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CH percolation: Leaching

We can now combine the percolation and diffusivity simulations and ideas to study the problem of CH (pore product) leaching in cement paste. As concrete is exposed to the elements, its underlying microstructure can be attacked by a variety of aggressive agents. For example, rainwater and groundwater can degrade the concrete by dissolving soluble constituents such as calcium hydroxide. Using computer simulation, we can study the effects of calcium hydroxide dissolution on: 1) the percolation properties of the capillary pore space, and 2) the relative ionic diffusivity.

The microstructure model shown in Fig. 3 for C₃S cement paste was used to produce a hydrated specimen which was subsequently subjected to the leaching process in which CH was randomly removed. This specimen can start with either a percolated or a disconnected capillary pore space. From a percolation perspective, the key point is the connectivity of the combined capillary porosity and calcium hydroxide phases. If these two phases together form a connected pathway across the microstructure, the capillary porosity will certainly be percolated when the calcium hydroxide is leached away, regardless of its initial percolation state. Based on computer simulation, a volume fraction of 16-20% for the combined capillary porosity and leachable calcium hydroxide phase is sufficient to form a percolated pathway after leaching has taken place [37].

The computer models have also been applied to computing the increase in diffusion coefficients due to the leaching of calcium hydroxide from fully hydrated C₃S pastes of various w/c ratios [37]. The removal of all the CH from a cement paste can result in a multiplicative increase in diffusivity of up to about 50 times that of the original paste, as computed from the model, if the unleached paste had a disconnected capillary pore space and leaching re- connected it. In Figure 10, model values are compared with experimental results [38] in which the diffusion coefficients of cesium and tritium were measured as a function of the amount of CH leached out of portland cement paste specimens. Reasonable agreement is exhibited between the model and the available experimental results.