Interaction Between Polycarboxylate Superplasticizers and Amorphous Ground Granulated Blast Furnace Slag

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.