 |
As mentioned above, hydrating cement paste is a material rich in percolation phenomena. The microstructure model can provide quantitative information on the evolution of these percolation processes during hydration. Figure 4 shows this evolution with degree of hydration for the two major percolation processes of interest: percolation of total solids (set and strength development) and depercolation of capillary porosity (transport properties). In this case, an initial starting microstructure for a typical OPC was created with a w/c=0.4 and all of the cement particles flocculated into a single floc structure, simulating the absence of any sort of dispersing agent (water reducer or superplasticizer). For set, two cement particles are considered percolated only if they are connected by hydration product (calcium silicate hydrate gel (C-S-H) or ettringite) and not simply if they are touching (weaker bonding forces) [41]. For this flocculated system, only about 2% hydration is required to achieve set, in agreement with experimental measurements of shear resistance [42]. For a higher w/c value, 0.5, 3% hydration is needed to achieve set, as the cement particles are more separated at the higher initial w/c ratio.
|
The other key percolation aspect of hydrating cement pastes is the depercolation of the capillary porosity. In Figure 4, one can see that the capillary porosity is highly connected throughout the early and middle stages of the hydration and then quickly loses connectivity and disconnects at about 70% hydration. This corresponds to a capillary porosity of about 21% in agreement with the values of 19-26% based on experimental measurements of permeability by Powers [43]. Previous results have shown that this percolation threshold on the order of 20% capillary porosity is independent of starting w/c ratio [27]. Thus, for a starting w/c ratio high enough such that, even at complete hydration, the capillary porosity exceeds 20% (e.g. w/c > 0.6) depercolation will never be achieved. Once the capillary porosity depercolates, transport processes will proceed at a much lower rate, as the primary pathway for transport will shift from the capillary pores to the much smaller gel pores present in the C-S-H.
Based on these percolation results and direct simulations of the electrical conductivity of model cement pastes, an equation relating diffusivity to capillary porosity has been developed and validated for OPC pastes [44] and used in a multi-scale modelling approach to develop an equation that predicts the chloride ion diffusion coefficient of concrete as a function of mixture proportions [45].