To bypass this limitation, the standard Gibbs energy of formation of the endmembers of the CSH-II solid solution was adjusted using GEMSFIT to better fit the experimental calcium concentration measured with the calcium ion selective electrode (Figure ) and silicon concentration measured by ICP (Figure ). The adapted solid solution model can reproduce the concentration curves with acceptable accuracy. This is achieved by increasing the solubility of the end members by less than 2 log units, which corresponds to an increase of less than 11.4 kJ mol -1 of the Gibbs energy of formation (Table ). However, the adjusted solid solution still overestimates the amount of Si that precipitates (Figure )