Effects of organic admixtures on the fresh and mechanical properties of earth-based plasters

As a building material, earth is known to be a natural humidity regulator and to improve comfort inside buildings, making it a good choice for indoor plastering. The clayey phase ensures the sorption capacity of the material and its global cohesion by acting as a binder for the sand grain skeleton. However, clay also induces drying shrinkage of the mortar, leading to cracking of the plaster. Additives or stabilizers are typically used to address issues such as mechanical resistance and drying shrinkage in earthen building materials. To promote the development of eco-building materials, this paper pays particular attention to the potential of organic admixtures (vegetal or animal). The effectiveness of such admixtures could depend largely on the type of soil used and on the amount and nature of the clay minerals involved. Four types of soils were investigated. Based on consistency, shrinkage, visual and numerical detection of cracking, and flexural, compressive and shear tests, the characteristics of mortars in fresh and hardened states were evaluated. The results show that the clay content is a key parameter of both fresh properties and mechanical performances of plasters but is not sufficient by itself, especially when organic admixtures are involved. The potential of seven admixtures was evaluated. Although the deflocculants enabled a water reduction in mixes as well as a mechanical improvement, and the cohesion strengtheners were able to enhance mechanical properties at the hardened state, only flax fibre-based mortars demonstrated sufficient adhesion to their application substrate and, above all, no cracking as they dried.

Automated summary

The study emphasizes the importance of clay content and organic admixtures in influencing the performance of plaster. While some additives can improve water content and mechanical properties, only flax fiber-based mortars demonstrate strong adhesion and prevent cracking during drying.