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. In order to better understand the role of water in mature concrete, a series of quasi elastic 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 interact 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 pico seconds to a few nano seconds, could be clearly differentiated from the data. While the observed motions on the pico second time scale are mainly stochastic reorientations of the water molecules, the dynamics observed on the nano second range can be attributed to long range diffusion. Diffusive motion was characterized by diffusion constants in the range of 0.6 2 10 9m2 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 wate