Organic Additives in Lime Mortar

There is a lot of talk about the use of organic additives as being potentially irresponsible. I would strongly disagree with this, The main principle for me is that people have always been idiots and if there are time proven methods and materials. they are probably as idiotproof as modern construction materials generally are. My other point in regards to this is the casual use of not time proven materials which have a major impact on mortars and plasters eg. PP fibres can make a plaster too porous. Metakaolin can make them too strong and severally reduce porosity. Or even materials which were rejected in certain areas and applications now being used such as hemp or chalk, both of which are very porous water holding materials. hemp shiv can also produce a much harder and less porous material because it is an organic additive which contains cellulose and lignosulfonates. 

So perhaps organic additives shouldn't be widely used but with the correct knowledge and experience they can vastly improve the functions of lime mortars and plasters within a safe margin unlike the aforementioned. In a similar manner to brick dust as a pozzolan which has an upper strength limit of 3,5N/mm2 as opposed to metakaolin which has no upper limit and can vastly increase in strength when used in conjunction with aggregates containing magnesium. 

Don't get me wrong there is huge potential to get things wrong when using organics casually but when used correctly and in an informed manner they are as safe as any other material. Arguably safer in a lot of cases. 

Organic additives in lime mortar are substances derived from natural sources that are added to the lime mix to modify its properties and enhance its performance. Lime mortar is traditionally made from a mixture of lime (often hydrated lime), sand, and water. Adding organic materials can improve various aspects of the mortar, such as workability, adhesion, and durability. Here are some common organic additives used in lime mortar:

1. Animal Hair (e.g., horse hair, cow hair)

• Purpose: The addition of animal hair helps reinforce the mortar. It improves the tensile strength and helps the mix hold together, especially in historical or heritage buildings. Hair fibres prevent the mortar from cracking as it dries and hardens.

• Benefit: Animal hair also contributes to better bonding between the lime and the masonry.

2. Casein (Milk Proteins)

• Purpose: Casein, a protein derived from milk, has been used historically to make mortar more flexible and waterproof. It is particularly useful in lime putty mixes.

• Benefit: Casein can enhance the durability of the mortar and its ability to resist moisture, while also providing a smoother texture and finish.

3. Seaweed Extracts (Carrageenan, Agar-agar)

• Purpose: Seaweed extracts, often in the form of carrageenan, can act as natural gelling agents. They are used to increase the workability and plasticity of lime mortars.

• Benefit: These additives help retain moisture during the setting and curing process, which can be beneficial in hot or dry climates, ensuring the mortar does not dry out too quickly.

4. Starch (e.g., wheat or corn starch)

• Purpose: Starch is sometimes added to improve the mortar’s adhesion and workability. It can be used to create a smooth paste and improve the consistency of the mix.

• Benefit: Starch helps increase the binding properties of the mortar and can make the application process easier and more efficient.

5. Sugars (e.g., molasses)

• Purpose: Sugars, like molasses, have been used in some lime mortar formulations to slow down the setting time. This can be particularly useful for large-scale applications where more working time is needed.

• Benefit: Sugars can also help in improving the hydration of lime and can provide a better bond with the masonry.

6. Animal Fats and Oils (e.g., tallow or linseed oil)

• Purpose: Animal fats or oils can help waterproof the mortar, making it more resistant to the elements. They are sometimes added to historic lime mortar to increase its durability and flexibility.

• Benefit: This addition also helps the mortar resist cracking due to temperature changes and provides better elasticity.

7. Plant-based Fibres (e.g., straw, hemp)

• Purpose: Plant fibres like straw or hemp can be mixed into lime mortar to act as reinforcement. The fibres help to prevent cracking and improve the overall structure of the mortar.

• Benefit: These fibres can improve the mortar’s resistance to tensile stress, making it more suitable for areas with movement or vibration.

• Warning; contains reactive materials

Benefits of Organic Additives in Lime Mortar:

• Enhanced Workability: Organic additives can improve the ease with which the mortar can be mixed, applied, and finished.

• Increased Durability: Some additives, like oils and fats, can help protect the mortar from water infiltration and weathering, enhancing the lifespan of the mortar.

• Flexibility: Organic materials can contribute to the flexibility of the mortar, allowing it to expand and contract with changes in temperature without cracking.

• Breathability: Lime mortar is naturally breathable, and organic additives usually don’t compromise this important property. In fact, certain additives like seaweed and casein can enhance moisture retention and release, maintaining the permeability of the mortar.

Considerations:

• Compatibility: The use of organic additives should be done carefully, as not all of them are compatible with every type of lime or in every building context. It's important to test mixes for the specific application.

• Long-term Behaviour: Over time, some organic additives can decompose or degrade, which may affect the mortar's performance. This is why modern lime mortar mixes often avoid using some of the more perishable organic additives. Lime suppliers also don't want the associated risk as the improved performance comes with the potential to go wrong. Every lime product currently on the market is also required to have very low skilled or automated production methods due to the manner in which those businesses are run. 

In summary, organic additives have a long history of use in lime mortar, especially in traditional and historical construction. They can help improve the mortar's workability, bonding ability, and longevity while contributing to the mortar's overall ecological and sustainable nature.