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Simulation of Leaching

Once a hydrated microstructure has been created, the calcium hydroxide is dissolved or leached away in a number of discrete cycles. After each leaching cycle, the connectivity of the capillary pore space and the diffusivity are computed, as described in the next subsection. This allows the determination of these material properties as a function of the amount of CH which has been leached away. In the model, the smallest capillary pore is one pixel in size. That means that all capillary pores in the model are no smaller than 1 µm, since the scale of the model is such that one pixel edge length is approximately equal to one µm.

The leaching process is modeled as a random dissolution process as shown schematically in two dimensions in Fig. 2. For each leaching cycle, the 3-D microstructure is scanned to identify all elements of CH which are in contact with pore space. All elements for which one or more of their six nearest neighbor pixels are pore space are identified during this scan. Following this identification, the microstructure is scanned once more with all elements identified in the first scan attempting to execute a one-step random walk to one of their six neighboring sites. If the step lands in a pore space pixel, the element of CH is dissolved and removed from the model. Otherwise, the element remains in place as CH. In this way, the CH dissolution process is equivalent to the dissolution of the C₃S which occurs during hydration.

Figure 2: Schematic diagram of the calcium hydroxide leaching model.

Since the CH elements that dissolve increase the capillary porosity, the dissolution process is likely to affect the percolation and diffusion properties of the pore space of the material. In this paper, the pore product material is treated as being made up entirely of leachable CH. For a portland cement paste, there are also aluminate phases, among others, that form as pore products. As long as any of these phases are more soluble than C-S-H, they will leach out before the C-S-H does, so that the analysis of this paper will still apply.