Plaster, setting

The inclusion of solvent molecules as part of the crystal lattice is another common phenomena in both organic and inorganic systems. Calcium phosphate used in modem building plaster, sets when it reacts with water and crystallizes as a stable deca-hydrate. In pharmaceutical systems it is common... 

You should not dig your fingers into the plaster because it may be difficult to remove them as the plaster sets. 

Plaster. setting is the reaction of calcium sulfate hemihydrate, anhydrite II or anhydrite III with water i.e. the opposite reaction to the manufacture of plaster ... 

Pastes made from plaster of Paris exhibit fast setting and hardening, and the hydration process is typically completed within hours. Small amounts of chemical additives may be added to the calcined material to modify its properties. These may include set accelerators (such as soluble sulfates or prehydrated plaster), set retarders (such as citric acid or ker-atin), or modifiers of rheology (such as methyl cellulose or caibo methyl cellulose). 

After the plaster sets, turn the mold over to expose the pattern and remove it from the mold. There may be a few air holes or other imperfectiom left in the surface of the mold cavity after you remove the pattern. Small holes won t matter, but it s best to fill anything that is much larger than 1/16" in diameter, then sand the repairs smooth. Sharp comers have a tendency to break off so sand a radius around the top edge of the mold cavity. 

Potassium alum, which also occurs naturally as the mineral kalinite [7784-24-9], KAl(SO 2 12H20, sp gr 1.75, is used ia tanning skins, as a mordant ia dyeiag, and ia the pharmaceutical and cosmetic iadustries (see Pharmaceuticals Cosmetics). It is used as a styptic pencil and as a hardening agent and set accelerator for cement and plaster. The ACGIH threshold limit value TWA is 2 mgAl/m (1 ). 

The reaction reverses when water is added to plaster of Paris and the mixture sets back to the dihydrate accompanied by a slight increase in volume and the evolution of heat. 

Concrete, Mortar, and Plaster. Citric acid and citrate salts are used as admixtures in concrete, mortar, and plaster formulations to retard setting times and reduce the amount of water requited to make a workable mixture (172—180). The citrate ion slows the hydration of Portland cement and acts as a dispersant, reducing the viscosity of the system (181). At levels below 0.1%, citrates accelerate the setting rate while at 0.2—0.4% the set rate is retarded. High early strength and improved frost resistance have been reported when adding citrate to concrete, mortar, and plaster. 

Each 100 g of calcined gypsum theoretically requires only 18.6 mL of water to complete the chemical reaction from the hermhydrate to the dihydrate. Any amount of water greater than 18.6 mL/100 g of powder is excess and reduces the strength of the hardened plaster. When a mixture of the hermhydrate and water hardens, linear expansion takes place. This expansion may amount to as much as 0.5% for plaster. Dental stones also expand on setting, but the amount is significantly less than that permitted in plaster, ie, 0.2% for type III, 0.1% for type IV, and 0.3% for type V. 

Impression plasters are formulated to produce a thin, fluid slurry when mixed with the proper amount of water. A satisfactory impression plaster should have a setting time of 4 1.5 min fineness, ie, 98% should pass a number 100 sieve (ca 0.15 mm), and 90% pass a number 200 sieve (ca 0.07 mm) setting expansion at 2 h should be <0.15% the compressive strength at one hour should be 5.9 2 MPa (855.5 290 psi) and testing consistency as determined by the diameter of the slump in the consistence test should be 90 3 mm. 

Impression plasters are manufactured from the finest finishing plasters, selected for color and purity. Setting time accelerators, setting expansion control agents, fillers, flavors, colors, or other special modifying agents may be added, eg, starch, to cause disintegration of the plaster impression when it is boiled. 

Model Plasters. Model plaster should have a setting time of approximately 10 minutes. The fineness of the powder should be such that 98% passes a number 100 sieve (ca 0.15 mm) and 90% passes a number 200 sieve (ca 0.07 mm). Setting expansion should be less than 0.30%, compressive strength at the end of one h should be a minimum of 8.8 MPa (1276 psi), and the consistency should form a disk during the slump test of 30 2 mm diameter. 

Model plasters are manufactured from select finishing plasters with special emphasis on a clean, white color. Setting-time control agents, setting-expansion control additives, fillers, and pigments may be added. 

All of the gluconates can be used as a cement or plaster additive where they retard setting times whilst increasing the strength and water resistance of the materials. 

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Into this category come the water-based plasters, mortars, cements and concretes which set at room temperature as the result of a chemical reaction between water and a powder. Some of these have been known... 

Hydraulic cements. These cements are formed from two constituents one of which is water. Setting comprises a hydration and precipitation process. Into this category fall Portland cement and plaster of Paris. 

Plaster of Paris has long been used as a casting material, a cement, and a mortar. If mixed with water, plaster of Paris forms a very soft and pliable mixture. After a very short time, lasting only 5-8 minutes, the wet, pliable mixture sets, that is, it hardens into a stable, firm solid. The setting process entails the incorporation of water molecules (a process known as hydration) into the calcium sulfate hemihydrate and the consequent formation and crystallization of hydrated sulfate of calcium. In other words, when water is added to plaster of Paris, the two combine, again forming gypsum, which soon crystallizes into a hard solid mass ... 

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