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The effects of high ambient temperatures on gypsum plasters
Build. Sci. Vol. 9. pp. 243 245. Pergamon Press 1974. Printed in Great Britain
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TECHNICAL NOTE
The Effects of High Ambient Temperatures on Gypsum Plasters A. E. M O U L D * D. W. WILLIAMS*
G)'psum plasterboard, widel)' used Jot internal partitions in building structures, has been shown to suffer rapid dehydration when heated between 6 0 to 125"C in the laboratory, under humidi O' conditions representatit,e of those jound in normal buildings ( ~ 13 mm Hg water t,apour pressure). This dehydration could have important consequences for the use of gypsum in some building situations, since loss qf water leads' to craeking, powdering and loss of fire resistance.
The thermodynamic stability of gypsum is limited to 42"C, above which anhydrous calcium sulphate (CaSO~) is the stable phase. When gypsum products are used in situations where they are continuously exposed to temperatures above 42' C, the gypsum decomposes to a metastable phase (the hemi-hydrate, CaSOg.½HzO ) and, at 400"C, to anhydrous calcium sulphate. The amount of calcination depends upon both temperature and exposure time, and because areas of gypsum plaster in buildings could be at sustained temperatures above 42°C, it is important to know to what extent water is lost from gypsum at temperatures likely to be found in use. Tests were therefore made to find out how much degradation could be expected in service conditions and what temperature limits need to be imposed on gypsum materials.
INTRODUCTION
T H E B U I L D I N G industry uses gypsum-based products extensively for internal plastering because, being based on calcium sulphate, they have a quick setting time. They are pre-mixed and can be applied wet to a variety of supporting surfaces. Considerable and increasing use is being made of dry fixed gypsum plasterboard for ceilings, wall and partition linings, and for protective panelling. Plasterboard is manufactured from calcium sulphate paste extruded to harden between paper facings. Two kinds of paper are used so that one board face can receive decoration direct while the other is suitable for a thin plaster finishing coat, applied after the board has been fixed. A great advantage of all gypsum products is their outstanding fire resistance, which is an inherent property of the material and so the degree of resistance is almost proportional to the thickness used. In both the set plaster and plasterboard all the calcium sulphate is present as the dihydrate, gypsum (Ca $ 0 4 . 2 H 2 0 ) , and it is the heat needed to decompose the gypsum and drive off the chemically-bound water which is responsible for its excellent fire-resistance.[I, 2] The loss of chemically-bound water is much more important in unbacked plaster which is only supported at intervals, such as by ceiling joists, than it is in plaster trowelled on to a solid backing, where the backing alone usually possesses the required fire resistance. Plasterboard, however is assumed to possess a certain degree of fireresistance which will remain constant and available throughout its life.
EXPERIMENTAL INVESTIGATION
The details of gypsum dehydration have been studied by many workers who have shown that the reaction is sensitive to water vapour pressure as well as temperature, but the experiments have been mostly of the short-term "dynamic" type such as d.t.a.[3,4]. Ball and Norwood have recently presented a detailed study of gypsum dehydration occurring between 80°C and 150~'C under a wide range of water vapour pressures[5]. Andrews has shown that gypsum plaster specimens exposed for a long period to an artificially dried atmosphere (approximately 0.3 mm Hg water vapour pressure) at 40°C behave very differently to those similarly exposed at 50~C[6]. The two curves shown as broken lines in figure I are his results. Whereas the samples held at 4 0 C lost only a
*Electricity Council Research Centre, Capenhurst, Chester. 243
244
A. E. M o u l d a n d D. W. W i l l i a m s
fraction of 1 per cent of their original weight, those at 50°C lost 15.7 per cent (the amount needed for complete conversion to hemi-hydrate) in 2-5 yr. It is clear from this evidence that under unusually dry conditions gypsum appears stable at 40~C, but that it dehydrates readily at 50°C. In buildings, however, water vapour pressure is invariably greater than in Andrews" experiments and temperatures may be higher. These new tests were made in a laboratory where the water vapour pressure was about 13 mmHg, typical of that found in many buildings throughout the year. The experimental material was taken from two commercially-obtained 9.5ram thick sheets of standard plasterboard. Samples (25 × 25 mm) were cut from the middle of the sheets so as to present uniform and representative surfaces of plaster edge and of paper facing. Samples were heated in ventilated ovens at fixed temperatures and weighed periodically to constant weight. The dehydration rates obtained are plotted in figure 1. It can be seen that the rate of dehydration is greatly accelerated by moderate increases in temperature above 50°C. Above 100°C complete
,~
/'----------/7~
. . . . .
conversion to hemi-hydrate occur~ rapidly, but between 60 and 100'~C the weight h)ss curve~ flatten off before this stage is reached. Andrew¢ results at 50°C show uninterrupted weight loss to full hemi-hydrate formation. The fact that all these results when plotted form a family of curves indicates that the initial dehydration rates are not particularly sensitive to water vapour pressure and, therefore, dehydration in a normal building environment at 50"C would be expected at first to follow Andrews' curve. Later stages of dehydration are sensitive to the ambient water vapour pressure. Thus, under very dry or very hot condition~ (> 100C) hemi-hydrate formation can be completed bul below I00~C, under the normal water vapour pressures prevailing in buildings, dehydration stops at approximately 1t-5 per cent weight loss. When weight loss tests were completed, the samples were exposed to normal room conditions in the laboratory (20-22~C, 50-60 per cent relative humidity) so that rehydration rates could be checked. Again samples were weighed periodically: during the first day those which had been heated above 100~'C regained about one third of their loss of weight, whereas those which had been heated to temperatures below 100°C did not change significantly in weight. Thereafter all samples stabilised at about 10 per cent below their original (unheated) weight, even alter many months. Loss of chemically-bound water from gypsum can result in powdering and cracking, and the indications are that the complete conversion of gypsum to hemi-hydrate will result in about 40 per cent loss of fire-resistance for a 9'5 mm thickness of plasterboard. CONCLUSIONS
~L~
0
C
~0 " hr
[ Cloy
I week
[
month
[yr
Fig. 1. Weight loss o f heated plasterboard and plaster samples. --E C R C Results, plasterboard, 13 mm Hg water vapour pressure. --Andrews (1951) results, plaster, 0"3 mm Hg water Fapour pressHre.
Below 42~C gypsum will not dehydrate under any humidity conditions but at about 13 mmHg water vapour pressure, typical of the humidity conditions normally found in buildings, gypsum plaster will decompose at a rate varying with temperature. The rate becomes appreciable within 2 h at 100'~C~ 12 h at 80°C or 3 weeks at 60'~C, and is such that l0 per cent weight loss will occur in less than I day at 100°C, 3 days at 80°C and 6 weeks at 60~'C. There is little or no rehydration of calcined samples on cooling in normal room conditions.
REFERENCES |. L. A. AsH'roN, Designing for protection against fire with plasterboard. Insulation. November, 223 (1973). 2. G.A. KING, J. BERETHKAand M. J. RIDGE, Chemical changes in slabs of cast gypsum during standard tests of resistance to fire. J. Appl. Chem. Biotechnol. 21, 159 (1971 ).
The Effects of High Ambient Temperatures on G3'psum Pla.~ter.~ 3. H. G. McADIE, The effect of water vapour upon the dehydration of CaSO4.2H20. Canadian J. Chem. 42, 792 (1964). 4., F. E. HANSENand H. J. CLAUSSEN, Dehydration of gypsum. Zement-Kalk-Gips. 26, 223 (1973). 5. M. C. BALL and L. S. NORWOOD, Studies in the system calcium sulphate-water. Pt. I. Kinetics of dehydration of calcium sulphate dihydrate. J. Chem. Sot., Seclion A, 11, 1633 (1969). 6. H. ANDREWS,Production, properties and uses of calcium sulphate plasters. Proc. Btdhlin~, Res. Con,C*. Div. 2, part F (1951).
Les panneaux de pl~tre commundment utilis~s pour les cloisons interieures de structures de bfitiments sont montrds subir une deshydratation rapide lorsqu'ils sont chauffes entre 60 et 125°C en laboratoire et sous des conditions d'humidit6 representatives de celles qui existent dans des bfitiments normaux ( ~ 13 mm de Hg de pression de vapeur d'eau). Cette ddshydratation pourrait avoir des consequences importantes dans l'emploi du gypse dans certaines conditions de construction, car la perte d'humidit6 mene au fendillement ~l la pulv&isation et ~ la diminution de la resistance "a l'incendie. Es wurde festgestellt, dab allgemein in Baukonstruktionen ftir lnnenzwischenw'ande verwandte Gipsbauplatten unter rapider Dehydrierung leiden, wenn sie im Laboratorium auf 60~'C bis 125°C unter Feuchtigkeitsverh/J.ltnissen erw/irmt werden, die in normalen Geb/iuden ( ~ 13 mm Hg Wasserdampfdruck) allgemein vorkommen. Diese Dehydrierung kann wichtige Folgen fiJr den Gebrauch yon Gips in bestimmten Baulagen haben, da Wasserverlust Risse, Zerst/iubung und Verlust an Feuerbest~ndigkeit verursacht.
245
]
,
, Rf7 j (J4)[
TECHNICAL NOTE
The Effects of High Ambient Temperatures on Gypsum Plasters A. E. M O U L D * D. W. WILLIAMS*
G)'psum plasterboard, widel)' used Jot internal partitions in building structures, has been shown to suffer rapid dehydration when heated between 6 0 to 125"C in the laboratory, under humidi O' conditions representatit,e of those jound in normal buildings ( ~ 13 mm Hg water t,apour pressure). This dehydration could have important consequences for the use of gypsum in some building situations, since loss qf water leads' to craeking, powdering and loss of fire resistance.
The thermodynamic stability of gypsum is limited to 42"C, above which anhydrous calcium sulphate (CaSO~) is the stable phase. When gypsum products are used in situations where they are continuously exposed to temperatures above 42' C, the gypsum decomposes to a metastable phase (the hemi-hydrate, CaSOg.½HzO ) and, at 400"C, to anhydrous calcium sulphate. The amount of calcination depends upon both temperature and exposure time, and because areas of gypsum plaster in buildings could be at sustained temperatures above 42°C, it is important to know to what extent water is lost from gypsum at temperatures likely to be found in use. Tests were therefore made to find out how much degradation could be expected in service conditions and what temperature limits need to be imposed on gypsum materials.
INTRODUCTION
T H E B U I L D I N G industry uses gypsum-based products extensively for internal plastering because, being based on calcium sulphate, they have a quick setting time. They are pre-mixed and can be applied wet to a variety of supporting surfaces. Considerable and increasing use is being made of dry fixed gypsum plasterboard for ceilings, wall and partition linings, and for protective panelling. Plasterboard is manufactured from calcium sulphate paste extruded to harden between paper facings. Two kinds of paper are used so that one board face can receive decoration direct while the other is suitable for a thin plaster finishing coat, applied after the board has been fixed. A great advantage of all gypsum products is their outstanding fire resistance, which is an inherent property of the material and so the degree of resistance is almost proportional to the thickness used. In both the set plaster and plasterboard all the calcium sulphate is present as the dihydrate, gypsum (Ca $ 0 4 . 2 H 2 0 ) , and it is the heat needed to decompose the gypsum and drive off the chemically-bound water which is responsible for its excellent fire-resistance.[I, 2] The loss of chemically-bound water is much more important in unbacked plaster which is only supported at intervals, such as by ceiling joists, than it is in plaster trowelled on to a solid backing, where the backing alone usually possesses the required fire resistance. Plasterboard, however is assumed to possess a certain degree of fireresistance which will remain constant and available throughout its life.
EXPERIMENTAL INVESTIGATION
The details of gypsum dehydration have been studied by many workers who have shown that the reaction is sensitive to water vapour pressure as well as temperature, but the experiments have been mostly of the short-term "dynamic" type such as d.t.a.[3,4]. Ball and Norwood have recently presented a detailed study of gypsum dehydration occurring between 80°C and 150~'C under a wide range of water vapour pressures[5]. Andrews has shown that gypsum plaster specimens exposed for a long period to an artificially dried atmosphere (approximately 0.3 mm Hg water vapour pressure) at 40°C behave very differently to those similarly exposed at 50~C[6]. The two curves shown as broken lines in figure I are his results. Whereas the samples held at 4 0 C lost only a
*Electricity Council Research Centre, Capenhurst, Chester. 243
244
A. E. M o u l d a n d D. W. W i l l i a m s
fraction of 1 per cent of their original weight, those at 50°C lost 15.7 per cent (the amount needed for complete conversion to hemi-hydrate) in 2-5 yr. It is clear from this evidence that under unusually dry conditions gypsum appears stable at 40~C, but that it dehydrates readily at 50°C. In buildings, however, water vapour pressure is invariably greater than in Andrews" experiments and temperatures may be higher. These new tests were made in a laboratory where the water vapour pressure was about 13 mmHg, typical of that found in many buildings throughout the year. The experimental material was taken from two commercially-obtained 9.5ram thick sheets of standard plasterboard. Samples (25 × 25 mm) were cut from the middle of the sheets so as to present uniform and representative surfaces of plaster edge and of paper facing. Samples were heated in ventilated ovens at fixed temperatures and weighed periodically to constant weight. The dehydration rates obtained are plotted in figure 1. It can be seen that the rate of dehydration is greatly accelerated by moderate increases in temperature above 50°C. Above 100°C complete
,~
/'----------/7~
. . . . .
conversion to hemi-hydrate occur~ rapidly, but between 60 and 100'~C the weight h)ss curve~ flatten off before this stage is reached. Andrew¢ results at 50°C show uninterrupted weight loss to full hemi-hydrate formation. The fact that all these results when plotted form a family of curves indicates that the initial dehydration rates are not particularly sensitive to water vapour pressure and, therefore, dehydration in a normal building environment at 50"C would be expected at first to follow Andrews' curve. Later stages of dehydration are sensitive to the ambient water vapour pressure. Thus, under very dry or very hot condition~ (> 100C) hemi-hydrate formation can be completed bul below I00~C, under the normal water vapour pressures prevailing in buildings, dehydration stops at approximately 1t-5 per cent weight loss. When weight loss tests were completed, the samples were exposed to normal room conditions in the laboratory (20-22~C, 50-60 per cent relative humidity) so that rehydration rates could be checked. Again samples were weighed periodically: during the first day those which had been heated above 100~'C regained about one third of their loss of weight, whereas those which had been heated to temperatures below 100°C did not change significantly in weight. Thereafter all samples stabilised at about 10 per cent below their original (unheated) weight, even alter many months. Loss of chemically-bound water from gypsum can result in powdering and cracking, and the indications are that the complete conversion of gypsum to hemi-hydrate will result in about 40 per cent loss of fire-resistance for a 9'5 mm thickness of plasterboard. CONCLUSIONS
~L~
0
C
~0 " hr
[ Cloy
I week
[
month
[yr
Fig. 1. Weight loss o f heated plasterboard and plaster samples. --E C R C Results, plasterboard, 13 mm Hg water vapour pressure. --Andrews (1951) results, plaster, 0"3 mm Hg water Fapour pressHre.
Below 42~C gypsum will not dehydrate under any humidity conditions but at about 13 mmHg water vapour pressure, typical of the humidity conditions normally found in buildings, gypsum plaster will decompose at a rate varying with temperature. The rate becomes appreciable within 2 h at 100'~C~ 12 h at 80°C or 3 weeks at 60'~C, and is such that l0 per cent weight loss will occur in less than I day at 100°C, 3 days at 80°C and 6 weeks at 60~'C. There is little or no rehydration of calcined samples on cooling in normal room conditions.
REFERENCES |. L. A. AsH'roN, Designing for protection against fire with plasterboard. Insulation. November, 223 (1973). 2. G.A. KING, J. BERETHKAand M. J. RIDGE, Chemical changes in slabs of cast gypsum during standard tests of resistance to fire. J. Appl. Chem. Biotechnol. 21, 159 (1971 ).
The Effects of High Ambient Temperatures on G3'psum Pla.~ter.~ 3. H. G. McADIE, The effect of water vapour upon the dehydration of CaSO4.2H20. Canadian J. Chem. 42, 792 (1964). 4., F. E. HANSENand H. J. CLAUSSEN, Dehydration of gypsum. Zement-Kalk-Gips. 26, 223 (1973). 5. M. C. BALL and L. S. NORWOOD, Studies in the system calcium sulphate-water. Pt. I. Kinetics of dehydration of calcium sulphate dihydrate. J. Chem. Sot., Seclion A, 11, 1633 (1969). 6. H. ANDREWS,Production, properties and uses of calcium sulphate plasters. Proc. Btdhlin~, Res. Con,C*. Div. 2, part F (1951).
Les panneaux de pl~tre commundment utilis~s pour les cloisons interieures de structures de bfitiments sont montrds subir une deshydratation rapide lorsqu'ils sont chauffes entre 60 et 125°C en laboratoire et sous des conditions d'humidit6 representatives de celles qui existent dans des bfitiments normaux ( ~ 13 mm de Hg de pression de vapeur d'eau). Cette ddshydratation pourrait avoir des consequences importantes dans l'emploi du gypse dans certaines conditions de construction, car la perte d'humidit6 mene au fendillement ~l la pulv&isation et ~ la diminution de la resistance "a l'incendie. Es wurde festgestellt, dab allgemein in Baukonstruktionen ftir lnnenzwischenw'ande verwandte Gipsbauplatten unter rapider Dehydrierung leiden, wenn sie im Laboratorium auf 60~'C bis 125°C unter Feuchtigkeitsverh/J.ltnissen erw/irmt werden, die in normalen Geb/iuden ( ~ 13 mm Hg Wasserdampfdruck) allgemein vorkommen. Diese Dehydrierung kann wichtige Folgen fiJr den Gebrauch yon Gips in bestimmten Baulagen haben, da Wasserverlust Risse, Zerst/iubung und Verlust an Feuerbest~ndigkeit verursacht.
245