The use of lime in plastering applications dates back to the construction of Egyptian pyramids about 4000 B.C. Plastering methods were refined by the Greeks and Romans. Early plaster systems could contain lime, gypsum, marble dust, fibers and sand in up to three coats.
Modern plasters can contain portland cement or gypsum products. Exterior plasters, often called stucco, normally contain portland cement because of its low solubility in water. Interior plasters usually contain gypsum. The advantages of these plasters in modern construction include the following:
- Lime is cementitious - Hydrated lime absorbs carbon dioxide from the air to chemically convert to limestone. This creates a hard finished surface and enhances long term durability by sealing hairline cracks.
- Type S Hydrated Lime is water retentive - This property enhances workability (plasticity) of the plaster as it is applied to walls. Since water is needed for gypsum or cement to set, the water retentivity of the lime maximizes the cementitious reaction of these materials.
Historically, prior to 1900, lime was used for all three coats of the plaster system. Lime-based basecoats typically contained a small amount of keene's cement or portland cement to assist with the development of initial strength. These plaster formulations often included hair or fibers as well.
Modern interior plaster can be applied in one to three coats. Basecoats are normally made with gypsum-based products. However, lime is an important ingredient in the final coat, often called the "finish coat." Common mix designs for interior plaster can be found in ASTM C842 (Specification for Application of Interior Gypsum Plaster). Interior plasters can either be sand finished or smooth trowelled.
Exterior plaster can also be applied in up to three coats. Exterior plasters or stucco can be made with portland cement, Type S hydrated lime and sand blends. Proportions of cement and lime are varied with each layer to provide a strong base and a flexible finished surface. Recommended mix designs for exterior stucco can be found in ASTM C926 (Specification for Application of portland cement -Based Plaster).
In both applications hydrated lime enhances the workability as well as the durability of the plaster. The fine particle size of lime products helps in the retention of water in the plaster. This enhances workability while providing water for proper gypsum or cement hydration to occur. Gypsum or portland cement provide the initial set and early hardness for both types of plaster. Hydrated lime absorbs carbon dioxide from the atmosphere to chemically change it into limestone. This reaction increases the strength of plaster over time.
The Graymont building lime products manufacturing plant is located in Genoa, OH. This area of Northwest Ohio is considered to have one of the best deposits of dolomitic limestone for the production of finish lime products in the world. Graymont Dolime (OH) Inc. offers several different types of lime products for plastering applications.
Ivory® Finish Lime and Snowdrift® Finish Lime are products which are autoclaved for immediate use with gauging plasters and portland cement, without soaking or slaking. These Type S hydrated lime products have the high plasticity, tight bonding and smooth troweling sought by plasterers.
Grand Prize® Finishing Lime is a single-hydrate (Type N), for use with gypsum or portland cement. Grand Prize requires a minimum of 16 hours of soaking to develop sufficient hydration for use. Grand Prize features high plasticity and exceptional body for smooth application.
Bondcrete® Stucco Lime is an air-entraining hydrated lime which is pressure-hydrated for immediate use in stucco applications. Bondcrete meets the requirements of ASTM C207 for a Type SA hydrated lime. When properly combined with portland cement and sand, it creates a stucco having superior performance, economical results and ageless durability. Bondcrete Stucco lime should not be used in smooth coat plaster applications.
Niagara® Mature Lime Putty is a high-purity dolomitic lime putty fully slaked and screened for immediate use. Niagara Lime Putty is slaked for a minimum of 2 months and complies with ASTM C1489. This product is available in 45 gallon drums or 3 gallon pails.