Cement Kilns

Chalk in cement manufacture

Chalk is a rock differentiated from other limestones by its softness, open structure and high moisture content. The latter characteristic is almost never mentioned in texts, although it is a major pre-occupation for cement manufacturers: the structure of chalk is very much like a sponge, and water continually flows through its body rather than through joints and cracks. Because the chalk of the Upper Cretaceous is the only significant young limestone in Britain, the term Chalk refers specifically to that geological formation in British parlance. The Chalk formation dominates the geomorphology of southeast England, with escarpments surrounding the Weald, and a long escarpment stretching from Devon to Yorkshire. Chalk once covered most of Britain and Ireland to the north and west of this, but has been eroded away, except in western Scotland and County Antrim, where it has been preserved, in a highly indurated condition, under a layer of Paleocene basalt (see Ireland).

In the southeast, the chalk (particularly the part closer to the surface) is very soft, with a moisture content up to 23% by mass (45% by volume). It can easily be ground by adding more water, when little more than a vigorous stirring action reduces it to a fine slurry. Further north, the chalk gets progressively harder and drier (7-14% water) and must be processed using hard rock techniques. The Ulster chalk, because of its history of contact with a huge basalt flow, is almost marble-like, and contains some metamorphic minerals. The upper layers of the chalk often contain layers of flint, constituting up to 10% by mass in places. This, because it is much harder and can’t be treated by “soft-rock” grinding techniques, is usually separated out by cement manufacturers. Fortunately, this is a trivial process when chalk is wash-milled. After removing flint, the rest of the mass is almost pure calcium carbonate, with MgO levels rarely rising above 1%, and often half that. Low iron contents (<0.05%) mean that most clean Upper Chalks can be used for white cement manufacture.

The British Chalk Formation occupies all six internationally-defined stages of the Upper Cretaceous. Traditionally, and still to some extent on BGS mapping, the chalk is divided into Lower, Middle and Upper divisions, divided at the hard bands that give rise to the chalk escarpment features of south-east England. The Lower Chalk consists of most of the Cenomanian. The Middle Chalk has as its base a hard band most often known as the Melbourne Rock, which is the topmost member of the Cenomanian, and includes the whole of the Turonian. The Upper Chalk has as its base a hard band most commonly called the Chalk Rock, which is the base member of the Coniacian and forms the lip of all the main chalk escarpments in southern England. The rest of the Upper Chalk occupies the dip slope of the escarpments. The three highest Cretaceous stages have mostly been eroded away, and the youngest British chalk, found only in a small area on the Norfolk coast, is Lower Maastrichtian.

Of the three divisions, the Lower Chalk is most variable, and is conventionally divided into four lithological subdivisions, as commonly found in the Chilterns: at the base the Glauconitic Marl, which is a thin green transitional bed, often sandy and/or phosphatic. Above this is the Chalk Marl, which is a bluish argillaceous limestone containing 50-80% calcium carbonate, and next the Grey Chalk, which is also argillaceous, containing 80-95% calcium carbonate. At the top are the very persistent Plenus Marls, typically a thin succession of yellow and dark grey marls. In a region between Buckinghamshire and Norfolk, the base of the Grey Chalk consists of a prominent hard band most commonly called the Totternhoe Stone, which sometimes produces a third, distinct chalk escarpment.

Almost everywhere, the lower layers of the Lower Chalk have successively lower calcium carbonate contents, often dropping to as low as 50% at the base. These have been used for centuries to make hydraulic limes, and many attempts were made to make them into “Natural Cements”. With addition of some higher-grade chalk, they have been used to make cements using only chalk as raw material, allowing a greatly-simplified process.

The Middle and Upper Chalks are both relatively pure calcium carbonate, apart from the occurrence of flint nodules. In the south, flint is found only in the Upper Chalk, where it may comprise up to 10% of the rock. The flint nodules, being extremely hard, can readily be removed undamaged from the chalk matrix by gentle wet grinding. Many of the larger cement plants developed flint sales as a sideline - the larger (and therefore more pure) flints were sold to the pottery industry for use, after calcination, as a tempering agent in ceramics. North of the Wash, the situation is reversed, with Upper Chalk containing relatively little flint, while flint is fairly abundant in the Middle Chalk.

The softness of chalk made it a natural choice of raw material for the early cement industry, when cement manufacturing technology was primarily limited by lack of effective grinding techniques. The fine grinding of hard materials was initially at best expensive, and at worst impossible. A second, more subtle reason for the early preference for chalk was its chemical dependability, amenable to the use of simple non-technical rules-of-thumb. If it’s white, it’s pure calcium carbonate. If it’s grey, it’s argillaceous, but still low in MgO and alkalis. All the other limestones are mineralogically variable, prone to be dolomitic, argillaceous or siliceous, and colour is no sure guide to these variations. Early attempts to use such limestones for cement were no doubt ended by nasty and unaccountable surprises. Their effective use required the services of a professional chemist, and 24-hour chemical monitoring. Before about 1890, this was an almost insuperable obstacle. The high moisture content of chalk has militated against its use in modern energy-efficient dry process plants, so the industry which initially used chalk almost exclusively because of its softness, has now disappeared in southeast England.

Although some plants using Chalk Marl needed no other secondary raw material, in general some sort of clay needed to be used in conjunction with the high purity chalks. Initially, the silty alluvial clays of the Thames and Medway estuaries were favoured, for a number of reasons: they could be obtained from the foreshore, avoiding mineral royalty costs; by their nature, they were amenable to water transport; and they contained plenty of fine-grained silica allowing easy burning. However, they were also necessarily high in salt content, which is unfavourable for use in rotary kilns, and so alternative clays were sought. In the London Basin, Eocene London Clay usually outcrops fairly close to the chalk quarries, while plants on the scarp slopes of the chalk tended to use the underlying Cretaceous Gault Clay or clays of the Upper Jurassic such as the Kimmeridge Clay.

Divisions of the English Chalk

chalk stages

The modern designations of Chalk Formations are divided into two provinces, the northern beginning in Norfolk, somewhat south of the Wash. In the northern province, the Hunstanton Formation represents a condensed part of the Lower Cenomanian and the Albian and Aptian (part of the Lower Cretaceous), and typically takes the form of the "Red Chalk".

Location of cement plants

Most of the chalk-based plants can be conveniently arranged into three fairly small geographical regions: the Thames Estuary, the Medway and Swale, and the Cam Valley: the rest are scattered in miscellaneous other areas, as shown on the map. (For the Irish Chalk see Ireland.)

Plants also fall into two distinct groups dependent on their position on the chalk outcrop: plants on the scarp slope used older, under-lying clays or marl as the secondary component, while plants on the dip slope used younger, over-lying clays.

The earliest chalk-based plant outside the core areas was probably Vectis in 1852, using chalk from Portsdown across the Solent. This plant, like Northam, Dovercourt, Waldringfield and Wilmington, began its life as a Roman Cement plant. Many of the other early plants in this group began by importing chalk from the Thames, and two (Dovercourt and Waldringfield) did so throughout their lives. Exploitation of the outer chalk areas started rather late: the first plant in the Chilterns was Saffron Walden (1877), the first in the South Downs was Shoreham (1883?), while the first Humberside plant to use local chalk was Adamant around 1888.

The chalk is at its softest and wettest in the south-east, and particularly to the north of the Wash, the chalk becomes relatively dry, hard and brittle. Because of this, simple washmilling was not an effective means of grinding, and it was only possible to process it after the tube mill had been introduced. The chalk is at its hardest in north-east Ireland and north-west Scotland, where it has been preserved in highly indurated form under a thick layer of Paleocene basalt. Although this chalk is of excellent quality, and dry enough for use in efficient processes in areas where other suitable raw materials are scarce, it was exploited only at Magheramorne, because, unless underground mining is used, massive amounts of hard basalt overburden have to be removed.

Because of purity and easy quarrying of chalk, a certain number of plants outside the chalk districts used brought-in chalk as a sweetener or even as their main raw material, using long-distance transport. Among these were Billingham and Warren in County Durham (treated here as waste-using plants), and Rugby and Southam in Warwickshire, treated here as Blue Lias plants. The latter two from the 1930s onwards brought chalk from the Dunstable area, initially by rail, and subsequently pumped as a slurry.

All the plants used Wet Process, until one kiln at Shoreham (and subsequently two more) was converted to semi-wet in 1955. A further semi-wet kiln was installed at Dunstable in 1966. South Ferriby was converted to semi-dry process from 1967, and a dry process kiln operated at Pitstone from 1970. With the closure of the last wet process plant at Westbury in 2009, South Ferriby, with a relatively dry chalk, is now the only plant on this map using chalk as a raw material. (Rugby, discussed under Blue Lias, continues to use chalk as its predominant raw material, pumped as a slurry from Kensworth near Dunstable, and this remains the last cement industry wet-chalk quarry.)

Links to plants in the outer Chalk areas

chalk map

Plants on the main chalk outcrop. Magheramorne was on the coast of County Antrim (see Ireland).

Evolution of capacity (annual clinker tonnes) in the outer chalk areas

Capacity by Kiln Type

© Dylan Moore 2011: last edit 10/10/14.