A humic acid graft copolymer possessing both water-retention and dispersing properties in cement slurry was synthesized by grafting lateral chains of 2-acrylamido-2-methylpropane sulfonic acid (AMPS (R)), N,N-dimethylacrylamide (NNDMA), and acrylic acid (AA) onto a backbone of humic acid using aqueous free radical polymerization. The graft copolymer is composed of 20 wt % humic acid backbone and 80 wt % graft chain (molar ratio AMPS/NNDMA/AA = 1 : 0.31 : 0.03), it exhibits a Mw of 323 kDa and is highly anionic in cement pore solution. The influence of this specific molecular design on cement flow properties is unraveled. When tested at 200 degrees C, the graft copolymer achieved very low cement fluid loss values (approximate to 50 mL) at low rheology. This behavior differentiates it from most common synthetic high temperature fluid loss additives which excessively viscosify cement slurries. The working mechanism of the graft copolymer was found to rely on adsorption onto the surface of hydrating cement. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013