Traditional:

Lime mortars prior to the industrial revolution were very complicated recipes with a half dozen different organic and mineral components carefully selected and prepared using hundreds if not thousands of years of local lore to produce a complicated series of crystal structures that all serve different functions perfectly suited to the local materials and environment. 

Nowadays they consist of industrially produced lime putties or NHL, washed graded sand, plastic fibres and occasionally an industrial by-product made in large industrial mixers. With little thought beyond ease of use and ticking a box marked lime. This description covers the majority of products currently available from lime suppliers.

Putty and sand are far from the most suitable materials for damp or salt rich conditions. And a hot mixed lime mortar with no consideration for aggregates or additives is no better. 

Modern:

Most of these solutions involve one of two things; a barrier to moisture or a cavity of some sort. A barrier destroys the existing masonry and a cavity can trap moisture behind it and hide problems that can cause very serious damage. And both solutions also require such a high standard of work from the installer that is becoming increasingly rarer by the day.

This is covering over the problem and the guarantees for these products cover their individual performance not the solution to the problem. So you could be left with a fully functional system with masonry that has rotten away behind it!

Why not a time proven method that utilises both?

Because Red Mortars have to be made slowly and to order by a Mortarman with more expensive materials which is less profitable than mass producing generic modern building materials in industrial machinery.

The highest performing mortars have to be made on-site with fresh organic materials, locally sourced aggregates/water, expensive lime and require an even higher degree of knowledge and craftsmanship. They often have to be used immediately too as they are usually complicated hot mixed lime mortars. And in a very specific manner otherwise failure can occur. 

However the performance is far superior and time proven in thousands of environments over thousands of years, they’re easier to apply and the aftercare is greatly reduced. So its more economical for the property owner in both initial costs and over a prolonged period compared to the majority of available solutions.

One could argue that this is just a precursor to NHL and that the attempts to reproduce a similar functioning mortar have caused a host of issues. NHL is lime and clay. Red mortar is Lime and crushed brick made from clay. The key difference between the two being how the porosity works. NHL and cement for that matter have very small pores which don't really work as an interconnected network to transport moisture to a drying face. Red mortar does, in fact its even more porous than an air lime which is part of it's success. in damp applications. There are essentially two types of red mortar as with plasters and renders. The first is a very porous material which will not allow water to be forced through it because its so porous, which is odd but true, and the second is a denser material which won't allow water through it in a different manner. More resistant to liquid water ingress which the former isn't. So you have a macro porous material which water falls down through and traps salt as it does so and a denser coating which stops most water coming through but will allow vapour to escape. So if the water does build up excessively it will be able to leave through the face which will generally be dry whilst the backing stays moist. An aerial lime plaster, hot mixed of course, will be mesa and macro porous enough to prevent wind driven rain and to allow water egress once enough moisture accumulates in the fabric. in a more efficient and thorough manner than red mortar which is perfect if you have limited water ingress but a problem if you don't. Such as a retaining or cellar wall. 

How Red Mortar Traps Salts: A Deeper Dive

Understanding the Components:

• Iron-Rich Brick Dust: This component provides iron oxides, which can react with salts and form insoluble compounds.

• Lime: Lime, when hydrated, forms calcium hydroxide, which can also react with salts to form insoluble compounds.

The Trapping Mechanism:

1. Physical Trapping:

   • The porous structure of the plaster can physically trap salt crystals within its pores.

   • Iron oxides and calcium hydroxide can provide additional surface area for salt to adhere to.

2. Chemical Reactions:

   • Salt Hydrolysis: Salts like chlorides, nitrates, and sulfates can hydrolyze in the presence of moisture, producing acidic or alkaline solutions.

   • Reaction with Iron Oxides: Iron oxides can react with these acidic or alkaline solutions to form insoluble compounds, effectively locking the salt ions within the plaster.

   • Reaction with Calcium Hydroxide: Calcium hydroxide can react with sulfate ions to form calcium sulfate dihydrate (gypsum), which can fill the pores of the plaster and trap other salts.

Visualizing the Process:

Imagine the plaster as a sponge. The salt solution seeps into the pores. As the water evaporates, the salt crystals are left behind. The iron oxides and calcium hydroxide act like glue, binding the salt crystals to the sponge's structure.

The red mortar's complex setting process involves multiple mechanisms:

• Protein-Based Bonding: Blood proteins, separated during lime slaking, act as a natural collagen-like glue.

• Pozzolanic Reaction: Pozzolanic materials in the mortar react with lime and water, forming a hydraulic cement.

• Carbonation: Lime reacts with carbon dioxide from the air to form calcium carbonate, further strengthening the mortar.

• Ettringite Formation: Potassium from potash contributes to ettringite formation, providing additional strength and crack-healing properties.

• Silicification: Potassium and silicate-rich materials in the mortar react, forming a silicate-based bond, enhancing durability.

• Sulfate-Based Setting: Sulfates in ash react with calcium to form gypsum, which sets faster and stronger than lime.

• Carbonate Hydroxyapatite: Phosphate ions from oxblood and lime react to form carbonated hydroxyapatite crystals, contributing to the mortar's strength and durability.

• Porosity Control: Oils and other materials help balance porosity, providing water and salt resistance through saponification.

The specific order of mixing, material quality, and temperature control are crucial for optimal performance. Fresh blood, proper lime slaking, and timely oil addition are essential. Using incorrect materials or techniques can hinder the mortar's effectiveness.

https://www.youtube.com/watch?v=S1t19oVJhCc&list=PLmJDknXr4_-JbgqQA5-kFdFkD4DL34XM4&index=17

https://www.mdpi.com/2075-5309/12/4/402