Effect of Ca²⁺ ion concentration on adsorption of poly(carboxylate ether)-based (PCE) superplasticizer on mica

Poly(carboxylate ether)-based (PCE) superplasticizers consist of a carboxylic acid backbone and grafted poly(ethylene glycol) (PEG) side chains. Ca²⁺ ion bridging mechanism is commonly purported to control PCE's adsorption on negatively charged cement particle surfaces in cement suspension, thus PCE was expected to adsorb on negatively charged surfaces in synthetic pore solutions via Ca²⁺/COO^- interactions.

Adsorption behaviors of a commercial PCE on negatively charged mica were studied in aqueous electrolyte solutions by a surface forces apparatus.

Direct force measurements indicated that the PCE adsorbed onto mica from 0.1 M K₂SO₄ due to K⁺ ion chelation by the ether oxygen units CH₂CH₂O on the PEG chains, but surprisingly did not adsorb from either 0.1 M K₂SO₄ with saturated Ca(OH)₂ or 0.1 M Ca(NO₃)₂. The adsorption in K₂SO₄ was weak, enabling the adsorbed PCE layers to be squeezed out under modest compression. Upon separating the surfaces, the PCE immediately achieved an identical re-adsorption. In high-calcium conditions, the PCE was highly positively charged due to Ca²⁺ ion chelation by PEG chains and backbone carboxylic groups COO^-, and mica also underwent charge reversal due to electrostatic adsorption/binding of Ca²⁺ ions. Consequently, the interaction between mica and PCE was electrostatically repulsive and no PCE adsorption occurred. These findings can be explained by the complex interplay of ion chelation by PEG chains, electrostatic binding and screening interactions with charged surfaces in the presence of monovalent and divalent counterions, and ultimately charge reversal of both the charged surfaces and polyelectrolyte in high divalent ion conditions.