GB2080788A

GB2080788A - Plaster coating composition

Info

Publication number: GB2080788A
Application number: GB8123572A
Authority: GB
Original assignee: BPB Industries PLC
Current assignee: BPB Ltd
Priority date: 1980-08-01
Filing date: 1981-07-31
Publication date: 1982-02-10
Also published as: GB2080788B

Abstract

The possible emission of hydrogen sulphide from building materials incorporating a sulphide-containing constituent is suppressed by applying a surface covering composition containing gypsum plaster and a metal compound e.g. zinc oxide capable of reacting with hydrogen sulphide to form a stable sulphide, and allowing such compound to react with hydrogen sulphide evolved by a sulphur containing component contained in the structure or in the composition itself. The sulphide containing constituent may be blast-furnace slag, or pozzolan.

Description

SPECIFICATION Improved cementitious coating composition The present invention relates to cementitious coating compositions suitable for use in covering the surfaces of building structures, for example walls or ceilings.

Interior surfaces of building structures, notably walls and ceilings, have traditionally been coated, prior to final decoration, with plastering compositions based on calcium sulphate plasters. Such compositions have the advantage of a rapid and controllable rate of set, which can be adjusted to an optimum value to ensure good working properties combined with a reasonably short setting time. They do, however, have the disadvantage that they dry from the surface of the applied layer, so that it is usual to apply the final decoration only when the coating is completely dry, and also that the strength of the coating remains low until it is dry.Rapid surface drying and thus earlier decoration can be achieved, without relinquishing the advantages of rapid and controllable set, by mixing another cementitious component with the plaster, notably a pozzolan or latent cement such as quenched or granulated blast-furnace slag. Unfortunately, such materials commonly include a sulphide impurity which in wet conditions can slowly release hydrogen sulphide gas. While the quantities of such gas released are very low and usually well below the permissible threshold limit values, they can give rise to an unpleasant smell in enclosed premises, and compositions containing such materials will not be acceptable unless this difficulty is overcome.Furthermore, there are other building products which incorporate cementitious materials which may also contain sulphide impurities, and are therefore liable to release hydrogen sulphide in damp conditions, for example slag blocks.

We have now found that the release of hydrogen sulphide into the atmosphere from building products containing sulphide impurities under wet conditions can be prevented by employing a cementitious or hydraulic surfacing composition containing a small quantity, preferably 0.01 to 1.00%, of a compound of a metal that forms a stable, and preferably insoluble, sulphide by reaction with hydrogen sulphide. The metal compound may be included in a coating incorporating a sulphide-containing cementitious component, to reduce or inhibit the release of hydrogen sulphide from such coating, or in a cementitious or hydraulic coating to prevent the release into the atmosphere of hydrogen sulphide produced from sulphide impurities in an underlying coating or structure.The coating is preferably based on gypsum plaster, by reason of the rapid and controlled set that can be achieved with this material, but the principles of the invention are applicable to other coating compositions, since the effectiveness of the metal compound does not depend on the presence of calcium sulphate.

According to one aspect of the present invention, an improved coating composition for building structures comprises a cementitious or hydraulic material and a small quantity of a compound of a metal forming a stable sulphide by reaction with hydrogen sulphide, the quantity of metal compound being sufficient to prevent release of hydrogen sulphide from a building structure surfaced with the coating.

The coating compositions of this invention may be employed to coat building structures constructed from sulphide-containing building components, such as slag blocks, orto coat building structures constructed from a wide variety of materials which have then been provided with an undercoat incorporating sulphide-containing cementitious components, such as the compositions of gypsum plaster and slag referred to above. Such plaster and slag compositions may themselves contain the metal compound to reduce or inhibit hydrogen sulphide emission, but are preferably faced with a finish coating containing the metal compound but no sulphurous component.

By virtue of this invention, sulphide-containing constituents may be safely used in building materials, whether in bricks, blocks panels or coating compositions, without the risk of unpleasant and undesirable release of hydrogen sulphide into the atmosphere in the building while the materials are subject to wet conditions. In particular, the advantageous properties of mixtures of gypsum plaster and quenched or granulated slag as coating compositions for building surfaces can be exploited without the disadvantages arising from hydrogen sulphide evolution.

Any metal compound that reacts promptly with hydrogen sulphide to form a stable metal sulphide can in principle be used for the purposes of this invention. However, it is preferable that a metal sh should be used that forms a colourless or only weakly coloured sulphide. A metal forming colourless sulphide is clearly preferable for a finish coating although it may be permissible to use a metal forming a more coloured sulphide for an undercoat composition. An insoluble, stable sulphide permanently removes the sulphur from the system. The present preferred compounds are zinc compounds, and it has been found that surprisingly little of these compounds is required, the quantity being much less than would be expected on a stoichiometric basis, in relation to the proportion of sulphide-containing material and the percentage of sulphide in the material.

It is also desirable that the metal compound should not interfere in the setting of the coating composition in which it is used, although it may be possible to offset such effects by the use of further additives. The use of a metal compound which is insoluble, although still being capable of reaction with hydrogen sulphide, minimizes the effect on the setting rate. For example, where zinc is the metal, zinc sulphate is an effective compound for preventing the release of hydrogen sulphide but has an accelerating effect on the setting of gypsum plaster. Since an accelerated plaster is unsuitable for ordinary plastering work, the use of zinc sulphate as the metal compound would require the addition of further retarder to compensate for the accelerating effect.Zinc oxide, on the other hand, has little effect on the setting rate of gypsum plaster and is accordingly the preferred metal compound for the purposes of this invention. It does, however, have some influence on the setting or hardening reactions of slag and, although we prefer to include zinc oxide in slag-containing surface coating compositions, the proportion of zinc oxide should be limited in relation to the proportion of slag to the extent that the setting of the latter is not impaired. Another zinc compound which is a possible metal compound additive for the purposes of the invention is zinc carbonate.

In accordance with a preferred aspect of the invention, we have found that compositions containing substantial quantities of quenched or granulated slag, to give coatings of quick surface drying and good wet strength properties, can only tolerate limited additions of metal compounds such as zinc oxide, ff the advantages of the slag component are to be retained, and that such limited additions may be insufficient to prevent hydrogen sulphide emission in the long term. This is particularly true in the case of compositions containing a high proportion of slag to gypsum plaster, which are particularly valuable for coating walls and other surfaces in buildings which by reason of external circumstances are in a wet condition when the coating has to be applied.In these circumstances, we prefer two keep the level of metal compound in the coating compositions to that at which the overall properties of the composition are not impaired, and to apply over coating a finish layer of a composition containing little or no sulphide-containing component, but a quantity of the metal compound sufficient to prevent passage of hydrogen sulphide from the layer beneath. A thin layer of such a finishing composition need not involve the disadvantages of slow surface drying which might otherwise arise with purely plaster based surface coatings under poor drying conditions.

For general application as an undercoat or rendering composition, a composition according to the invention should contain gypsum plaster (and more especially hemihydrate) in the approximate range from 20 to 84%, ground blast furnace slag in a quantity of from 79 to 15%, and zinc oxide in an amount of from 0.02 to 1.00% depending on the proportion of slag (all these percentages being by weight), preferably together with small proportions of portland cement and lime. The resulting compositions can be extended with light-weight aggregates and their setting time may be adjusted by the use of conventional retarders.

Within such broad ranges of proportions may be distinguished two classes of coating composition. The first is an undercoat composition for general application, which preferably contains from 64 to 84% hemihydrate plaster, 15 to 35% ground blast furnace slag, 0 to 5% portland cement, 0 to 5% lime, and 0.02 to 0.10 zinc oxide, ignoring for the purposes of these proportions such additives as retarders, viscosity control additives, for example cellulose ethers, and also possible aggregates such as expanded perlite and exfoliated vermiculite which may be added in the proportions of 5 to 20% of the total composition.

For use under very wet conditions, for example in the renovation of decayed buildings, the increased wet strength afforded by a higher proportion of slag is desirable. Such compositions preferably contain from 64% to 40% hemihydrate plaster, from 35 to 59% ground blast furnace slag, from 0 to 5% portland cement, from 0 to 10% lime and from 0.05 to 0.2% zinc oxide, again with the addition of additives and aggregates at similar levels to those already mentioned.

For the purposes of a finish coating, to be applied over a coating of the undercoat compositions already described, but also suitable for providing a hydrogen sulphide resistant finish on other sulphide-containing building materials, the preferred compositions comprise from 97.8 to 99.9% hemihydrate plaster, from 0 to 2% lime, and from 0.05 to 0.2% zinc oxide. Retarders and other additives may be included in the usual way, and a light-weight finish plaster can be provided by the inclusion of a light-weight aggregate such as exfoliated vermiculite in a proportion of from 0.5 to 5.0% of the total composition.

The preferred slag component in these compositions is quenched or granulated blast furnace slag, ground to a suitable particle size, which has been found to contain sulphides leading to the slow release of hydrogen sulphide gas as a result of chemical reaction between the sulphide and the aqueous component of the coating compositon. Such a slag material is available under the trade designation "Cemsave". Such materials are used as extenders for cements, and the protective aspects of the present invention can be utilised in association with coatings, castings or prefabricated building components incorporating such slag products or other materials having a sulphide content which is liable to react under wet conditions with the release of hydrogen sulphide.

A particularly preferred system in accordance with this invention comprises a wall, ceiling or other building structure having a first coating of a cementitious composition comprising gypsum plaster and ground blast furnace slag together with a small quantity of a metal compound such as zinc oxide, for example in accordance with the undercoat formulations given above, and a second or finish coating of a cementitious composition containing gypsum plaster and zinc oxide, for example according to the finishing composition formulation given above. In such a system, the amount of zinc oxide in the undercoat is sufficient to prevent the emission of undesirable hydrogen sulphide during the initial stages, while the undercoat is exposed, although insufficient to neutralise the total hydrogen sulphide emission in the long term. The release of hydrogen sulphide into the atmosphere in the long term, however, is effectively prevented by the zinc oxide present in the finish layer.

The following are examples of specific preferred formulations for the various types of composition discussed above. All percentageS are given by weight.

Example 1 The following are formulations for an undercoat plastering composition for general use (A) and one for use in wet conditions (B), to which may be added the usual aggregate and additives, which have been omitted in calculating the percentages.

A B Stucco 76.95 56.51 Slag 19.19 37.67 Portland Cement 1.90 3.80 Lime 1.90 1.90 Zinc Oxide 0.06 0.12 100.00 100.00 Example 2 The following are examples of formulations for general use (A) and for use under wet conditions (B) including retarder and light-weight aggregate.

A B Stucco 64.95 47.7 Slag (Cemsave) 16.2 31.8 Portland Cement 1.6 3.2 Lime 1.6 1.6 Keratin 0.6 0.6 Perlite 15.0 15.0 Zinc Oxide 0.05 0.1 100.00 100.00 Example 3 The following is an example of a finish coating composition which does not contain slag but contains zinc oxide for the purposes of the invention: Stucco 97.84 Lime 0.98 Retarder 0.09 Zinc Oxide 0.05 Vermiculite 1.04 100.00 In these examples, the zinc oxide employed may be a general purpose grade as sold or use in the rubber, paint, ceramic and glass industries, for example having a purity of 99.8% and not more than 0.15% lead, and a surface area of about 5 m2g-1 (by the air permeability method). Analytical Reagent grade zinc oxide was also found to be satisfactory.

Claims

1. A method of suppressing the emission of hydrogen sulphide from building materials incorporating a sulphide-containing constituent, which comprises applying to a building structure a surface covering composition containing gypsum plaster and a metal compound capable of reacting with hydrogen sulphide to form a stable sulphide, and allowing such compound to react with hydrogen sulphide evolved by a sulphur-containing component contained in the said structure or in the composition itself.

2. A method according to claim 1 in which the metal compound has no substantial effect on the setting rate of the gypsum plaster.

3. A method according to claim 1 or 2 in which a colourless, insoluble metal sulphide is formed by reaction of the said compound with the said component.

4. A method according to claim 3 in which the metal compound is a zinc compound.

5. A method according to claim 4 in which the metal compound is zinc oxide.

6. A method according to any of claims 1 to 5 in which the composition contains from 0.01 to 1% by weight of the metal compound.

7. A method according to any of claims 1 to 6 in which the gypsum plaster is calcium sulphate hemihydrate.

8. A method according to any of claims 2 to 7 in which the composition contains a latent cement or pozzolan.

9. A method according to any preceding claim wherein the composition comprises by weight from 20 to 84% gypsum plaster, from 15 to 79% blast furnace slag and up to 1% of the metal compound.

10. A method according to any preceding claim in which a layer of a composition including a sulphide-containing constituent is first applied to the structure and a layer of the said metal compound containing composition is applied thereover.

11. A method according to any of claims 1 to 7 or 10 wherein the said structure or a first applied layer includes a sulphide-containing component and the said composition comprises by weight 97.8 to 99.9% gypsum plaster, Oto 2% lime and 0.05 to 0.2% zinc oxide.

12. A method according to claim 11 in which the composition is in admixture with a light-weight aggregate in a proportion of 0.5 to 5% by weight of the total composition.

13. A surface covering composition for building structures comprising by weight from 20 to 84% hemihydrate plaster, from 15 to 79% aggregate including sulphide-containing material and from 0.01 to 1% of a metal compound capable of reacting with hydrogen sulphide to form a stable sulphide.

14. A composition according to claim 13 in which the metal compound is capable of reacting to form an insoluble colourless sulphide.

15. A composition according to claim 13 or 14 in which the metal compound is an insoluble zinc compound.

16. A composition according to any of claims 13 to 15 in which the aggregate includes a latent cement or pozzolan.

17. A composition according to claim 16 in which the latent cement is ground blast furnace slag.

18. A composition according to claim 17 comprising by weight from 64% to 84% hemihydrate from 15% to 35% blast furnace slag, from 0 to 5% portland cement, from 0 to 5% lime, and from 0.02 to 0.10% zinc oxide.

19. A composition according to claim 18 in admixture with a light-weight aggregate in a proportion of from 5 to 20% by weight of the total.

20. A composition according to claim 16 comprising by weight 40 to 64% hemihydrate 35 to 59% ground blast furnace slag, 0 to 5% portland cement, 0 to 10% lime and 0.05 to 0.2% zinc oxide.

21. A surface covering composition for a building structure substantially as described in any of the examples herein.

22. A building structure incorporating a sulphide-containing material having a tendency to release hydrogen sulphide in wet conditions and including a surface covering formed by a method according to any of claims 1 to 12.