Regulating chemical foaming reaction to control porosity of geopolymer foams

Significance Statement

Geopolymer concretes are appearing as environmentally friendly construction materials as compared to the conventional cement-based concretes. The production of geopolymers is not as capital and energy intensive as ordinary cement manufacturing processes and if their manufacture is controlled, geopolymers can curtail carbon-dioxide emission.

Foam concrete or cellular concrete is a lightweight concrete with air voids trapped in its matrix making it light and sustainable by reducing its embodied energy. Cellular concretes come with a number of benefits such as, reduced construction cost, and improving thermal and acoustic insulation. They have also been applied as nonstructural elements such as in-situ or precast walls, facades, insulation screeds and slabs. Geopolymers and alkali-activated binders have also been applied in making foam concretes in a bid to offer environmentally construction materials compared to the Portland cement-based concretes.

Mechanical as well as chemical foaming has been identified to introduce air voids within the concrete matrix. However, chemical foaming seems to give better results with regards to dropping density. The two methods can however be combined to bring together their benefits. Aluminum metal powder has been applied widely in lightweight concrete as a foaming agent. Targeting low densities in geopolymer concretes, higher percentages of aluminum powder has been used raising environmental concerns.

It is therefore necessary to explore efficient methods towards geopolymer formulation with appreciable porosity and low densities without extensive aluminum powder use. Australian researchers Ailar Hajimohammadi, Tuan Ngo, and Priyan Mendis at University of Melbourne and in collaboration with Jay Sanjayan at Swinburne University of Technology synthesized geoploymer foams with varying porosities with the same amount of aluminum foaming agent. They also implemented manipulated amounts of alkaline activators well known as accelerators and inhibitors of the aluminum reaction. Their research work is now published in Materials and Design.

The authors prepared two alkali-activating solutions by dissolving sodium metasilicate and sodium hydroxide in water. To prepare the geopolymer foams, they activated fly ash by alkali solution with varying ratios of the alkali activators. The geopolymer mixture was mixed and a similar amount of aluminum powder added as a foaming agent.

The foamed pastes were poured into cubic molds and cured at 60 °C for 24h. The cured samples were then removed and kept at room temperature ready for testing.

Sodium silicate and sodium hydroxide solutions were alkali activators and acted as an inhibitor and a catalyst, respectively. Therefore, by manipulating the ratios of the two alkali activators, the authors were able to regulate foaming extent. They realized that the total porosity was increased without adding extra amount of aluminum powder. High aluminum oxidation led to formation of larger voids in the matrix.

Reduced circularity of the voids was observed by increasing porosity. Wider size pore distribution was observed in samples with higher aluminum reaction. Formation of more fine voids was responsible for density reduction in the samples. Increasing sodium hydroxide to sodium silicate solution ratio affected the nanostructure of the binding skeleton. The amount of unreacted fly ash was noted to reduce in the sample with a higher ratio.

“Aluminium metal powder which is widely used in the lightweight concrete industry as an aerating agent has very high embodied energy, and its extensive usage in foamed geopolymers will diminish the environmental benefits of geopolymeric materials” Dr. Hajimohammadi said “The developed method can successfully increase the porosity of geopolymers and drop their density without adding more aluminium powder to the mix design. This has significant impact on commercial applications of foamed geopolymers and remarkably improves the energy rating of the products.”

Regulating chemical foaming reaction to control the porosity of geopolymer foams-Advances in Engineering

About the author

Dr Hajimohammadi is a postdoctoral research fellow in the Department of Infrastructure engineering at the University of Melbourne.  She received her PhD from The University of Melbourne supported by many prestigious scholarships and awards. Her PhD was focused on development of the new class of sustainable construction materials known as ‘geopolymers’. Her fundamental studies on geopolymers provided a new important structural insight into these amorphous heterogeneous materials, and her research instantly sparked wide interest in the research community.

Her research have been cited 229% more than the global average in this field (SciVal data for Field-Weighted Citation Impact). The methodological and technical developments she made in her PhD and postdoctoral research put a significant step forward in the custom design of geopolymer materials which is vital for optimising their performance and for enhancing the prospects of their wider commercial adoption.

Dr Hajimohammadi has taken up a postdoctoral position at the University of Melbourne working on an ARC-Linkage Project, where she collaborated with the industry partner to bridge the gap between experimental research and practical applications of geopolymers.

About the author

A/Prof Tuan Ngo is the Director of the Advanced Protective Technologies for Engineering Structures (APTES) Group at the University of Melbourne. He is also the Research Director of the ARC Centre for Advanced Manufacturing of Prefabricated Housing and the Leader of the CRC-P for Innovative Prefabricated Building Systems. A/Prof Ngo has played a key role in the PrefabAUS, a peak industry body, involving more than 180 companies in the prefabricated manufacturing of modular buildings.

He has carried out significant research collaborations with industry and government agencies to perform large-scale experiments of prefabricated modular components of building structures (facades, wall panels and floor slab systems). He is leading the ARC Centre for Advanced Manufacturing of Prefabricated Housing funded by ARC and industry. The national and international research interactions have given A/Prof Ngo widespread recognition amongst the scientific community and industry as an expert in design and manufacturing of building components and systems using high-performance materials.

About the author

Prof Priyan Mendis is a Professor in the Department of Infrastructure Engineering. He is one of world’s leading expert in structural design especially tall buildings and concrete structures.
Prof Mendis is world’s leading expert in analysing structures for extreme events such as wind, fire, blast, earthquakes, etc. His work related to High-strength/High-performance concrete design has been used in many world’s tallest buildings.

Prof Mendis is a leading authority in sustainable development, and he leads the Advanced Protective Technology of Engineering Structures Group at the University of Melbourne. He was the Convener of the ARC Research Network for a Secure Australia (RNSA) from 2004 -2011, and currently, he is a member of ARC College of Experts and the managing director of the ARC training centre ‘Centre for Advance Manufacturing of Prefabricated Housing (CAMPH)’. The CAMPH aims to unlock the potential for growth of Australia’s prefabricated building industry by developing and enabling the next generation of engineers and architects.

About the author

Prof Jay Sanjayan is the Director of Swinburne’s Centre for Sustainable Infrastructure and Professor of Concrete Structures. His current projects include 3D printing of concrete, low carbon concretes, geopolymer concretes, use of phase change materials in construction and monitoring the performance of geopolymer concrete in the field.

He has published extensively in these topics and is also involved in a number of consulting projects related to concrete materials technology and structural design. In 2015, RILEM (International Union of Laboratories and Experts in Construction Materials, Systems and Structures) appointed Professor Sanjayan Honorary President and he also served as Chairman of the Concrete 2015 conference held in Melbourne. He has also chaired the editorial board of Concrete in Australia and was a member of the editorial board of Construction Materials Journal and Materials and Structures.


Ailar Hajimohammadi1, Tuan Ngo1, Priyan Mendis1, Jay Sanjayan2. Regulating the chemical foaming reaction to control the porosity of geopolymer foams. Materials and Design 120 (2017) 255–265.

Show Affiliations

1 Department of Infrastructure Engineering, University of Melbourne, Victoria 3010, Australia.
2 Faculty of Science, Engineering and Technology, Swinburne University of Technology, Victoria 3122, Australia. 


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