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A cement paste microstructure model has been combined with a simple dissolution model for CH leaching and a direct algorithm for computing diffusivities to study the effects of CH leaching on cement paste percolation and diffusion properties. The study has shown that leaching can increase the relative diffusivity by an order of magnitude or more, and change the connectivity of the capillary pore space of neat portland cement paste. Silica fume has been shown to be effective in minimizing these effects by reducing the calcium hydroxide content of cement paste. In designing cement-based materials whose diffusivity is relatively stable with respect to leaching of CH, the concept of a critical CH volume fraction + capillary porosity of about 18% has been developed. Depending on the w/s ratio and expected maximum degree of hydration, it has been shown that different amounts of silica fume will be required to produce a system in which this critical threshold is not exceeded. If this threshold is not exceeded, then any increase in the relative diffusivity of a paste due to leaching will be minimal. At a constant degree of hydration, improvements in reducing the detrimental effects of leaching on transport properties can be achieved by either reducing the w/s ratio or increasing the silica fume content up to but not exceeding the values at the minima in Fig. 8.
The results of this paper are believed to be at least qualitatively correct, as they are based on fundamental principles of cement paste microstructure and percolation theory. In order to quantitatively test these theoretical results, carefully controlled experiments need to be carried out. Cement pastes of various w/s ratios, incorporating various amounts of silica fume, could be prepared, and carefully monitored with respect to degree of hydration, not just age. If a fast electrical method of measuring diffusivity, like AC electrical conductivity measurements, were to be used [3,16], in combination with quantitative x-ray or SEM analysis to measure CH remaining after leaching, the experimental equivalents of Table 1 and Fig. 6 could be produced and compared to the theoretical predictions.
This study demonstrates the need in mix design to consider not only the structural design requirements for a cement-based material, but also the durability design requirements for a material to be exposed to the intended in-service environment. Failure to allow for durability design requirements results in materials that may initially serve their intended purpose, but which can then fail due to degradation process-induced microstructural changes.