Thermally insulating lightweight cement-based composites incorporating glass beads and nano-silica aerogels for sustainably energy-saving buildings

•Glass bead (GB) and nano-silica aerogel (NSA) were used to fabricate GB-NSA cement-based composites;•The thermal conductivity of the GB-NSA composites is as low as 0.08 W/mk;•Increasing NSA augments total porosity, specific surface area and intensity of PSDs;•A hybrid use of GB and NSA is more rational to retain material strength;•The findings of this study help develop new cement-based materials for sustainably energy-saving buildings. [Display omitted] Thermally insulating lightweight cement-based composites with excellent mechanical properties are urgently needed for the development of sustainably energy-saving buildings. Herewith two types of thermally insulating admixtures, glass bead (GB) and nano-silica aerogel (NSA), were used as the coarse and fine fillers, respectively, to fabricate thermally insulating cement-based composites (GB-NSA). The chemical components, micro-and-pore structures, mechanical and thermal properties of the GB-NSA composites were investigated by various experimental tests comprehensively. The results indicate that NSA can accelerate the hydration of cement and the carbonation of calcium hydroxide due to the loosely compacted matrix microstructure. The GB-NSA composites show complex and multi-scaled material structures due to the distinct characteristic sizes of GB, NSA and cement particles, and their interactions after cement hydration. Increasing the NSA content makes the material matrices more loosely compacted and the fracture surfaces much rougher due to the highly porous structure of the NSA particles and agglomerations, and the local W/C promotion supplied by the adsorbed water in their surfaces and pores. As a result, increasing the NSA content decreases the density and augments the total porosity, specific surface area and pore size distribution of the GB-NSA composites. Due to the fact that NSA weakens the matrices and interactions between the matrices and GB spheres, both the compressive and flexural strengths decay with the NSA percentage, and change linearly against the density and porosity. Also the thermal conductivity varies linearly to the density and porosity, but decays exponentially to the BET specific surface area. A strategy of incorporating NSA and other fillers in cement matrix may be an appropriate and effective way to design new types of thermally insulating cement-based composites for sustainably energy-saving buildings.