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The history of hydraulic lime mortars In
the 4th century B.C., the Greeks discovered that specific layers of
volcanic deposits, mixed with slaked lime and sand, formed mortars, which could
harden under water and were afterwards water-resistant.
Roman architects experienced that the tuff layers at the base of the
Vesuvius, near the village Pozzuoli, gave an extremely hydraulic reaction when
combined with slaked lime. The
mortar, composed of these components, was called “Pozzolana”, a still
current term. The
Romans exported the knowledge of the pozzolanas and the hydraulic lime mortars
towards their ruled regions. Consequently,
it was necessary to look for new exploitations for materials with the same
hydraulic characteristics. On the
Greek isles, the volcanic ashes of Santorini were suitable, and near the present
Syria, pumice was used to compose mortars.
In north and central Europe, “trass” of the German Eiffel region was
used for this purpose. The
Romans, also invented another type of hydraulic mortar, based on lime and
crushed ceramics. Nowadays, this mortar type is known as “cocciopesto”, and
gave the Romans the opportunity, to produce hydraulic mortars over the entire
empire. With
different mortar mixtures, the Romans developed a new construction technique : a
mixture of lime, cocciopesto, pozzolanas and aggregates was poured into wooden
framings to construct colonnades and vaulting.
Nowadays, this product is known as “Roman lime”, although this is a
wrong terminology; specialists prefer the specification “Roman cement”. Domes
and cupolas were constructed with Roman cement, in which the pozzolanas were
replaced by lightweight pumice. Another
technique was to arm the concrete by means of crushed bricks or silt poured in
prefabricated structures. Although
Vitrivius has left the detailed description of these techniques, and though this
information was still known in the Middle Ages, the knowledge and the use of
materials and technology started to faint in the 18th century, as
mainly the hydraulic components seemed to be exhausted.
Only the use of crushed ceramics and bricks survived, and often the
material was empirically mixed with lime, leading to a doubtful quality. Modern mortars The
post-war reconstruction caused a gradual replacement of lime mortars by cement
mortars, which are still the basis construction materials.
The growing importance of the new cement binder summarizes the desired
qualities in a construction sector, transforming from a traditional activity to
an industrial business. As the use
of cement increased, and the specific characteristics of this material allowed
the fast and cheap production of huge quantities, the traditional knowledge
using lime mortars became blurred, often to disappear completely.
The
development of cement mortars distorted all concepts to such an extent, that
nowadays a “traditional mortar” is composed of a sand-cement mixture.
But is tradition nothing more then a deliberated business plan?
The
damage however, caused by this plan, is highly visible in modern buildings.
The quest for alternative solutions, calls to mind the craftsmen which
could work with the traditional materials, but which will also be capable to
translate professional skill into a modern approach concerning industrial yards.
Luckily,
some remnants of real tradition and of manuscripts have been preserved.
This makes it possible to develop specific products, which further supply
the lack in recuperating traditional methods and the use of natural products.
Hydraulic lime Plinius
was the first to mention that burning bituminous rocks with lime, should give a
binder, which could harden under the waterline.
Vitruvius confirmed the existence of a hydraulic reaction, but attributed
this property to the mixing of lime with pozzolanas.
In
the Middle Ages, impure limestones, from selected quarries, were used to produce
lime, which had to be slaked at the very last moment on the yards.
The reason was a very fast hardening reaction, even with and under water. During
the Renaissance time, numerous manuals, like Palladio’s, described the
hydraulic characteristics of the limestones of the “Colli Euganei”.
The study on hydraulic lime mortar mainly took place during the second
half of the 18th century, when it was discovered that marly
limestones (septaria), burnt at 800°C, resulted in a hydraulic lime, also
called “Roman cement”. In
the 19th century, the investigators Smeaton, Forst and Vicat produced
a lime out of burnt marl, describing the exact chemical properties of this type
of rock. The resulting product was
defined as a “natural hydraulic lime”.
The name was used to distinguish from the artificial limes, developed out
of Portland cement. Portland-derived
limes were defined as “eminently” hydraulic limes, but it is really a
different product with the same characteristics (chemically and physically) as
Portland cement. As
scientific investigation would not stagnate, also “artificial” hydraulic
lime was commercialised, a well-balanced mixture of limestone and clay, with the
same properties as natural hydraulic lime.
Most manufacturers will however ignore the rules and the fabrication
procedure, and will compose a mixture after burning and slaking.
Natural hydraulic lime Burning
marly limestones between 800°C and 1000°C produces natural hydraulic lime.
At this temperature, hydraulic reactive components like
calcium-silicates, calcium-aluminates, and calcium-aluminium-ferrates will be
formed. At
these temperatures, no other salts will be released, and the hardening and
setting behaviour will be the same as with air-hardening lime.
These properties are also important for the elasticity and porosity of
the products, but natural hydraulic lime will be more weather- and
impact-resistant then air-hardening limes, and is insensible to moisture
infiltration and more harsh weather conditions during the carbonation of the
product. San Romedio natural hydraulic lime This
natural hydraulic lime is produced in the quarries “Miniera di San Romedio”,
since the late 19th century. The
limestone quarry has been managed by the firm Tassulo S.p.a. since the beginning
of the 20th century, and has been developed into a high-tech and
industrial group, still improving the qualities of the product.
Thanks
to the scientific equipment of the laboratory, the physico-chemical aspects of
the raw material have been studied to depth, to develop specific products,
appreciated by everyone for the outstanding qualities and application
possibilities. Chemically,
natural hydraulic lime for rendering, plastering and masonry construction,
consists of bicalcium-silicates (C2S).
This specific binder will not suffer of moisture rising in walls, but
will even withdraw water for the chemical hardening reaction. The
product is produced by burning marls from specific geological strata at
relatively low temperatures (900-1000°C).
After burning, the material is partially slaked, crushed and pulverized,
to obtain a very porous structure. The
amount of hydraulic components will eventually be increased by the addition of
pozzolanas, if necessary. |
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Arte Constructo bvba - Molenberglei 18 - B- 2627 Schelle (Belgium) tel: +32 (0)3 880.73.73 fax: +32 (0)3 880.73.70
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