Templating effect of alginate and related biopolymers as hydration accelerators for calcium alumina cement - A mechanistic study

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 Al-27 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 (CAH(10) and C(2)AH(8)). No further reaction products like intercalation compounds were detected. An indepth 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. (c) 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).