The formation of C-S-H consumes the silicon ions, and calcium is continuously added. Therefore, after nucleation, the free calcium concentration rises once the silicon concentration has reached the solubility limit of C-S-H. We could not accurately model the experimental C-S-H solubility curve using the C-S-H thermodynamic attributes available in the Cemdata18 database regardless of the complexity of the solid solution (Figure ) . All solid solutions underestimate the calcium and the silicon concentration compared to the values measured experimentally. This is not unexpected: