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Effect of silica fume addition

Since CH is more easily dissolved than the other phases in cement [1], the addition of a pozzolan such as silica fume will tend to reduce the effects of leaching simply by reducing the CH content of the cement paste below the 20-25% volume fraction typically found in fully hydrated specimens [15]. To investigate the effects of silica fume addition on leaching and changes in material properties, 5 and 10% by weight of the cement was replaced with silica fume while maintaining a constant water-to-solids (w/s) ratio of 0.45.

The effects of the addition of silica fume on the values of the relative diffusivity produced during leaching for the 0.45 w/s ratio pastes are shown in Fig. 6, plotted against capillary porosity. The 0.45 w/c ratio neat portland cement paste data from Fig. 5 is put on the graph for purposes of comparison. The main effect of silica fume is to reduce the amount of CH available to be leached, which reduces the overall increase in that is possible due to leaching. Thus, the addition of silica fume limits the effects of the leaching process by maintaining the relative diffusivity in the flat part of the curve in Fig. 6, not allowing it to continue into the sharply increasing part of the curve. The 10% silica fume addition is particularly effective, since the capillary pore space never re-percolates during the leaching process, as after hydration, the CH and capillary pore space together do not form a spanning cluster across the 3-D microstructure.

Figure 6: Relative diffusivity D/D_o vs. capillary porosity for systems with w/s = 0.45 and variable silica fume contents, subjected to leaching.

The reduction of the effects of leaching on diffusivity that is achieved by adding silica fume to cement paste may be even more pronounced in mortar and concrete. In these composite systems, the silica fume addition will greatly reduce the capillary porosity of the interfacial zone by consuming the large crystals of CH and replacing them with a higher volume of secondary C-S-H [11]. Simulation studies have shown that a 10-20% by weight silica fume addition can produce a nearly homogeneous distribution of C-S-H (primary plus secondary) throughout the microstructure [11], including the interfacial zone. It should be noted that there is some concern as to the stability of the C-S-H phase and the depassivation of the steel reinforcement bars in systems lacking enough CH to buffer the pore solution and maintain a high pH [1]. This could be a problem for underground structures exposed to acidic groundwater. Fortunately, more moderate silica fume additions, such as 4-10%, which eliminate only a portion of the CH phase, may still be adequate to reduce the effect of leaching on diffusivity in conventional, 0.3-0.5 w/s ratio, cement paste systems. This point will be discussed further below.