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| Traditional French NHL stucco, mortar |
As we considered in a previous post, lime is perhaps the
most prized and exceedingly versatile building material of the modern world.
Early civilizations such as the Egyptians, Greeks and Romans used lime
extensively. Many of their works in lime have survived to the present day
testifying to its durability and intrinsic beauty.
Today, lime is processed into plasters, stucco coatings,
paints, mortars and cements. Chemically defined, pure limestone is a carbonate
of calcium or calcite having the formula CaCO3. However, there exist
several categories of impure limes (dolmitic, magnesium, natural cement i.e.). The
subject of our post today focuses on one of these: natural hydraulic lime or
NHL. As we shall discover, sometimes impurities result in interesting and very
useful properties.
Hydraulicity
What makes a lime “hydraulic”? As you might guess it has to
do with water. Perhaps we can start by considering a non-hydraulic example,
pure lime*. When water is added to pure lime it forms a putty. As long as the mix
is kept covered the lime will stay in a putty state. Pure lime only reacts
chemically when exposed to air, reabsorbing carbon dioxide and returning to its
original state of calcite, CaCO3.
Hydraulic materials however, exhibit a chemical change with
water or in the presence of water. Moulding plaster and Portland cement are
common examples of hydraulic materials. NHL’s mixed with water quickly transform
from a putty to a hardened state, even underwater.
Geology
Where do hydraulic limes come from? Limestone is a
sedimentary rock that forms from skeletons of marine creatures that have
accumulated on the sea floor. With time and pressure these skeletons are
pressed together in beds of stone. Nevertheless, limestone remains relatively
porous and under certain geologic conditions impurities can leach into or
infiltrate the stone over time. A valued impurity for NHL is silica.
Common silica’s like quartz are very prevalent, highly
crystalline and non-reactive. Amorphous, chemically active silica’s on the
other hand don’t tend to last very long in nature because they are very
reactive, especially with lime. The most useful limestones for producing NHL’s
have a high amorphous silica content. These limestones are cooked a little
hotter than pure limestone, approximately 1100 to 1200 °C, to
drive off the carbon dioxide. Once the carbon dioxide is driven off the lime is
available to react with the amorphous silica, just add water!
History
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| Tadelakt objets d'arte, Marrakech souk |
The Romans were famous for their great works of
architecture. They were the first to have a level of understanding and to make
widespread use of hydraulic limes for ports, aqueducts and monumental
architecture. Many of these works were accomplished with additions to lime to
make them hydraulic. Pozzolanic lime is a subject we will consider in a future
post. However, there is also evidence to support that the Romans exploited
limestone deposits in the province of Gaul, modern day Languedoc and Provence
regions of France, which produced limes that were inherently hydraulic without
additions.
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| Eddystone Lighthouse, 1756 |
The Romans brought their lime technology in the conquest of
North Africa. The tradition of using natural hydraulic limes continued for
water cisterns, stucco and objets d’arte. During the Renaissance Palladio makes
mention of hydraulic limes in his architectural treatise. By the 18th
century English and French engineers were hard at work identifying quality
mineral deposits and exploiting them for public works. Advances in modern
chemistry led to the 1807 discovery that lime was not an element but an oxide
of calcium. With this knowledge established, French engineer Louis Vicat conducted
an exhaustive study and published a landmark, comprehensive paper in 1818 classifying
limes on the basis of hydraulicity and compressive strength.
Contemporary Use
With the advent of Portland cement in the 19th
century, NHL production decreased dramatically. The faster set, harder
compressive strengths and impermeability of Portland cement were considered
superior qualities that allowed buildings to be constructed faster and cheaper.
However, with the passage of time and a large inventory of buildings using both
materials, advantages of NHL have become clear and production is once again on
the increase.
The lower compressive strength of NHL is now appreciated as
a good quality for mortar and stucco. The flexibility of natural hydraulic lime
reduces cracking, allowing wall assemblies often to bend rather than break when
subject to typical settling over time. The increased porosity of NHL stuccoes facilitates
water that penetrates the coating to readily escape through the surface. This
same porosity is also of great benefit to masonry work permitting soluble salts
to slowly deteriorate the mortar (which can be re-pointed), protecting the more
valuable brick or stone supports. NHL’s thus preserve many of the benefits of
pure lime mortars and stuccoes whilst allowing masonry and stucco work to be
conducted at a faster rate and under a greater range of weather conditions.
*Technically there is a chemical reaction of quicklime, CaO,
with water to form slaked lime Ca(OH)2.
However, this article refers to the common definition of a hydraulic
lime reaction, the formation of calcium silicates.
Contributed by Patrick Webb
Contributed by Patrick Webb
