*3.1. 13C*

The 13C NMR spectrum of the anhydrated cement (Figure 1 (top)) shows a single broad asymmetric line at a chemical shift of about 168.7 ppm. Most likely, this line originates from amorphous calcium carbonate and, possibly, small amounts of calcite and dolomite, which are contained in cement, according to the phase composition obtained from the XRPD analysis (Table 1). An effort to perform component deconvolution in this spectrum was not attempted, because its line shape is quite broad and the 13C chemical shifts of the carbonate compounds fall close to each other [14]. The 13C NMR spectra obtained at different ages of cement hydration show two (except for **CAA** and **CSA** at 1 d) clearly distinguished peaks (Figure 1 (bottom) is an example of the 13C NMR spectrum of the sample C07; the other spectra are provided in Supplementary Figures S1a–S3a). Moreover, the peak at about 168.5 ppm consists of unresolved narrow and broad signals; the deconvolution into two peaks is justified in Supplementary Figure S1c,d. We note, however, that the precision of the deconvolution of unresolved signals might suffer from larger errors and a certain degree of caution should be exercised when analyzing these results. The position of the signal at about 171 ppm remains virtually unchanged over time. However, the narrow component of the signal at 168.5 ppm was slightly shifted to the weak field at the initial stage of hydration, and the broad component was shifted to the strong field throughout the entire hydration period, with the exception of the **CSA** sample, for which this component was shifted, on the contrary, to the weak field. The signals mentioned are contributed also by the presence of calcium monocarboaluminate hydrate, forming during the hydration process. The time dependences of the 13C chemical shift values are shown in Supplementary Figures S1b–S3b in Supporting Information.

**Figure 1.** 13C NMR spectra of the (**top**) anhydrated Portland-limestone cement, and (**bottom**) cement paste sample without additives on the 7th day of hydration.

According to the literature, all the observed signals correspond to CO3 <sup>2</sup><sup>−</sup> structural units of various amorphous and crystalline modifications of calcium carbonate and dolomite [14–16]: the broad signal at about 168.5 ppm corresponds to amorphous calcium carbonate and to metastable ikaite, which is possibly contained in the samples; the narrow signal corresponds to calcite, dolomite and vaterite; and the signal at about 171 ppm corresponds to carbon atoms in aragonite and vaterite. According to Reference [17], the two narrow signals from carbon nuclei in the structure of vaterite correspond to its two most probable crystal structures.
