BINDER COMPOSITION This invention relates to a cement binder suitable for use in construction products. Emissions of 'greenhouse gases', and predominantly carbon dioxide (C0 2), are thought to contribute to an increase in the atmospheric and surface temperatures of the Earth - a phenomenon commonly referred to as 'global warming'. Such temperature increases are predicted to have serious environmental consequences. The main contributor to this increase in man-made C0 2 is the burning of fossil fuels such as coal and petroleum. Portland cement is the most common type of cement in general use at this time. It is an essential element of concrete, mortar and non-speciality grouts. Portland cement consists of over 90% Portland cement clinker, up to 5% gypsum and up to 5% other minor constituents. Portland cement clinker is a hydraulic material consisting mainly of di-caicium silicate {2CaO.Si0 ), tri-calcium silicate (3CaO.Si0 2), tri-calcium aluminate (3CaO.A! 0 3) and calcium aluminoferrite (4CaO.AI20 3 Fe20 3) phases. Magnesium oxide (MgO), can also be present in Portland cement, although its amount must not exceed 5% by mass as its delayed hydration is believed to give rise to unsoundness in concrete. Gypsum (CaS0 .2H20 ) is added to Portland cement clinker to control its setting time, and the mixture is ground to give a fine powder. On reaction with water, the constituents of the cement hydrate forming a solid complex calcium silicate hydrate gel and other phases. The manufacture of Portland cement is a highly energy intensive process that involves heating high volumes of raw materials to around 1450°C. n addition to the C0 2 generated from burning fossil fuels to reach these temperatures, the basic raw material used in making Portland cement is calcium carbonate (limestone, CaC0 3) , and this decomposes during processing to calcium oxide, releasing additional geologically sequestered C0 . As a result, the manufacture of Portland cement typically emits approximately 0.8 tonnes of carbon dioxide for every tonne of cement produced and is responsible for approximately 5% of all anthropogenic C0 2 emissions. Apart from the intrinsic benefit of reducing C0 emissions, it is likely that C0 2 emissions by the cement industry will be regulated in an attempt to reduce environmental damage. Therefore, there is a real need to develop a new range of cementitious binders that are associated with minimal or even negative C0 2 emissions. Binders based on systems other than calcium oxide and silicates are known. For example, Sorel cement (magnesium oxychloride cement or magnesia cement) is an example of a cement binder that comprises a mixture of magnesium oxide (burnt magnesia, gO) and magnesium chloride together with filler materials like sand or crushed stone. It sets to a very hard abrasive-resistant material and so is used for grindstones, tiles, artificial stone (cast stone) and cast floors, in which application it has a high wear resistance. However its chief drawback is its poor resistance to water, making it unsuitable for external construction applications. Other magnesium based cements include magnesium oxysulfate cement and magnesium phosphate cements but both these have drawbacks, the former having a poor water resistance and the latter sets very fast so that it is difficult to work with. GB 1160029 discloses cements based on mixing magnesium oxide (MgO), sodium chloride (NaCI) or sodium nitrate (NaN0 3) and calcium carbonate (CaC0 3). CaC0 3 is used as a "moderating substance" to enable the salt and the MgO to perform the chemical reactions necessary to set, which are similar to those of the other Sorei cements. These cements require the use of hard-burnt MgO, which is generally produced by high-temperature treatment (~1000°C) of magnesite (MgC0 3), which causes C0 2 emissions not only from the calcining of magnesite but also from the burning of fossil fuel. US 5897703 discloses binder compositions based on mixing MgO with a hardening agent, propylene carbonate. The magnesium oxide used can be any mixture of soft-burnt and hard-burnt MgO. It is known that in the presence of water, propylene carbonate decomposes to carbon dioxide and propylene glycol and so the addition of the propylene carbonate provides a source of C0 to carbonate the magnesium oxide. US 6200381 discloses a dry powdered cement composition derived from dolomite (a magnesium and calcium carbonate mineral; MgC0 3 aC0 3) . The dolomite is heated to decarbonate the MgC0 3 so that the composition contains CaC0 3 and a partially decarbonated MgC0 3, i.e. a mixture of MgC0 3 and MgO. Certain additives may be included in the composition (e.g. aluminium sulphate (AI2(S0 4)3) , citric acid, sulphuric acid (H2S0 ) , NaCI, etc.), which assist the composition to set on addition of water; the water may be contaminated water, e.g. sea water. The CaC0 3 component of the cement composition reacts with several of the specified additives that are used. For example, the addition of H S0 wi!l react with CaC0 3 yielding hydrated CaS0 4 (e.g. CaS0 .2H20 ) and C0 2. The C0 2 released assists the carbonation of MgO and Mg(OH)2. NaCI may be added before the thermal treatment of dolomite to decrease the decarbonation temperature of MgC0 3, and in the binder composition as an additive, where it appears to assist in achieving an early strength to the composition, which is probably due to reactions with MgO (Sorel cement type reactions). CaC0 3 acts as a "moderating substance" to enable NaCI and the MgO to perform the necessary chemical reactions (see GB 1160029 above). US 1867180 describes a cement composition based on slaked lime (Ca(OH)2) that contains less than 1% MgO and NaCI. US 1561473 discloses that, when a wet mixture of aggregates and magnesium oxide is treated with gaseous or dissolved C0 2, its tensile strength is improved. The composition must be exposed to C0 2 when wet and the patent discloses the exposure of the wet mixture to a special atmosphere of moist C0 2. WO 01/55049 discloses a dry powdered cement composition containing MgO, a hydraulic cement component, such as Portland cement, Sorel cements or calcium aluminate cements, and optionally various pozzolanic materials. The cement composition taught can also contain various additives such as ferrous sulphate (FeS0 ), sodium or potassium silicates or aluminates, phosphoric acid (H3P0 ) or phosphoric acid salts, copper sulphate (CuS0 4) , and various other organic polymers and resins, such as polyvinyl acetate (PVA), vinyl acetate-ethylene, styrene-butyl acrylate, butyl acrylate-methylacrylate, and styrene-butadiene. The magnesium oxide is obtained by low temperature calcining. GB 529128 discloses the use of magnesium carbonate as an insulating material; it is made from concentrated sea water containing magnesium salts by precipitating the salts with alkali metal carbonates, which forms needle-like crystals that can set. A slurry of such crystals, when paced in a mould, will set to provide a slab or block that is useful as insulation. If there are any bicarbonate ions in the alkali metal carbonate, magnesium bicarbonate will form in the above reaction, which slows down the setting reaction. In order to counteract this, 1-5% magnesium oxide may be added, which will precipitate the bicarbonate as magnesium carbonate. US 819893 and US 1971909 both disclose the use of magnesium hydroxide or a mixture of magnesium hydroxide and calcium carbonate as an insulating material since such magnesium hydroxide is light and highly flocculated. US 5927288 discloses thai a mixture of hydromagnesite and magnesium hydroxide, when incorporated into a cigarette paper, reduces side-stream smoke. The hydromagnesite/magnesium hydroxide compositions have a rosette morphology and the hydromagnesite/magnesium hydroxide mixture is precipitated from a solution of magnesium bicarbonate and possible other soluble magnesium salts by adding a strong base, e.g. potassium hydroxide. EP 03938 3 and WO 01/51554 relate to flame retardants for plastics. EP 0393813 discloses that a mixture of a double salt of calcium and magnesium carbonate (e.g. dolomite), hydromagnesite, and magnesium hydroxide can provide flame resistance to thermoplastics, e.g. a sheath of an electric wire. WO 01/51554 teaches the addition of various magnesium salts, including hydromagnesite and magnesium hydroxide, to polymers. US 2009/0020044 discloses the capture of carbon dioxide by sea water to precipitate carbonates, which can be used in hydraulic cements; up to 10% of a pH regulating material, including magnesium oxide or hydroxide, can be added to the cement to regulate the pH. JP 2006 076825 is concerned with reducing the amount of C0 2 emitted from power stations and by the steel industry. It proposes capturing the C0 2 by reacting with ammonium hydroxide to form ammonium carbonate: 2NH4OH + C0 ® (NH4) C0 3 +H20 Meanwhile magnesium chloride is made by reacting magnesium oxide and hydrochloric acid: MgO + 2HCI MgCI + H20 The magnesium chloride is reacted with the ammonium carbonate, which precipitates magnesium carbonate leaving a liquor containing dissolved ammonium chloride: