What is lime plaster?

There is a lot of misconception or misunderstanding concerning the definition of what is actually a lime plaster.

Lime Plaster refers to a mortar using only pure lime as a binder and sand as an aggregate.
Too often common mortars made out of lime, cement, and sand are improperly called lime plasters simply because they include some kind of lime. The correct name for such a product is stucco, cement stucco, or lime-based stucco. The addition of cement to lime dramatically changes the physical, mechanical, and chemical properties of the lime by reducing its elasticity, breathability (vapor exchange), and durability as well as altering the color rendering. As a matter of fact, lime is commonly added to cement in order to mitigate these properties. Mixing Hydraulic Lime or Hydrated Lime, or both, with aggregates, achieves a true lime plaster.

Hydrated Lime is the most commonly used and known lime, also called (high) calcium lime or air lime. It can only set through carbonation (re-absorption of CO2), and has very strong limitations in construction use.  Hydraulic Lime has an initial set with water, much like cement, and a second set through carbonation, like hydrated lime.  This allows for simplicity in application, identical to ordinary stucco.

Some kind of Hydraulic Lime was used for most of our old structures—many dating back several centuries. Its durability or longevity has been unsurpassed by any modern material, including cement stucco or “lime-based plasters.” This is simply due to its composition (calcium carbonate or limestone), resistance to salts (no sulphate attack or alkali-silica reactions), elasticity (reduces the risk of cracking and water intrusion), and breathability (does not trap water and allows its elimination through vapor exchange). A true lime plaster has the unique quality of reflecting multi-nuances of color, enhanced by the varied angles of sunlight reflected throughout the day.

In today’s world, having finally realized our impact on global warming, the use of lime plaster will save approximately 80% of the CO2 release compared to ordinary stucco. One single residence will save between 5,000 and 10,000 lbs of CO2 emissions. Each year in the US alone, environmentally conscientious builders are saving several millions of pounds of CO2 release by simply avoiding the use of cement-based products, and choosing Natural Hydraulic Lime instead.