The present invention relates to a combustion additive comprising a colloidal solution containing dispersed fine metal particles. The present invention also relates to a method for producing the colloid. More particularly the present teaching relates to a combustion additive having a colloid, wherein the colloid comprises metal particles providing in an alkaline aqueous solution, the metal particles being dispersed within that solution and having an average diameter in the range of 30 nm to 30 μm. The colloid can partly/fully substitute water of a water injection system or used as an air humidification component for combustion.