Natural Mortars, Grouts, and Plasters

Conventional mortars and plasters are made from cement, water, and fine aggregates, but cement manufacturing is one of the most energy intensive industrial processes, and produces massive amounts of carbon dioxide. Manufacturing cement produces a ton of carbon dioxide for every ton of cement, and in total, the production of cement is presently responsible for over 8% greenhouse gases generated by human activity annually.

Any plastering material used for cob or light clay walls must be breathable and have similar vapor permeability properties with the substrate.  When earth walls have been plastered in concrete, water permeates into the wall, resulting in deterioration of the wall, which is hidden by the concrete plaster.  Instead, lime and clay plasters are natural, breathable alternatives.  Both have been used throughout the world for centuries.

Clay Plaster: Clay naturally absorbs moisture during high humidity then dries out when the humidity decreases and creates negative ions in the process.  Researchers have shown that negative ions have a positive effect on the health and happiness of humans, which partly accounts for the good feeling one feels outside in nature, where the concentration of negative ions is significantly higher,[1] than in an office building stacked with electronics. It is naturally abundant, non-toxic, and totally recyclable.

Image courtesy of Tyler Walter

Lime Plaster: A mixture of lime or calcium hydroxide (binder) and sand (aggregate). [2]   Lime is made by heating calcium carbonate then adding water.  Lime plasters are more elastic, breathable and durable than concrete plasters and therefore don’t crack as easily. [3]   In fact lime plasters were used on most old buildings and have lasted for centuries, surpassing any modern alternatives such as cement stucco.  Using lime in place of cement reduces carbon dioxide emissions by 80%, which amounts to thousands of pounds of savings in just a single residence![4]

Lime Mortar:  A mixture of lime and sand that is breathable and elastic (does not crack easily).  When little cracks eventually develop, the material seals itself with water that seeps down the crack and dissolves a little mortar, which then fills in the crack.  Mixing one part lime with three parts sand typically gives the right consistency, although the proportions vary depending on the type of sand.  In general the lime should just coat the sand.