A micro-reactor approach to study the hydration of cementitious materials has been developed in our laboratory.The method involves milling gaps that are a few microns in dimension in grains using a Focused Ion Beam.These gaps are then filled with solution, leading to dissolution, nucleation, and growth in the gaps.Hydration is stopped at selected time intervals, and a Scanning Electron Microscope is used to image the gaps.Information is obtained about dissolution-growth kinetics and hydrate morphology.Using this technique, we were able to obtain significant insights into the factors influencing early age hydration of tricalcium silicate and alite, and the effect of selected chemical admixtures on the same.