Silica aerogels are lightweight and highly porous materials, with a three-dimensional network of silica particles, which are obtained by extracting the liquid phase of silica gels under supercritical conditions. Due to their outstanding characteristics, such as extremely low thermal conductivity, low density, high porosity and high specific surface area, they have found excellent potential application for thermal insulation systems in aeronautical/aerospace and earthly domains, for environment clean up and protection, heat storage devices, transparent windows systems, thickening agents in paints, etc. However, native silica aerogels are fragile and sensitive at relatively low stresses, which limit their application. More durable aerogels, with higher strength and stiffness, can be obtained by proper selection of the silane precursors, and constructing the silica inorganic networks by compounding them with different organic polymers or different fiber networks. Recent studies showed that adding flexible organic polymers to the hydroxyl groups on the silica gel surface would be an effective mechanical reinforcing method of silica aerogels. More versatile polymer reinforcement approach can be readily achieved if proper functional groups are introduced on the surface of silica aerogels and then copolymerized with appropriate organic monomers. The mechanical reinforced silica aerogels, with their very open texture, can be an outstanding thermal insulator material for different industrial and aerospace applications. This paper presents a review of the literature on the methods for mechanical reinforcing of silica aerogels and discusses the recent achievements in improving the strength and elastic response of native silica aerogels along with cost effectiveness of each methodology.

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The experimental study herein presented was conducted aiming to evaluate the influence of nanosilica (nS) addition on properties of ultra-high performance concrete (UHPC). Thermo gravimetric analysis results indicated that nS consumes much more Ca(OH) 2 as compared to silica fume, specifically at the early ages. Mercury intrusion porosimetry measurements proved that the addition of nS particles leads to reduction of capillary pores. Scanning electron microscope observation revealed that the inclusion of nS can also efficiently improve the interfacial transition zone between the aggregates and the binding paste. The addition of nS also resulted in an enhancement in compressive strength as well as in transport properties of UHPC. The optimum amount of cement replacement by nS in cement paste to achieve the best performance was 3 wt.%. However, the improper dispersion of nS was found as a deterrent factor to introduce higher percentage of nS into the cement paste.

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Cross-linking of silica aerogels with organic polymers is an effective way to overcome their fragility and poor mechanical properties. It has also been shown that adjusting the nanoskeleton of silica aerogels using different silica precursors can lead to improved mechanical properties, mesoporosity and thermal conductivity. In this paper, the effect of the incorporation of 1,6-bis(trimethoxysilyl)hexane (BTMSH) and 1,4-bis(triethoxysilyl)-benzene (BTESB) into the underlying silica structure of tri-methacrylate crosslinked silica aerogels is examined. In order to attain a simple, cost and time effective procedure, the sol-gel process and addition of organic monomers are performed in one single step. The effect of the amount of silicon derived from alkyl-linked and aryl-linked bis-silane precursors as well as the effect of the cross-linker concentration on the mechanical strength, thermal conductivity, porosity and other properties of the synthesized aerogels are studied. Different reinforced silica aerogels with density range from 0.13 to 0.39 g cm À3 , compression strength from 11 to 400 kPa and thermal conductivity of 0.039-0.093 W m À1 K À1 were obtained. The cross-linked aerogels made by replacing 5-10 mol% of total silicon by BTESB showed a drastic improvement in their surface area and thermal insulation properties along with an increase in the mechanical strength. The surface area and thermal insulation improvements obtained for aryl-bridged polysiloxanes were attributed to the aryl spacer within the aerogels body, which leads to aerogel with high extent of porosity or pore volume when compared to the alkyl-bridged polysiloxanes. Therefore, for the first time we were able to show the dependency of thermal conductivity values on the silica structure by proper designing the mesoporosity of the resulting aerogels.