On the interpretation of 1H 2-dimensional NMR relaxation exchange spectra in cements: Is there exchange between pores with two characteristic sizes or Fe3+ concentrations?

We discuss the effect of paramagnetic impurity content (thought to be predominantly Fe3+) in cement pastes on the interpretation of 1H NMR T2–T2 exchange spectra. Through measurements on synthesised C–S–H with greatly reduced paramagnetic impurity concentration, we show that the spectra cannot be explained by exchange between regions of different Fe3+ concentration but rather are explained by exchange between regions of different pore size.