S.I. No. 240/1950 - Standard Specification (Gypsum Plasters) Order, 1950.

S.I. No. 240 of 1950.

STANDARD SPECIFICATION (GYPSUM PLASTERS) ORDER, 1950.

I, DANIEL MORRISSEY, Minister for Industry and Commerce, in exercise of the power conferred on me by subsection (3) of section 20 of the Industrial Research and Standards Act, 1946 (No. 25 of 1946), hereby order as follows :

1. This Order may be cited as the Standard Specification (Gypsum Plasters) Order, 1950.

2.—(1) The specification set forth in Part II of the Schedule to this Order is hereby declared to be the standard specification for the commodity described in Part I of the said Schedule.

(2) The said standard specification may be cited as Irish Standard 27 : 1950.

SCHEDULE.

PART I.

GYPSUM PLASTERS.

PART II.

SPECIFICATION.

SCOPE.

1. This specification applies to gypsum plasters, suitable for use in building operations and in the manufacture of building materials, which set due to the hydration of calcium sulphate.

DEFINITIONS.

2. For the purpose of this specification the following definitions shall apply :—

(a) Hemihydrate. The term hemihydrate shall mean an intermediate phase in the dehydration of gypsum, CaSO₄.2H₂O, wherein the amount of combined water corresponds approximately to the formula 2CaSO₄.H₂O, calcium sulphate hemihydrate.

(b) Test sieve of any particular designation shall mean the test sieve of that designation as prescribed by Irish Standard 24: 1950.

CLASSES.

3. Gypsum plaster shall be one of the three classes : Common Plaster of Paris, Superfine Plaster of Paris, and Hardwall Plaster, designated respectively as,

Common Gypsum Plaster,

Superfine Gypsum Plaster, and

Retarded Gypsum Plaster.

CHEMICAL COMPOSITION.

4. Gypsum plaster shall consist mainly of hemihydrate of calcium sulphate, but may also contain added materials to improve the working characteristics or to impart anti-corrosion properties.

Materials for the purpose of retarding the set shall not be added to common gypsum plaster and superfine gypsum plaster.

Added materials and materials arising from impurities in the original gypsum shall not be of a type having a deleterious effect on the set plaster or on other building materials in contact with it or adjacent to it when in use.

The content of sulphuric anhydride expressed as a percentage by weight of the plaster, when determined by the method described in Appendix A, shall be,

for common gypsum plaster not less than 35 per cent.,

for superfine gypsum plaster not less than 40 per cent., and

for retarded gypsum plaster not less than 35 per cent.

The content of calcium oxide when determined by the method described in Appendix A shall be not less than two-thirds of the content of sulphuric anhydride by weight.

The content of soluble sodium salts expressed as sodium oxide Na₂O, when determined as described in Appendix B, shall be not greater than 0·1 per cent. by weight of the plaster.

The content of soluble magnesium salts expressed as magnesium oxide, MgO, when determined as described in Appendix B, shall be not greater than 0·1 per cent. by weight of the plaster.

LOSS OF WEIGHT ON IGNITION.

5. The loss of weight on ignition, when determined by the method described in Appendix C, shall be not greater than 9 per cent., and not less than 4 per cent. by weight of the plaster.

CONTENT OF COARSE PARTICLES.

6. The content of coarse particles expressed as a percentage by weight, when determined in the manner described in Appendix D, shall not exceed the values set out for each class in Table 1.

TABLE 1.

SETTING TIME.

7. The setting time of common gypsum plaster and of superfine gypsum plaster, when determined by the method described in Appendix G, shall be not less than 10 minutes and not more than 30 minutes.

The setting time of retarded gypsum plaster, when determined by the method described in Appendix G, shall be not less than one hour and not more than three hours.

SOUNDNESS.

8. The set plaster pats, when prepared and tested by the method described in Appendix H, shall show no signs of disintegration, popping or pitting.

MECHANICAL RESISTANCE.

9. The mechanical resistance of the set plaster, when determined in the manner described in Appendix K, shall be such that the impact of the dropping ball produces an impression not greater than 4·5 mm. and not less than 3 mm. in diameter.

TRANSVERSE STRENGTH.

10. The transverse strength (modulus of rupture) of the set plaster, when determined by the method described in Appendix L, shall be not less than 70 lb. per sq. in. for common gypsum plaster and superfine gypsum plaster and not less than 300 lb. per sq. in. for retarded gypsum plaster.

MARKING.

11. Each package of plaster shall be marked to show clearly the class to which the plaster belongs, the nett weight of the plaster and the manufacturer's name or mark.

TESTING.

12. The manufacturer shall ensure by regular periodical testing that the plaster conforms to the requirements of this specification.

If the purchaser requires independent tests, the tests shall be carried out in accordance with this specification, and the cost of the tests shall be borne,

by the vendor in the event of results showing that the plaster does not comply with this specification,

by the purchaser in the event of results showing that the plaster complies with this specification.

SAMPLING AND SIZE OF SAMPLES.

13. Samples of plaster required for the purpose of testing conformity with this specification shall be taken in approximately equal portions from not less than 12 different positions when the plaster is loose or from not less than 12 different packages from each consignment of 5 tons of plaster or less. Samples shall be withdrawn by means of a sample spear, about 1 in. in diameter, and long enough to penetrate the full depth of the package. The sample portions shall be thoroughly mixed and shall be quartered down to give a sample of not less than 10 lb. in weight and shall be enclosed immediately in a clean, dry, air-tight container. The container so filled shall be sealed and shall be marked with the date of sampling, and with sufficient information to identify the sample.

TESTING CONDITIONS.

14. When carrying out the tests described in the appendices the temperature of the materials, including water, and the temperature of the test room at the time when the tests are being carried out shall be between 12° C. and 20° C.

Except where otherwise specified the water used in all testing operations shall be pure and free from more than traces of dissolved salts.

Appendix A.

DETERMINATION OF SULPHURIC ANHYDRIDE AND OF CALCIUM OXIDE.

One g. of the plaster, ground if necessary to pass a No. 52 test sieve, shall be mixed with 50 ml. of distilled water in a beaker. Five g. of ammonium carbonate shall be added and the whole gently boiled for 15 minutes. A small excess of hydrochloric acid shall then be added and the boiling continued for a further 10 minutes.

The insoluble matter shall be filtered off and washed with hot water, and the combined filtrate and washings diluted to 250 ml.

The amounts of sulphuric anhydride and of calcium oxide in this filtrate shall be determined, and the contents of sulphuric anhydride and of calcium oxide in the plaster calculated as percentages by weight.

Appendix B.

DETERMINATION OF SOLUBLE SODIUM AND MAGNESIUM SALTS.

Five g. of the plaster, ground if necessary to pass a No. 52 test sieve, shall be weighed in a beaker and 50 ml. of boiling distilled water added. The mixture shall be well stirred and then maintained at a temperature of just below boiling point for fifteen minutes with frequent stirring. The insoluble matter shall be allowed to settle, and the solution carefully decanted through a paper filter. This process shall be repeated twice, but extracting for five minutes instead of fifteen. The residue shall finally be washed on a filter paper with about 50 ml. of boiling water and then rejected.

The amounts of sodium salts and of magnesium salts in this filtrate shall be determined and from the results, the contents of soluble sodium salts and of soluble magnesium salts in the plaster shall be calculated.

Appendix C.

DETERMINATION OF LOSS OF WEIGHT ON IGNITION.

One g. of the plaster shall be weighed and spread out in a thin layer in a small porcelain or silica boat and heated to constant weight in a slow stream of dry, carbon-dioxide-free air, at a temperature of 500 ° C. to 600° C. The initial heating shall be carried out at a suitable slow rate to avoid loss of material by " boiling." The plaster shall then be weighed in a closed weighing tube as expeditiously as possible, so as to avoid absorption of water-vapour or of carbon-dioxide. The percentage loss of weight shall be calculated.

Appendix D.

METHOD FOR THE DETERMINATION OF THE CONTENT OF COARSE PARTICLES.

One hundred g. of the plaster shall be sifted continuously for 5 minutes on a No. 14 test sieve and the residue on the sieve weighed. In the case of superfine gypsum plaster and retarded gypsum plaster, the plaster which has passed through a No. 14 test sieve shall be sifted continuously for 5 minutes on a No. 100 test sieve and the residue on this sieve weighed. Air-set lumps in the sample may be broken down with the fingers, but nothing shall be rubbed on the sieve.

Appendix E.

STABILISING OF SAMPLES FOR TESTING.

The plaster shall be spread in a layer not more than ½ in. in thickness and exposed for 24 hours to an atmosphere of 76±3 per cent. relative humidity, at a temperature between 12° C. and 20° C., and with vigorous air circulation over the specimen throughout this period.

The requisite degree of humidity may be obtained by placing a wide open dish, containing a saturated solution of common salt (NaCl), together with solid salt, in a tightly-closed cabinet along with a shallow dish containing the sample of plaster, the air in the cabinet being constantly stirred and having free access to the brine-salt mixture and to the plaster sample.

Appendix F.

DETERMINATION OF NORMAL CONSISTENCY.

The apparatus used for the determination of normal consistency shall be the Vicat apparatus, the plunger of which shall consist of a metal cylinder 44·4 mm. in length and 19 mm. in diameter, and the top of the sliding arm shall carry a metal shelf. The combined weight of the plunger, sliding arm and shelf shall be 50 g.

The moulds used shall be ring moulds consisting of a slightly tapered split ring the two parts of which are held together by an outer narrow ring. The mould shall be 8 cm. in internal diameter and 4 cm. in height. The moulds shall be made of brass, bronze, or other corrosion resistant metal, and shall be used well-greased and resting on well-greased non-absorbent plates.

Two hundred g. of the plaster which has been stabilised in accordance with the procedure described in Appendix E shall be weighed and mixed with a measured weight of water, and if the material is common gypsum plaster, 0·25 per cent. by weight of glue, size or other suitable retarder shall be added and the mixture gently stirred to form a smooth paste free from lumps and air bubbles. The ring mould resting on a non-absorbent plate shall be filled with the paste and struck off smooth and level with the top of the mould, care being taken to prevent the formation of voids or air bubbles in the plaster. The mould shall be placed under the plunger of the Vicat apparatus and the plunger shall be wetted and shall be lowered gently to touch the surface of the plaster in the mould. The millimetre scale shall thenbe read and the plunger released and allowed to sink into the plaster under the weight of the moving parts of the apparatus only. When the plunger has come to rest the millimetre scale shall be read. At least two such operations shall be carried out in quick succession, care being taken that the plunger is wet and the mould full. The plaster shall be regarded as mixed to normal consistency when the average of two penetrations of the plunger as read on the millimetre scale is 30 mm. The weight of the water in grams required to be added to one hundred g. of the plaster in order that a penetration of 30 mm. of the plunger shall occur under the above conditions shall be calculated.

Appendix G.

DETERMINATION OF SETTING TIME.

The apparatus used for the determination of setting time shall be the Vicat apparatus. The vertical moveable rod of the apparatus shall have at its lower end a needle 1 mm. sq. in cross-section with a flat end, and at its upper end a shelf loaded so that the weight of the needle, sliding arm and shelf is 300 g.

Two hundred g. of the plaster stabilised in accordance with the procedure described in Appendix E shall be weighed. The weight of water required to be mixed with the plaster to produce a mix of normal consistency in accordance with the procedure described in Appendix F shall be determined. The weighed quantity of plaster shall be added to the calculated quantity of water by sprinkling or dusting on in small quantities and the mixture stirred to produce a smooth paste free from lumps and air bubbles. The mixture shall then be poured into a ring mould resting on a non-absorbent plate as described in Appendix F and struck off level with the top of the mould in such a manner as to eliminate voids or air-bubbles. The mould with its base-plate shall be placed under the needle of the Vicat apparatus. The needle shall then be lowered gently until it is in contact with the surface of the plaster and then suddenly released and allowed to sink into it. The operation shall be repeated until the needle just fails to reach the base-plate of the mould. The time from the moment of adding the plaster to the water until the needle just fails to pierce completely the plaster pat in the mould shall be regarded as the setting time of the plaster.

Appendix H.

DETERMINATION OF SOUNDNESS.

The soundness shall be determined by steaming 6 pats of neat plaster prepared in ring moulds. The moulds shall be 4 in. in diameter and ¼ in. deep, well greased before use and resting on well-greased non-porous base plates.

A weighed quantity of the plaster stabilised in accordance with the procedure described in Appendix E shall be added in small quantities by sprinkling or dusting on to the quantity of water which will produce a paste of normal consistency as described in Appendix F. The pats shall be formed by pouring or pressing the material into the moulds, in such a manner as to avoid air bubbles, and smoothing off level with the top edge of the ring with a broad flexible palette-knife.

The six pats shall be allowed to set undistrubed in air for between 16 and 24 hours and then subjected to the action of saturated steam at atmospheric pressure for a period of 3 hours, without removal from the moulds, and finally examined in good light. The steamer shall be arranged so that condensed water cannot drip back on the surface of the plaster pats.

Appendix K.

DETERMINATION OF MECHANICAL RESISTANCE.

A weighed quantity of the plaster stabilised in accordance with the procedure described in Appendix E shall be added in small quantities, by sprinkling or dusting on to the quantity of water which will produce a paste of normal consistency as described in Appendix F. The plaster shall then be filled into 4 moulds using only a flexible palette knife and introducing small quantities at a time and in such a manner as to eliminate voids or air bubbles. The surfaces shall be smoothed down and struck off level with the top of the moulds.

The plaster shall be allowed to rest undisturbed in the moulds for 24 hours, in an atmosphere of at least 90 per cent. relative humidity. The blocks of plaster shall then be removed from the moulds and dried for three days separately in a well ventilated drying oven maintained at a temperature of 35° C. to 40° C. and then tested dry.

The moulds used shall be of a size and shape such as will produce rectangular blocks 1 in. sq. in cross-section with a tolerance of 1/64 in. in each direction, and 4 in. long.

To carry out the test a plaster block shall be supported firmly on a smooth, substantial and unyielding horizontal surface. A clean and polished hard steel ball ½ in. in diameter weighing 8·33 g. shall be released from rest and allowed to fall down a smooth clean dry tube on to the surface of the plaster block. The tube used shall be about 5/8 in. in diameter and not more than 68 in. in length and shall be supported firmly in a vertical position. The top of the tube shall be 6 ft. above the upper surface of the specimen under test, the ball being released at the top of the tube. A smooth side surface of the specimen which was in contact with the internal face of the mould when casting shall be placed uppermost to receive the impact of the falling ball. One impression shall be made on each of the opposite faces of the four specimens ; eight impressions being made in all. The indentations shall be made in areas free from blemishes and air bubbles and not more than ¼ in. from the centre line of the block nor within a ½ in. of the end of the block. Any impression obviously irregular in shape shall be neglected and the test repeated on that face. For each impression two diameters approximately at right angles shall be measured and the mean of the sixteen measurements obtained.

Appendix L.

DETERMINATION OF TRANSVERSE STRENGTH.

In the case of common gypsum plaster and superfine gypsum plaster, 240 g. of the plaster stabilised as described in Appendix E shall be weighed and mixed with 160 g. of hydrated lime, conforming to Irish Standard 8 : 1949, by shaking together in a closed containerTwo hundred and eighty ml. of water shall be taken and the plaster mixture shall be distributed lightly on the surface of the water with continuous stirring, the operation occupying 45 to 60 seconds. The whole shall then be stirred for another minute and immediately poured into six moulds. The surfaces shall be struck off level with the top of the moulds in such a manner as to eliminate voids or air bubbles. The whole operation should be completed within five minutes of the first addition to the water. The plaster shall be allowed to set undistrubed in the moulds for two hours and the specimens then removed and tested immediately.

In the case of retarded gypsum plaster, 240 g. of the plaster stabilised as described in Appendix E shall be weighed and the quantity of water required to produce a paste of normal consistency as described in Appendix F shall be taken. The plaster shall be sprinkled or dusted on to the surface of the water with continuous stirring and the whole then stirred for about 1 minute and poured into six moulds. The surfaces shall be struck off level with the top of the moulds in such a manner as to eliminate voids or air bubbles. The plaster shall be allowed to set undisturbed in the moulds for 24 hours in an atmosphere of at least 90 per cent. relative humidity. The plaster shall be removed from the moulds and dried for three days in a well-ventilated drying oven maintained at a temperature of 35° C. to 40° C. and then tested.

The moulds used shall be of a size and shape such as will produce rectangular blocks 1 in. sq. in cross-section with a tolerance of ± 1/64 in. in each direction and 4 in. long ; a six-gang mould is convenient. The moulds shall be made of brass, bronze or other corrosion resistant metal and shall be well-greased before use and resting on well-greased non-absorbent plates.

The transverse strength of each specimen shall be determined by placing it symmetrically on two parallel metal rollers ½ in. in diameter and 3 in. apart centre to centre and applying a load by a third parallel roller of the same size at a point midway between the supporting rollers. The load shall be applied steadily and uniformly, starting from zero, and increased at a rate of 50 to 200 lb. per minute. No packing shall be used between the rollers and the specimen.

The mean value of the breaking load in lb. of the six specimens shall be determined. The value obtained shall be multiplied by 4·5 to obtain the transverse strength.

GIVEN under my Official Seal this 15th day of September, 1950.

DANIEL MORRISSEY,

Minister for Industry and Commerce.