Geopolymer

About: Geopolymer is a research topic. Over the lifetime, 6776 publications have been published within this topic receiving 157991 citations. The topic is also known as: geopolymers.

Preparation, structure and hydrothermal stability of alternative (sodium silicate-free) geopolymers

Abstract: In this contribution, we present the preparation and structural characterization of a new type of alternative (sodium silicate-free) geopolymer system. A new procedure of geopolymer synthesis based on the preparation of a reactive geopolymer precursor by direct calcinations of low-quality kaolin with Na/K hydroxides is introduced. The subsequent formation of geopolymer matrix does not require activation by alkaline silicate solution. The compact and hardened material was prepared only by adding a small amount of water. Besides the introduction of a new synthetic procedure, we focused also on the systematic study of chemical structure, mineralogical composition and hydrothermal stability of the prepared geopolymer systems as seen by solid-state NMR spectroscopy and powder X-ray diffraction (XRD). An important part of our contribution is the demonstration of structural and mineralogical changes induced by hydrothermal treatment and long-term aging of the prepared geopolymers. It was found that redistribution of basic structural units (SiO44− and AlO45−) and gradual formation of zeolite fractions can be related to the observed changes in mechanical properties. Up to a certain level, the presence of zeolites enhances the mechanical properties of the prepared geopolymer systems. However, the additional formation of a new generation of zeolite fractions, occurring over the long-term period causes an inversion of this trend and a dramatic reduction of mechanical strength. Nevertheless, formation of the geopolymer matrix by alkaline and thermal activation of low-quality kaolin has the potential to be used in ecological problems solving (solidification of powdered and dangerous waste materials).

The Effects of Temperature on the Local Structure of Metakaolin-Based Geopolymer Binder: A Neutron Pair Distribution Function Investigation

Abstract: Neutron pair distribution function (PDF) analysis is utilized to advance the understanding of the local atomic structural characteristics of geopolymer binders derived from metakaolin, specifically the nature and amount of the water associated with these materials. Samples were heated in air to temperatures up to 1200°C, then analyzed ex situ by high momentum transfer neutron total scattering and PDF analysis. Water contained in large pores, along with water associated with hydration of potassium cations in the geopolymer framework structure, comprise the majority of water in this material. The remaining water is situated in small pores and as terminal hydroxyl groups attached to the Si–Al framework. The Si–Al framework structure undergoes only subtle rearrangement upon heating, but maintains a tetrahedral aluminosilicate framework environment. After crystallization with heating beyond 1000°C, the geopolymer gel is predominantly converted to leucite, with small amounts of amorphous mullite and glassy silica, which have never before been observed in heated geopolymers. This demonstrates the value of neutron PDF analysis to probe the local structure of these important geopolymeric materials.

Properties of inorganic polymer (geopolymer) mortars made of glass cullet

Abstract: This study presents the results of experiments aiming to produce geopolymers from glass cullet, a non-traditional material compared to those usually found in the manufacture of geopolymers (e.g., metakaolin and fly ash). The study gives the principal formulation parameters affecting the behavior of glass cullet geopolymers. The glass used comes from recycled glass bottles. The parameters studied are the fineness of the glass (Blaine of 1000 to 4000 cm2/g), the temperature of synthesis (20, 40 and 60 °C), and the nature and concentration of the activation product (KOH, NaOH). The properties are evaluated in terms of compressive strength and durability. The results show that cullet of soda-glass can be used as a base material for the production of geopolymers and, contrary to metakaolin-based geopolymers, no waterglass is necessary for its setting and hardening since cullet glass already contains a high proportion of alkalis. Thermal activation at 40 or 60 °C is necessary but sufficient to obtain strength of more than 50 MPa, especially for the finer glass (4000 cm2/g). The durability of glass cullet geopolymers is affected by water conservation.

Effects of aluminates on the formation of geopolymers

Abstract: The mechanisms of Al speciation and hydrolysis in geopolymer systems are investigated based on the partial charge model together with preliminary experimental validation. Calculation of the partial charges of species discloses that the positive partial charge of the Al atom is always higher than that of Si atom under conditions of similar pH values, suggesting greater tendency of [Al(OH)4]− tetrahedra to attract negatively charged groups from other species. Given Al atom with four hydroxyl groups under alkaline conditions and the potential to expand its coordination number with greater ease, condensation reactions involving aluminate species appear to occur much more readily. Thus, the model essentially predicts that aluminate species promote condensations reaction owing to factors such as their partial charge and the number of hydroxyl groups, implying that the solubility of aluminate sources (i.e. metakaolin) and the distribution of [Al(OH)4]− ions have an important influence on the mechanical properties of inorganic polymers. Corresponding experimental data on aluminate dissolution obtained from progressive milling of metakaolin particles demonstrated that varying particle sizes and different degrees of aggregation of metakaolin can have an important influence on inorganic polymer properties such as setting time, microstructure and compressive strength. The results indicate that milled metakaolin powders with high specific surface area have faster setting characteristics, higher compressive strength, and possess a more homogeneous microstructure, attributable to Al availability as predicted by the partial charge model.

Effect of nanosilica-based activators on the performance of an alkali-activated fly ash binder

Abstract: This paper assesses the effect of the use of an alternative activator based on nanosilica/MOH (M = K + or Na + ) blended solutions on the performance of alkali-activated fly ash binders. Binders produced with commercial silicate activators display a greater degree of reaction, associated with increased contents of geopolymer gel; however, mortars produced with the alternative nanosilica-based activators exhibited lower water demand and reduced permeability, independent of the alkali cation used. Na-based activators promote higher compressive strength compared with K-based activators, along with a refined pore structure, although K-activated samples exhibit reduced water demand. Zeolite type products are the major crystalline phases formed within these binders. A wider range of zeolites is formed when using commercial silicate solutions compared with the alternative activators. These results suggest that there are variations in the availability of Si in the system, and consequently in the alkalinity, depending on the silicate source in the activator, which is important in determining the nanostructure of the geopolymer gel.