Nuclear magnetic resonance (NMR) relaxation time mapping in partially dried mortar is presented. Relaxation times in concrete are typically multi-exponential, and exhibit highly effective relaxation mechanisms (T-1L approximate to 3 ms, Deltav(1/2) approximate to 2500 Hz). One-dimensional mapping of the spin-lattice relaxation time is used to study relative changes in the water-occupied surface-to-volume ratio, as a function of mixture conditions, initial hydration period, and drying time. These results provide evidence for a penetrating microcracking phenomenon, due to local drying-induced tensile stresses. This conclusion was further supported by a relaxation time study of samples which were preconditioned so as to be at equilibrium with various relative humidities.