Hydration retarded cellulosics

SPECIAL CELLULOSIC THICKENERS 4.1 Hydration Retarded Cellulosics... 

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). 

Sodium Borates (11). These water-soluble salts, made by U.S. Borax and others, have been known since the eighteenth century as nondvu-able flame retardants for cotton, rope, canvas, and paper. The hydrated salts can act as endothermic heat sinks and moreover the salt fuses to a nonflammable glassy barrier layer. Sodium borates, in particular borax (sodium tetraborate decahydrate [1303-96-4]), are used in disposable paper and nonwoven cellulosic products, and as wood flame retardants where their water solubility can be tolerated. The borate salts are effective on flaming combustion but allow severe afterglow they are best used in formulations with afterglow preventatives, and often used with boric acid. 

Another problem is the increased water absorption of the autoclaved aerated concrete elements, therefore water retarding additives, e.g., cellulose esters, are used in the mixture of plasters. Due to an insufficient amount of water in the mixture of plasters, the hydration of the cement minerals slows down. The influence of natural zeolite and clinoptilolite on the properties of plaster has been investigated (45). Clinoptilolite is a natural zeolite comprising a microporous arrangement of silica and alumina tetrahedra (46). It commonly occurs as a devitrification product of volcanic glass shards in tuff and as vesicle fillings in basalts, andesites and rhyolites. 

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