Welcome to the Secondary Resources for Engineered Materials, SREMat, and the section dedicated on work performed in Inorganic Polymers (geopolymers).
Background and motivation
Inorganic Polymers are materials generally rich in aluminosilicates, conventionally formed by alkali hydroxide or alkali silicate activation of a solid precursor. In literature different names have been used to describe these materials, namely "inorganic polymer glasses", "alkali-bonded ceramics", "soil cements" and more. Usual precursors are coal fly ash, calcined clay, metallurgical (blast furnace and steel) slag and more. Their synthesis involves dissolution of the solid source by alkaline hydrolysis, gelation and finally rearrangement and reorganization, to a three-dimensional network. The Inorganic Polymer is typically XRD amorphous and can have different coordination/structure for Al and Si. A subset of the inorganic polymers is defined as "geopolymers". In this case, the binding phase is almost exclusively aluminosilicate, containing tetrahedral Al and tetrahedral Si.
The first objective is to understand the microstructure-reactivity relationship. For this cause, the effect of chemistry and mineralogy of different inorganic polymer precursors is being evaluated. The intention is to investigate the role of other elements besides Al and Si, and to correlate structural characteristics of precursors with potential to synthesise inorganic polymers. Experimentally, the above is addressed by producing precursors at different cooling rates. This affects the solidification path and the structure of the resulting material, demonstrating variations both in the nature and the amount of crystalline and amorphous phases. Ultimately, this affects the dissolution behaviour and the synthesis of inorganic polymers. Importantly, although there is consensus that the content of amorphous phase is significant in the synthesis of inorganic polymers, a clear correlation has not been established and it appears that the structural heterogeneity within the glass is also a decisive factor.
A second objective is to understand the synthesis kinetics and microstructural evolution of inorganic polymers. Inorganic polymers typically exhibit very fast setting. This characteristic, apart from limiting the analytical possibilities, is also linked to the presence of (high) internal stresses and possible crack formation. To study the above, in-situ and ex-situ experimental set ups are used to study the interaction of the different precursors with variable solutions of MOH (M: alkali metal, K or Na) and M-silicate. The combination of high analytical power on specifically synthesised precursors in terms of chemistry and mineralogy allows the detection of incremental changes and provides a coherent image of the occurring transformations. The use of different activating solutions also facilitates the detection of the synthesis steps and clarifies the kinetics of the incubation, dissolution and network formation periods. The control of synthesis kinetics will permit a gradual microstructural evolution, avoiding the build up of high internal stresses and crack formation, and ultimately will deliver materials with superior properties. Controlling the speed of setting is yet an important contribution to longevity and durability of the material.
Machiels, L., Arnout, L., Jones, P., Blanpain, B., Pontikes, Y. (2014). Inorganic polymer cement from Fe-Silicate glasses: Varying the activating solution to glass ratio. Waste and Biomass Valorization, 5(1), 12-29.
Pontikes, Y., Machiels, L., Onisei, S., Pandelaers, L., Geysen, D., Jones, P., Blanpain, B. (2013). Slags with a high Al and Fe content as precursors for inorganic polymers. Applied Clay Science, 73, 93-102.
Onisei, S., Pontikes, Y., Van Gerven, T., Angelopoulos, G.N., Velea, T., Predica, V., Moldovan, P. (2012) Synthesis of inorganic polymers using fly ash and primary lead slag. Journal of Hazardous Materials, 205-206, 101-10.