I am ESR 11, He Niu, in University of Oulu. I am really happy to write my second blog since posting my last blog about my home town early this year. This time I would like to give a scope of my research about inorganic polymer and use of mine tailings as the precursors. Let us see the picture below, which give the mine tailings precursors and the end product of inorganic polymers. The blog is to some extent reflecting part of my resent research; however, I would like to simply explain the general professions.
Let us start with Q&A frame throughout this blog.
What is inorganic polymer?
The inorganic polymer is opposite to organic polymer which is also a three-dimensional framework structure. In the book named Geopolymers, edited by John L. Provis and Jannie S.J. van Deventer, indicates that inorganic polymers attracts fewer citations than their importance deserve 1. They use this terminology interchangeably with ‘geopolymer’ throughout the book, which is a subset of alkali-activated materials. Then, another question appears that what is alkali-activated materials? when giving another term to you, you may fell more familiar: Portland cement (high Ca), which is another subset of alkali-activated materials. Cement is overwhelmingly used throughout our life such as buildings, roads, railway sleepers and bridges. We cannot imagine the life without such important substances; however, one thing we need to concern is that cement industry contributes to huge amount emission of carbon dioxide. Therefore, inorganic polymers came to the public attention due to its environmentally friendly and sustainability. Now, it is time to go some deeper into the structure of inorganic polymer.
The term ‘geopolymer’ was coined in the 1970s by the French scientist and engineering Prof. Joseph Davidovits, who successfully synthesized solid materials by reacting aluminosilicate powder (e.g. metakaolin) with the alkaline solution (e.g. sodium hydroxide or sodium silicate) 2. From a micro view, the negatively charged alumina units and silica units are balanced by sodium cations, forming a three-dimensional framework. Alkali-activated metakaolin geopolymers is a useful ‘typical system’ for geopolymer research. The Fig 1 shows the 1:1-layer lattice structure of kaolinite, in which aluminium octahedral sheet is oxygen bridged with silicon tetrahedral sheet and this double layer is repeated stacking along major principle axis. When the alkaline activator is introduced, the reaction between aluminosilicate source and alkaline solution starts (Fig 2). The highest temperature of geopolymer manufacturing can be lower than 100 °C which is quite energy efficiency compared with Portland cement industry.
How mine tailings become inorganic polymers?
As we all know that the mine tailings from SULTAN project are from three different site including Neves Corvo containing muscovite in Portugal, Plombières containing calcite in Belgium and Freiberg containing muscovite in Germany. The mine tailings are also aluminosilicate rich, thereby they can be potential candidates for generating inorganic polymers according to the proceeding description. Furthermore, the cumulative storage of mine tailings in those three sites is practically over several million tonnes. Those mine tailings are currently underutilised all over the world! Apart from the considerable storage of tailing waste, it also causes tremendous environment issues such as acid mine drainage and air contamination. (If you are not familiar with the Acid Mine Drainage, please follow the link to Acid Mine Drainage: Let that mine flood) Therefore, the valorisation and recycling of mine tailings including hazardous metal immobilization appeals extensive public attention. Nevertheless, the mine tailings are mostly chemically inner, therefore, the pre-treatment is necessary before alkaline activation. The common way to activate precursors can be thermal treatment and mechanical treatment. In order to maximize the local availability and versatility, it is significant to develop the utilization of mine tailings especially in the section of sustainable construction materials 5.
What are the traditional methods for the pre-treatment of mine tailings?
Here we will take a typical case kaolinite to introduce heat treatment for minerals in mine tailings. As described in the Figure 1, in which the hydroxyl removal happened becoming metakaolin after calcination of kaolinite at 750 °C. the original 6-fold coordination of aluminium transferring to a combination of 6, 5, 4-fold coordination6. Thereafter, the conceptual diagram in Figure 2 can explain how metakaolin reacts with sodium hydroxide forming a dense and hardened solid state.
2) Mechanical activation
Mechanochemical treatment, more accurately, induces plastic deformation and internal stress resulting in high kinetic energy, during which the materials experience the transformation from crystalline to amorphous state i.e. the structure of mineral disrupted. Crystalline state means a stable state, while amorphous one means an unstable state, which means the minerals can be easily attacked by caustic liquids such as sodium hydroxide solution to promote geopolymerization. In the Figure 3, we can clearly see the difference between crystalline and amorphous structure which shows ordered and disordered structure respectively. Basically, the mechanical activation has the same effect as heat treatment aiming to disrupt the structure of minerals in mine tailings.
My current work is to make those crystal mine tailings amorphous. Let’s see there may be some interesting things happening.
- Provis, J. L. & Van Deventer, J. S. J. Geopolymers: structures, processing, properties and industrial applications. (Elsevier, 2009).
- Davidovits, J. Inorganic polymerie new materials. 24.
- Grim, R. E. Clay mineralogy. vol. 76 (LWW, 1953).
- Duxson, P. et al. Geopolymer technology: the current state of the art. J. Mater. Sci. 42, 2917–2933 (2007).
- Provis, J. L. Alkali-activated materials. Cem. Concr. Res. 114, 40–48 (2018).
- Chapter 6 – 17O NMR. in Pergamon Materials Series (eds. MacKenzie, K. J. D. & Smith, M. E.) vol. 6 333–395 (Pergamon, 2002).