Building materials and techniques used by architects and artisans at the time of the Republic of Venice were substantially based on those described by Vitruvius in his book De Architectura at the time of the Roman Empire. However, the Roman art of building was the heritage of the Greek culture and the construction technique know-how, including building materials, came from the Egyptians and Persians through a slow transmission process of the oriental culture (particularly from East Iran) through the Mesopotamian and the Mediterranean civilization.
Lime may be seen as a commodity but it is the noblest building material ever used by humankind. It has an impressive tradition in the history of architecture. The Greeks, Egyptians, Mayas, Incas as well as the first Chinese and Indian dynasties are a just a few examples of cultures that left architectural treasures essentially composed of lime. The earliest plasters known to us were lime-based. Around 7500 BC, the people of ‘Ain Ghazal in Jordan used lime mixed with unheated crushed limestone to make plaster which was used on a large scale for covering walls and floors. In ancient India and China, renders in clay and gypsum plasters were used to produce a smooth surface over rough stone or mud brick walls, while in early Egyptian tombs, walls were coated with lime and gypsum plaster and the finished surface was often painted or decorated. The Romans used mixtures of lime and sand to build up preparatory layers over which finer applications of gypsum, lime, sand and marble dust were made. Following the fall of the Roman Empire, the addition of marble dust to plaster allows for the production of fine details. Hard, smooth finishes in hand-modeled decoration were not used until the Renaissance. Later the Italians and the French used stucco techniques to produce prestigious frescoes and precious wall textures we can still admire today. In the last 20 years we have rediscovered these proven ways to protect and embellish buildings inside and out.
Lime plaster is composed of calcium oxide (lime), obtained by heating calcium carbonate (limestone, marble, chalk and shells) in a kiln to between 900°C and 1200°C. Carbon dioxide (and any water) is driven off leaving anhydrous calcium oxide or quicklime, sometimes described as non-slaked lime or, misleadingly, as lump lime. Quicklime soaked in water changes to calcium hydroxide or slaked lime. When dried and ground to a fine powder, it is called hydrated lime or lime hydrate.
Types of lime
Even though in common language the word “lime” is used in a generic way, it designates differences depending on chemical structure, properties, and applications. eco stucco™ is composed of two types of lime: air lime and hydraulic lime.
Air lime: It is obtained by heating calcareous stones that do not contain more than 5% of clay matter. This type of lime when very pure in carbonates (over 95%) and because of its creamy texture when mixed with water was also called “fat lime” in the past. When purity is inferior and the level of clay reached 5%, this types of lime is called “lean lime” because the obtained paste is not a creamy. The pastes are therefore mixes of sand and/or marble powder and lime in a variety of ratios. Air lime may be raw (calcium oxide or quicklime) or hydrated (calcium hydroxide). The setting agent is carbon dioxide which gets absorbed and sequestered during the curing process.
Hydraulic lime: When the stone is rich in clay, iron, and alumina we obtain a lime called “hydraulic”. The setting agents are both water and carbon dioxide. The clay content determines the hydraulic ratio of the lime. Natural Hydraulic Lime (NHL) is then classified for different uses: Feebly hydraulic lime NHL 2 (<8% clay content)is used in interior and external sheltered areas; moderately hydraulic lime NHL 3.5 (8 to 19% clay content) is used in most exterior areas, and eminently hydraulic lime NHL 5 (>20% clay content) is used in harsh environments.
The preparation of lime plaster involves a calcium carbonate (limestone) as the raw material and its conversion into lime, which results in a workable compound binding all the ingredients. First quicklime is obtained from the heating of limestone at 1600° F (versus 2700° F for Portland cement). At that stage, quicklime (calcium oxide) may be converted into calcium hydroxide (hydrated lime) by adding a controlled amount of water to adjust the chemical affinity. This process disintegrates the lumps, pebbles, and granules found in quicklime to obtain a fine white powder ready to be mixed with the appropriate aggregates and mineral admixtures. As the lime plaster is applied onto the wall it cures and hardens slowly reabsorbing and sequestering carbon dioxide. In the process the calcium hydroxide is converted back into calcium carbonate (limestone), producing an durable, protective mineral crust.