Monomers of 2-acrylamido-2-methylpropane sulfonic acid (AMPS (R)), N,Ndimethyl acrylamide (NNDMA) and acrylic acid (AA) were grafted on humic acid as backbone by aqueous free radical copolymerization in such a manner that a graft copolymer possessing lateral terpolymer chains was obtained. Molar ratios between AMPS (R), NNDMA, and AA were found to be 1 : 1.54 : 0.02 and the ratio between backbone and graft chain was 20 : 80 wt %. The synthesized fluid loss additive (FLA) was characterized by size exclusion chromatography (SEC), charge titration, and Brookfield viscometry. Thermogravimetric and SEC analysis revealed stretched backbone worm architecture for the polymer whereby humic acid constitutes the backbone decorated with lateral graft chains. Grafting was confirmed by SEC data (Rg) and by ineffectiveness of a blend of AMPS (R)-NNDMA-AA copolymer with humic acid. Their performance as high temperature FLA was studied at 150 degrees C by measuring static filtration properties of oil well cement slurries containing 35% bwoc of silica fume and 1.2% bwoc AMPS (R)-co-itaconic acid retarder. At this temperature, 1.0% bwoc graft copolymer achieves API fluid loss value of 40 mL, thus confirming high effectiveness. The graft copolymer viscosifies cement slurries less than other common synthetic FLAs. The working mechanism of the graft copolymer was found to rely on adsorption onto surface of hydrating cement, as was evidenced by adsorption and zeta potential measurements. Adsorption is hardly affected by temperature and results in constriction of the filter cake pores. The study provides insight into performance of cement additives under the harsh conditions of high temperature and high pressure. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012