Earthen plaster


Earthen plaster is made of clay, sand and often mixed with plant fibers. The material is often used as an aesthetically pleasing finish coat and also has several functional benefits. This natural plaster layer is known for its breathability, moisture-regulating ability and ability to promote a healthy indoor environment. In the context of stricter indoor air quality regulations, earthen plaster shows great potential because of its properties as a building material.

Physical composition

All plasters and stuccos have several common features: they all contain a structural component, a binding element, and some form of fiber. Usually the term plaster refers to a wall covering made from earth, lime or gypsum, while stucco uses a cement or synthetic binding element.

Clay: the binding agent

, a crucial soil component with particles smaller than 2 micrometers, exhibits glue-like properties in the presence of water due to its extremely small particle size and high surface-to-volume ratio. This allows it to bind effectively with sand and fibers, playing a key role in holding the mixture together and securing it to the wall. Additionally, when clay is wet, its plasticity enhances the workability of plaster mixtures.
Within the domain of earthen building materials, clay particles act as primary binders. These particles not only provide workability during the plastic phase but also ensure cohesion after drying, contributing to the structural integrity of the construction. Notable clay minerals involved in this process include montmorillonite, chlorite and illite, each adding distinct properties to the composition. Despite the chemical variation among clays, their prevailing crystalline phases primarily consist of phyllosilicates, such as the mentioned clay minerals. The colloidal component further includes poorly crystalline hydrous aluminum silicates, along with iron and aluminum oxides.
The clay proportion significantly influences mixture characteristics, impacting strength, shrinkage, and mixing water requirements. However, it's essential to note that the recommended maximum clay content in the earth mixture is 25%.

Sand: structural strength

Sand, the granular skeletal component, provides structure, durability, and volume to earthen plasters. Consisting of tiny mineral particles derived from its original rock material, sand is predominantly made up of silicon dioxide and is recognized as a non-reactive substance.
Sand is incorporated into the plaster mixture not just for structural purposes but also plays a vital role in minimizing the likelihood of cracks during the drying process. Moreover, the presence of sand not only helps in preventing cracks but also results in a reduction in the sorptive capacity of the mixture. This dual impact indicates the careful balancing act required in soil composition to achieve both structural integrity and controlled moisture absorption.
Given that sand naturally occurs in various subsoils, there's a possibility that all the necessary sand is already inherent in the soil.

Fiber: tensile strength and reinforcement

In the context of improving adhesion and compatibility with different substrates, fibers may be introduced to earthen plasters without compromising their environmental profile. Various natural fibers, such as dry straw, hemp, cattails, coconut fiber, shells, and animal hair, prove to be suitable choices for reinforcing earthen plasters.
Research indicates that the inclusion of natural fibers, moderately increases open porosity, facilitating improved pore interconnection. A meshwork is formed within the plaster, enhancing cohesion and providing flexibility to the dried mixture.
Clay tends to shrink and crack during drying, the added fibers effectively counteract these issues. The presence of fibers in the mixture significantly reduces drying shrinkage, with larger fibers exhibiting a more pronounced effect than finer ones. This reduction is attributed to the increased water content required for workability when adding more and finer fibers.
Exploring the physical performance changes resulting from the addition of natural fibers reveals a reduction in material density. The bulk density decreases with higher fiber content, while adhesion strength experiences a positive trend with the addition of fibers, particularly when more and finer fibers are incorporated.
The addition of fibers to plasters is observed to have various benefits, including reduced density, minimized shrinkage cracks, and improved adhesion strength. While the general influence on compressive strength and tensile strength may vary depending on base materials and fibers, the overall conclusion of the research affirms the positive impact of adding fibers to earthen plasters. This enhancement encompasses reduced heat conduction, decreased drying shrinkage, and an improved hygienic buffering capacity.

Water: viscosity

Water plays a crucial role in the formulation and application of clay plaster, impacting both its workability and structural integrity. As mentioned earlier, clay exhibits adhesive properties in the presence of water, emphasizing the waters vital role in providing structural support. The amount of water added is determined by the specific characteristics of the clay and the overall mixture proportions.
However, the balance between water content and plaster performance becomes apparent in the compressive strength of the material. An increase in initial water content can negatively affect compressive strength. Striking a balance is crucial. For optimal plasticity, the water requirement for plasters should fall within the liquid and plastic limits of the soil. Opting for a water-clay content close to the liquid boundary can enhance ease of application and mitigate surface cracking. The recommended approach is to maintain an initial water content between 30% and 40% of the clay's weight.
It's noteworthy that as the clay content in the mixture increases, so does the demand for water. However, a delicate equilibrium must be maintained to prevent potential shrinkage cracks associated with higher water content. Achieving an optimal water-clay ratio is crucial for utilize the benefits of clay plaster while preserving its structural integrity.

Additives

Additives can be incorporated into the composition of clay, sand, water, and fiber to enhance various properties of the plaster. Depending on the application, these additives may be selectively applied to the final coat or included in all layers. Many commonly used additives originate either from natural sources or result from industrial and agricultural processes, providing a cost-effective means to refine the characteristics of clay plaster. The diversity of additives allows for their blending in various proportions, each inducing distinct alterations in the plaster. Due to the absence of a comprehensive theoretical model explaining these effects, predicting the impact of a specific additive in a particular plaster mixture relies on empirical testing for each combination.
The primary utilization of additives revolves around addressing inherent weaknesses in clay plaster, such as dry shrinkage, mechanical strength, or adhesion. Furthermore, certain additives aim to enhance properties crucial for indoor applications, including thermal resistance and moisture buffering capacity.

Biopolymers

Biopolymers are a broad group of additives that are produced from plants or animals. They can serve many purposes: some biopolymers can act as a glue holding the matrix together, while others help fill cavities and supplement the particle distribution, both will increase the cohesion. This can cause multiple benefits: increased density often leads to an increase in overall strength, while less porous plasters prove more water resistant and durable. Some biopolymers also influence the viscosity and processability of the plaster, requiring less water and therefore reducing the dry shrinkage.
Some of the most common biopolymer additives are wheat flour paste, manure, cactus juice, casein and various natural oils such as linseed oil. Other additives include: stearate, tallow, tannin, leaves and bark of certain trees, natural gums and glues, kelp, powdered milk, or the blood of livestock.
Flour paste
Cooked flour paste is a cheap natural glue that is easy to make from common ingredients. The water and flour slurry is cooked until the gluten binds the elements of the mixture, creating a durable glue. In plaster, the flour paste serves as a binding agent and a hardener.
Manure
Manure serves as a binding agent and gives plaster more body. Manure also contains small natural fibers that provide additional tensile strength as well as reduce cracking and water erosion. Different types of manure have different effects. Horse manure has a high microfiber content, but cow manure has more hardening enzymes. People have reported success with llama and alpaca dung. Manure should be fresh or fermented when mixed with plaster, as composted manure loses its enzymes and adhesive qualities. Manure should be sifted before use.
Prickly pear cactus juice
The liquid from prickly pear cactus used to be one of the most common additives in the Americas.
The juice from the prickly pear cactus leaf pads will serve many functions. According to some sources, it helps the plaster set and increases its stickiness or adhesion. Cactus juice also serves as a stabilizer in that it helps make earthen plasters more water-resistant and more durable. It also prevents dusting.
Cactus juice can increase plaster's workability and its ability to be formed into the desired shape. Workability depends on the water content, the shape and size distribution of its aggregate, the age of the plaster, and the amount of other natural binder Altering the water content, changing the aggregate mix, soaking the clay, or changing the binders will increase or decrease the plaster's workability. Excessive water will lead to increased bleeding and/or segregation of aggregates, with the resulting plaster having reduced quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix design with a very low workability, which cannot be readily made more workable by addition of reasonable amounts of water or binder.
Cactus juice works well because it contains pectin, a water-soluble long-chain carbohydrate that acts as the binding agent to increase the adhesion of an earthen plaster. Pectin is also responsible for increasing the water resistance of an earthen plaster and has been used to augment lime plasters in both Mexico and the southwestern United States for hundreds of years.
Cactus juice is extracted by immersing cut leaves in water for as long as two weeks.