H-1 nuclear magnetic resonance characterization of Portland cement: molecular diffusion of water studied by spin relaxation and relaxation time-weighted imaging

Water molecular dynamics in a hardened Portland cement were characterized b y proton Fourier transform nuclear magnetic resonance (NMR) at 400 MHz. Thr ee different types of water molecule (physically bound, chemically bound an d porous trapped) were observed. When the hardened cement sample was heated at 105 degrees C, the physically bound water diffused out of the sample as a function of the heating time while the chemically bound water remained i n a stable form. A trace amount of the porously trapped water was also dete cted to remain in the cavities of the hardened cement even after heating fo r up to 20 h at th is temperature. The loss of the physically bound water p roved to be a diffusion-controlled process as evidenced from the NMR data a nd from a gravimetric technique. A Fake doublet was observed in the NMR spe ctra. This is a result of the oscillation of the water molecules with hinde red molecular motions due to their entrapment in the cement pores. Soaking the dried samples in water resulted in the diffusion of water back into the hardened cement as physically bound water. Nuclear spin-spin relaxation ti me, T-2-weighted imaging showed that the distribution of the physically bou nd water inside the cylindrical sample formed a doughnut shape after overni ght soaking. The residual air in the cement pores may have slowed down the diffusion rate of the water molecules back into the dried cement. (C) 1998 Kluwer Academic Publishers.