CLASSIFICATION OF' PLASTERS (a) Produced by the incomplete dehydration of gypsum, the calcination being carried on at a temperature not exceeding 190° C.
(1) Plaster of paris, produced by the calcination of a pure gypsum, no foreign materials being added either during or after calcination.
(2) Cement plaster (often called patent or hard wall plaster) produced by the calcination of a gypsum containing certain natural impurities, or by the addition to a calcined pure gypsum of certain materials which serve to retard the set or render more plastic the product.
(b) Produced by the complete dehydration of gypsum, the calcination being carried on at temperatures exceeding 190° C.
(3) Flooring plaster, produced by the calcination of a pure gypsum.
(4) Hard finish plaster, produced by the calcination at a red heat or over of gypsum to which certain substances (usually alum or borax) have been added.
Plaster of paris and cement plaster are usually burned at tem peratures from 140° to 1S0° C., the difference between them being due to the substances added to the gypsum in making the cement plaster. Flooring plaster and hard-finish plaster are burned at 400° to 500° C. for three or four hours. If the heat be too high or too prolonged, the plaster may be injured, becoming very slow in action, and is called dead-burnt plaster.
Keene's Cement is a Nvell-known hard-finish plaster made by the double calcination of gypsum, alum being added between the two heatings.
Cement plaster, after being calcined, requires the addition of some material as a retarder to decrease the rapidity of set. This is usually a very small quantity (0.1 to 0.2 per cent) of organic matter such as blood or glue. Hydrated lime or clay is usually added to gypsum wall plasters to increase their plasticity and make them work better. With plasters made from gypsum earth contain ing clay this is unnecessary.
38. Properties and plasters when mixed with water set and harden through the combination of the water with the plaster to again force gypscun. The setting of plaster of paris
is rapid, requiring from about five to fifteen minutes. Cement plaster sets more slowly, requiring from one to three hours. Floor plaster and hard-finish plaster are slow setting.
Very few data are available concerning the strength of gypsum plasters, which usually gain strength rapidly for a few days, reach ing a maximum in three or four weeks, and then suffer retrogression in strength for a time. A series of tests made by Professor Marston of Iowa State College on hard wall plasters indicate a strength for neat plaster of 300 to 500 one month after mixing. About 80 per cent as much for 1 to 1 mortar and 50 per cent as much for 1 to 2 mortar with sand. These strengths would not be reached under the conditions of ordinary use. The strength is much less when the mortar is kept clamp during the period of hardening.
Plaster of paris sets too rapidly for use in construction, although it is used to some extent combined with other materials, as in hard finish, composed of plaster of paris, lime putty, and marble dust. It is commonly employed for casting plaster, where quick set is desired.
Cement and hard wall plasters are used for making various wall plasters, being usually mixed with hair, asbestos, or wood fiber, and clay or hydrated lime. They are received upon the work ready for use and do not require the time or space for preparation needed for lime plaster, but are not so plastic and smooth to work.
Hard-finish plasters are used in a number of ways in making solid or hollow blocks and tiles for use in construction of partitions and in finishing floors and ceilings. Mixed with sawdust, blocks are formed which may be nailed into place. Blocks reinforced with steel are now being made for use in supporting roofs. (Sec Art. 18.)