Lime: a building material with history


Natural Hydraulic Limes - NHL

NHL is the abbreviation for Natural Hydraulic Lime. NHL is available in different strength groups. NHL 2 is the softest among the natural hydraulic limes. It is used for relatively soft plaster and for soft wall joints, such as those found in a half-timbered house. NHL 3.5 is a little stronger and is used to mix plaster or mortar in brick construction. The NHL 5 is much firmer. It is ideal for use in solid, modern masonry.

The building materials made of lime, i.e. plaster or mortar, always consist of a mixture of sand and an NHL. The strength class of the NHL and the mixing ratio between lime and sand determine the strength mixture. Additional additives can be added to influence the properties. For example, charcoal ash accelerates the setting process and citric acid slows it down, brick dust improves staying power and lightweight aggregates have an insulating effect.


The chemical composition of the limestone at the mining site determines the strength of the natural limes. Different silicates in limestone are responsible for the respective strength.

Furthermore, the limes differ in their color: depending on the storage location of the raw rock and its purity, the limestone is sometimes white or gray. A particularly light limestone such as Calix Blanca, which is mined in northern Italy, is popular for the production of fine plasters and lime paints because of its color.

So-called air limes are extremely pure limes which, however, can only achieve very low strengths. The firmness of white lime hydrate, a classic air lime, is below that of an NHL 2.


Be careful in the jungle of abbreviations: An NHL is a naturally hydraulic lime. An HL, in turn, is a so-called Hydraulic Lime. You will not get this from us, as cements and / or pozzolans, for example fly ash, are added to an HL. Fly ash is fine dust trapped in the chimney.

Lime binder

Trass lime is an exception to the natural lime.  Trass lim/trass is formulated as an NHL. That means: the NHL 2 is mixed with a substance that improves its function. The surcharge is also here a pozzolan, but in this case a natural trass flour. Trass is a rock consisting of clays, silicates, limestone and iron oxide, the components of which are fused together by the action of heat. In connection with the NHL 2 and water, an enormous binding capacity and compressive strength can be achieved. Trass lime is mostly used for the foundations of half-timbered buildings or in the preservation of monuments.

Cement is not an ecological building material. Products with cement achieve a strength that in most cases is not necessary in house construction. A plaster with cement additives is too hard for most substrates - and for the resident too. Due to the hardness, the substrate and the plaster do not work with each other, but against each other. Inevitably, cracks occur, which then lead to further damage.

Manufacture of lime and cement. For the production of building lime, the rock is mined in the limestone quarry, crushed and then burned. Put simply: the temperature during firing determines whether building lime or cement is produced. During the production of cement, the temperature is significantly higher - but also the required energy. Unfortunately, waste often also functions as fuel in cement production, as it has a high calorific value; these are the so-called FLUFF fuels. Due to the high temperature, the limestone is "burned to death" during the production of cement. Moisture absorption and regulation of the room climate hardly take place at all in cement-based plaster.

Swamp lime, burnt lime and air lime

Various materials can be extracted during the lime cycle. The lime that has not yet been slaked is the so-called burnt lime. This reacts strongly with water. Careless work with quicklime is grossly negligent! Extinguishing burnt lime is exciting, but also very dangerous. It should be urgently ensured that there is enough water available as security and that not too much burnt lime is extinguished at once. Incidentally, a plastic bucket is not a suitable container for extinguishing burnt lime (even if it may result in interesting sculptures).

Before ready-mixes became common, the plaster was mixed directly on the construction site. Often a burnt lime mortar was mixed for this, which is also called lime lumps (Kalkspatzen in German). For this purpose, sand and burnt lump lime are layered. This cake is then poured with water, rises in a violent chemical reaction and with strong heat development and is either used immediately as a hot plaster or as a lime plaster over the next few days. The small, light-colored lime inclusions that can be seen in the solid mortar are characteristic of this. These are the so-called lime lumps.

Swamp lime is produced by slaking lime. With swamp lime, burnt lime (i.e. either a white fine lime or a lump lime) is slaked in water. The lime remains in the water for up to two years, but the time factor depends on the size of the lumps of lime: the coarser the pieces, the longer the lime has to soak in. If you use a white fine lime, the swamp lime is ready to use the following day.

The lime can be further processed into plaster or paint for the interior. Since a plaster made from swamp lime is comparatively soft, it is not recommended to use it as a facade plaster.


Hydrated lime or swamp lime- what are the differences?

After the lime has burned, it must be extinguished with plenty of water to become a binding agent for plaster or mortar. A sump lime is created. The swamp lime now lies in the water bath for a long time and breaks down naturally there. It gets finer and finer over time.
Hydrated lime, on the other hand, is extinguished with steam. As little water as possible is used here, so that the lime is completely slaked and dry after this process. The lime slaked in this way is then mechanically crushed in a ball mill.

The swamp lime is said to have a more noble appearance. Some people swear about swamp lime: it is the purest form of lime. Swamp lime is described as having a very special depth effect and an almost "lustrous" sheen. However, the dry version of lime, i.e. hydrated lime and the finished, dry lime plasters, often work better technically. Swamp lime is not only beautiful, swamp lime is also a bit of a Diva. Perfect surfaces are usually better achieved with the mature ready-mixed plasters.

Roman cement

Roman cement, or Roman lime, is an NHL that hardens particularly quickly due to its ingredients, becomes extraordinarily hard and is a real magic agent due to its diverse application possibilities. 

Contrary to its name, however, Roman cement is free of cement: The name RomanZEMENT comes from the fact that this pure and natural lime can be harder than conventional cement.

Various types of construction can be implemented with Roman lime, which require rapid setting, high strength and watertightness. Rapid setting is necessary for setting mortars for plaster-free installation of plaster rails. A Hessler HP 9 KH, which is a base plaster with hemp fibers and 10% Roman lime, is suitable for setting mortar.

Setting cans without gypsum? An ideal case for Roman lime. The setting time is sometimes so short that you cannot use more than one can without a setting retarder. So you should only mix small amounts if you want to use the novel lime pure.

The quick repair of broken drill holes works just as well with Roman cement: after a setting time of 5 minutes, you can drill into the wall again and insert the dowel. The Roman lime can also be used to attach pins as a suspension or support.

Lime cycle - the alkalynity of lime

The mildew-inhibiting effect of a lime plaster is based on the alkalinity of the lime. The alkalinity is best explained using the lime cycle.

At the beginning there is the limestone, which is mined and burned. When burning, the stone loses the CO² it has stored. Then it is extinguished: the water is added. In the case of sump lime, the lime remains in the water; in the case of a hydrated lime, the lime is slaked, crushed in a drum mill and dried.

Mixing it with sand and water creates the plaster that is applied to the wall. During the drying process, the plaster loses its water and absorbs CO² from the air again. The lime absorbs exactly the amount of CO² that it previously lost during burning.

The absorption of the CO² is a process that, depending on the layer thickness and the available CO², can take many years. The thicker the layer, the slower the CO² penetrates into the deeper layers. The surface hardens through the absorption of the CO², but with increasing hardness it becomes more and more difficult for the CO² to penetrate the upper layers. The lower layers are flexible and firm at the same time: firm enough to adhere to the surface and flexible enough to endure the work of the surface.

The alkalinity of the lime surface lasts until the plaster has hardened, i.e. is saturated with CO² down to the deep layers. Once the surface has set, it is no longer alkaline. At this point the lime plaster loses its natural protection from unwelcome settlers such as mold or moss. The process starts all over again by applying new lime, for example when a coat of lime paint follows.

Swamp lime paint