Investigation of failures of white-coat plaster

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 failures 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 CaS04.2H20, CaCOs, Ca(0H)2, Mg(0H)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 CaS04.2H20, Mg(0H)2, Ca(0H)2, and CaCOa 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 precautions that should be exercised in specifying a proper lime in order to prevent future failures are discussed. Publication of National Bureau of Standards Research Paper RP1022, "Hydration of Magnesia in Dolimitic Hydrated Limes and Putties" [2], referred to in the letter to "Builduag and Building Management", soon led to consideration of the actual effect (if any) of unh^'drated magnesia (MgO) in partially hydrated dolomitic limes on the performance of mortars and plasters containing such limes, and the National Bm-eau of Standards soon became interested in this problem as it relates to plaster.1 to which it is to be applied. After it is applied, but before it hard...

Properties of some masonry cement

“…In the investigation of white-coat plaster failure by W ells, Clarke, Newman, and Bishop [5], it was shown that the failures were caused by the delayed hydration and resulting expansion of the magnesia in the dolomi tic lime. It was also shown by Wells, Clarke, and Levin [6], in their study of th e expfinsive characteristics of hydrated limes and the development of a test for soundness, that expansion of the autoclaved bars was dependent not only on the amount of unhydrated magnesia but also on the 14 strength of the test specimens.…”

Nomograms for obtaining the compound composition of hydrated limes from the oxide analysis/

“…Studies [4] have been in progress for several years at the National Bureau of Standards of a particular type of plaster failure, associated only with regularly hydrated dolomitic limes and characterized by the formation of bulges, or large blisters, in the white-coat. It has been found that this type of failure results from a delayed expansion of the set, white-coat plaster.…”