Carbonation front in cement paste detected by T ₂ NMR measurements using a low field unilateral magnet

Unilateral magnetic resonance was used to obtain the CPMG T₂ decay at different positions along 6 cm long cement paste samples. The aim was to detect the carbonation front based on changes in the pore structure caused by accelerated carbonation and to compare the results with the phenolphthalein test. Cement pastes at water-to-cement ratios of 0.60, 0.50, and 0.40 were prepared using ordinary Portland cement. After moist curing and conditioning at 65% RH and 35 °C, the pastes were subjected to accelerated carbonation with 4% by volume CO₂ at 65% RH and 35 °C. Carbonation increases the magnetic resonance T₂ lifetime compared with the control and compared with the noncarbonated region of the samples. A reduction in porosity and changes in the pore size distribution are caused by carbonation. The position with the highest rate of T₂ change was consistent with the fully carbonated front determined by the phenolphthalein test.