Mechano-chemical processing of kaolins for the synthesis of potassium-based poly-siloxo sialate (K-PSS) geopolymeric binders

Metakaolins (MKAs) are the most used materials in the synthesis of Si-Al based geopolymers [1], which are nanostructured materials with a general formula X [-(SiO2)z-AlO2-]n wH2O, where X= K+, Na+, Li+, Ca+2; z=1, 2, 3; n is the degree of polycondensation and w the number of water molecules in the nanocrystal structure. They are obtained by alkaline attack using solutions with different cations and Si/Al ratios. Depending on the Si/Al ratio, 3-D or 2-D geopolymers are obtained. Both of them act as strong binders for other materials, including natural stones, carbon fibers and metals [1]. These geopolymers are able to resist to temperatures higher than1200°C. In building industry and cultural heritage they can replace different types of Portland cements [1], in car and aeronautic industry composite materials obtained using geopolymeric fibers are used to make parts that need to resist to high temperatures [1]. MKAs are amorphous materials obtained by dehydration of natural kaolins, where kaolinite crystals are, by far, the dominant mineral. Dehydration of kaolinite is usually performed by heating kaolins at temperatures ranging from 650 to 800°C for more than 2 hours. Losses of water from kaolinite, where Al is octahedrically coordinated (Al- VI), leads to amorphous MKAs where substantial portions of Al are converted into tetrahedrally and pentahedrally coordinated forms, indicated as Al-IV and Al-V, respectively [1]. According to Davidowits [1], the formation of Al-V is one of the key factors that makes MKAs the most suitable materials for the synthesis of Si/Al based geopolymers. The production of highly reactive MKAs through thermal processing is expensive and strongly depends upon the type of kiln used, and the mineral impurities present in the parent kaolin. Although was known from long time that kaolins can also be converted into MKAs by grinding them in mills whose moving masses rotate at high speeds (800 to 2.000 r.p.m.) [2], only recently it has been shown that dehydration occurs in the OH located in the inner layer of kaolinite, thus altering the Al coordination [3]. However, very limited information exist on the formation Al-V from mechanically activated MKAs [4], and no attempts have been made to synthesize geopolymers from them. The potential of mechanochemical processing of kaolins for the synthesis of Si/Al geopolymers was investigated using a kaolin whose thermal behavior was known. The material was grinded under different conditions and times, and the various MKAs obtained were characterized by XRD, FTIR-AR, TGA-DTG-DTA, 27Al, 29Si and 1H MAS NMR, and other techniques. Results obtained show that mechanochemically produced MKAs are quite different from those obtained by thermal processing, especially in terms of (Al-V)/(Al-IV+Al-VI) ratios and the content of free and bonded water. The ability of mechanically activated MKAs to make K-PSS geopolymers was thus investigated, and the chemical, mechanical and thermal properties of the obtained products determined. The results of this investigation will be presented and critically discussed.