Hydrothermal carbon dioxide fixation in magnesium hydroxide and serpentine: Effects of temperature and pH

•Hydrothermal fixation of CO2 in serpentine and Mg(OH)2 at temperatures up to 623 K.•Main product in fixation of CO2 is MgCO3.•Carbonation of Mg(OH)2 enhanced with increasing temperature and pH.•Both low and high initial pH enhanced serpentine carbonation at 573 K.•Serpentine dissolution and magnesite formation are key reactions in fixation.