Understanding Gypsum, Lime, and Cement: Properties and Uses
Types of Casts
Gypsum
Gypsum Black: Obtained from algez and contains many impurities. It is calcined directly, often containing ashes and blackened smoke. It is used in works where high quality is not required.
Tendido: Black chalk.
- Guarnecido
- White gypsum plaster
Gypsum White: In cases of the greatest purity and fineness of grind, it is much higher in quality than black chalk and requires more careful calcination. It is used in coatings and is also called plaster and stucco.
Escayola: White plaster of the best quality, superior firm employs it for emptying, moldings, ornamentation, false ceilings, etc.
Properties and Characteristics of Gypsum
- Solubility: Slightly soluble in fresh water, more soluble in water with salt. Should never be used outside.
- Adhesion: Adheres well to many materials, even tenacious stony iron.
- Corrosion: Corrosion occurs in cast iron and steel in the presence of moisture. It is important to protect any element in contact with plaster. Cement offers the best protection for iron and steel against oxidation.
- Fire Resistance: Gypsum provides good fire protection due to its chemical composition.
- Fineness of Grind: Exerts a very direct influence on the different properties. The finer the grind, the more complete the chemical reaction between gypsum plaster and water.
- Setting (Fraguado): When plaster is mixed with water, a film forms around the fine particles, initiating rapid gypsum crystallization.
Yeso Killed: This occurs when the plaster is dented when touched or when a nail is easily pushed through, leaving a large hole.
- Expansion: The reaction of plaster with water is exothermic and causes an increase in volume.
- Permeability: Plaster should not be exposed to water, as it loses resistance and absorbs water rapidly. The solution is to waterproof it with resins or resin paints.
Using Plaster
Commissioning work is usually done in plastic form, though it is also normal to use prefabricated elements. Plaster was used as a binder in the construction of brick and Rasillo slabs. Pasta = binder = H2O.
Kneading
To knead, pour the plaster into the pastera, add the necessary water, and sprinkle it slowly while stirring with the plaster trowel.
Hydraulic Binders
Lime
There are two types of lime: aerial lime and hydraulic lime.
Aerial Lime
The product resulting from the decomposition of limestone by heating to 900°C, calcium oxide, is called quicklime. It is white and unstable. Amorphous calcium hydroxide is slaked lime, which is white, amorphous, slightly soluble, very sticky, and forms a creamy and fluid paste.
Hydraulic Lime
Characterized by solidifying or forging in contact with air and water. Lime is mixed with other binders for use as coatings.
Cement
A hydraulic material that hardens with water, composed of lime and silica. It is irreplaceable in the manufacture of mortars and concretes.
Types of Cement
- Natural: Patented in 1792 by Parker.
- White: Contains no color impurities and is used when a predominant color is desired in mixtures.
- Coloring: These are mass coloring additives.
- Alumina: Created when mixed with clay containing 5-10% alumina.
- Puzolamicos: Uses pozzolans, which have a high silica content.
- Portland: Most used in construction.