BACKGROUND TO THE INVENTION It is known to internally line pipes such as steel water main pipes with a cement lining. The cement acts to provide a barrier between water being transported in the pipe and the steel of the pipe, thus preventing corrosion of the steel by the transported water. Typically pipes used as water mains are lined with Portland Cement to a depth of approximately 1 Omm.

Cement, when initially set, includes within its structure molecules of calcium oxide CaO (lime) present mainly in hydrated form as Ca (OH) 2. Where the cement is in contact with water, lime leaches through pores in the cement into the surrounding water. If the water is stationary, it soon reaches an equilibrium concentration of lime and the leaching slows. Where the cement is used in a water main, however, the leaching of lime into the transported water is continuous.

The leaching of lime into the water being transported causes a degradation in water quality, notably an increased pH and calcium ion concentration. Where water is transported over long distances in new water mains this effect can be significant. One recorded rise in pH was from an initial level of 8.1 at the start of a water main to 12.6 at the end of the main. This is well in excess of acceptable levels, such as the

European Community standard of pH having an upper limit at 9.5, and Australian guidelines requiring maximum pH of 8.5 (with 9.2 being"tolerated"in new pipes).

The presence of significant concentrations of calcium ions in the water is also undesirable. The resulting water hardness can be above 1000mg/L.

A further problem resulting from the leaching of lime into the water is a degradation of the water pipe itself. If the cement structure is significantly weakened by the loss of calcium then corrosion of the steel of the water main may occur.

A known solution to the above problem is to ensure that the transported water includes a sufficient concentration of carbonate (Co32~) and bicarbonate (HC03-) ions in order to promote the formation of calcium carbonate deposits at the pores of the cement. Over time, the formation of these deposits effectively seals the pores and prevents the further leaching of lime into the water.

Whilst this process leads to stabilising of the pipe, it can require a long time and the use of a lot of water. In the above example, excess water was drawn through the pipe and then discharged near the end of the main. It required nine months for the pipe to be sufficiently stabilised.

The present invention attempts to alleviate at least in part some of the aforementioned problems associated with cement lined water pipes.

SUMMARY OF THE INVENTION In accordance with one aspect of the present invention there is provided a method for curing a cement lining of a pipe, characterised in that the method comprises exposing the cement lining of the pipe to air having an increased concentration of carbon

dioxide gas relative to ambient atmosphere conditions, the exposure occurring during a curing period.

DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION The present invention will now be described by way of example.

During the manufacture of cement lined pipes, cement is applied to the interior of the pipes by known means, typically a centrifuge action. In prior practice, the pipes are then sealed at both ends for a few days in order to permit the cement to cure and harden in a moist environment before being exposed to the atmosphere.

In the method of the present invention, carbon dioxide gas is introduced into the pipe following the initial hardening period, and the cement is permitted to cure in the presence of the carbon dioxide gas. In another application of the invention the gas may be introduced during the initial hardening period.

The calcium hydroxide present in the cement reacts with carbon dioxide to form calcium carbonate according to the equation Ca (OH) 2 + C02 = CaC03 + H20.

The calcium carbonate acts to block the pores in the cement. In practice, calcium carbonate formation penetrates within the cement lining to a depth determined by the partial pressure of the carbon dioxide gas and the length of time during which exposure takes place. The calcium carbonate acts as a barrier to prevent the subsequent leaching of lime from the inner surface of the cement layer into water being carried in the water main.

It will be understood that the formation of calcium carbonate reduces the partial pressure of carbon dioxide gas within the pipe. In a preferred embodiment of the invention carbon dioxide gas is continuously supplied to the interior of the pipe during the curing time. In another embodiment of the invention, additional carbon dioxide gas is supplied to the interior of the pipe at intervals during curing.

Preliminary testing of the invention has been undertaken according to the following process.

Water was applied to the surface of cement samples in order to simulate the properties of wet cement. The samples were then placed within a carbon dioxide rich atmosphere for a certain time period.

The samples were then immersed in water for a period of eight days, with some samples having the water changed every second day.

The water was then measured to determine its pH and calcium content.

The results can be tabulated as follows: pH of water samples after eight days. Length of C02 exposure Water changed every two days Water unchanged No C02 exposure 10. 65 11. 42 One hour 10. 41 11. 31 Six hours 10. 27 11. 02 24 hours 9. 6 10. 45 48 hours 9. 59 10. 29 Calcium concentration (mg/L) of water samples after eight days. Length of C02 exposure Water changed every two days Water unchanged No C02 exposure 21 63 One hour 19 44 Six hours 18 29 24 hours 16 21 48 hours 16 14

These results suggest that the exposure of the cement lining to carbon dioxide for a period of around 24 hours is sufficient to obtain the maximum effect.

It will be appreciated that the length of exposure to carbon dioxide gas required will be dependent on the partial pressure of carbon dioxide gas in the pipe. The above experiments were conducted in an atmosphere which was close to 100% carbon dioxide gas. In use, exposure to air having a carbon dioxide concentration increased relative to atmospheric conditions will produce a useful result. Air with a carbon dioxide concentration of at least 5% will enable the designed reaction to proceed at an enhanced rate. Air with a carbon dioxide concentration in the range of 10% to 80%, preferably 40% to 60%, is believed to have a useful result.

Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention. For instance, it will be immediately apparent that the invention can be applied to mortar linings other than Portland Cement.