Unilateral low-field magnetic resonance measurements of carbonation depth in unsaturated and saturated Portland cement mortars

Unilateral Nuclear Magnetic Resonance is used to characterize the carbonation depth in 0.60 w/c ratio unsaturated/saturated Portland cement mortars. A three-magnet array combined with a surface elliptical coil was utilized to obtain the Nuclear Magnetic Resonance signal at different positions along mortar samples. The T₂ lifetime and signal intensity behaviors in the carbonated and non-carbonated zones were analyzed. Scanning Electron Microscopy images were used to identify the macro-porosity and micro-cracking in the samples. Results confirmed that the crossover point between profiles of T₂ lifetime and signal intensity agree with the carbonation depth obtained by the phenolphthalein test. The carbonation depth at the position where the T₂ lifetime begins to behave nearly constant is about 1.44 times the depth indicated by the phenolphthalein test. The spatial resolution was improved four times in comparison with previous research. Scanning Electron Microscopy images analysis confirmed a significant decrease in local porosity and crack area in the carbonated zone.