Interaction mechanisms between Na montmorillonite clay and MPEG-based polycarboxylate superplasticizers

Sodium montmorillonite clay is shown to negatively impact the dispersion force of two methacrylate based polycarboxylates (PCEs) in cement paste. The PCEs tested consist of methacrylic acid/MPEG methacrylate-ester with molar ratios of 6:1 and 1.5:1. It was found that the PCEs sorb both chemically and physically onto clay. The sorbed amounts are ~ 100 times more than on cement. Chemisorption occurs via intercalation of the poly(ethylene oxide) side chains into the interlayer region between the alumosilicate layers, while physisorption occurs on clay surfaces which are positively charged through uptake of Ca2+. PCEs possessing high grafting density predominantly intercalate and show less surface adsorption, and vice versa. Also, the type of sorption is dosage dependent, whereby side chain intercalation dominates at higher PCE dosages, while electrostatic attraction via the anionic backbone prevails at lower dosages. Polyglycols can be utilised as sacrificial agents when highly grafted PCEs are employed at high dosages.