Changes in the solid and liquid phases during hydration
Both the composition of the solid and of the liquid phases changes during hydration. The changes in the solid phase can be followed by XRD, TGA or NMR measurements and by ESEM studies. The composition of the aqueous phase of hydrating cement can give important insights into the chemical processes and the interactions between solid and liquid phases. The composition of the liquid phase determines which hydrate phases are stable and can thus (potentially) precipitate. The measured composition of the pore solution and its changes during hydration can be considered in many cases as a prerequisite for a good thermodynamic model.
Measured and modeled composition of the pore solution during hydration

Pore solutions can be collected during the first hours of hydration by vacuum filtration. After hardening, pore solution can be gained using a high pressure device to extract the pore solutions.

During the first hours the composition of pore solution of Portland cement is dominated by potassium, sodium, sulfate, hydroxide and calcium. A significant change in the composition of the pore solution is observed between 6 and 24 hours; calcium and sulfate concentrations decrease as solid gypsum and anhydrite are depleted due to the formation of ettringite, while hydroxide concentrations increase at the same time.

Saturation indices during the hydration of a Portland cement
Comparing the measured concentrations in the pore solutions with the theoretical solubility products (KS0) of different solids gives the degree of over- or undersaturation and thus information whether a solid can potentially form or not.
Important publications

In the last years we have carried out hydration studies in pure Portland cement systems, in blended systems, in calcium sulfoaluminate cements, in alkali activated slag and fly ash and supersulfated slag systems.