Thermal insulation materials are critical for reducing the energy consumption and carbon emissions associated with buildings. A good insulation material must not only have low density and sufficient mechanical properties but also resist high temperatures and fires. In addition, its production process must be simple and inexpensive. This study describes the production of very low density (>85 kg/m(3)) inorganic foams with high porosity (<94%). Silica fume and NaOH solutions are mixed to prepare a geopolymer paste that is cured in a laboratory oven at 60-100 degrees C, and expanded in a furnace at similar to 200-500 degrees C. The expansion mechanism and pore structures of the foams, as well as their physical, mechanical, and thermal properties were investigated with microscopy, spectroscopy, and measurement of compressive strength and thermal conductivity. Foaming of cured geopolymer pastes is due to loss of water vapor resulting from the condensation of silanol groups. Porosity increases and bulk density decreases with increasing NaOH concentration and higher furnace temperatures. The foams have 0.15-0.75 MPa compressive strength, and 0.04-0.10 W/mK thermal conductivity. Addition of 1-2% chopped basalt fibers increases strength of the low-density foams similar to 3-6 times while only doubling their densities. Microscopy reveals that the foams are made of irregular-shaped, fragmented cells, mu m to mm in size. (C) 2020 Elsevier Ltd. All rights reserved.