Effect of Al anodizing waste on the final properties of porous geopolymers

Porous geopolymers have become increasingly popular in recent years because of their low thermal conductivity, excellent mechanical strength, high thermal and chemical stability, and ability to immobilize heavy metals. Due to these properties, porous geopolymers are tailored to suit various applications as membranes or catalysts, insulating materials, coatings, and molecular adsorbents and filters.

The direct foaming method is the most widely used porous geopolymers production technique. It involves adding a chemical foaming agent, for example, hydrogen peroxide or metallic powders of aluminium or silicon, during the stirring process. Difficulty in controlling pore size and distribution is one shortcoming of the direct foaming method for producing porous geopolymers. Also, the porosity of the obtained materials is mostly closed, therefore limiting some of their applications e.g. as filters and molecular adsorbents.

The use of commercial surfactants or thickening agents has been fronted as a possible solution to overcoming the shortcomings of the direct foaming method. According to literature, carbon fibers, Portland cement, proteins, cellulose fibers, and rice starch are the commonly used thickening agents.

Researchers L.H. Buruberri, M.P. Seabra, and J.A. Labrincha from the University of Aveiro in Portugal, collaborating with L. Senff from the Federal University of Santa Catarina in Brazil, produced porous geopolymers by the direct foaming method using aluminium powder as the foaming agent. Aluminium-anodizing sludge was added into the geopolymeric mixture to control porosity type and pore size distribution. For the first time, pore architecture control was realized using a waste – aluminium-anodizing sludge instead of the expensive commercial surfactants. The use of aluminium anodizing sludge reduced the coalescence phenomenon by increasing the viscosity of the mixture. Their work is currently published in the journal,Construction and Building Materials.

The authors used the direct foaming technique to prepare porous geopolymers with aluminium powder as the chemical foaming agent. Aluminium anodizing sludge was then added into the mixture for porosity control. The obtained porous geopolymers were then characterized in terms of porosity, pore morphology, apparent density, water vapor permeability, compressive strength, and thermal conductivity.

The addition of aluminium anodizing sludge into samples without aluminium powder didn’t significantly change the evaluated properties except the thermal conductivity that decrease with the aluminium anodizing sludge amount rise. As expected, the addition of aluminium powder increased the sample’s total porosity, but further increase induced a slight decrease in total and open porosity, and as a result, of the water absorption. Higher gas released and a tendency to form larger pores that coalesce and collapse are to blame for the inversion of the observed properties.

Simultaneous addition of aluminium powder and aluminium anodizing sludge helped obtained materials with higher porosity, and as a consequence, higher water absorption, lower apparent density, thermal conductivity, and compressive strength. Thermal conductivity is strongly affected by the material’s porosity, and as expected, a lower thermal conductivity value was recorded. The value was comparable to certain insulating materials such as foamed concrete, expanded clay, and reed board. Still, it was smaller than values found in literature for materials with similar porosity.

Irrespective of aluminium powder content, the addition of aluminium anodizing sludge always resulted in the decrease of water vapor diffusion resistance coefficient. Compressive strength is strongly affected by porosity. Therefore, the less porous samples (those prepared without aluminium powder) showed higher compressive strength. The obtained porous geopolymers exhibited low compressive strength, with the values tending to drop with porosity enhancement.

In summary, the addition of aluminium-anodizing sludge modified the viscosity of the geopolymeric paste and reduced the formation of large voids and pore coalescence. The obtained porous geopolymers had narrow pore size distribution and exhibited a more homogenous microstructure. The new approach reported in the study is indeed promising for the preparation of highly stable filters, lightweight structures, and insulating materials.

Acknowledgement: This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the Portuguese Foundation for Science and Technology/MCTES.