The effect of an activation solution with siliceous species on the chemical reactivity and mechanical properties of geopolymers

Precursors are critical parameters in geopolymerization mechanisms because they govern the reaction kinetics as well as the working properties of the final materials. This study focuses on the effect of alkaline solutions on geopolymer formation. Toward this end, several geopolymer samples were synthesized from the same metakaolin and various alkaline solutions. First, the solutions were characterized by thermogravimetric analysis as well as DTA–TGA, infrared spectroscopy, and MAS-NMR spectrometry. The structural evolution of the formed geopolymers was investigated using infrared spectroscopy. The measurement of mechanical strength was tested by compression. The results provide evidence of relationships between the chemical composition, the extent of depolymerization of the alkaline solutions, the kinetics of Si–O–Si bond substitution by Si–O–Al and the compressive strength. For a given aluminosilicate source, the nature and the quantity of siliceous species in the activation solution appear to lead to variation in the reactivity and, consequently, to the formation of various networks that control the kinetics of formation of geopolymers and their mechanical properties.

     

Role of alkaline cations and water content on geomaterial foams: Monitoring during formation

The synthesis of an in situ inorganic foam based on alkaline polysialate was achieved at a slightly elevated temperature by alkaline activation of raw minerals and industrial waste based on a geopolymer synthesis process. To understand the formation of the foam, the differences induced by the variations in the alkaline element (sodium or potassium) and water content were investigated throughout XRD, in situ FTIR-ATR, volume expansion and thermal analysis coupled with mass spectrometry. The amount of water is the determining parameter for volume expansion but does not interfere with the structure formation of foams. The comparison between geopolymers and foams has elucidated the formation of at least two amorphous networks in the case of sodium foam and three in the particular case of potassium as the alkaline element.     

Influence of raw materials and potassium and silicon concentrations on the formation of a zeolite phase in a geopolymer network during thermal treatment

The development of porous materials based on geopolymers allows the benefits of eco-consumption, good insulating properties and good mechanical properties to be combined. For geopolymers to be useful, the development of an understanding of their properties under various conditions is important. Attention was given to the structural evolution of porous materials with heating. The structural evolutions were investigated using thermal analysis and infrared spectroscopy. The formation of a crystalline phase was observed and identified as a zeolite. To elucidate the roles played by the raw materials, silicon concentration and potassium concentration on the formation of the crystalline phase, three parameters in the foam synthesis process were modified. The influence of silica fume and metakaolin on the appearance of the crystalline phase was examined through the reactivity of the raw materials in contact with a potassium solution. Different behaviors directly linked to the network composition were observed. Silica-fume-based samples showed phase crystallization and dehydroxylation due to the large concentration of silicon, which likely led to the formation of hydrated silica species. The formation of the zeolite-phase network upon the application of heat depended only on the silica dissolution and was linked to the composition of silica in terms of the silicon species and grain size. Studying the crystalline phase formation by variation of the amount of potassium allowed the determination of a maximum silicon concentration and a minimum potassium concentration. The formation of the zeolite phase occurred at a potassium concentration greater than 3.5 mol·L^? 1 and a silicon concentration lower than 16.5 mol·L^? 1, which confirmed the presence of F-zeolites.     

Evidence of a gel in geopolymer compounds from pure metakaolin

During the synthesis of geopolymer materials, all of the phenomena and reactions that occur should be included in the analysis of the synthesis. This work aims to study the role that siliceous species play in an activation solution in the presence of pure metakaolin. The formation of a gel phase during the synthesis of a K-geopolymer was shown. Different mixtures were analyzed by infrared spectroscopy. During material consolidation, there is always competition between the geopolymer network and the gel that is governed by a change in the siliceous species. The heat treatment of various gels and solids provided evidence of various networks in geopolymer materials. Finally, the change in the crystallinity of the silica in the activation solution led to a change in the kinetics of the polycondensation reactions, in agreement with previous work.     

Geomaterial foams: role assignment of raw materials in the network formation

The geomaterial foams studied is based on geopolymerization reactions, which is a type of geosynthesis that involves silico-aluminates. Its study during formation has however revealed a different behavior than geopolymer, suggesting the formation of various networks. This work investigates the interaction between initial compounds (metakaolin, silica fume, potassium-based solution) by a kind of mixture decomposition to ultimately understand the formation mechanism of foam. The structural evolution was determined using thermal analysis, FTIR spectroscopy and ²⁷Al and ²⁹Si MAS-NMR measurements. The use of different raw materials in combination with various solutions demonstrates the formation of various species in solution. The reactivity of the solution will then evolve in different ways. The Si/K ratio controlled the type of species created and, particularly, the reactivity in the mixture. From the various reactions of dissolution and polycondensation that were deduced, we could identify the composition of the four networks (K_0.5SiAl_0.75O_6.8H_8.6; K₂Si₂O₅; KAlSi₂O₄,1.5H₂O; and amorphous silica) constituting the foam.     

Structural characterization of geomaterial foams — Thermal behavior

The structural evolution of in-situ inorganic foam based on sodium or/and potassium is investigated. The synthesis of foam based on an alkaline polysialate, is achieved at slightly elevated temperature by the alkaline activation of raw minerals and industrial waste. The structural evolution is studied through differential thermal analysis/thermogravimetric analysis (DTA-TGA) coupled with mass spectrometry, in-situ X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and ²⁹Si, ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR). The role of alkali cations is observed, and they are shown to have a significant effect on structure changes with temperature, leading to the onset of a crystalline phase in potassium-based foam. Shifts of MDI reflect the competition that takes place during heating between the evolution of the zeolite phase and the amorphization of the material.     

Reply to the J. Provis and S.A. Bernal comment about the article “Structural characterization of geomaterial foams. Thermal behavior” published in J. Non-Cryst. Solids, 2011

In this paper, we discuss the alternative interpretation of NMR spectra from (i) “Structural characterization of geomaterial foams—Thermal behavior” by Prud'homme et al. published in Journal of Non-Crystalline Solid, 2011, and from (ii) the reply proposed by J. Provis and S.A. Bernal in this journal. In this contribution, other alternatives for the NMR bands assignment are discussed and compared with infrared spectroscopy and thermo-gravimetric results since closed relationship between NMR-IR-TG is effective. In agreement with these authors, the analysis of prepared foams requires additional work addressed to detect the presence of two phases with different tetrahedral condensation. Relative arrangement of these two phases could affect in a significant way the physical chemical properties of prepared foams.     

Role of the silica source on the geopolymerization rate

The synthesis of geopolymer-silica composites was achieved at room temperature to determine the role of the silica source (quartz or amorphous silica) on the polycondensation rate and the mechanical properties of synthesized materials. Then, samples with a composition range from 100% quartz to 100% amorphous silica were formed, compared and characterized by XRD, infrared spectroscopy, thermal analysis, SEM, and compression tests. The results give evidence that the increase of amorphous silica in the mixture favors the polycondensation reaction (i.e., “geopolymerization”) to form consolidated materials whereas quartz led to heterogeneous materials without cohesion. These facts are explained by the modification of the Si/Al ratio in the geopolymer matrix due to the increase of quartz in the mixture.     

Synthesis of consolidated materials from alkaline solutions and metakaolin: existence of domains in the Al–Si–K/O ternary diagram

The activation of metakaolin using an alkaline solution is a method for producing high quality consolidated materials. Si, Al and K are the main factors that influence the nature of the consolidated materials. This study focuses on the effects of the composites on the final network formation. Two types of alkaline solutions, one (S_c) based on a commercial potassium silicate solution and the other (S _l ) synthesized by mixing KOH and amorphous silica, were studied, and their effects on the network formation of consolidated materials were compared and discussed. Four types of materials were observed and identified based on different Si, Al and K contents, including a geopolymer, gel, sedimented materials and hardening materials. These behaviors are manifested in the various polycondensation mechanisms and microstructures, which were characterized using FTIR spectroscopy, thermal analysis, X-ray diffraction measurements and Scanning electron microscope observations. The influence of Si, Al and K is discussed according to network formation.