At present there is a pressing need to find an environmentally friendly grouting material for the construction of tunnels. Silica nanoparticles hold great potential of replacing the organic molecule based grouting materials currently used for this purpose. Chemically, silica nanoparticles are similar to natural silicates which are essential components of rocks and soil. Moreover, suspensions of silica nanoparticles of different sizes and desired reactivity are commercially available. However, the use of silica nanoparticles as grouting material is at an early stage of its technological development. There are some critical parameters such as long term stability and functionality of grouted silica that need to be investigated in detail before silica nanoparticles can be considered as a reliable grouting material. In this review article we present the state of the art regarding the chemical properties of silica nanoparticles commercially available, as well as experience gained from the use of silica as grouting material. We give a detailed description of the mechanisms underlying the gelling of silica by different salt solutions such as NaCl and KCl and how factors such as particle size, pH, and temperature affect the gelling and gel strength development. Our focus in this review is on linking the chemical properties of silica nanoparticles to the mechanical properties to better understand their functionality and stability as grouting material. Along the way we point out areas which need further research.

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With the rapid development of nanotechnology, there is urgent need of characterizing techniques; especially determining the particle size distribution directly from solution. Dynamic light scattering is often used but presence of a small number of aggregates can greatly influence the size distribution. Electrospray scanning mobility particle sizer (ES–SMPS) is rapidly emerging as an alternative method in colloidal science. However, a major limitation is the use of silica-coated capillaries, which are negatively charged at pH > 3, and therefore making its use difficult for positively charged nanoparticles. In this work, we have developed the polyether ether ketone (PEEK) capillary for ES–SMPS, which removes this limitation because it carries no charge. We have shown that the new capillary not only produced equally good particle size distributions for negatively charged particles (SiO 2 , Au, and latex) as obtained with silica capillaries, but also precise particle size distributions for positively charged particles (TiO 2 ). Moreover, the PEEK capillaries are much cheaper than the silica capillaries. Thus, the results shown in this paper strengthen the development of the ES–SMPS method as a versatile method for determining the particle size distributions of colloidal sols directly from solution.

Aqueous silica nanoparticle suspensions are widely available and used within a number of industries. A relatively new area of application is as a grouting material for sealing narrow fractures in tunnels. While silica sols/gels have been used successfully to grout sections of tunnels the continued reliability of the grouting requires knowledge of gel formation, long