Due the increasingly stringent environmental regulations and sustainable development-induced pressures to increase efficiency, the world faces new challenges. For this reason, the production of structural materials with controlled properties has become urgent. Due to their excellent mechanical and chemical properties and energy-efficient production methods, geopolymers have remarkable potential in higher value-added applications. This potential application exceeds their historical use as environmentally friendly substitutes for Portland cement. The aim of the present research is to produce metakaolin-based geopolymer foams using the combined technique of saponification/peroxide decomposition/gel casting. Concerning physical properties, the effect of H₂O₂ concentration, the type and amount of stabiliser and the addition of slag were investigated. XRD, FTIR, SEM, CT and thermogravimetric analyses were performed to characterise the material structure. Multifunctional geopolymer foams were developed that can be made free of organic matter and cracks through the optimisation of technological parameters (heat treatment, washing and firing). Such high strength (8.9 MPa), lightweight (~499 kg/m³) and low thermal conductivity (~0.041 W/mK) foams can be used as thermal insulating materials to reduce the energy loss of buildings. Due to their high open porosity (~77.7 vol%) and specific surface area (~4.3 m²/g), foamed geopolymers can function as catalyst supports in many physicochemical applications.

Nowadays, the construction industry is challenged not only by increasingly strict environmental regulations, but also by a shortage of raw materials and additives. It is critical to find new sources with which the circular economy and zero waste approach can be achieved. Promising candidates are alkali activated cements (AAC), which offer the potential to convert industrial wastes into higher added value products. The aim of the present research is to develop waste-based AAC foams with thermal insulation properties. During the experiments, pozzolanic materials (blast furnace slag, fly ash, and metakaolin) and waste concrete powder were used to produce first dense and then foamed structural materials. The effects of the concrete fractions, the relative proportions of each fraction, the liquid/solid ratio, and the amount of foaming agents on the physical properties were investigated. A correlation between macroscopic properties (strength, porosity, and thermal conductivity) and micro/macro structure was examined. It was found that concrete waste itself is suitable for the production of AACs, but when combined with other aluminosilicate source, the strength can be increased from 10 MPa up to 47 MPa. The thermal conductivity (0.049 W/mK) of the produced non-flammable foams is comparable to commercially available insulating materials.

Due to their favorable production conditions and promising properties (e.g., low shrinkage after foaming, mechanical and chemical stability, high-temperature resistance), geopolymer foams are suitable for heat- and sound insulating refractory building materials. Another promising field of application may be their use as catalyst supports in water treatment. Metakaolin-based foams were prepared by a direct foaming process with high total porosity (> 75 vol%), low bulk density (< 500 kg/m3), approximately 1 MPa compressive strength, and low thermal conductivity (0.095 W/mK). By varying the concentration of foaming agent (H2O2 solution) and stabilizing agent (sodium oleate), it is possible to produce foams with designed porosity and pore size distribution. Foams with mainly closed pores are suitable for thermal insulation, while those with significantly open pores can use as catalyst supports. The computed tomography images showed that the concentration of stabilizing agent is a key parameter in forming a homogeneous pore structure and open pores; up to 24 vol% open porosity can be achieved without significantly affecting other properties. The physical properties of the foams are equally influenced by the thickness of cell walls and the size of the cells themselves.