Natural Hydraulic Limes: A Plaster in a League of its Own

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What distinguishes Natural Hydraulic Lime (NHL) from lime in general? As we previously considered, early civilizations such as the Egyptians and Greeks used lime extensively. Many of their works in lime have survived to the present day testifying to its durability and intrinsic beauty.

Patrick Webb

NHL stuccoes and mortars are used widely in historic preservation, courtesy of Lafarge

The Romans of course are famous for their widespread, imposing and durable works of architecture. They were the first to evidence a more technical understanding of lime generally and to make widespread use specifically of hydraulic limes (limes that would set with water) for ports, aqueducts and monumental architecture. Many of these works were accomplished with additions to lime to make them hydraulic. These “pozzolanic” limes are a subject we will consider in a future article. However, the Romans also exploited limestone deposits in the province of Gaul, the modern day Languedoc and Provence regions of France, which produced limes that were “naturally” hydraulic without pozzolanic additions.

The Romans brought their lime technology in the conquest of North Africa. The tradition of using natural hydraulic limes (NHL) continued for water cisterns, stuccoes and objets d’arte. NHL's continued to be used throughout the medieval period. At the height of the Renaissance, Palladio made specific 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, rather an oxide of calcium. With this scientific knowledge established, French engineer Louis Vicat conducted an exhaustive study and published a landmark, comprehensive paper in 1818 classifying limes based on properties of hydraulicity such as rapidity of set with water and corresponding compressive strength.

Chemistry & Manufacture

Patrick Webb

The NHL Cycle

What makes a lime “hydraulic”? As you might guess it has to do with water. Perhaps we can start by first reviewing 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, slowly reabsorbing carbon dioxide and returning to its original state of calcite, CaCO3.

By contrast, hydraulic materials exhibit a chemical change in the presence of water. Moulding plaster and Portland cement are common examples of hydraulic materials. NHLs mixed with water likewise transform from a putty to a hardened state, even underwater.

Limestone is a sedimentary stone that forms primarily though not exclusively 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 that remain relatively porous. Under certain geologic conditions impurities can leach in, deeply infiltrating the limestone over time. There exist several categories of impure limes (dolmitic, magnesium, natural cement i.e.); however, the subject of this article focuses on just one of these: natural hydraulic lime or NHL whose valued impurity is silica.

Common silicas like quartz are very prevalent, highly crystalline and non-reactive. Amorphous, chemically active silicas on the other hand don’t tend to last very long in nature because they are very reactive. The most useful limestones for producing NHL’s have a high infiltration of this amorphous silica.

These limestones are fired a little hotter than pure limestone, approximately 1800 degrees F to 1900 degrees F, to both drive off the carbon dioxide and fuse a portion of the available calcium with silica. After firing the lime is given just enough steam to “slake” or “hydrate” it, converting the highly reactive calcium oxide into the more stable calcium hydroxide. The resulting dry hydrate lime can be bagged and is available to react with the silica compounds by the addition of more water.

The terms hydrated and hydraulic can be a little confusing as they both relate to water. A hydrated material is chemically bound to water in a dry state whereas a hydraulic material is one that can set with water. The terms are not mutually exclusive and as it turns out NHLs are made both hydrated and hydraulic during manufacture.

Properties & Specifications

Patrick Webb

An early scientific specification of NHL mortar in England based on tested hydraulic properties and compressive strength: Smeaton's Tower, 1756.

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 and correspondingly high flexural strength of NHLs are now recognized as fantastic properties for mortar and stucco. The flexibility of natural hydraulic lime as a binder in mortar and stucco mixes reduce cracking, allowing wall assemblies often to bend rather than break when subject to typical settling over time.

Likewise the increased porosity of NHL stuccoes permits water permeating the coating to readily escape again through the surface. This same porosity is also of great benefit to masonry work permitting soluble salts to slowly deteriorate the sacrificial mortar (which can be re-pointed), protecting the more valuable brick or stone supports.

NHLs thus preserve many of the benefits of porosity and flexibility of pure lime mortars and stuccoes. However, the hydraulic properties imbue mortars with increased early compressive strength allowing masonry work to be carried out more quickly. Likewise for stuccoes, hydraulicity helps provide protection from early freeze thaw cycles and improved long term resistance to erosion for especially exposed exterior surfaces.

In our next article we'll consider a limestone that has a slightly different “contamination” that results in a mortar with very interesting properties: Natural Cement.