- Author
-
Wells, L. S.
|
Clarke, W. F.
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Newman, E. S.
|
Bishop, D. L.
- Title
- Investigation of Failures of White-Coat Plaster.
- Coporate
- National Bureau of Standards, Washington, DC
- Report
-
BMS 121
February 1, 1951
44 p.
- Keywords
-
plaster
|
failure
|
surveys
|
blisters
|
thermal analysis
- Abstract
- An investigation was made of a particular kind of plaster failure that is characterized by the formation of blisters or bulges in the finish coat of plaster. Extensive surveys revealed that the failures are widespread; that several years elapse before the bulges appear and thereafter the number and extent of filures increase as the age of the buildings increases; that the failures occur more extensively and rapidly during warm humid weather; and that there is a marked similarity in the failures regardless of the kind of plaster base coats, type of construction base, varying job conditions, and type of workmanship. The usual white coat is prepared from a lime putty gaged with plaster of paris (gypsum gaging plaster). Chemical analysis of 88 samples of white-coat plaster that had failed showed that in every instance a dolomitic lime had been used in preparing the white coat. Regularly hydrated dolomitic limes contain about 32 percent of total MgO (by weight), of which only about 5 percent is hydrated and the remaining 27 percent is still present as unhydrated MgO. This highly incomplete hydration is due to the fact that the magnesia has been badly overburned in the production of dolomitic quicklimes (which contain CaO and MgO in nearly equal molecular proportions) and thus is inactive toward hydration in the usual hydrators. Furthermore, only about 20 percent of the hydration of the magnesia is completed at the end of the customary 1-day soaking period. The amounts of CaSO4.2H2), CaCO3, Ca(OH)2, Mg(OH)2, and MgO in the hardened plasters were calculated from the chemical analysis and closely checked by the method of heat of solution. The presence of CaSO4.2H2O, Mg(OH)2, Ca(OH)2, and CaCO3 was confirmed by thermal analysis. The average amount of hydration of the MgO for 88 samples of failed white coat was 59.0 percent - much in excess of the 20 percent attained after the customary 1 day of soaking the lime as a putty. Hydration of magnesia must, therefore, occur on the wall. The hydration of MgO is attended by marked expansion. Failures were also observed in base coats where dolomitic lime containing considerable unhydrated MgO was used in their preparation. It was shown that suggested causes other than hydration of MgO with its attendant expansion cannot account for the bulges that occur in plaster several years after a building has been erected. The precaustions that should be exercised in specifying a proper lime in order to prevent future failures are discussed.