The Lime-Fake

Especially in the field of exterior lime plasters and hemp bricks, innovative lime products are currently being marketed and the technically untrained people are being sold for fools.

Whenever “lime” is mentioned, it should be clear which lime is being referred to. Two types of lime are commonly used in the construction industry and could hardly be more opposite.

The first is aerial lime, usually in the form of hydrated lime or lime putty. The other is hydraulic lime. There is a natural hydraulic lime which differs from normal hydraulic lime only in that no artificial hydraulic factors are added.

Limestone, which occurs often in nature, is the basic material for all limes and cements. For aerial lime the purest (white) lime/calcium carbonate stone is taken, for hydraulic lime and cement the limestone contaminated with clay to benefit from the hydraulic effect of the clay.

The main difference between aerial lime, hydraulic lime and cement, apart from the purity of the limestone, is its burning temperature for its production.
Aerial lime between 900-1,000°C; hydraulic lime 1,000-1,200°C; cement up to 1,450°C.

First of all, it is interesting to know that the highest possible temperature of a natural fire is 970°C . That is why, for example, the rock wool used for building fire protection is stated to have a melting point of more than 1,000°C. The historic lime kilns, which had been used for centuries, were wood-fired with temperatures of … maximum 970°C.

The hydraulic limes and cements burnt at well over 1,000°C undergo an irreversible material transformation that would be impossible in nature. Due to the energy required for this and the processes associated with the material transformation, the cement industry is responsible for 8% of global CO₂ emissions. Hydraulic lime does not play a role in terms of its production volume in relation to cement, but because of the similar production process it differs only insignificantly in its damage to the environment.

Additional hazards come from the fuels used and the addition of industrial ashes as additional hydraule factors (natural hydraule factors could be volcanic ashes or the flour of low-fired clay bricks). The addition of additives necessary for production for natural hydraulic limes is limited to 0.5%, but this does not make it a natural building material.

Recycling is only possible to a limited extent due to the material transformation at temperatures above 1,000°C. Recycling always means a compromise in contrast to the possibility of unlimited reuse of earth building materials: Recycling means “shredding” and using for secondary tasks such as fill material in road construction, for example. I have also made an interesting contribution to the CO₂ footprint of lime plasters in my article “Clay plasters”.

At the latest, the effects of material transformation on building material behaviour should be of interest.
Traditional aerial lime has gained its good reputation through a whole series of typical properties: good plastic working properties, more “elastic”, antiseptic, breathable, CO₂-binding. Hydraulic binders shine first and foremost because they are stronger.

Aerial lime can only be used in layers up to 1.5 cm thick because CO₂ has to be extracted from the air for carbonation=hardening. This is why air-lime is not suitable for use in greater thicknesses such as in rammed earth construction or hemp bricks.

Advantages and disadvantages in the use of the two types of lime are characterised in particular by their pore size.

Aerial lime has large pores. Water penetrates less deeply because the capillary strength is lower. Hydraulic lime has small pores. Water penetrates more easily (a small diameter straw makes the transport of liquid easier than with a thick straw, which requires much more force). At the same time, large pores allow better air-flow and thus drying of penetrated moisture.

Moral of the story: Hydraulic lime is not much different from cement, just less hard than cement. Of course, natural hydraulic lime is not a natural building material but hydraulic lime without artificial hydraulic factors. Just look at who the producers of (natural) hydraulic limes are. In most of the cases it is the cement industry.
As an exterior plaster, hydraulic lime is not comparable with the properties of a traditional air-lime plaster because of its small pore size.
Hempcrete blocks have a hydraulic lime content of about 15%. To call this building material ecological or 100% natural is miss-leading and irresponsible.