By Tim Jordan, California Wall Design
I spend a lot of time working with plaster. Apparently, I also spend my free time asking questions about it.
This one started with Tadelakt. When we work olive-oil soap into a lime surface, what actually happens? And does the lime have to be hydraulic for that reaction to work?
That second question opens up a much bigger conversation about what makes this Moroccan finish so interesting.
The traditional treatment uses soap made from olive oil. The oil has already been converted into soap before it reaches the wall. Pouring straight olive oil onto plaster is a different treatment.
On a fresh, sufficiently alkaline lime surface, the soap can react with available calcium to form calcium soaps. These compounds help the surface repel water. That is a chemical change at the surface, alongside the physical work of polishing. [1]
For an applicator, this makes the soaping stage much more interesting. The material under your hand is still reacting while you are bringing the finish together.
No. A particularly useful experiment appears in a 2018 study of Marrakech Tadelakt. The researchers mixed black soap with high-calcium air-lime putty, allowed it to age for about a month, and detected evidence of calcium stearate, a calcium soap. They demonstrated the chemistry using non-hydraulic lime. [2]
That answers the chemistry question. It does not establish that every lime putty, aggregate mix, and application method will perform equally well in a shower.
It also changes how I think about the label on a bag. “Hydraulic” tells us something important about the binder. We still need to understand the complete formulation and how it is meant to be used.
Air lime develops strength mainly through carbonation: calcium hydroxide reacts with carbon dioxide, forming calcium carbonate. Moisture is part of that process. The plaster becoming dry to the touch does not mean the reaction has finished. [3]
Hydraulic lime also contains minerals that react with water and contribute to hardening. In traditional Marrakech lime, researchers identified calcium silicates, including belite. The lime can therefore develop strength through hydration as well as carbonation. [2]
There are separate jobs happening here: the binder develops strength, the plaster is compressed and polished, and the soap changes how the surface interacts with water. Understanding those jobs makes the whole process easier to evaluate.
The Marrakech stone examined in the 2018 study contained carbonate minerals, clay, and other mineral constituents. Firing caused calcium to react with silica and alumina, forming hydraulic compounds. Underburnt material also remained in the mixture and contributed aggregate. The resulting lime carried ingredients for both binding and texture. [2]
Its composition depended on the original stone and the firing. A separate 2024 study described its Marrakech lime samples as slightly hydraulic. These studies support hydraulic behavior, while also showing why we should avoid assigning one fixed strength or composition to every traditional batch. [4]
That is what holds my attention: the finished wall begins with geology, then passes through fire, water, and the hands of the person applying it.
There is another route to hydraulic behavior. Start with air lime and add a reactive material such as metakaolin, a pozzolan.
In the presence of water, metakaolin reacts with calcium hydroxide to form additional binding compounds, including calcium silicate and calcium aluminate hydrates. Researchers studying lime-metakaolin pastes found that the mixture proportions influenced the reactions and the compounds formed. [5]
This means a formulation can begin with non-hydraulic lime and still develop hydraulic binding products after mixing. The starting lime and the finished plaster are two things worth asking about separately.
It does not tell us what is in any particular manufacturer's product. That requires the actual technical information. Nor does it give us a universal recipe: changing a plaster mix means evaluating its working behavior, finish, curing, and performance again.
Anyone watching Tadelakt being polished can see how much of the finish comes from the applicator. Burnishing compresses and closes the surface, developing its sheen and characteristic movement. Timing depends on the plaster's moisture, the substrate's suction, and the conditions in the room. [6]
The chemistry helps explain why the process works. It also gives me more appreciation for the judgment involved in doing it well.
A surface that beads water tells us something useful about its surface treatment. It does not, by itself, prove that an entire shower assembly is watertight.
Even products sold as Tadelakt have different permitted uses. For example, Kreidezeit describes its finish as water-repellent after soaping but currently advises against using that particular product inside showers, basins, or bathtubs. Product-specific instructions matter. [6]
For a CWD project, the conversation needs to include the selected finish system, substrate, waterproofing details, curing, and maintenance. A photograph cannot answer those questions.
Care belongs in that conversation from the beginning. Follow the chosen system's instructions for cleaning and any repeat soap treatment. Acidic cleaners and abrasive tools can damage a lime finish. [7]
This is part of why I keep coming back to these materials. You see a soft, polished wall. Behind it are mineral reactions, careful preparation, and decisions made by hand. The more I understand about the chemistry, the more interesting the craft becomes.
If you are considering Tadelakt for a project, explore our Tadelakt finishes or talk with California Wall Design about the space and how it will be used.
A little lime plaster research sends Tim down a rabbit hole about self-healing materials, Qigong, and whether the wall needs quiet time.
A personal update from Tim Jordan on a busy season at California Wall Design, his appearance on The Aggregate podcast, a week learning Tadelakt in Bozeman, and remembering fellow plasterer Rik Cirignano.
