Fundamental mechanisms for polycarboxylate intercalation into C3A hydrate phases and the role of sulfate present in cement

The fundamental reactions leading to the intercalation of polycarboxylate (PC) superplasticizers into calcium aluminum hydrates were studied by hydration of pure C₃A in the presence of PC at 75 °C. It was found that the amount of dissolved sulfate present in cement pore solution determines whether organo-mineral phases are formed or not. In the absence of sulfate, PCs easily intercalate during C₃A hydration in alkaline solution. Under these conditions, only excessive steric size of the PC will prevent intercalation. At low sulfate concentrations (SO₄²⁻/C₃A molar ratios of 0.1-0.35), PC intercalates with intersalated alkali sulfate, are formed. At high sulfate concentrations (SO₄²⁻/C₃A molar ratios of 0.7-2), PC can no longer intercalate. Instead, sulfate, because of its higher negative charge density, fills the interlayer space and monosulfoaluminates with different water contents are formed.Anion exchange experiments confirm that from the initially formed C₄AH₁₃, PC will exchange the interlayer OH⁻ anion whereas with monosulfoaluminates, no replacement of sulfate by PC was found. Consequently, in alkaline solution, PC intercalates will not exchange their PC against OH⁻ anions whereas sulfate will gradually replace the PC.Generally, intercalation of PC is an unwanted process because it consumes superplasticizer which is effective only when it adsorbs onto the cationic surfaces of AF_m and AF_t phases. Our experiments demonstrate that intercalation can be avoided by using PCs with long side chains or highly sulfated cements (SO₄²⁻/C₃A molar ratio ≥ 0.75) containing alkali or calcium sulfates which dissolve fast. In undersulfated cements, however, PC intercalates can be formed, either directly during the stacking process of the Ca₂Al(OH)₆]⁺ main layer, with PC acting as the template which determines the interlayer distance, or by anion exchange between initially formed aluminate hydrates (e.g. C₄AH₁₃ or C₂AH₈) and the PC anion.