History of Prompt Natural Cement


At the beginning of the 19th century throughout all of Europe, a substitute was created to replace the then widespread lime and pozzolan mixes dating back to Roman times. Pozzolan, imported from Italy, was expensive and highly inconsistent in quality. In keeping with the spirit of the 18th century and for reasons of economics, as well as national independence and the pursuit of technical advances to enhance hydraulicity, a good number of lime kiln operators turned to firing limestone with greater impurities using empirical formulations with widely varying results.


In 1818, Louis-Joseph Vicat developed for the first time the theory of hydraulicity (i.e. the property of binders to harden when exposed to water) by scientifically demonstrating the influence of clay content found in the limestone of that period. By focusing on the clay/lime ratio, he defined the "hydraulicity index", which allowed classifying the lime and natural cements being produced at the time (see below) and inventing artificial cements by recomposing the lime and clay mixes.



This same foresight led L. Vicat to oversee the first industrial fabrication of PROMPT NATURAL CEMENT in 1842 at the same Grenoble site where lime was being quarried from a clayey limestone outcropping he found in 1827. His son Joseph would continue producing this natural cement in the nearby Chartreuse region at the "Perelle" site, where to this day operations are ongoing. Due to the intrinsic attributes of the raw material input, the VICAT PROMPT NATURAL CEMENT has become today's only natural cement produced in industrial-scale quantities throughout the world. At the time however, natural cements, also called "Roman cements" or "Roman lime" or simply "hydraulic lime", were being produced all over using limestone with variable clay contents from one deposit to the next and even in some instances within the same deposit.



The natural cement produced by Vicat can be distinguished from commercially-labeled "Roman cements" by its raw materials, a clayey limestone with an outstanding and unique composition featuring the double characteristic of:
  • A constant chemical composition, yielding cement of a consistent quality; and
  • An ideal mineralogical composition, thanks to optimal clay content, making the cement resistant to higher-temperature firing, given that at the time it was possible to produce, using the same parent rock, a high-quality natural Portland cement. As such, this material was a precursor of today's artificial Portland cements.

There is some anecdotal evidence of concretes made from Natural Cements having been used hundreds of years ago. Whatever the origins, it seems unlikely that lime manufacturers, who have been plying their trade since at least ancient Egyptian times, would not have come across suitable limestone laced with 'impurities' and made, from time to time, materials that we now call Natural Cement.

There are references, particularly in France, to Natural Cements and to Water Cements (later called Hydraulic Limes) in the 18th Century.

PARKER'S CEMENT 

In 1796, at the Isle of Sheppey on the North Kent coast the Reverend James Parker disclosed the use of "Cement-Stones" found on the shores of the Thames Estuary.  These "Cement-Stones" were of a limestone that contained an amount of clay within them. When they were fired and ground in a particular way a NATURAL CEMENT was produced.  This, Parker patented and marketed as "Roman Cement".



At the same time (1796) a French military engineer, Lesage, found similar "cement-stones" on the shoreline at Boulogne, and he too manufactured a type of Natural Cement from them.  Other sources of Natural Cements were found at St Petersburg in Russia and at several sites in the Appalachians in the USA.  As well as "nodules" or stones found on shorelines, mines were started to exploit seams of material.

This material was a commercial success.  Much military construction was going on at this time of revolution and Napoleon; fortifications, docks, wharves, barracks etc were being built at a greater rate.  Parker's Cement was strong, waterproof, set quickly (in hours), set under water, and was used in concretes, mortars, and, very extensively, in stucco work.

COMPETITION 

The Reverend Parker's patent ran out in 1811, and similar materials were produced almost immediately in Swanscombe, Harwich and Whitby and later at other sites throughout the UK. The same search for different sources was happening in France and the rest of Europe, in Russia, the USA and Canada.  Each source produced different cement with different properties, characteristics and consequent uses reflecting the different "Natural" limestone.  Some are similar to Hydraulic Limes in make up, others are rich in Magnesium, some set in hours, others within seconds, some have relatively high compressive strengths when set, others have little strength.  Colours varied from brown to beige.

At that time, it was not so easy to move bulk products around and these cements, from natural sources, were not necessarily found and manufactured near the cities.  Parker had been lucky, he was on the Thames with easy access to London, and his cement rapidly became the standard product to be used for the stucco work of Georgian London which had become the vogue, the use of wood having not long been banned.  In other places, an alternative was needed.

PORTLAND CEMENT

Louis Vicat working in France had the alternative, to make cement industrially, an artificial cement that would duplicate the combination of minerals and thus the properties of Natural Cements and Hydraulic Limes.


Louis Vicat
Messrs Aspdin, Frost etc in the UK, and other pioneers in France and Germany rapidly took up the development and so the "artificial" Portland cement industry was borne from the efforts to emulate the properties of the Natural Cements, which had begun to change the way civil engineers worked.  It is no coincidence that North Kent and Boulogne, where the first cement-stones were found, later became important centres of artificial Portland Cement production.  The local limestone contains clay, reducing the need for further addition in the factory.

Although Vicat himself pursued the "artificial lime" option with vigour, he remained aware of the superior qualities of Natural Cements, recommending their use where they were available.  Much later, in 1855, he helped his son, Joseph, set up his first factory  at Grenoble (and the company "Ciments Vicat") and that produced a Natural Cement as well as an artificial 'Portland' type cement, and a type of "Marine" (Sulphate Resisting) cement.

For most of the 19th Century in Europe, and for longer in the USA, the two products (Natural and Artificial) were sold alongside each other at different prices and for different purposes.

In the USA, Natural Cements were used to build first the Erie and then other canals and by the turn of the century the production of Natural Cements was a very extensive industry indeed.  Rosendale cement from New York State was among the most successful with some 25 mines exploiting a very large, consistent and homogenous seam of material.  There were many other producers throughout the country.  Rosendale had the advantage of access to New York, then growing rapidly.

It was not until the end of the 19th Century that the "Artificial" cement plants introduced higher firing temperatures and rotary kilns enabling the use of lower grade limestone.  This produced a much lower cost Portland cement with far greater compressive strength.  With the higher firing temperatures the C3S reaction of modern Portland cement becomes the predominant reaction, hence the greater strength.  Prior to then it was the C2S reaction (as in Prompt Natural Cement) that was the most important.  This change in process allowed the use of a wider variety of limestone thus enabling limestone's in more convenient localities to be used.  With improvements in the transport systems, it was soon universally available.  The decline in use of Natural Cements began.  The last production in the UK seems to have been at Harwich in the 1951's.  In the USA production did not cease until 1971.

PRESENT DAY PRODUCTION

In France, Spain, Italy and Switzerland, however, the story was a little different. Small scale production of Natural Cement has never stopped and continues to the present day.  All but one are products that are similar to Hydraulic Limes and are used, particularly, in restoration work.

The one exception is a particular Natural Cement mined in the French Alps near Grenobles.  This is quite different from the others that are available or have been available in the past, and is categorised in its own right under a French Norm (NF P15-314) as PROMPT NATURAL CEMENT.  It cannot be classified as a lime.  It is very much faster setting and rapid hardening than others.  Some 211,111 MT of this material are produced annually, plus a further quantity of derivative binders.  Commercial production started in 1842, at a factory called PORTE DE FRANCE on the then border with Italy, which is now owned by the company Ciments Vicat.  At one time there were many other centres of production of this peculiar cement, throughout South and South-east France, and in the Lot and Dordogne areas.  It was first manufactured at Poully en Auxois in 1827, and then Vassy in 1832.  Only the mine at Grenoble survives today.

The seam of argillaceous limestone exploited by this production is large, stable and consistent.  Firing is at moderate temperatures across a wide range (500-1200°C) in traditional vertical kilns using generations of skill and judgement in charging each kiln and balancing the results to produce a consistent standard product from the natural limestone.  Most of the rock is never sintered, and consequently the cement remains chemically stable and predictable.  The complex chemistry thus produced gives the cement a number of properties, including the very fast setting and hardening reactions (thus no curing period) and no release of "Free Lime".  This enhances its resistance to acid attack, waterproof properties, safety with drinking water and the maintenance of a high level of alkalinity which in turn means that any rusting of reinforcements is totally inhibited.

This particular Prompt Natural Cement continues in use to this day throughout France and Italy, to a lesser extent in Spain and Germany and very recently in the UK and is exported around the world, as far a field as Korea and Hawaii.  It is valued as a water stopping material, as a cement resistant to sea water and acid attack, as a quick repair material and as a constituent of proprietary "fast" mortars and concretes, especially for grouting and for shotcreting mixes.




RENEWED INTEREST 

Some 21 years ago, the SNCF (French Railway) engineers were designing tunnels for the TGV train network.  With the most sophisticated resources of the major cement and chemical admixture producers at their disposal, there were considerable problems meeting the designers' specifications.  High strengths were possible, but not without the brittleness that comes with ultra fast strength gain.  Fast setting to combat water ingress was possible, but not without a cocktail of admixtures whose interactions were not totally understood and had Health and Safety implications.  So it was that Prompt Natural Cement was "rediscovered".  Changes to the production methods increased the compressive strengths to exceed the engineering requirements.  The mine at Grenobles was modernised to increase production.It is interesting to see this traditional material helping the modern engineer overcome some of the most complex and difficult problems of the late twentieth century. The qualities of Prompt offer real benefits in expanding industries like eco-construction or in contemporary construction.


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