We report the first nuclear magnetic resonance ͑NMR͒ two-dimensional correlation T 1 -T 2 and T 2 -T 2 measurements of hydrating cement pastes. A small but distinct cross peak in the two-dimensional relaxation spectrum provides the first direct evidence of chemical exchange of water between gel and capillary pores occurring over the first 14 days of hydration. A correlation of features along the line T 1 =4T 2 provides strong supportive evidence for the surface diffusion model of 1 H nuclear spin relaxation in cements and for a multimodal discrete pore size distribution. Differences in detail of the results are reported for white cement paste and white cement paste with added silica fume. Both the method and the theory presented can be applied more widely to other high surface area materials with other reactive surface areas.

ABSTRACT:The nano-scale morphology of, and pore water interactions in, calcium-silicate-hydrate (C-S-H), the active component of cement, remain uncertain. 1 H nuclear magnetic resonance (NMR) can fully characterise the nano-porosity of C-S-H in as-prepared material without the need for damaging sample drying. We use NMR to follow the density of C-S-H in sealed cured pastes as a function of degree of hydration (α) and water to cement ratio. We show clear evidence for C-S-H densification. The C-S-H "solid" density, exclusive of gel pore water, slightly decreases from ! ! = 2.73 g/cm 3 at α ≈ 0.4 to 2.65 g/cm 3 at α ≈ 0.9 due to an increase in the number of layers in the nanocrystalline aggregates. In the same range, the C-S-H "bulk" density, including gel water, increases from around 1.8 to 2.1 g/cm 3 . The increase corresponds to a transition from growth of low density product containing gel pores to higher density product devoid of gel pores.We update Powers' classical model from 1947. In contrast to the single "hydrate" of Powers', NMR differentiates between C-S-H and calcium hydroxide and separates out the inter-layer water within the C-S-H. It shows a clear non-linearity in the growth of the different fractions with α.

Abstract1 H nuclear magnetic resonance (NMR), supported by a measurement of the degree of hydration using X-ray diffraction, has been used to fully characterise the nano-scale porosity and composition of calcium-silicate-hydrate (C-S-H), the active component of cement. The resultant "solid" density and composition are 2

The first detailed analysis of the two-dimensional ͑2D͒ NMR T 2 -T 2 exchange experiment with a period of magnetization storage between the two T 2 relaxation encoding periods ͑T 2 -store-T 2 ͒ is presented. It is shown that this experiment has certain advantages over the T 1 -T 2 variant for the quantization of chemical exchange. New T 2 -store-T 2 2D1 H NMR spectra of the pore water within white cement paste are presented. Based on these spectra, the exchange rate of water between the two smallest porosity reservoirs is estimated for the first time. It is found to be of the order of 5 ms −1 . Further, a careful estimate of the pore sizes of these reservoirs is made. They are found to be of the order of 1.4 nm and 10-30 nm, respectively. A discussion of the results is developed in terms of possible calcium silicate hydrate products. A water diffusion coefficient inferred from the exchange rate and the cement particle size is found to compare favorably with the results of moleculardynamics simulations to be found in the literature.

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