A porous geopolymer based on aluminum-waste with acoustic properties

Paval, a solid waste stream from the aluminum industry, is used as a pore generation agent in geopolymers. Paval was mixed with coal combustion fly ash, as a geopolymeric precursor, and activated with alkaline solution with the aim of obtaining porous geopolymers to be used as noise barriers. Both geopolymeric and pore generation reactions happen simultaneously. Aluminum from Paval can react with water and OH¯ from the geopolymerization activating solution, producing hydrogen. The hydrogen gas released generates a highly porous material. The influence of the fly ash-paval proportion and the setting temperature on open porosity, compressive strength and noise-absorbing properties were evaluated. To better understand these influences, the setting time, volume expansion and mineral composition were also studied. The obtained results showed that a higher Paval content (fly ash-Paval ratio 50:50) and setting temperature (70 °C) produced a lower setting time and higher volume expansion, increasing the open porosity and improving acoustic properties, but reducing the compressive strength. The material manufactured under these conditions showed similar amorphous phase content to the non-porous geopolymers made without Paval. On the other hand, the obtained materials did not raise environmental concerns in a normalised leaching test.