Improving the microstructure of ITZ and reducing the permeability of concrete with various water/cement ratios using nano-silica

The interfacial transition zone (ITZ) between an aggregate and a cement matrix is known to be the weakest component of concrete; hence, its microstructure is the key factor determining its performance in terms of permeability. The objectives of this study are to strengthen the ITZ and reduce the permeability of concrete using nano-silica, and explore its improvement features under different water-to-cement ratios (W/Cs) between 0.3 and 0.5. An innovative testing and characterization methodology is adopted to quantitatively evaluate the properties of the ITZ. The results indicate that nano-silica can reduce both the width and formation of abrasion cracks in the ITZ. Further, the role of nano-silica is especially effective when the W/C of concrete is reduced. Small hydration products growing on the surface of the aggregate and cement matrix of nano-concrete with a reduced W/C can overlap each other and effectively fill the gap, resulting in enhanced densification of the ITZ by the nano-silica at reduced W/C ratios. Further, the enhancement rate of the anti-permeability of concrete also becomes significant. It can be concluded that nano-silica can effectively improve the performance of concretes, especially those with reduced W/C.