The free phosphate solution displays a unique broad resonance centered at 2.7 ppm, which is in agreement with the reference Pi solution at the same pH (Figure ). By increasing the reaction mixing time from 20 ms to 60 sec, we observe the progressive broadening of the resonance on the right side ( +2 ppm) associated with a shift of the maximum toward lower frequency ( -1.8 ppm) (Figure ). Such signal evolution is characteristic of the interaction of phosphate with Ca 2+ as supported by calcium potentiometry and pH monitoring where a dramatic drop in [Ca 2+ ]free is evidenced (down to 7.5 mM) associated to a decrease in pH (down to 6.5) within the first seconds of reaction (Figure ). This process leads to the formation of amorphous calcium phosphate (ACP) as evidenced by cryo-TEM observation showing the formation of spheres after a few seconds of reaction (30-40 s) (Figure ).