Adsorption Affinity of Linear and Aromatic Organic Anions Competing for Cationic Cement Surfaces

Competitive adsorption of chemical admixtures onto cement is of critical importance in delivering bulk performance requirements of cement slurries employed in constructing high-performing structures, like oil wells. This challenge is complex to investigate, because of the many variables that include the heterogeneity, high pH, and ionic strength of cement fluids; the multiple crystalline phases present in unhydrated and set cement; and the high number of admixtures required to meet performance criteria in commercial operations. The purpose of this study is to relate chemical structures to relative adsorption behavior of admixtures onto cement when present together and classify such interactions as beneficial (synergistic) or detrimental (antagonistic). Adsorption characteristics of single admixtures were examined by total organic carbon analysis, FT infrared spectroscopy, scanning electron microscopy, calorimetry, and UV/vis spectrophotometry. Results show that the adsorption of single chemical admixtures follows the order of monomeric hydroxycarboxylate molecule > sulfonated linear polymer > sulfonated aromatic polymer > carboxylated/sulfonated linear polymer > carboxylated branched polyether polymers. The observed adsorption behavior of polymers correlates extremely well with the order for cement hydration retardation, with carboxylated polymers being the most powerful retarders. Results correlate closely with the proposed mechanism that sulfonated polymers adsorb onto aluminate phases, presumably the tricalcium aluminate phase; and the carboxylate polymers onto silicate phases, particularly the predominant tricalcium oxysilicate phase. The hydroxycarboxylic monomeric molecule was the strongest retarder of all and has the highest adsorption level, presumably on tricalcium oxysilicate. The competitive adsorption behavior in binary mixtures was studied by monitoring the displacement of a signaling polymer by a second admixture. Results indicate that, for similar functional groups, shorter polymers are competitively more strongly adsorbed than longer chain molecules and that the shorter chain polymers were not desorbed significantly by longer chain polymer molecules. Rheological measurements correlated admixture adsorption behavior to the observed slurry fluidity.