**4. Conclusions**

Chemical additives play a significant role in several stages of the formation and transformation of CaCO3 crystals. In the present paper, the fast controlled precipitation method was used to study the influence of chemical additives on the CaCO3 crystallization process. The additives employed were three foreign salts (MgCl2, Na2SO4 and MgSO4) and two antiscalants (sodium polyacrylate and sodium-tripolyphosphate). The results showed that the nucleation time was retarded after the addition of each foreign salt, regardless of its ionic strength value. The MgSO4 ion greatly delayed the precipitation of CaCO3 compared to Mg2+ and SO4 <sup>2</sup><sup>−</sup> ions. Moreover, the sulfate ion had more impact on the pH of prenucleation and precipitation. As for magnesium ion, it influenced the nucleation time. Thus, the presence of magnesium and sulfate ions affected nucleation and growth through different ways. At the macroscopic scale, it would seem that both magnesium and sulfate ions, although presenting opposite charges, had the same effect on the orientation of CaCO3 precipitation to the heterogeneous precipitation, regardless of their modes of action in solution. Furthermore, both antiscalants RPI and STPP had a great effect on the crystallization kinetic by greatly delaying the precipitation time. Indeed, the prenucleation stage duration (Δt=tprec − tprenuc) was longer when the antiscalant concentration was larger. Therefore, the distinction between the prenucleation and precipitation thresholds became easier. In addition, the presence of each antiscalant affected the precipitation threshold remarkably, leading to a large variation in the supersaturation coefficient. In addition, the chemical inhibition favorably promoted precipitation on the cell wall, detrimental to the bulk solution scaling. Finally, the X-ray diffraction patterns of deposit scale formed in the absence and presence of each chemical additive revealed the formation of different crystal forms of calcium carbonate, which were calcite, vaterite and aragonite.

**Author Contributions:** Conceptualization, R.H. and M.M.T.; methodology, R.H. and M.M.T.; software, R.H.; validation, R.H. and M.M.T.; formal analysis, R.H. and M.M.T.; investigation, R.H.; resources, R.H. and M.M.T.; data curation, R.H.; writing—original draft preparation, R.H.; writing review and editing, R.H. and M.M.T.; visualization, R.H.; supervision, R.H. and M.M.T.; project administration, R.H. and M.M.T.; funding acquisition, R.H. and M.M.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Deputyship for Research and Innovation, Ministry of Education in Saudi Arabia, Project number (IF2/PSAU/2022/01/21605).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are available on reasonable request.

**Acknowledgments:** The authors extend their appreciation to the Deputyship for Research and Innovation, Ministry of Education in Saudi Arabia for funding this research work through the Project number (IF2/PSAU/2022/01/21605).

**Conflicts of Interest:** The authors declare no conflict of interest.
