Water dynamics in hardened ordinary Portland cement paste or concrete from quasielastic neutron scattering

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

Water dynamics in cement pastes

Cement has been used for centuries as a cost effective construction material,as well as for the conditioning of lowlevel radioactive waste. The interaction of water with calcium silicate hydrate the main component of cement paste,CS H is one of the prime factors for controlling the stability of the cement paste in the environment. However,much of the material science of cement hydration,including the bonding and interaction of water with C S H,remains unknown. Here we used high resolution quasi elastic neutron scattering to investigate the water dynamics on cement pastes hydrated for more than 28 days with different levels of hydration on the nanosecond time scal

Cracks and Pores - Their Roles in the Transmission of Water Confined in Cementious Materials

Cement paste is formed through a process called hydration by combining water with a cementitious material. Concrete, the worlds most versatile and most widely used material, can then be obtained when aggregates (sand, gravel, crushed stone) are added to the paste. The quality of hardened concrete is greatly influenced by the water confined in the cementitious materials and how it is transmitted through cracks and pores. Here we demonstrate that the water transport in cracks and capillary pores of hardened cement pastes can be approximately modeled by simple equations. Our findings highlight the significance of arresting the development of cracks in cementitious materials used in repository barriers. We also show that neutron scattering is an advantageous technique for understanding how water transmission is effected by gel pore structures. Defining measurable differences in gel pores may hold a key to prediction of the reduction of water transport through cement barriers

Using x-ray powder diffraction as a cost effective tool in cement industry

Rietveld Analysis of cement diffraction patterns have been used to determined the composition of cement since John Taylor\u27s pioneering work in the 1990\u27s. Since then many workers have used this techniques to analyse cement and supplementary cementitious materials and their hydration products, both for research and production control purposes. Nevertheless there are a number of factors, including the amorphous content of the cement and relative proportion of mineral polymorphs present in the initial clinker, whose impact on analysis are still not completely understood. X-ray powder diffraction beamlines from the Brazilian Synchrotron Light Laboratory (LNLS) and the Australian Synchrotron, which produce more intensity and better resolution than normal x-ray diffraction sources, were used to investigate cement diffraction patterns and the hydration products of a range of cement pastes cured for up to 28 days. This study highlights the information that can be obtained from X-ray diffraction analysis for controlling and optimizing cement production and concrete durability.<br /

Hindered Water Motions in Hardened Cement Pastes Investigated over Broad Time and Length Scales

We investigated the dynamics of confined water in different hydrated cement pastes with minimized contributions of capillary water. It was found that the water motions are extremely reduced compared to those of bulk water. The onset of water mobility, which was modified by the local environment, was investigated with elastic temperature scans using the high-resolution neutron backscattering instrument SPHERES. Using a Cauchy-Lorenz distribution, the quasi-elastic signal observed in the spectra obtained by the backscattering spectrometer was analyzed, leading to the identification of rotational motions with relaxation times of 0.3 ns. Additionally, neutron spin echo (NSE) spectroscopy was used to measure the water diffusion over the local network of pores. The motions observed in the NSE time scale were characterized by diffusion constants ranging from 0.6 to 1.1 x 10(-9) m(2) s(-1) most likely related to water molecules removed from the interface. In summary, our results indicate that the local diffusion observed in the gel pores of hardened cement pastes is on the order of that found in deeply supercooled water. Finally, the importance of the magnetic properties of cement pastes were discussed in relation to the observation of a quasi-elastic signal on the dried sample spectra measured using the time-of-flight spectrometer