Water Dynamics in Hardened Ordinary Portland Cement Paste or Concrete:  From Quasielastic Neutron Scattering

Abstract

Portland cement reacts with water to form an amorphous paste through a chemical reaction called hydration. In concrete the formation of pastes causes the mix to harden and gain strength to form a rock-like mass. Within this process lies the key to a remarkable peculiarity of concrete: it is plastic and soft when newly mixed, strong and durable when hardened. These qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks and superhighways, houses, and dams. The character of the concrete is determined by the quality of the paste. Creep and shrinkage of concrete specimens occur during the loss and gain of water from cement paste. To better understand the role of water in mature concrete, a series of quasielastic neutron scattering (QENS) experiments were carried out on cement pastes with water/cement ratio varying between 0.32 and 0.6. The samples were cured for about 28 days in sealed containers so that the initial water content would not change. These experiments were carried out with an actual sample of Portland cement rather than with the components of cement studied by other workers. The QENS spectra differentiated between three different water interactions: water that was chemically bound into the cement paste, the physically bound or "glassy water" that interacted with the surface of the gel pores in the paste, and unbound water molecules that are confined within the larger capillary pores of cement paste. The dynamics of the "glassy" and "unboud" water in an extended time scale, from a hundred picoseconds to a few nanoseconds, could be clearly differentiated from the data. While the observed motions on the picosecond time scale are mainly stochastic reorientations of the water molecules, the dynamics observed on the nanosecond range can be attributed to long-range diffusion. Diffusive motion was characterized by diffusion constants in the range of (0.6-2) 10(-9) m(2)/s, with significant reduction compared to the rate of diffusion for bulk water. This reduction of the water diffusion is discussed in terms of the interaction of the water with the calcium silicate gel and the ions present in the pore water.

Translational diffusion of water and its dependence on temperature in charged and uncharged clays: A neutron scattering study

“…The data obtained ͑see Table III͒ for bulk water are in good agreement with other neutron scattering data 11,47 as well as other measurements. 48 In summary, the diffusion coefficients obtained for the charged clays were strongly reduced, whereas those for the uncharged kaolinite were identical or slightly higher compared to those for bulk water ͑see Table III͒.…”

Cement: A two thousand year old nano-colloid

“…bulk-like water modeled by two Gaussians and surface-interacting water by one Gaussian). The second approach to model QENS spectra has been mainly implemented by Fratini et al [11] and Bordallo et al [60] that used a delta function for the immobile water fraction and explicitly considered the dependence of the quasi-elastic broadening as a function of Q. Two different physical frameworks have been applied: Bordallo et al combined jump diffusion for translations with rotationdiffusion models while Fratini et al used the well-know relaxing cage model (RCM) either for translation alone, Q below about 1 Å À1 , and a combination of translational and rotational motions in the case of a wider interval of the momentum transfer [61].…”

Nano-scale hydrogen-bond network improves the durability of greener cements

“…In the case of bulk water, the potential of QENS has been exploited for many years1819. However, it is now also well established that neutron scattering, and QENS in particular, is a powerful technique that can examine the interactions of the pore water with the C-S-H in cement paste202122232425.…”