| Summary: | In earlier publications it has been demonstrated that alginate and similarly structured biopolymers unexpectedly accelerate the hydration of calcium alumina cement. Here, this effect is investigated by applying 27Al MAS NMR spectroscopy and XRD technique to track the hydration reaction and identify the hydration products. In the presence of these biopolymers an earlier consumption of clinker phases was observed indicating a shortened induction period. The subsequent formation of hydration products occurs earlier, resulting in the formation of C-A-H phases (CAH10 and C2AH8). No further reaction products like intercalation compounds were detected. An in-depth mechanistic study revealed that alginate does not adsorb on cement, but captures Ca2+ ions from the pore solution, thus resulting in a positively charged biopolymer chain which then attracts [Al(OH)4]− ions to its surface. Through this binding of calcium and aluminate ions and alignment to the alginate molecule, a precursor for calcium aluminate hydrates is formed which acts as a nucleation seed and triggers continued growth into large C-A-H crystals, similar as in related biotemplating processes which are known for calcite or brushite formation.
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