The surface chemistry of slag dispersed in synthetic cement pore solution (SCPS) was studied in absence and presence of polycarboxylate (PC) superplasticizers. Three different slag samples and two PCs based on methacrylic acid–co–ω–methoxy poly(ethylene glycol) methacrylate ester were employed. Zeta potential measurement of slag suspensions in SCPS revealed that all slags initially possess a negative surface charge owed to the highly alkaline pH (formation of silanolate groups). Onto this surface, at first, calcium and subsequently sulfate ions present in SCPS adsorb, thus forming a double ion layer; the first positively charged layer consisting of Ca2+ ions and the second negatively charged layer containing SO42− anions. Consequently, the final surface charge of slag dispersed in SCPS is always negative. Upon addition of PC superplasticizer, competitive adsorption between the sulfate ions and PC occurs. Highly anionic PCs can desorb SO42− and adsorb in high amount onto the Ca2+ ion layer, thus producing a strong dispersing effect. However, less anionic PCs cannot adsorb and do not fluidify the slag suspension. The total adsorbed amount of PC is determined by the packing density (positive charge) of the Ca2+ ion layer present on the surface of slag.