• 100%. We observe a general trend of decreasing nanoparticle size and size distribution with increased precipitation pH and increasing Mg 2+ concentration, as illustrated in Figure C and summarized in Table . In contrast, others did not observe an effect of Mg 2+ on ACC nanoparticle size. To elucidate the formation mechanism of Mg-ACC nanoparticles, we first determine the kinetics of precursor nucleation and growth. We analytically describe the transformation from phase A, the solution containing all solutes, to phase B, the phase-separated precursor particles. By assuming the presence of only phase A and phase B, we exclude time points at which Mg-ACC nanoparticles are formed. We infer the appearance of condensed Mg-ACC nanoparticles from the time-dependent surface-to-volume ratio (Figure SI 5-7 D) of the formed structures. Initially, less dense, fractal-like precursor particles form, transforming into denser Mg-ACC nanoparticles. This transformation reduces the system's overall surface-to-volume ratio. The time the S/V ratio approaches a constant value is used as the critical time, t c marking the appearance of Mg-ACC nanoparticles. The volume fraction (Φ) of the formed particles was determined following SI 2.3 and normalized to Φ(t = 0) = 0 and Φ(t = t c ) = 1 representing the transformed fraction (T). Figure shows the transformed fraction as a function of time for all Mg:Ca ratios for varying pH, showing a clear shift to longer nucleation times with decreasing pH and increasing Mg 2+ concentrations. The time-dependence of the transformed fraction can be analytically described by an Avrami-like equation T (t) = 1 -e -kt n , where k is a kinetic constant and n is a growth exponent related to the mechanism of nucleation and growth. We obtain the best fits of the kinetic constant k, summarized in Table for a fixed growth exponent of n=3, observing a general trend of an increase of k with increasing pH and decrease Mg 2+ concentration. In the case of Mg-ACC nanoparticles formed at pH 8.7 with a 50:50 Mg ratio, the phase separation is instantaneous and inadequately described by the employed analytical growth model. This quasi-instantaneous formation of particles is also evident in the SAXS data (Figure SI 2 C), where only one intermediate structure could be temporally resolved after 20 ms, and the reaction was completed within 45 ms.