Technology of lime

The curing of lime based mortars depends largely on temperature and moisture content.

Historical use of lime

Mortars have been used in building for at least the past 8500 years. Lime mortars have been found at archaeological sites in Jordan and Serbia dating back approximately to 6000 BC. The abundance of the raw material and easy production have been the key to its success. The "modern" and sophisticated mortar technology was a Roman innovation, with evidence still surviving in many cities throughout Southern Europe.

Production of lime binders

Limestone (Calcium carbonate) is the raw material for all production of lime. Lime is the binder in any lime-based mortar. Depending on the purity of the limestone the final chemical and hardening properties will vary. Impurities affecting its final properties are mainly clays based on Silica-, Ferrous- and Alumina minerals (eg quartz, plagioclases, mica). The applied burning technique (temperature, time and fuel) will affect the lime quality and will be carefully adapted to obtain the planned finished product. Limestone lumps are burnt at apx 950 degrees Celsius in up to 24 hours in a vertical or horizontal lime kiln. When heated, the calcium and magnesium carbonates will be reduced to oxides, also called unslaked lime (calcium oxide, burnt lime, quicklime). This unslaked lime reacts easily with water to form the binder used in mortars. In chemically correct amounts of water a dry powder is formed: hydrated lime. This one used in most modern applications. Todays dry slaked limes are of high purity with up to 99,9% Ca(OH)2). Impurities and unslaked residue removed before bagging.

When slaking in excess amounts of water, wetslaked/limeputty is formed. Different techniques and traditions for the latter may produce wet slaked limes with high plastic properties. Aged slaked lime is a high value material used for conservation and high end applications.

Lime hardening

Pure lime hardens in contact with CO2 dissolved in moisture. This hardening process happens from the outside and inwards- the carbonation process depending on temperature and moisture content. The resulting mortar obtains apx 1MPa without any additives- although this strenght might take time (a few months to several years) to obtain. NHLs harden with both water AND with the CO2 form moisture and ressembles the hardening process of cement. NHL gains strenght more rapidly that pure lime mortars, but retains the flexibility and dampdiffusion abilities of lime. The amount of impurities in the raw material decides the end strenght - heras NHL2, NHL3,5 and NHL 5 represents the minimum strenght after 28 days of curing in controlled mixtures and hardening process (2MPa, 3,5MPa and 5 MPa, ISO EN 459-1). The final strenght of the mortar is depending on the composition of the mortar (sand, binders and water) as well as the hardening process.

Lime behaviour

 So called NHLs (natural hydraulic Lime, noted NHL 2, NHL3,5 and NHL 5) are produced with a high amount of impurities in the raw material and burnt differently to obtain not only calcium oxide but also Alite and Belite (higher content in NHL 5 than NHL 2, but always at least 15% free lime/Ca(OH2)) that contributes with the hydraulicity and ability to react also with water in the hardening process. NHLs are always produced as a dry powder as its hardening process starts in contact with water. All building limes  are regulated by ISO-EN 459-1

Carbonation and hardening of lime mortars

Pure lime hardens when in contact with CO2 dissolved in moisture. This carbonation process depends on the CO2 supply, temperature and moisture content and happens from the outside and inwards. The resulting mortar obtains approximately 1 MPa without any additives, although it might take time (a few months to several years) to obtain this strength. This process may lead to thick mortars remaining uncarbonated for several hundred years due to the lack of soluble CO2. Lime putty can mature in the bucket and have undefined storage time.

NHLs harden with both water and CO2 from moisture, resembling the hardening process of cement as the alite and belite react with the water. NHL gains strength more rapidly than pure lime mortars, but retains most of the flexibility and damp diffusion abilities of lime. The amount of impurities in the raw material decides the final strength - here NHL 2, NHL 3.5 and NHL 5 represent the minimum compressional strength after 28 days of curing in controlled mixtures and the hardening process (2 MPa, 3.5 MPa and 5 MPa, ISO EN 459-1). The final strength of the mortar depends on the composition of the mortar (sand, binders and water) as well as the hardening process. The different types of lime binders makes it suitable for designing mortars for specific projects and it is designed to be compatible with the materials originally used.

Lime properties and self healing crystallization in microcracks

The curing of lime-based mortars is largely dependent on the temperature and moisture content. Optimal hardening is obtained at approximately 15 degrees Celsius, with regular moistening of the surfaces. Lime-based mortars are flexible over time, have self-healing properties (carbonation/recrystallisation in microcracks) and allow for damp-diffusion. The chemical process involved turns the hydrated lime into carbonates naturally occurring in nature. Lime-based mortars , as with all carbonates (limestones), are vulnerable to pollution (acids) and will dissolve.