Natural or Roman cement
Natural cement, initially – though improperly – called “Roman cement”, is quick-setting. It was the first cement in the modern sense of the term, and at the time was a technological breakthrough.
Natural or Roman cements were used in many 19th-century constructions. They were favoured for 4 main reasons: speed of setting, strength of mortar, aesthetics and the durability of the resulting constructions.
Since the famous Roman mortars – mixtures of sand, lime and pouzzalana – no progress was made in hydraulic binders until the end of the 17th century.
The first progress was made in England. Parker filed a patent in 1796 concerning the baking of marl nodules (septaria). This was a major invention. It showed that a highly hydraulic binder, compared to the lean limes and lime-pouzzalana mixtures used at the time, was possible by low temperature firing (below malting point) of lime with a greater-than-normal clay content, without hydrating the fired stone – just by grinding it. At the beginning of the 19th century, this marl (argillaceous limestone) firing process spread across the whole of continental Europe. This cement was called “Roman cement”. The term “Roman” is a misnomer, since it was not the reinvention of the mortar used in Roman times.
In the 19th century, a confusion occurred between names on a single production site. The following terms were used: natural cement, quick-setting cement, Prompt cement, Roman cement, and elsewhere, plaster cement.
The correct name would be “natural quick-setting cement”.
Reasons for the increasing popularity of this cement throughout the 19th century :
- it brought an economic and lasting solution for façade decoration. It perfectly imitates stone without the cost of stone, and produces a warm, ocher yellow–brown colour. It is used either on bricks in elements (cornices, etc.), applied in situ or in prefabricated mouldings or in imitated stone cement,
- its rapid hydraulicity properties brought effective solutions for bridges and suchlike, especially in contact with water,
- it laid the foundations for the prefabrication industry, especially for water pipes: natural cement pipes were more resistant to corrosive water than pipes made of the first artificial Portland cements of the period,
- it is easy to manufacture.
It is easy to manufacture because :
- Of the availability of raw material thanks to the fact that firing is done at low temperatures, below the fusion point, allowing marl or argillaceous limestone to be used, with an argillaceous content of between 22% and 35%. It is therefore possible to use various sources from different geographical locations. Even though Louis Vicat showed in 1817 that it was possible to make an equivalent cement with a mix of artificial clay and limestone, the grinding tools and the energy costs of the period made it difficult to make this clay limestone mixture cheaply. For this reason the “natural” combination of these two components in marls was preferred.
- The firing technology was that of the original limestone kilns, which are simple and still in service today. In contrast to lime, Roman cement is not subject to extinction, because it practically contains no quicklime. It is simply ground.
Upper part of the kiln at the La Pérelle plant circa 1900, France