# Immobilization of (Aqueous) Cations in Low pH M-S-H Cement
The contaminants are encapsulated during the hydration and the hardening of the cementitious materials. The immobilization of metal ions using Portland cement (PC) as encapsulating agent includes chemical and physical mechanisms [10; 11; 12]. The physical processes include the immobilization of contaminants through sorption at the surface of the calcium silicate hydrate (C-S-H), the main hydrated product of PC. This mechanism is playing a relevant role in cements that present high surface areas (50-200 m\({}^{2}\)/g) and high number of unsaturated sites [13; 14]. On the other hand, chemical immobilization relies on the incorporation of the ions into the cement matrix/crystal structure itself. This approach aims to change the chemical configuration of the pollutant to achieve materials with lower solubility and reduced hazardousness [15; 16; 17]. Reducing the potential for harmful environmental impact of waste disposal through chemical fixation represents a sustainable approach. Nevertheless, the distinction between purely chemical and purely physical encapsulation is not straightforward, since heavy metals can interact with the cement paste in different ways (e.g., adsorption, chemisorption, precipitation, ion-exchange, passivation, surface complexation, inclusions, and chemical incorporation) [18]. To develop an appropriate technology for immobilization of toxic materials, parameters like the resistance against an aggressive environment and the chemical compatibility with the waste are essential [19]. As the properties of the wastes vary regarding their composition, the requirements of the cement must be adapted to the specific chemical characteristics. For instance, the highly alkaline porewater in commonly used Portland cement causes high corrosion of metals leading to an undesired H\({}_{2}\) generation [20]. Although Portland cement (PC) is known to immobilize metal wastes as precipitated hydroxides, the stability of those highly depends on the internal pH values and therefore, preventing amphoteric substances from leaching is a severe issue for PC due to its high pH (>12) [3]. For instance, the optimal pH value where cement shows the highest retention of metals from fly ash waste lays in between 9 and 11 [6], and therefore the alkaline character of the pore solution during hydration of Portland cement could be unfavorable. Another important issue regarding the use of Portland cement as encapsulating material is that metal ions have been shown to retard the hydration process that yields calcium silicate hydrate (C-S-H) [21]. Regarding the mechanical properties at macroscopic scale, the addition of doping ions to PC has been suggested to alter the bonding strength of the silicate chains of the formed C-S-H, leading to significant reduction of the compressive strength [22]. A potential encapsulating material that meets the pH requirement is magnesium silicate hydrate (M-S-H). Its relatively low pH (- 9 to 11) makes it a promising host material as most heavy metals typically show the lowest solubility in the pH range between 9 and 12 [23]. M-S-H is a magnesium-based cementitious binder analogue to the main products of PC hydration reactions (C-S-H). In the last years, it has caught the attention of researchers as a potential environmentally friendly alternative to ordinary PC [24; 25; 26; 27; 28]. In addition to its potential use as a low carbon cement, the suitability of M-S-H for the encapsulation of amphoteric species of waste have also been in focus of various investigations [20; 29; 30; 31]. The existence of natural magnesium silicate hydrates formed through dissolution and reprecipitation processes of ultramafic rocks suggests the sufficient durability needed for long-term encapsulation [32; 33]. M-S-H is also formed upon perturbation of low-pH cement in contact with clayey environment [34]. Regarding incorporation of contaminants, natural magnesium silicate minerals such as sepiolite have demonstrated their ability to entrap heavy metal ions and hazardous organic compounds [35; 36].