Effect of Ions and Admixtures on the Growth of Synthetic Calcium Silicate Hydrate (C-S-H)

This thesis investigates the effect of different parameters such as Fe (III), D-gluconate and heterogeneous particles on the nucleation and growth of synthetic C-S-H by using the dropwise precipitation method. Firstly, the effect of Fe (III) on the synthetic C-S-H system was investigated. This was achieved by controlling pH and also Fe (III) ion concentration. The experiments at pH higher than 11.0 of initial solutions containing Fe leads to the pre-precipitation of an amorphous Fe phase (ferrihydrite) which is then transformed to the more stable crystalline Fe phase of siliceous hydrogarnet. This condition is closely related to the phase formation pathway in real systems where the formation of ferrihydrite precludes the formation of the more thermodynamically stable iron containing siliceous hydrogarnet is also observed. To investigate the possible inclusion of Fe into the C-S-H structure, the experimental conditions were designed to avoid the pre-precipitation of ferrihydrite. This was achieved by controlling ionic speciation of the solution containing Fe with controlled pH (2.5). Magnetic resonance data strongly supported that the Fe (III) is incorporated into the C-S-H structure.

Secondly, this thesis investigates the effect of D-gluconate on the nucleation and growth of synthetic C-S-H. One effect of D-gluconate on synthetic C-S-H was a surface interaction which led to the formation of bi-pyramidal cage-like structures from the assembly of the precipitated C-S-H nanofoils. Characterization confirmed the adsorption of D-gluconate on the C-S-H surface but no changes in the underlying C-S-H structure could be discerned. From calorimetry data, D-gluconate not only interacts with newly precipitated C-S-H surfaces but also strongly on the dissolving solids in Portland cement e.g. alite, retarding hydration significantly. The complexation between calcium and D-gluconate in solution showed that increasing D-gluconate concentration gave more complexation with calcium and modified the kinetics of nucleation and growth. When collecting kinetics data to look at influences of D-gluconate on nucleation and growth mixed C-S-H morphologies nanoglobules, nanfoils and nanofibrils were observed. For a quantitative analysis by the population balance method recently developed in our lab such a mixed morphology cannot be analyzed. The experimental conditions for the collection of kinetics data for uniform morphology of precipitated synthetic C-S-H were found by controlling pH and D-gluconate concentrations. This new and unique kinetics data will in the near future be evaluated by using population balance modeling in order to discern the quantitative effect of nucleation and growth of precipitated synthetic C-S-H formation in the presence of D-gluconate.

Finally, to approach more real system conditions, heterogeneous particles (e.g. quartz and calcite) were introduced in the synthetic C-S-H system. Quartz and calcite were able to act as heterogeneous substrates for the nucleation and growth of precipitated synthetic C-S-H. The elemental maps confirmed uniformly distributed Ca, Si and O in nanofoils at the heterogeneous surfaces corresponding to the C-S-H composition. Without calcium ions in the silicate solution, calcite partially dissolved to give Ca2+ and CO32- ions which then formed C-S-H on its surface. Kinetics data shows different growth of C-S-H formation and will be evaluated in the future by using population balance modeling.