Thermal resistance of argillite based alkali-activated materials. Part 2: Identification of the formed crystalline phases
Graphical abstract
Introduction
Currently, fire resistant materials are required for various applications and domains (aerospace, construction and building materials, refractory materials, foundry, etc.). Geopolymer or more generally alkali activated materials are known to have good thermal stability [[1], [2], [3]]. However, due to the variety of the potential used raw materials (metakaolin, fly ash, blast furnace slags, industrial co-products, etc) and the variability of their chemical compositions, the thermal resistance may change considerably from one material to another. The used alkali cation influences also the thermal behaviour. It was demonstrated that the viscous sintering onset temperature, regardless of the Si/Al ratio, decreases when using sodium or mixed alkali cation (sodium and potassium) compared to potassium [2,4]. Indeed, the thermal behaviour is linked to structural variation and phase transformation, which are controlled by thermodynamics, ie, the most thermodynamically stable phases will be formed with the increase of the temperature [2]. For example, Buchwald et al. [5], have demonstrated that the thermal behaviour of sodium-based geopolymer is controlled by the dehydration at a temperature below 400 °C and the transformation of the geopolymer network to nepheline between 850 and 950 °C. In the case of potassium based geopolymers, the thermal behaviour is related to their recrystallisation to feldspars, leucite and kalsilite at a temperature of 1000 °C [4].
Other authors [6] have studied the thermal behaviour of fly ash based geopolymers using sodium and potassium based alkaline solutions. Sodium-based materials exhibit a drastic decrease of strength at 800 °C in relation to an increase of the average pore size. The amorphous structure is transformed to crystalline Na-feldspars. However, for potassium-based materials an increase of strength until 1000 °C was evidenced. These facts were explained by the decomposition of the geopolymer leading to the presence of free Na, K, Si and Al. This decomposition is favored using Na cation which lower the melting temperature. Moreover, it has been proven that in a multicomponent melt, silicium will complex in preference with K > Na > Ca > Mg [7]. The thermal behaviour also depends on the thermal treatment conditions such as the reached temperature, heating rate and exposure time. A rapid rise of the temperature was evidenced to induce an important thermal gradient between the surface and the core of the sample leading to stresses, cracks and deterioration of the material [8]. That is why, many authors [9] focused on the resistance to thermal shock. For instance, Nazari et al. [10], have studied fly ash based alkali-activated materials after thermal shock at 1000 °C. They prove the formation of zeolite.
The existing literature generally focusses on the evaluation of the thermal resistance and mechanical behaviour after temperature exposure but there is a lack of knowledge concerning the structural variation understanding, which is responsible of this behaviour especially in complex systems (raw materials composition, mixed alkali cations, etc). In previous work [11], alkali activated materials based on COx argillite which conserve or even increase their mechanical properties after heat treatment were successfully obtained. In the present work, the motivation of this study is not only the valorization of argillite and the obtaining of fire resistant materials, but, also a further understanding of the structural evolution leading to this thermal behaviour. For this, the structural changes in argillite based alkali activated materials under different thermal treatment conditions will be examined. Thus, this study will contribute to the transition to the industrial scale and the widespread of these materials as fire resistant binders.
Section snippets
Raw materials and sample preparation
The samples are prepared using two aluminosilicate sources and two different alkaline silicate solutions. The aluminosilicate sources are argillite calcined with furnace (Ac) or flash process (Af) [12]. The first alkaline solution, denoted as S, is based on an aqueous solution of potassium silicate and potassium hydroxide (63 wt % of water and Si/K molar ratio = 0.58). The second alkaline solution, P, is based on a sodium silicate powder, water and potassium hydroxide (58 wt % of water and Si/M
Structural changes after thermal treatments
It has been previously shown that the alkali activated samples based on various aluminosilicate sources (Ac and Af) and alkaline solutions (S and P) present different resistances to thermal treatment and to thermal shock at 800 °C [12]. The physical behaviour of the different samples to the thermal treatments (slow or fast increase of the temperature) are reported in Table 2. On one hand, all samples exhibit good thermal resistance to calcination at 800 °C (dimensional stability and no or small
Conclusion
Thermal behaviour of argillite based alkali-activated materials was investigated. The studied samples were synthesized using thermally heated argillites with furnace or flash processes at 750 °C and two alkaline solutions based on potassium or on a mix of potassium and sodium. Regardless of the sample, a good resistance to thermal treatment at 800 °C (ramp 1 °C/min) was obtained (1≤σafter/σbefore ≤ 2.2). However, all samples exhibit low resistance (σbefore/10) to thermal shock (800 °C, 10 min).
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