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Gauging   plasters   are   obtainable   in   slow-set, quick-set,  and  special  high-strength  mixtures. Gypsum molding plaster is used primarily in casting and ornamental plasterwork. It is available neat (that is, without admixtures) or with lime. As with Portland cement mortar, the addition of lime to a plaster mix makes the mix more “buttery.” Keene’s  cement  is  a  fine,  high-density  plaster capable  of  a  highly  polished  surface.  It  is  customarily used with fine sand, which provides crack resistance. LIME  PLASTER Lime  is  obtained  principally  from  the  calcining  of limestone,  a  very  common  mineral.  Chemical  changes occur that transform the limestone into quicklime, a very caustic material. When it comes in contact with water, a violent reaction, hot enough to boil the water, occurs. Today, the lime manufacturers slake the lime as part of the process of producing lime for mortar. Slaking is done in large tanks where water is added to convert the quicklime  to  hydrated  lime  without  saturating  it  with water. The hydrated lime is a dry powder with just enough  water  added  to  supply  the  chemical  reaction. Hydration is usually a continuous process and is done in equipment similar to that used in calcining. After the hydrating  process,  the  lime  is  pulverized  and  bagged. When received by the plasterer, hydrated lime still requires soaking with water. In mixing medium-slaking and slow-slaking limes, you should add the water to the lime. Slow-slaking lime must be mixed under ideal conditions. It is necessary to heat the water in cold west.kr. Magnesium lime is easily drowned, so be careful you don’t add too much water to quick-slaking calcium lime. When too little water is added to calcium and magnesium limes, they can be burned. Whenever lime is burned or drowned, a part of it is spoiled It will not harden and the paste will not be as viscous and plastic as it should be. To produce plastic lime putty, soak the quicklime for an extended period, as much as 21 days. Because  of  the  delays  involved  in  the  slaking process of quicklime, most building lime is the hydrated type. Normal hydrated lime is converted into lime putty by soaking it for at least 16 hours. Special hydrated lime develops  immediate  plasticity  when  mixed  with  water and  may  be  used  right  after  mixing.  Like  calcined gypsum, lime plaster tends to return to its original rock-like  state  after  application. For interior base coat work, lime plaster has been largely replaced by gypsum plaster. Lime plaster is now used mainly for interior finish coats. Because lime putty is the most plastic and workable of the cementitious materials used in plaster, it is often added to other less workable  plaster  materials  to  improve  plasticity.  For lime plaster, lime (in the form of either dry hydrate or lime putty) is mixed with sand, water, and a gauging material. The gauging material is intended to produce early strength and to counteract shrinkage tendencies. It can be either gypsum gauging plaster or Keene’s cement for interior work or portland cement for exterior work. When using gauging plaster or Keene’s cement, mix only the amount you can apply within the initial set time of the material. PORTLAND  CEMENT  PLASTER Portland cement plaster is similar to the Portland cement mortar used in masonry. Although it may contain only  cement,  sand,  and  water,  lime  or  some  other plasterizing  material  is  usually  added  for  “butteriness.” Portland cement plaster can be applied directly to exterior and interior masonry walls and over metal lath. Never  apply  portland  cement  plaster  over  gypsum plasterboard  or  over  gypsum  tile.  Portland  cement plaster is recommended for use in plastering walls and ceilings of large walk-in refrigerators and cold-storage spaces, basements, toilets, showers, and similar areas where an extra hard or highly water-resistant surface is required. AGGREGATES As we mentioned earlier, there are three main aggregates  used  in  plaster:  sand,  vermiculite,  and perlite. Less frequently used aggregates are wood fiber and  pumice. Sand Sand for plaster, like sand for concrete, must contain no  more  than  specified  amounts  of  organic  impurities and harmful chemicals. Tests for these impurities and chemicals are conducted by Engineering Aids. Proper  aggregate  gradation  influences  plaster strength  and  workability.  It  also  has  an  effect  on  the tendency of the material to shrink or expand while setting. Plaster strength is reduced if excessive fine aggregate material is present in a mix. The greater quantity   of   mixing   water   required   raises   the water-cement  ratio,  thereby  reducing  the  dry-set 7-2

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