Abstract
The results of a 1H double-quantum-filtered (DQF) nuclear magnetic resonance (NMR) study of water in cement pastes are reported. It is shown that the DQF signal increases with curing time and in sympathy with the loss of mobile single-quantum signal, suggesting strongly that a signal from 1H in chemically combined and strongly confined water is selectively observed. The DQF signal in white cement comprises at least two components: the first is assigned to portlandite (Ca(OH)2); the second is assigned to water in the planar, nanometre-wide, calcium–silicate–hydrate (C–S–H) gel pores. The pore water signal is significantly broader than that expected for bound water. The width is interpreted in terms of the water undergoing a two-dimensional walk in the vicinity of Fe3+ impurities. A simple model is presented and found to be consistent with experiment and the known Fe3+ concentration. In grey cements, a third component is identified and associated with Fe-rich phases. The analysis places a lower bound on the lateral extent of planar C–S–H pores. The change in DQF signal components upon drying a sample mirrors the loss of the singlequantum components observed in a parallel study.