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 (dolomitic, 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.
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.
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!
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.
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.
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