New insights into physicochemical interactions occurring between polycarboxylate superplasticizers and a stabilizer in self-compacting concrete

Abstract

The interaction between two methacrylate ester-based polycarboxylate (PCE) superplasticizers and an self-compacting concrete (SCC) stabilizer based on an AMPS®/N,N-dimethyl acrylamide copolymer was studied via concrete, cement paste, and adsorption experiments. Surprisingly, it was found that the stabilizer, and not the PCE, determines the overall flow behavior and rheological profile of a SCC. When only stabilizer was added to the paste, first at low dosage (0–0.06% bwoc) fluidity decreased, signifying that the stabilizer acted as viscosifier. However, at higher dosages, fluidity increased gradually and even exceeded that of the reference paste without stabilizer, suggesting that the stabilizer now acted as dispersant. Adsorption measurements revealed that the mechanism underlying the negative effect of the stabilizer on PCE performance is partly based on competitive adsorption between the two admixtures: The AMPS®/NNDMA copolymer possesses a much higher anionic charge than the PCE samples studied here, and thus adsorbs preferentially on the surface of cement. Therefore, the rheological behavior of the SCC is at first governed by the stabilizer. Only when higher PCE dosages are applied, then sufficient amounts of these superplasticizers are adsorbed which now can develop their dispersing effect, enhance concrete fluidity, and control the flow properties of this concrete. The study reveals that the interaction between PCEs and a stabilizer is a complex one and that for optimum results, the combination of both admixtures needs to be fine-tuned carefully to each other.