**1. Introduction**

Crystallization in the calco-carbonic system CaCO3-CO2-H2O and the parameters that significantly control the polymorph's formation have been studied for more than a century by several researchers in different scientific disciplines ranging from industrial crystallization [1,2] to biomineralization [3–5].

Calcium carbonate exists in three crystalline forms, which are calcite, aragonite and vaterite, in the order of stability at earth surface conditions. These crystalline phases have different crystal structures and morphologies. Calcite, aragonite and vaterite crystals have rhombohedral, orthorhombic, and hexagonal structures, respectively. In addition, crystalline monohydrocalcite (CaCO3.H2O) and ikaite (CaCO3.6H2O) are known. The observation of an intermediated or transient amorphous phase (amorphous calcium carbonate ACC) has also been reported [6–9]. In accordance with the Ostwald's step rule [10], the transformation of the unstable phase (ACC) to a metastable phase (vaterite or aragonite) is followed by the formation of the more stable phase (calcite) through dissolution– crystallization reactions of various degrees of complexity.

The nucleation and growth of these forms are generally related to kinetic and thermodynamic factors such as the degree of supersaturation, solvent, organic, and inorganic additives. CaCO3 crystallization is intensely affected by the presence of such additives, which delay or inhibit nucleation and growth, and change the polymorph morphology [11–13]. The additives' impact is detected in the nucleation and growth steps [14–18]. In the current

**Citation:** Hamdi, R.; Tlili, M.M. Influence of Foreign Salts and Antiscalants on Calcium Carbonate Crystallization. *Crystals* **2023**, *13*, 516. https://doi.org/10.3390/ cryst13030516

Academic Editor: José L. Arias

Received: 28 February 2023 Revised: 13 March 2023 Accepted: 15 March 2023 Published: 17 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

work, the effect of different types of chemical additives such as magnesium, sulfate, and antiscalants on the CaCO3 crystallization process was studied.

The presence of dissolved ions, for instance magnesium and sulfate, is supposed to produce modification in the kinetic and the driving force of calcium carbonate precipitation [19,20]. They have long been considered as additives in calcium carbonates, usually known to affect calcite growth and dissolution. Magnesium ions have a significant role in the formation and transformation phases of CaCO3 crystals. They inhibit calcite formation [18,21] by decelerating its growth rate [22] and favoring the appearance of aragonite [23] and monohydrocalcite [24], which are less stable phases. They extend the duration of the amorphous phase life [25], and determine the polymorph formation. Magnesium interacts directly with calcium carbonate crystals and affects their morphologies [26]. The presence of magnesium and sulfate, separately and together, modifies the shape of the calcite crystal [27]. Sulfate affects the nucleation even at lower concentrations, by suppressing the formation of CaCO3 crystals and modifying their morphology [28]. It reduces the crystallization rate and favors the formation of the aragonite form [29].

The addition of antiscalants, also known as chemical inhibitors, is an effective method to inhibit the formation of scale [30]. Indeed, antiscalants delay calcium carbonate crystallization through growth sites blocking [31]. They have three mechanisms of action, which are dispersion, chelation, and crystal modification, consisting of the distortion of the crystal to become irregular and less adhesive, thus inhibiting crystal growth at calcite surfaces [32]. The polymer adsorption on the crystal growing surface suppresses the growth rate and displaces the precipitation mode from the most thermodynamic stable phase (calcite) to the metastable phases (aragonite and vaterite) [33]. The addition of the inhibitor resulted in (i) an important inhibition of crystallization, and (ii) a porous or an unconsolidated consistency of precipitates [34]. Therefore, the incorporation of the chemical additives in the structure of the crystal phase form modified its thermodynamic properties.

The present paper aims to improve our understanding of the effect of chemical additives on the CaCO3 crystallization. The changes induced in the kinetic, thermodynamic, and morphology of CaCO3 phases formed after the use of diverse types of chemical additives will be evaluated to explore the complex behaviors of the crystallization described in the calco-carbonic system. Consequently, the experimental process for CaCO3 crystallization based on the fast controlled precipitation (FCP) method [35,36] will be used. The chemical additives employed are three foreign salt (MgCl2, Na2SO4, and MgSO4) and two antiscalants (sodium polyacrylate and sodium-tripolyphosphate).
