Interaction between polycarboxylate (PCE) comb polymers and microsilica suspended in a highly alkaline, Ca2+-loaded model cement pore solution was studied via zeta potential, adsorption, and paste flow experiments. Zeta potential measurements reveal that in an alkaline suspension, microsilica possesses a negative surface charge stemming from deprotonated silanolate groups. Addition of soluble calcium salts (e.g., CaCl(2)) was found to cause a charge reversal to positive, owing to the adsorption of Ca2+ ions forming a monolayer on the microsilica surface. Further experiments demonstrate that through Ca2+ mediation, anionic PCE graft polymers adsorb in high amount on the microsilica surface. Polymers possessing a stronger anionic character exhibit a higher affinity to the positive microsilica surface, and consequently, exercise a more powerful dispersing effect than PCE showing less anionic character. A model summarizing the processes occurring at the surface of microsilica in this fluid system is proposed. The study suggests that the high fluidity of concrete containing microsilica depends on the effective dispersion of microsilica, and not of cement.