This invention relates to a cement binder composition and to the use of the binder composition in construction products. The cement binder composition is based on MgO that absorbs CO2 when hardening. The binder composition of the present invention also comprises at least one magnesium carbonate (either hydrated or unhydrated) having the formula xMgCO3-yMg(OH)2-zH2O, wherein x is at least 1, and at least one of y or z is greater than 0. The binder composition may optionally comprise a hydroscopic material, for example NaCl. The MgO when mixed with water in the presence of the magnesium carbonate produces magnesium hydroxide that has a rosette-like morphology (as shown).