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.

Effect of Al anodizing waste on the final properties of porous geopolymers - Advances in Engineering

About the author

João Labrincha is Associate Professor at the Materials and Ceramic Engineering Department of the University of Aveiro. He is expert in ceramics processing. Following his PhD in Materials Science and Engineering (University of Aveiro, 1993) he implemented a research line on Wastes Recycling and Sustainable Use of Resources, teaching disciplines on related topics. He supervised 14 PhD and 54 MSc theses and has now 5 new students under progress.

João Labrincha participate/d in 24 R&D projects (9 as leader/responsible) and in 16 Contracts financed/in cooperation with industries, has 22 patent applications (two of those as International PTC), and 310 papers (SCI). He was cited more than 8700 times and has h = 49 (Scopus). Between December/2012-2016 he was member of the Operational Group #2 of the European Innovation Partnership on Raw Materials.

The main scientific interests include: (i) development of multifunctional eco-materials; (ii) waste-based products (geopolymers, cements and mortars, pigments) for a circular economy; (iii) novel photocatalytic materials active under visible-light.

About the author

L Senff Graduated in Brazil as Civil Engineer (1995 – 2000). Master’s in Material and Metallurgical Engineering was obtained from the State University of Santa Catarina, Centre of Technology Sciences, UDESC, Brazil (2002 – 2004). Afterwards, the PhD in Material and Metallurgical Engineering in the Federal University of Santa Catarina, UFSC, Florianopolis, Brazil (2005 – 2009). Title: Effect of Micro and Nanossílica addition on the rheological behavior and fresh and hardened state of mortars and cement paste. Area of scientific activity involves the development of cementitious materials using with particular emphasis on nanoparticles and rheology.

Regarding the academic and research activities at University, He serves as an Adjunct Professor at the Federal University of Santa Catarina and has experience in materials science and engineering, focusing on Experimental design (DOE), as a statistical methodology for formulation of mixtures, Nano and microparticles added on cement/lime mortars and Rheological characterization of reactive pastes and suspensions.

About the author

L.H. Buruberri has a Master’s degree in Materials Science and Engineering (2013) and a PhD in Materials and Science Engineering (2021), both from the University of Aveiro. Currently she is in the Research & Development Department of GRESTEL S.A. – a Portuguese company dedicated to manufacturing and designing tableware and accessories made of fine stoneware, in an original concept of quality, durability and exclusive design.

She worked on a European project, ECOSEE – ECO-innovative, Safe and Energy Efficient wall panels and materials for a healthier indoor environment, participated in 2 International Conferences and published 8 articles, some of them in high impact factor Journals, like Hazardous Materials, Cleaner Production and Construction and Building Materials.

Her current research interests include geopolymer processing, porous geopolymers, valorization of industrial waste and ceramic materials processing.

About the author

Maria Paula Seabra got her PhD in Materials Science and Engineering (University of Aveiro, 2004). She is Researcher at the Materials and Ceramics Engineering Department of the University of Aveiro and member of the Aveiro Institute of Materials (CICECO).

She has published ca. 127 SCI papers and 1 book chapter (h-index 30), and 3 patents (one international). Paula participate/d in 22 R&D projects (7 as responsible). Currently she participates in 5 ongoing projects and supervise 2 PhD and 2 MSc students.

Her main scientific interests are: i) Wastes recycling/valorisation – development of several products, namely, ceramic pastes, inorganic pigments, geopolymers and clinker from industrial wastes; (ii) Inertization of hazardous materials and (iii) Eco-building materials – development of innovative building materials for indoor air quality improvement and passive indoor environment control.


L.H. Buruberri, L. Senff, M.P. Seabra, J.A. Labrincha. Effect of Al anodizing waste on the final properties of porous geopolymersConstruction and Building Materials, issue 263 (2020), 120160.

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