Lime plasters used in construction are a mix of lime putty, sand and water. They slowly harden in contact with air (known as carbonation), by re-absorbing the CO₂ from the air, thus being a CO₂ -neutral, environmentally friendly building material.
Limestone in nature often contains other minerals such as clay, manganese etc, impurities which in small quantities (less than 10%) are negligible, but if applied in larger quantities they can significantly alter the properties of lime, resulting in lime mortars with special qualities.
Being outstanding architects and builders, the Romans have experimented extensively with lime, formulating many plaster mixes that stood the test of time. Cocciopesto plasters and roman cement are probably the most known traditional roman plasters.
The Romans have discovered that by adding volcanic soils and other minerals to lime, they can significantly alter its properties, especially its strength and water resistance, while retaining its breathability.
The most commonly added materials were pozzolans (volcanic soils or rock fragments) and cocciopesto (milled bricks or terracotta fragments). They reacted chemically with the free lime, forming water resistant compounds. Such mortars were able to harden quickly not only in the presence of water but even underwater in the total absence of air, and they are known as hydraulic mortars.
Cocciopesto plasters – the ancient mix of lime, sand and cocciopesto were one of the first hydraulic mortars in history, lasting for centuries. They have been extensively used by the Romans in very challenging environments including sewers, ports, spas and aqueducts. For structural waterproofing – for harbour works and foundations – the sand has been partially or fully replaced with pozzolans or cocciopesto.
Cocciopesto plasters have also been extremely popular in Venice, well suiting the humid and aggressive environment of the Venetian lagoon. Many palaces of the old Venetian Republic have been built and plastered with Cocciopesto (lime and cocciopesto mix) which peculiarly, while developing good mechanical strength, has also retained the full breathability of aerial lime plasters.
It is important to understand that the degree of porosity and breathability of lime plasters is primarily determined by the properties of lime and not by its hydraulic additives, an important factor being the firing temperature of lime. By firing limestone at low temperatures (at around 850-900 °C), the breathability of lime is retained, while the carefully selected hydraulic additives make the mortar fully waterproof. These are different from today’s NHL mortars, whose much higher firing temperatures (~1200 °C) impair their breathability.
Roman cement, as mentioned by Roman architect Vitruvius in his writings “The Ten Books on Architecture”, is also an entirely lime-based mixture, made of aerial lime, pozzolans, cocciopesto, sand and water.
Natural Hydraulic Limes (NHLs)
The first appearance of natural hydraulic lime plasters (NHLs) is marked by the Italian Renaissance. Between the late 1500s and early 1600s – contrary to Vitruvius’ recommendations – some architects used lime stone with up to 20% impurities fired at higher temperatures (up to 1200 °C). Increasing the firing temperature and the amount of impurities resulted in a “stronger” (more eminent) type of hydraulic lime.
The increased availability of natural hydraulic lime started replacing traditional aerial lime and pozzolan mixes for hydraulic works, especially when natural pozzolans were not easily available. But aside from a few sporadic uses of NHL in the historical period, the modern discovery and commercial production of NHLs can be attributed to the English engineer John Smeaton in the year 1750.
The exact transition date between Smeaton’s natural hydraulic lime (NHL) and cements is uncertain. From the late 1800s, but even more so in recent years, the industrialization and increased mass-production gradually lead to a shift in building philosophy: away from traditional, long-lasting, quality materials, in the direction of low-cost, often inferior quality, mass-produced materials that can be applied fast and set quickly.
Cements pushed up the percentages of impurities up to 40% and firing temperatures were raised up to 1500 °C, resulting in a complete melt-together of silica and alumina in a product called clinker.
Cement-based plasters or eminently hydraulic NHLs (namely cements), have a practically absent pore structure, so they are non-breathable and as a result they trap humidity, resulting in the degradation of the wall fabric.
Their high mechanical strength makes them 5 to 8 times more rigid than lime mortars, so temperature and humidity variations that generate high tensile stresses lead to the cracking and debonding of cement from the surface of the walls.
As such, for technical reasons, they are absolutely not compatible with old buildings built and plastered with lime. In conclusion, in our opinion, the use of cement and eminently hydraulic lime mixes should be limited to foundations or other works that require high mechanical strengths, otherwise they should not be used for the plastering of old buildings.
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