Stucco Classico: Lime Stucco for Pennsylvania

Why Lime Stucco?

Lime stucco, also known as known as lime plaster when used on interiors, with variations like marmorino, venetian plaster, tadelakt, etc., dates back to 14,000 (or more) years ago.  Portland cement has been in production in this country only since 1871.  Pennyslvania in particular, due to its early settlement and excellent lime deposits, had numerous kilns for the production of lime in the pre-Portland cement era.  Although Portland cement has overtaken lime due to its advantages of rapid setting and a cheap price (due to economy of scale and energy subsidies), it has numerous drawbacks: lack of moisture permeability, inflexibility, poor appearance, chemical complexity, and relative lack of stability.

Lime Stucco is Moisture-Permeable

Though more moisture-permeable than acrylic (existing as a synthetic stucco under the names EIFS and Dryvit), compared to lime stucco, cement stucco passes little moisture.  On an old building, the results can be disastrous.  It is generally not advisable to resurface a softer, moisture-permeable surface with a harder, less-moisture-permeable one.  The latter can trap moisture, causing deterioration in the substrate of the surroundings.  This is easier to see in masonry, where modern cement mortars, when applied to old, soft masonry, cause spalling of the face of the brick as well as cracking and other evidence of cement "cancer."  In stucco, the evidence is usually concealed for some time until the results reveal themselves.  The high moisture retention of Portland cement stuccos also means they have a high thermal conductivity (i.e., poor insulating value) compared to lime stuccos.

Lime Stucco is Self-Healing

Lime stuccos set primarily or at least partially by slow reaction with air (absorption of carbon dioxide in the presence of water), remaining active for active for hundreds or thousands of years.  This unreacted or "free lime" is water-soluble, meaning that the microfissures that develop are able to heal autogenously.  In addition, lime stucco is more prone to develop these small microfissures rather than the large cracks characteristic of Portland cement.

Lime Stucco is a Diverse Product

Lime exists in numerous types.  Its most basic form is class A, which is usually found as bright white putty.  This is typically close to pure calcium hydroxide, and in its ordinary form sets only by its reaction with the air.  Limes made from different types of limestones have different setting properties; some have varying degrees of "hydraulic" set, or the ability to set underwater.  The degree of hydraulicity varies with the quantity of clay or sand present in the limestone.  Natural hydraulic lime (such as that made by Saint-Astier from siliceous limestone), is a harder-setting material with a hydraulic set, while still containing free lime.  Hydraulic set along with higher compressive strengths can also be obtained by the addition of reactive "pozzolana" such metakaolin and pulverised fly ash.  We prefer to use a natural hydraulic lime when hydraulicity is needed since the results are more predictable.

Lime Stucco is Visually Appealing

Another big advantage lime stucco has over Portland cement stucco is aesthetic: in contrast with Portland cement's rather dead appearance, lime stucco has a much more visually-appealling appearance.  This is because as the free lime on the surface sets, calcite crystals are formed.  Calcite is birefringent, meaning it has a dual refractive index (further explanation at this external link).  The resulting light scattering is what gives lime stucco its unmatched "old world" look.  It takes new lime stucco months or years to develop the patina that results from the carbonation of surface free lime.

Lime Stucco is Chemically Stable

Chemically, lime stucco differs from Portland cement in that it is simpler.  Grey Portland cement has four phases: alite, approximately tricalcium silicate; belite, approximately dicalcium silicate; celite, very roughly tricalcium aluminate; and ferrite (see the Understanding Cement Web site for more explanation of Portland cement and its phases).  White Portland does not have ferrite (very roughly tetracalcium aluminoferrite).  The primary setting phases are alite and belite.  Alite is the early set, while belite is the slower, late-setting material.  The setting time of the various phases, particularly those containing aluminate, must be controlled, which is done at the factory by an admixture of calcium sulfate (plaster or gypsum).  The gypsum is used to prevent external sulfate attacks by reacting with the vulnerable aluminates, but it can make the cement more vulnerable to internal sulfate attack or, in particular, delayed ettrigite formation.  Ettringites, fine needle-like structures, are expansive, and weaken the material as well as decreasing its freeze/thaw resistance.  Portland cement composition has changed since its manufacture began; early forms had no gypsum and are sometimes called Natural Portland Cements.  More recently, Portland cements in the earlier part of the twentieth century contained much less gypsum.  The alites are the main cause of its inflexibility.  For more on this as well as photographs of the crystalline structure, please see this interview with Andy DeGruchy.  Please note that our lime putties do not have most of the drawbacks he ascribes to his.  Natural hydraulic lime stucco relies solely on belite for its hydraulic set.  In contrast, as long as no additives are used and kiln temperatures are controlled, aerial (class A) lime has none of these phases, but relies only on slow reaction with the air.  We are able to assist this reaction by natural, time-tested additives.

Lime Stucco is Time-Tested

Ancient Roman concretes, masonry, and stuccos, which contained no Portland cement, and relied on natural limes and pozzolana, are still serviceable after thousands of years.  Thus, lime stucco has an impressive time-tested track record of its longevity.  We believe that a major reason for its longevity is that lime stucco is natural and chemically simple compared to Portland cement.  It is for the reasons of superior moisture permeability, flexibility, autogenous healing, excellent aesthetics, and chemical simplicity, that we use natural limes, both hydraulic and aerial.

For expert lime stucco work, contact Stucco Classico.