The role of capillary pores percolation and gel pores are investigated to explain the underlying differences between relative diffusivity obtained from different experimental techniques using microstructures generated from two different types of hydration model viz., CEMHYD3D (a voxel based approach) and HYMOSTRUC (a vector based approach). These models provide microstructures with different capillary pore connectivity for the same degree of hydration and the same porosity due to the underlying assumptions. In order to account for a C-S-H diffusivity at the micro-scale, a continuum micro-mechanics based model has been proposed. These simulations show that deperolation of capillary pores at around 20% of capillary porosity is essential in order to correctly predict diffusivity of cement paste with water-cement ratio by mass (w/c) in between 0.4 and 0.5. Furthermore from our analysis we present a viable postulate that the higher diffusivity measured by electric resistivity compared to other methods is due to differences in contribution from gel pores. For electrical resistivity measurement it is proposed that all gel pores are diffusive whereas for ion and tracer transport it is proposed that only nitrogen accessible gel pores are diffusive.

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In this paper, a newly developed lattice Boltzmann method based reactive transport model to simulate changes in microstructure of ordinary Portland cement paste due to calcium leaching is presented. The model takes three-dimensional digitized cement paste microstructure as input and is capable to capture an evolution of microstructure due to leaching, accounting for the dissolution of portlandite and corresponding increase in capillary porosity and the decalcification of C-S-H resulting in increase in gel porosity. The developed model has been applied to microstructures generated using two cement hydration models, CEMHYD3D and HYMSOTRUC, for three water-to-cement ratios. It was observed that the rate of leaching is directly proportional to ability of microstructure to transport calcium ions and higher fraction of percolated capillary pores result in higher rate of leaching. The model qualitatively reproduces experimentally observed changes in cement paste porosity and pore size distribution due to leaching. The quantitative validation of model at this scale is not possible by comparison of leaching obtained experiments and simulations which can be attributed to several factors including the differences in the scales of experiment and modelling study presented in this paper.

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This paper provides a comprehensive overview of existing experimental and modelling approaches to determine effective diffusion coefficients of water saturated ordinary Portland cement-based materials. A dataset for diffusivity obtained from different experimental techniques have been presented for cement paste, mortar and concrete. For cement paste at low porosities, diffusivity reported by different authors varies up to a factor of five and electrical resistivity measurements for low capillary porosity are up to one order of magnitude higher compared to other techniques. Experimental data of mortar and concrete reveals predominant influence of increasing tortuosity due to aggregates and limited influence of interface transition zone. Hence, a particular emphasis has been placed on assessing predictability of diffusivity models for cement paste on a larger dataset collected in this paper. It has been observed that all predictive models have similar level of accuracy and fail to predict electrical resistivity data at low capillary porosity as these models are not calibrated using electrical resistivity data.

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Material microstructure defines its physical and mechanical properties. With this idea, different models have been developed to effectively determine the physical and mechanical properties of a given microstructure. When the microstructure alters as a consequence of chemical processes, coupling of transport with geochemical models is essential for further extension of predictive capabilities. In order to simulate alteration of the microstructure of cement paste during chemical degradation, a new threedimensional pore scale reactive transport code has been developed. The model has the capability to account for reactive transport processes in two types of pore space representations: (1) an explicit representation of pores and solid phases for capillary pores and (2) a continuum representation for unresolved pores (pores with sizes below the spatial discretization) embedded in the solid phases. Multicomponent transport is modelled using a lattice Boltzmann (LB) method. The transport model is coupled with an established geochemical solver (iPHREEQC). Due to dissolution and precipitation reactions, both the explicit pore and the continuum domains are updated using, respectively, static update rules and constitutive relations. The model is applied to a virtual cement microstructure under the process of calcium leachinig. For calcium leaching in cementitious systems we further show the results where the chemistry is represented by a abstracted chemical model. This model abstraction allows for numerically more efficient simulations which allows for parametric studies. Finally, comparison between the simulated results and an experiment carried out under accelerated conditions is presented. @en