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Two metakaolins exhibiting different reactivities were used to study geopolymers obtained via phosphoric acid activation. The study of consolidation of samples with various Si/Al and Al/P showed that the Al- and Si-rich and P-poor compositions were more likely to consolidate properly. The geopolymers formed at 20 or 70 °C kept the same crystalline impurities as the starting metakaolins, the samples remaining mostly amorphous. The microstructure of geopolymers formed with less reactive metakaolin was smooth and dense, while the geopolymers prepared from the most reactive one were more porous and rough. Finally, tests on water and fire resistances of all the samples were reported. [ABSTRACT FROM AUTHOR]

Geopolymers are obtained from an alkali silicate solution and aluminosilicate sources. The source commonly used geopolymer is metakaolin. The chemical composition, extraction site or calcination process of metakaolin influence its reactivity and thus the properties of the consolidated samples. This work focused on clarifying how the properties of aluminosilicate-based raw materials evolve when different metakaolin sources are mixed. The study involved mixing different metakaolins to evaluate their physico-chemical properties. The different samples were characterized by measuring their granulometry, wettability and zeta potential. Structural data were obtained from X-ray diffraction and 27Al nuclear magnetic resonance spectroscopy. It appears that the properties of the mixtures can be expressed as a function of different parameters. Granulometric properties directly depend on the quantity of each source, wettability is related to the amount of available amorphous aluminum in the sources, and zeta potential is strongly influenced by the source with the highest amount of siliceous-based impurities. This methodological approach can be applied to geopolymer synthesis. [ABSTRACT FROM AUTHOR]

Acid-based geopolymers are interesting materials, as they usually present higher mechanical and thermal properties than their alkali-based counterparts do, but they are difficult to use for complex shapes due to the high viscosity of their reactive mixture. This can be overcome by the addition of water in the formulation. In this study, geopolymers have been synthesized from metakaolin and phosphoric acid with variable [P] concentration (by a modification of either the water content or Al/P ratio). Measurements of the viscosity, density and consolidation times have been performed on reactive mixtures differentiated by their water content. Furthermore, a study of the geopolymers' microstructure and mechanical properties confirmed their evolution with [P]. Finally, a characterization by thermal and structural analyzes of their structure (Fourier transform infrared spectroscopy and X-ray diffraction), have been carried out in order to assess the influence of the water content on the different amorphous networks. It appears that the addition of water influences the structure of the material, as it leads to the formation of new amorphous networks. More specifically, for Al/P = 1 and variable water amounts, the networks are mainly based on the coexistence of Al–O–P with an hydrated silica network, while for variable Al/P, the Al–O–P network coexists with the unreacted metakaolin. In addition, it seems that all the properties are governed by the parameter n P /n Al + n H2O. Image 1 • Addition of water and Al/P change lead to the formation of new amorphous. • The properties of acid geopolymer are governed by the parameter n P /n Al + n H2O. • The availability of reactive aluminous or phosphate species control the setting time and viscosity. [ABSTRACT FROM AUTHOR]