Field of the invention The present invention relates to the treatment of structures, such as sewers and associated structures such as manholes, to prevent bacterial corrosion. More particularly, the present invention provides compositions for such treatment and to treated structures.

Background of the invention

The most common form of sewerage asset corrosion is acid corrosion due to the activity of bacteria. This form of corrosion is a significant problem and millions of dollars are spent worldwide on the repair and maintenance of affected structures. One example underlying mechanism that may be involved relates to sulfur oxidising bacteria typically present in biofilm on the concrete surface. These bacteria oxidise hydrogen sulphide produced by anaerobic bacteria in untreated sewage, to sulfuric acid. The sulfuric acid dissolves carbonates in the concrete and causes loss of strength and possible structural failure.

A variety of techniques have been employed to inhibit corrosion of concrete in this context. One approach involves pre-treating sewage in order to increase pH or to precipitate or oxidise sulfides that are present thereby inhibiting the activity of sulfur oxidising bacteria. However, such approaches tend to have only a temporary effect, and repeated and continuous treatment is necessary for prolonged corrosion protection. This can be labour intensive and expensive.

Another approach involves providing a layer of magnesium hydroxide onto a concrete surface susceptible to bacterial corrosion. This may be done by application to the surface of a magnesium hydroxide slurry. The aim of the layer is to provide a highly alkaline environment that is acid neutralising and that substantially reduces the density and activity of corrosion causing bacteria. Once applied the magnesium layer can maintain a suitably high H at the concrete surface for a period of time, after which re-application is necessary in order to maintain corrosion resistance.

In relation to this approach it has been found that the thickness of the applied layer has a practical impact on efficacy. Generally, for practical reasons, the thickness applied ranges from 1 to 2 mm. If the layer applied is less than about 1 mm it may be difficult to achieve even coverage of a concrete surface and this can lead to localised corrosion over time. On the other hand, if the thickness is too great, cracking can result due to shrinkage of the layer as it dries. The cracking may be attributable to rapid water loss from the layer as it dries. Practically, when the layer applied is from 1 to 2 mm thick, corrosion protection for a period of up to about 3 years may be expected. Re-application of the layer is then required for continued corrosion protection.

Against this background it would be desirable to provide means for preventing bacterial corrosion of a substrate, especially a concrete substrate, that avoids the drawbacks of conventional approaches, and that in particular may achieve a significant duration of protection, for example in excess of 3 years, preferably in excess of 5 years.

Summary of the invention

Accordingly, in one embodiment the present invention provides a magnesium hydroxide slurry suitable for preventing bacterial corrosion of a substrate, the magnesium hydroxide slurry comprising fibres that impart crack resistance when a layer of the slurry is provided on the concrete substrate.

The present invention also provides a method of treating a substrate to minimise or prevent bacterial corrosion, the method comprising applying to the substrate a magnesium hydroxide slurry in accordance with the present invention. The present invention also provides a substrate treated in accordance with this aspect of the invention. The present invention may be applicable to any substrate that is susceptible to bacterial corrosion. However, it is believed to have particular applicability in relation to treating concrete substrates. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Detailed discussion of the invention

In the following reference is made to treating a concrete substrate as this is believed to be an area of particular applicability of the invention. However, the invention should not be read as limited to treatment of concrete substrates, as noted above.

In accordance with the present invention it has been found that incorporation into a magnesium hydroxide slurry of fibrous material can improve the crack resistance of a layer formed from the slurry when applied to a concrete substrate in order to impart resistance to bacterial corrosion. In turn this may allow the slurry to be applied as a layer having a greater thickness than conventional magnesium hydroxide slurries, thereby achieving increased duration of corrosion protection. Indeed, it is believed that embodiments of the invention may provide suitable resistance to bacterial corrosion for in excess of 5 years. This is believed to be a significant advance in the art. The precursor magnesium hydroxide slurry into which the fibres are added may be of conventional type. Thus, the invention may be applied to modify an existing magnesium hydroxide slurry to provide increased crack resistance thereby allowing the slurry to be applied to a concrete substrate as a layer with a greater thickness than otherwise practical. The magnesium hydroxide slurry is usually an aqueous suspension (at least 50-60 wt%) of magnesium hydroxide. It is a thixotropic non-Newtonian liquid that is pumpable and exhibits a gelling characteristic. The slurry should show good adherence (especially under the influence of gravity) to a substrate in a single application and at an effective thickness. As noted this is currently up to about 2 mm. By way of example, magnesium hydroxide slurries that may be modified in accordance with the invention are described in International Patent Publication No. WO 96/003346, the contents of which are hereby incorporated by reference.

The slurry of the present invention must exhibit suitable rheological properties for application to a concrete substrate. Typically, the substrate will be the internal surfaces of a pipe. This means that the slurry in accordance with the invention must be pumpable for application by spraying as is usually done for coating such substrates. After pumping desirably the slurry develops a yield stress (i.e., a gel) that prevents the slurry from falling under gravity. If the applied slurry does not exhibit this characteristic it will not be suitable for application to the internal surfaces of a pipe. With these properties in mind the magnesium hydroxide slurry into which liquid organic material is to be added may be as described in WO 96/003346.

Incorporation of fibres into a magnesium hydroxide slurry may change the rheological properties of the slurry, and this should be considered given the intended use and application method. In an embodiment of the invention the magnesium hydroxide slurry into which fibres are to be added may be unsuitable for the intended use because of its rheological properties, but addition of the fibres yields a final slurry that is useful. Thus, in such embodiments the invention may increase the range of precursor slurries that may be used for a given application. In another embodiment however a precursor slurry (i.e., without addition of fibres) may need to be modified with one or more additives in order to provide properties useful given the context of use/application. For example, a magnesium hydroxide slurry may be modified in order to provide desirable properties prior to addition of fibres (taking into account also the effect of that addition). In this case, addition of the fibres should not impact on the usefulness of the final slurry.

The fibres may be natural or synthetic and should be chemically unreactive or relatively unreactive with respect to the slurry components. The fibres should also not present problems given the intended context of use of the slurry. The fibres may be formed of nylon, polyester, acrylic, olefin, carbon etc. Mixtures of one or more types of fibre may be used. Typically, the fibres will have a diameter of less than 100 μιη. The fibres are usually less than 3 mm in length. Typically, the fibres will be used in an amount of less than 10 wt% based on the total weight of the slurry. If mixing is not adequate and/or if the proportion and/or the size of fibres is excessive, lumps of fibres may form in the slurry. This may present application problems, especially when spraying is to be employed.

Inclusion of fibres in the slurry may impact on the rheological properties of the slurry, and on the intended application methodology (e.g., spraying). Such effects should be taken into account as they may influence implementation of this embodiment of the invention.

A magnesium hydroxide slurry in accordance with the invention may be produced by mixing of the individual ingredients. The extent of mixing (intensity, duration and speed) may be optimised by experimentation. The slurry of the invention may also include other functional additives, provided that this does not compromise the efficacy of the slurry in terms of bacterial corrosion resistance and crack resistance.

After formation, the slurry of the invention should maintain sufficient fluidity for application to a chosen substrate. After application the slurry should form an adherent gel layer on a substrate. The slurry of the invention may be applied to any concrete substrate that is susceptible to bacterial corrosion. Typically, the substrate is a structure, such as a pipe, that comes into direct or indirect contact with sewage. The substrate may also be (the internal surfaces of) a manhole. The slurry may be applied to the substrate by spraying. This would be done in conventional fashion. For an in situ structure, the substrate surface to be treated is usually cleaned with water to remove any slime layer and/or loosely bound corrosion products. For substrate surfaces that have not been in service, cleaning may not be required prior to coating with slurry. In accordance with the invention it may be possible to achieve a magnesium hydroxide layer having a thickness of 2 mm or greater, and this may provide corrosion resistance for a longer duration than has been achieved using conventional layers of magnesium hydroxide slurry. In accordance with the invention it may be possible to achieve corrosion resistance in excess of 3 years, preferably in excess of 5 years. The invention may be applied to treat a concrete substrate that has not been treated before or to remediate a concrete substrate that has previously been treated.

In accordance with another embodiment of the invention it has been found that incorporation into the magnesium hydroxide slurry of a low volatility liquid organic material (also referred to as "liquid organic material" for simplicity) may also improve the crack resistance of a layer formed from the slurry when applied to a concrete substrate in order to impart resistance to bacterial corrosion. In turn this may allow the slurry to be applied as a layer having a greater thickness than conventional magnesium hydroxide slurries, thereby achieving increased duration of corrosion protection. Indeed, it is believed that embodiments of the invention may provide suitable resistance to bacterial corrosion for in excess of 5 years. This is believed to be a significant advance in the art.

The expression "low volatility liquid organic material" means a liquid organic material that does not evaporate to any significant extent when included in the magnesium hydroxide slurry of the invention and the slurry then applied to a concrete substrate as intended. To be effective it is believed that the liquid organic material should remain in liquid form in the slurry when the slurry is applied as a layer. The low volatility liquid organic material is believed to enhance crack resistance of the layer by retarding water loss from the layer. This leads to reduced stresses in the layer and reduced cracking. Typically, the liquid organic material will exhibit a volatility of 10 ^"1 to -10 ^~6 g/cm ² /sec or lower at 13°C.

Additionally, the liquid organic material should be compatible with the slurry into which it is added and the fibres included in the slurry, and the liquid organic material should not impede or interfere with the intended corrosion resistant activity of the resultant magnesium hydroxide layer. The liquid organic material should also be cost-effective to use.

By way of example, the liquid organic material may be a synthetic or naturally-occurring oil. As synthetic oils it may be possible to employ a fully synthetic or semi- synthetic lubricant base oil, for example a Group I, II, III, IV or V base oil. Conceivably, the invention may be implemented with a new or used lubricant, for example a motor oil or a gear oil. Lubricants contain a variety of additives and the compatibility of a given lubricant in the context of the present invention may need to be assessed.

The liquid organic material may also be selected from vegetable oils, that is a triglyceride extracted from a plant. These oils are liquid at room temperature. Examples of natural vegetable oils include soy bean, canola, avocado, mustard, walnut, sunflower, safflower, peanut, cottonseed, coconut, palm, rice bran, olive, sesame and almond, oils and combinations thereof. Of these, the use of rice bran oil and peanut oil may be preferred. Generally, the liquid organic material will be present in the magnesium hydroxide slurry in an amount of up to about 30% by weight based on the total weight of the slurry. Desirably, the liquid organic material is used in a minimum amount to achieve the desired crack resistance based on the intended context and parameters of use. Incorporation of liquid organic material into the magnesium hydroxide slurry may change the rheological properties of the slurry, and this should be considered given the intended use and application method. In an embodiment of the invention the magnesium hydroxide slurry into which liquid organic material is to be added may be unsuitable for the intended use because of its rheological properties, but addition of the liquid organic material yields a final slurry that is useful. Thus, in such embodiments the invention may increase the range of precursor slurries that may be used for a given application.

Embodiments of the invention are illustrated with reference to the following non-limiting examples. Example 1

Materials

The following material was used in the testing

-High shear impeller (0-2000 rpm)

-2 mm Nylon fibres. 10 μιη diameter

-MHL : Magnesium Hydroxide slurry, 55-65% solid w/w

-Spatula

-Scraper (Aluminium plate)

-Viscometer (Sheen Rotathinner Viscometer 455)

-Concrete bricks

-Metal plates of 3 mm thickness (Same length as bricks at minimum)

Methods

395 gram of MHL is added into a beaker. The impeller is adjusted to 300 rmp. 8 grams of Nylon fibre is added slowly. Application of the slurries to bricks was done as follows:

- Put cardboard on workbench and place three concrete bricks on top. The bricks must closely touch each other over their length (23 cm).

- Place metal sheets on the left and right brick in such a manner that they exactly board the inner brick. Based on the required slurry thickness of 3 mm place one metal plate onto each of the outer bricks (Metal plates are 3 mm thickness).

- Take slurry sample and vigorously stir with a spatula for 1 minute. - Then pour slurry onto middle brick and scrape it by pulling the scraper from top to bottom over the metal plates which frame the sides of the middle brick. This will give a thickness of 3 mm.

- The bricks are then let to dry for 24 hours at a temperature of 25 C and 50 % relative humidity.

Very little cracking was observed when the slurry is dry.

Example 2

388 gram of MHL is added into a beaker. The impeller is adjusted to 300 rmp. 8 grams of Nylon fibre is added slowly. Very little cracking was observed when the slurry is dry.