The early stages of ordinary Portland cement (OPC) hydration were studied by cold field emission scanning electron microscopy and isothermal conduction calorimetry. Particular attention was paid to samples from the end of the induction period, where an additional peak in the tricalcium silicate hydration reaction has recently been discovered. Higher resolution images were obtained than is possible with current in situ imaging techniques, allowing for the observation of features on the 5-nm scale. The peak in the hydration at the end of the induction period was associated with both the formation of pores in the surface of the cement grains and the formation of nanoscale calcium silicate hydrate (C-S-H) structures on those surfaces. The effects of grinding of OPC and additions of tricalcium aluminate and tetracalcium aluminoferrite on the peak at the end of the induction period were also examined. The results of the study were compared with the models in the literature used to describe the causes of the induction period and its end.