. 

It may, therefore, have been a happy coincidence that the materials being used by Aspdin and also James Frost, James Parker and others to produce various hydraulic limes and cements were in a state which permitted mixing on a very intimate level with relatively little physical effort.  A key element of the various patents in the nineteenth century was that the raw materials were finely ground before being fired.  Joseph Aspdin’s patent for Portland Cement related to Carboniferous Limestones of the Pennines and required that the material be “puddled” or “powdered” before use.  When his son William, as well as the other Kentish cement producers, established cement manufacturing facilities using soft chalks and clays, all that was required for size reduction and blending of the various components was washing to create a slurry from which coarse material dropped out, leaving only the ideal sized materials for cement clinkering reactions to occur. 

It was perhaps the relative ease of working these materials which led to the development of what we now call the wet method of cement clinker manufacture.  This involves mixing the materials with water and making a slurry which goes into a long rotary kiln to be dried, calcined and combined to make cement clinker. 

Based on the wet process kilns of Southern England, similar kilns were constructed around the world.  Where the raw materials were harder limestone, the stone was crushed and ground, mixed with water and made into a slurry before being put into the wet process kiln, in which the water needed to be evaporated before any calcining or clinkering could take place.

For the wet process the size reduction of the chalk and clay by washing takes place in one of two milling systems.  These are known as the washmill and the washdrum.  The washmill is a circular tank into which the chalk is loaded then mixed with water.  The comminution of the chalk is achieved by the  movement of a series of harrows through the mix rotating around the centre of the tank, producing a slurry of about 40% moisture content.

 The washdrum is similar to a ball mill without the steel media.  The action of the chalk with water while the drum is rotated is sufficient to break down the structure of the chalk components.  In some chalk deposits, as described above, there are nodules of silica which are appreciably harder than the chalk.  These assist in the grinding process by acting as autogenous grinding media.

The water needs at some stage to be lost from the slurry, in the final event by evaporation in the cement kiln.  As this is expensive of energy, any reduction in the water content before the kiln is advantageous. This can sometimes be achieved by the use of water reducing additives but while these may be successful with the purer chalks, any clay content may react adversely with the additives making the slurry more, rather than less, viscous when energy is applied to move it.

The energy requirements using chalk based slurry are evidently very high by modern standards and it was the drive to reduce these that led to the investigation of other methods of size reduction and minimising the heat used in evaporation in times when all fuels were won from mining or drilling and the costs of production were directly related to the use of fuel, before the arrival of alternative fuels with negative or minimal costs.  

The development of more efficient dry process kiln systems with preheaters and precalciners did not, however, solve the problem that the chalk (and usually also the clay component of the mix) are taken from the ground frequently with over 20% moisture and are not easily milled in conventional ball mills or vertical roller mills.  An early solution to the problem of poor efficiency of wet kilns was to put the slurry through a drying process before the kiln.  A common practice in the 1970s  and 1980s was to place a filter press between the raw material preparation and the cement kiln.  This provides the opportunity to reduce the moisture from the slurry (about 36-40% moisture) to about 18% moisture out of the filter press.  The filter cake can be fed into a semi-wet Lepol type kiln, where drying takes place on a moving grate, or directly into a shortened wet process kiln.                                                                                                                                                                                                                                             

While this achieved some level of success the use of filter presses is, itself, of poor efficiency and the use of electricity to operate the presses is generally outside the control of the cement plant.  The advent of alternative fuels does not encourage the use of such energy.  The persistence of washmills is therefore still a feature of relatively new cement plants using these materials, such as those at Rugby in England and Aalborg in Denmark, even though the slurry is then to be fed into a dry process cement kiln.  The development of the flash drier in a semi-wet kiln system has allowed these processes to be combined while still maintaining relatively efficient combination at about 1000 kcals/kg clinker.

A further development of this use of chalk in cement making has been accomplished at Chelm in Eastern Poland where a 5000 tpd cement kiln operates with no raw material grinding at all.  Chalk and Marl are dug from the quarries then, after a primary crusher, they are stockpiled in linear stacker reclaimer sheds.  The materials are put through hoppers and fed directly into a drier crusher at the base of the preheater tower.  Waste gases from the kiln pass through the chamber where a giant hammer mill breaks up the agglomerates from the chalk and marl and the fines are carried to the top of the tower from where they progress through a conventional dry process kiln with precalciner.  The need to maintain enough heat to dry the materials limits the number of cyclones in such a kiln to two or three, but the fuel consumption can be as low as 800 kcals/kg.

These developments are a result of the ability to use water as the medium for size reduction of the chalk raw material.  With harder raw materials the size reduction is of necessity in a raw mill and the use of the wet process seems today totally irrational for designing a cement plant.  There are, however, a considerable number of cement plants around the world where materials have been won from the ground with a moisture component, put through a raw mill where they are dried, then more water is added to make a slurry after which the water is evaporated in the long wet cement kiln.  It seems quite likely that, had William Aspdin not left his native Northern English home with its Carboniferous Limestone and had he continued to make his revolutionary cement clinker with its integral flux phase, he or his successors may have worked out a more efficient way to use the scarce fuel resources a little sooner than in practice was the case.

If the early cement manufacturers had concentrated more on the development of the early shaft kilns, modified into “bottle kilns” to increase draft and therefore temperatures, with the efficient heat transfer characteristic of this type rather than developing a system for evaporating water, then it may be that a dry process would have developed earlier, with the rotary kiln being an add-on to achieve better clinkering.

A Harrisson 2011