density. The cementitious material becomes over-extended since it must coat a relatively larger overallaggregate surface. An excess of coarse aggregateadversely affects workability-the mix becomes harshworking and difficult to apply.Plaster shrinkage during drying can be caused by anexcess of either fine or coarse aggregate. You canminimize this problem by properly proportioning theraw material, and using good, sharp, properly size-graded sand.Generally, any sand retained on a No. 4 sieve is toocoarse to use in plaster. Only a small percentage of thematerial (about 5 percent) should pass the No. 200 sieve.VermiculiteVermiculite is a micaceous mineral (that is, eachparticle is laminated or made up of adjoining layers).When vermiculite particles are exposed to intense heat,steam forms between the layers, forcing them apart.Each particle increases from 6 to 20 times in volume.The expanded material is soft and pliable with a colorvarying between silver and gold.For ordinary plasterwork vermiculite is used onlywith gypsum plaster; therefore, its use is generallyrestricted to interior applications. For acoustical plaster,vermiculite is combined with a special acoustical binder.The approximate dry weight of a cubic foot of 1:2gypsum-vermiculite plaster is 50 to 55 pounds. The dryweight of a cubic foot of comparable sand plaster is 104to 120 pounds.PerliteRaw perlite is a volcanic glass that, when flash-roasted, expands to form irregularly shaped frothyparticles containing innumerable minute air cells. Themass is 4 to 20 times the volume of the raw partlicles.The color of expanded perlite ranges from pearly whiteto grayish white.Perlite is used with calcined gypsum or portlandcement for interior plastering. It is also used with specialbinders for acoustical plaster. The approximate dryweight of a cubic foot of 1:2 gypsum-perlite plaster is50 to 55 pounds, or about half the weight or a cubic footof sand plaster.Wood Fiber and Pumiceas wood fiber and pumice, are also used. Wood fiber maybe added to neat gypsum plaster, at the time ofmanufacture, to improve its working qualities. Pumiceis a naturally formed volcanic glass similar to perlite,but heavier (28 to 32 pounds per cubic foot versus 7.5to 15 pounds for perlite). The weight differential givesperlite an economic advantage and limits the use ofpumice to localities near where it is produced.WATERIn plaster, mixing water performs two functions.First, it transforms the dry ingredients into a plastic,workable mass. Second, it combines with the binder toinduce hardening. As with concrete, there is a maximumquantity of water per unit of binder required forcomplete hydration; an excess over this amount reducesthe plaster strength.In all plaster mixing, though, more water is addedthan is necessary for complete hydration of the binder.The excess is necessary to bring the mix to workableconsistency. The amount to be added for workabilitydepends on several factors: the characteristics and ageof the binder, application method, drying conditions,and the tendency of the base to absorb water. A porousmasonry base, for example, draws a good deal of waterout of a plaster mix. If this reduces the water content ofthe mix below the maximum required for hydration,incomplete curing will result.As a general rule, only the amount of water requiredto attain workability is added to a mix. The water shouldbe potable and contain no dissolved chemicals thatmight accelerate or retard the set. Never use waterpreviously used to wash plastering tools for mixingplaster. It may contain particles of set plaster that mayaccelerate setting. Also avoid stagnant water; it maycontain organic material that can retard setting andpossibly cause staining.PLASTER BASE INSTALLATIONLEARNING OBJECTIVE: Upon completingthis section, you should be able to associate thenames and purposes of each type of lath usedas a plaster base. You should also be able todescribe the procedures used in plastering,including estimating materials and theprocedures for mixing and applying plasterbases.Although sand, vermiculite, and perlite makeup theFor plastering, there must be a continuous surfacegreat majority of plaster aggregate, other materials, suchto which the plaster can be applied and to which it will7-3
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