Ground granulated blast furnace slag produced in iron‐making is an amorphous, glassy material, which is widely used in blended cements. Here, the surface chemistry of slag dispersed in water and its behavior in cement paste were studied in the absence and presence of anionic dispersants. Three different slag samples possessing different oxide compositions and two polycarboxylate (PC) dispersants based on methacrylic acid—co—ω‐methoxy poly(ethylene glycol) methacrylate ester were investigated. When suspended in deionized water, all slag samples released different amounts of Ca2+, K+, Na+, and OH− ions, thus producing pore solutions possessing high pH. Electrokinetic properties of slag suspensions were determined by zeta potential measurement, revealing that initially negatively charged slag adsorbs considerable amounts of Ca2+ ions on its surface until saturation is reached. Through this mechanism, slag attains a strongly positive zeta potential in pore solutions containing Ca2+ ions. Onto this positively charged layer of adsorbed Ca2+ ions, anionic PC dispersants adsorb, producing a Langmuir‐type adsorption isotherm. PC consumption generally correlates with the absolute value of the positive zeta potential of slag. The study demonstrates that when blended into cement, slag is not inert relative to anionic superplasticizers. Instead, competition occurs between cement and slag for the dispersant.

It is well established that the performance of polycarboxylate (PCE) superplasticizers can be severely affected by the composition of individual cements. Here, a novel allylether/maleic anhydride (APEG)-based PCE was synthesized using allyl maleate monomer as a new, additional building block. When polymerized into the PCE main chain, this building block was found to form a cyclic lactone structure. The resulting PCE molecule was tested with respect to the dispersing force in cements possessing different phase compositions and alkali sulfate (K 2 SO 4 ) contents. These data were compared with those from conventional APEG-and methacrylate ester (MPEG)-type PCEs. Results obtained from cement paste flowability and adsorption measurements suggest that the modified PCE disperses all cement samples well and hence is more robust against variations in cement composition. Apparently, the new building block induces a higher affinity of the polymer to the surface of cement and can form a denser polymer layer.

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.

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