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Modern approach to surface layer modifications of hydrated cement paste for improving permeability estimates of virtual concrete

Piet Stroeven 1, Kai Li 2

1 Delft University of Technology, Faculty of Civil Engineering and Geosciences, the Netherlands
2 Hunan University, College of Civil Engineering, Changsha, China

Permeability estimates obtained by physical experiments and by simulations differ quite significantly. When biases at the experimental side would be eliminated, the expected modest gap could be bridged by surface layer modifications of the hardened cement paste, discussed herein. This involves the formation of a fractal-like nano-particle structure in the outer hydration layer, instead of the smooth-surfaced layer obtained by the vector approach. This study firstly applies a DLA method for simulating the fractal structure of nodules in a low-density 2D setting of mono-size cement particles. Herewith it is confirmed that density of the outer hydration layer will diminish almost linearly away from the inner one, inevitably leading to permeability decline. This approach yielded also a realistic range of modifications for the outer layer, i.e., a 0.1 to 0.5 µm increased thickness. Therefore, in the normal 3D set up, the smooth vector-based hydration layer is given a stepwise thickness increase in this range, whereupon the consequences for pore geometry and topology, as well as for permeability can be investigated. The outcomes of this study are laid down in a separate publication. When further structural information on nano level will become available in the near future, the simulations can be appropriately adapted. This paper specifically proposes as a practical methodology to slightly step-wise enlarge at the packing simulation stage the fresh cement grains in proportion to their respective sizes. The herein introduced standard methodology for porosimetry and permeability estimation can readily be employed for simulating these cases. This is demonstrated for cement paste with w/c = 0.5, whereby packing density is increased by about 1% per step. This can be associated with a fictitious reduction in w/c from 0.49 to 0.45. It could be demonstrated this way that a permeability decline of one order of magnitude can be realized, i.e., at least enough to bridge the modest gap with corrected experimental permeability data. So, the final solution can be expected in this range.

Key words: Cement, hydration, surface modification, nano-packing, permeability, simulation