A hydraulic binder composition in the form of a dry powder comprising: (a) 10% to 95 % MgO; and (b) 5% to 80% magnesium carbonate; (c) 0% to 10% chloride salt based on the total weight of components (a) to (c) wherein either: the magnesium carbonate is in the form x MgCO 3 · y Mg 2 · z H 2 O wherein x is a number that is at least 1, and both y and z are numbers greater than 0 and the binder composition comprises MgO and optionally Mg 2, magnesium carbonate and optionally chloride salt in an amount selected from one of the following ranges: i) (a) 30% to 80% magnesium oxide (MgO); (b) 20% to 60% of one or more magnesium carbonate; and (c) 0% to 10% chloride salt based on the total weight of components (a) to (c); ii) (a) 10% to 40% magnesium oxide (MgO); (b) 60% to 80% of one or more magnesium carbonate; and (c) 0% to 10% chloride salt based on the total weight of components (a) to (c); iii) (a) 40% to 85 % magnesium oxide (MgO); (b) 5% to 30% magnesium hydroxide (Mg 2); (c) 10% to 20% of one or more magnesium carbonate; (d) 0% to 10% chloride salt based on the total weight of components (a) to (d); iv) (a) 20% to 75% magnesium oxide (MgO); (b) 5% to 30% magnesium hydroxide (Mg 2); (c) 20% to 40% of one or more magnesium carbonate; and (d) 0% to 10% chloride salt based on the total weight of components (a) to (d); v) (a) 10% to 60 % magnesium oxide (MgO); (b) 30% to 50% magnesium hydroxide (Mg 2); (c) 10% to 30% of one or more magnesium carbonate; and (d) 0% to 10% chloride salt based on the total weight of components (a) to (d); or the magnesium carbonate is in the form x MgC03 · z H 2 O wherein x is a number that is at least 1, and z is a number that is greater than 0 (a) 10% to 95 % MgO; and (b) 5% to 80% magnesium carbonate; (c) 0% to 10% chloride salt the magnesium carbonate is in the form x MgCO 3 · y Mg 2 · z H 2 O wherein x is a number that is at least 1, and both y and z are numbers greater than 0 and the binder composition comprises MgO and optionally Mg 2, magnesium carbonate and optionally chloride salt in an amount selected from one of the following ranges: i) (a) 30% to 80% magnesium oxide (MgO); (b) 20% to 60% of one or more magnesium carbonate; and (c) 0% to 10% chloride salt based on the total weight of components (a) to (c); ii) (a) 10% to 40% magnesium oxide (MgO); (b) 60% to 80% of one or more magnesium carbonate; and (c) 0% to 10% chloride salt based on the total weight of components (a) to (c); iii) (a) 40% to 85 % magnesium oxide (MgO); (b) 5% to 30% magnesium hydroxide (Mg 2); (c) 10% to 20% of one or more magnesium carbonate; (d) 0% to 10% chloride salt based on the total weight of components (a) to (d); iv) (a) 20% to 75% magnesium oxide (MgO); (b) 5% to 30% magnesium hydroxide (Mg 2); (c) 20% to 40% of one or more magnesium carbonate; and (d) 0% to 10% chloride salt based on the total weight of components (a) to (d); v) (a) 10% to 60 % magnesium oxide (MgO); (b) 30% to 50% magnesium hydroxide (Mg 2); (c) 10% to 30% of one or more magnesium carbonate; and (d) 0% to 10% chloride salt based on the total weight of components (a) to (d); or i) (a) 30% to 80% magnesium oxide (MgO); (b) 20% to 60% of one or more magnesium carbonate; and (c) 0% to 10% chloride salt based on the total weight of components (a) to (c); (a) 30% to 80% magnesium oxide (MgO); (b) 20% to 60% of one or more magnesium carbonate; and (c) 0% to 10% chloride salt ii) (a) 10% to 40% magnesium oxide (MgO); (b) 60% to 80% of one or more magnesium carbonate; and (c) 0% to 10% chloride salt based on the total weight of components (a) to (c); (a) 10% to 40% magnesium oxide (MgO); (b) 60% to 80% of one or more magnesium carbonate; and (c) 0% to 10% chloride salt iii) (a) 40% to 85 % magnesium oxide (MgO); (b) 5% to 30% magnesium hydroxide (Mg 2); (c) 10% to 20% of one or more magnesium carbonate; (d) 0% to 10% chloride salt based on the total weight of components (a) to (d); (a) 40% to 85 % magnesium oxide (MgO); (b) 5% to 30% magnesium hydroxide (Mg 2); (c) 10% to 20% of one or more magnesium carbonate; (d) 0% to 10% chloride salt iv) (a) 20% to 75% magnesium oxide (MgO); (b) 5% to 30% magnesium hydroxide (Mg 2); (c) 20% to 40% of one or more magnesium carbonate; and (d) 0% to 10% chloride salt based on the total weight of components (a) to (d); (a) 20% to 75% magnesium oxide (MgO); (b) 5% to 30% magnesium hydroxide (Mg 2); (c) 20% to 40% of one or more magnesium carbonate; and (d) 0% to 10% chloride salt v) (a) 10% to 60 % magnesium oxide (MgO); (b) 30% to 50% magnesium hydroxide (Mg 2); (c) 10% to 30% of one or more magnesium carbonate; and (d) 0% to 10% chloride salt based on the total weight of components (a) to (d); (a) 10% to 60 % magnesium oxide (MgO); (b) 30% to 50% magnesium hydroxide (Mg 2); (c) 10% to 30% of one or more magnesium carbonate; and (d) 0% to 10% chloride salt the magnesium carbonate is in the form x MgC03 · z H 2 O wherein x is a number that is at least 1, and z is a number that is greater than 0