Very low w/p values are attained by the particle packing attributes of MS, whereby very fine silica particles pack between the much coarser cement grains. By using large quantities of a dispersing agent (usually a superplasticizer), a low water demand is achieved. The particle packing can be further optimized to get a more ideal packing.

A microsilica/cement ratio of 0.45 is needed for a complete reaction. Initially CH is formed as a reaction product before the pozzolanic reaction has advanced but is reduced to very low levels. Large massive crystals of CH are not observed, the microstructure is very uniform in contrast to regular cement paste. Only about 50% of the cement is able to hydrate because of the low w/c used. The pore structure is also quite different. Mercury intrusion porosimetry curves shown in Figure 15.7 indicate a very low pore volume accessible to mercury. This is a good reason to believe that no capillary pores are present in DSP.

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