*2.2. Experimental Setup*

Ten custom-built incubation chambers made of Teflon [31,36] were used to conduct three cross-factor experiments with two Ca:CO3 scenarios (high and low) in parallel with three Mg concentrations (0 mM, 26.5 mM, and 53 mM) in series (Figure 2; Table 1). Each chamber was attached with Tygon and Marprene® tubing to two 1-L Tedlar® gas sampling bags filled with either a calcium chloride [CaCl2] or sodium bicarbonate [NaHCO3], the preparation of the stock solution is detailed below. Five replicates were run for each treatment. The volume of each incubation chambers was 0.25 mL. The initial flow rate of the solutions into the chambers was accelerated to quickly fill the incubation chambers and then reduced to a constant flow (10 μL min<sup>−</sup>1) via a 24-channel peristaltic pump (Ismatec®). The seed material was placed in each of the incubation chambers and the experiments were run between 32–70 days in a temperature-controlled Rumed® climate cabinet maintained at a constant 25 ◦C (±0.5 ◦C). The variation in experimental duration was due to unexpected health and safety issues of the authors not being allowed into the laboratory.

**Figure 2.** Schematic design of the experimental conditions showing (**a**) the separate stock solutions of Ca and CO3 passing through the peristaltic pump and mixing in the reaction chambers, and (**b**) the physicochemical conversions of CaCO3, bicarbonate [HCO3 −] and CO3 occurring during mineral precipitation and dissolution phases. Each of the six experimental scenarios is outlined in Table 1.
