Cold reaction sintering for preparation of ultra-dense geopolymer products

In the wake of climate change and global warming and their catastrophic impacts on planet earth, the need to reduce carbon dioxide emissions (CO₂) has been of great significance. Reducing CO₂ emissions is a collective effort that requires the input of all stakeholders. To date, a number of CO₂ reduction activities targeting high emitting industries have been developed. Among these industries is the cement industry, which accounts for about 7% of global CO₂ emissions.

Recently, geopolymers have attracted significant research attention for CO₂ reduction applications due to their contribution to carbon neutrality, which is better than traditional Portland-based cement manufacturing. Generally, the formation of geopolymers involves reacting amorphous aluminosilicate (mostly industrial wastes like fly ash) with alkaline solutions without using cement. Thus, their application in construction has attracted considerable interest. However, geopolymer exhibits low mechanical properties and takes relatively long (several days to a month) to cure. Thus, the development of effective and efficient hardening methods is highly desirable.

Currently, a method using a warm press is known for the production of geopolymer products as a higher strength and shorter time method than the conventional curing method described above. The hardening process involves two stages. In the first stage, a conventional curing method based on slurry is used to form the geopolymers followed by hardening and crushing. In the second stage, the fabrication of highly dense geopolymer is achieved via re-hardening with warm pressing together with the crushed geopolymer powder. Despite its ability to improve the densification and strength of the hardened products, this process is complicated. Thus, developing a simpler and more effective method is highly desirable.

Herein, PhD candidate Kosuke Nishikawa, Professor Shinobu Hashimoto and Mr. Haruo Imai from Nagoya Institute of Technology together with Professor Sylvie Rossignol from the University of Limoges proposed a new hardening method, named as cold reaction sintering, to prepare ultra-dense geopolymer products. This was a single-step method because both the hardening and reaction occurred simultaneously. The authors examined the applicability of hardening geopolymers made from solid starting materials like fly ash and sodium metasilicate hydrate using a warm press device, the physical properties as well as the formation mechanisms of the resulting geopolymer products. Their work is currently published in the journal, Construction and Building Materials.

In their approach, this method comprised used all solid starting materials in the powder form. Specifically, fly ash was used as the aluminosilicate and sodium metasilicate hydrate was used as the alkaline activator instead of sodium hydroxide solution. This meant no addition of aqueous solution or water because the hydrate supplied the water needed during the hardening process. During the cold sintering process, the amount of the supplied water was precisely controlled by the mixing ratios of sodium metasilicate hydrate (Na₂SiO₃·9H₂O) and sodium metasilicate anhydrite (Na₂SiO₃). The deformation behavior of the starting powder mixtures and geopolymerization during cold sintering were investigated.

The authors showed the possibility of achieving both geopolymerization and densification without requiring conventional slurry approach attributed to the benefits of using sodium metasilicate hydrate as an alkali activator. Under optimal conditions, the obtained geopolymers exhibited the desirable ultra-density and significantly improved compressive strength. For example, maximum compressive strength of 425 MPa was achieved when hardening for 10 mins with fly ash: Na₂SiO₃·9H₂O: Na₂SiO₃ (68:16:16 wt%, respectively) at a temperature of 130 °C and uniaxial pressure of 100 MPa. Furthermore, the hardening was obtained in a relatively shorter time than those obtained via conventional slurry methods.

In summary, the successful fabrication of geopolymer products from starting materials in powder via cold reaction sintering was reported. The free water released from the Na₂SiO₃·9H₂O played a crucial role in enhancing the geopolymerization and densification processes. The underlying geopolymer hardening mechanism was revealed and detailed. In a statement to Advances in Engineering, Professor Shinobu Hashimoto noted that the study provided useful insights that would contribute to developing high-performance geopolymers for use in the construction industry.