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Molecular-Dynamics and Monte-Carlo Simulations of Water Dynamics in CementS.-H. P. Cachia 1 , J. S. Bhatt 1 , S. V. Churakov 2 , N. C. Howlett 1 , D. A. Faux 1 , P. J. McDonald 11 Department of Physics, University of Surrey Guildford Surrey GU2 7XH United Kingdom 2 Laboratory forWaste Management, Paul Scherrer Institute, 5232 Villigen-PSI, SwitzerlandINTRODUCTION:1 H Nuclear MagneticResonance (NMR) relaxometry has been used tocharacterise water dynamics in cement pastes onthree timescales: (i) nanosecond hopping of watermolecules across pore surfaces; (ii) microsecondresidency time for water molecules adsorbed atpore surfaces and (iii) millisecond exchange ofwater between C-S-H interlayer and gel pores. It isthe purpose of this work to seek to corroboratethese measurements using a combination ofMolecular-Dynamics (MD) and Monte-Carlo (MC)simulations. The MD is able to assess waterdynamics in nano-pores from the pico to nanosecond timescale in great detail. MC is required toextend this to longer length and timescale.METHODS: Atomic trajectories of water inconfined geometries have been obtained with MDand MC simulations. These are used to calculatethe nuclear dipole-dipole correlation functions:G * (t) = 1 (3cos 2 (! 2 ! ij)"1) / (r 3 ij(t)r 3 ij(0))where the sum is over all pairs of 1 H nuclei withseparation r ij (t) and θ is the angle between thisvector at time zero and t.The correlation functions are Fourier transformedto yield the spectral density function J(!) fromwhich the NMR spin-lattice and spin-spinrelaxation times, T 1 and T 2 respectively, arecalculated:1 = A J(!T1( 0)+ 4J(2! 0))1 = A 3J(0)+ 5J(!T2( 0)+ 2J(2! 0))where A is a constant characteristic of the materialand ! 0is the NMR frequency.RESULTS: MD simulations were performed forbulk water, water confined between (100)-facets ofSiO 2 , Tobermorite’s basal plane and other C-S-Hanalogues.Figure 1 shows a snapshot of an MD simulation ofwater confined between two planes of SiO 2 . Theplanar spacing is d = 3.1 nm. Simulations havebeen performed for separations as small as 1 nm.Water confined to a pore can be identified as“surface” or “bulk” dependent upon its initiallocation with respect to the SiO 2 . Figure 2 showsthe contribution to the correlation function arisingfrom bulk-bulk, surface-bulk and surface-surfaceinteractions from which the relaxation times arecalculated. There are significant differencesbetween the correlation functions of confinedwater and those for bulk water that give rise to thereduction in relaxation times. Further simulationshave followed the contribution of, for instance,surface paramagnetic impurity contributions andinter- and intra-water molecule contributions.Fig. 1: An MD snapshot of water confined between(100) SiO 2 facets. O-atoms are red, Si-atoms areyellow, H-atoms are white.Fig. 2: Total and partial G * (t) correlationfunctions. Black: total; Green: bulk-bulk; red:surface-bulk; blue: surface-surface.REFERENCES:1A. Abragam, The principle of nuclear magnetismOxford, (1961)2 Sholl, J. Phys. C. 7, 3378 (1974)3 Faux, et al, J. Phys. C. 19, 4115 (1986)ACKNOWLEDGEMENTS:This work is supported by the European UnionSeventh Framework Programme (FP7 / 2007-2013) under grant agreement 264448 and by theU.K. Engineering and Physical Sciences ResearchCouncil, grant number EP/H033343.50