Mg-ACC nanoparticles were synthesized by coprecipitating CaCl 2 and MgCl 2 at molar ratios of 50:50, 70:30, and 80:20 at a total concentration of 0. Mg:Ca ratio. Figure shows SEM images of ACC nanoparticles rapidly dried with acetone after the synthesis at different pH and a constant 70:30 Mg:Ca ratio. For low pH (8.7 and 8.8), the formation of monodisperse, spherical ACC nanoparticles is observed, whereas at pH 8.9, less particulate and more continuous ACC structures are formed. SAXS data were recorded at two sample-to-detector distances (10 m and 30 m) covering q-ranges of 0.003-0.15 nm -1 and 0.01-0.6 nm -1 . Time series were collected after a dead-time of 20 ms at logarithmic time intervals (smallest interval 10 ms) to enhance temporal resolution during the early stages of particle formation while avoiding beam damage effects at later time points. The reaction was monitored for not more than 120 seconds to prevent artifacts from the gravitational precipitation of dense, large particles. For each new set of parameters, the capillary was cleaned with a low-concentration citric acid solution followed by a water rinse. Before each dataset, a fresh water background was taken to ensure no fouling of the capillary, enabling accurate background subtraction. Time-resolved and absolutely scaled complete datasets for all pH values and Mg ratios are