Abstract
Geopolymers and other alkali-activated materials were investigated in detail as alternatives to ordinary Portland cement because of their reduced CO2 emissions, high (radionuclide) binding capacities, and low permeabilities. The last two properties make them potential materials for the immobilization of several types of chemical waste. In this context, the direct immobilization of liquid waste streams would be a useful application. This study aimed to develop geopolymers with high water-to-binder ratios, but with good mechanical strengths, while elucidating the parameters that dictate the strengths. Three potential metakaolin geopolymer recipes were cast and cured for 28 days, after which their strengths, mineralogy, and microstructures were determined. The results show that it is possible to attain acceptable mechanical strengths at water-to-binder ratios that vary from 0.75 to 0.95, which is a significant increase from the ratio of 0.55 that is commonly used in the literature. It was found that the most important parameter that governs the mechanical strength is the dilution of the activating solution, which is represented by the H2O/Na2O ratio, while the microstructure was found to benefit from a high SiO2/Al2O3 ratio.
Highlights
Alkali-activated materials (AAMs) have been studied intensively as an alternative to ordinary Portland cement (OPC) systems, from the perspective of reducing the CO2 emissions that are associated with OPC production, or for superior heat and acid resistance
In Ca-rich precursors, such as ground granulated blast furnace slag (BFS) and type C fly ash, a CASH-type gel is formed, which is similar in structure to the CSH gels that are formed in OPC systems
The goal of this study is the manufacture of a metakaolin-based geopolymer with a high W/B ratio that still satisfies the mechanical strength characteristics expected of a stabilized waste form, as defined by the Belgian waste acceptance criteria (ACRIA)
Summary
Alkali-activated materials (AAMs) have been studied intensively as an alternative to ordinary Portland cement (OPC) systems, from the perspective of reducing the CO2 emissions that are associated with OPC production, or for superior heat and acid resistance. Most AAMs are produced by dissolving an aluminosilicate precursor in a strongly basic solution, which is followed by the condensation of small aluminosilicate structures into a 3D network gel. The structure of this gel depends on both the chemistry of the activating solution and the type of precursor used. In Ca-poor precursors, such as metakaolin and type F fly ash, a NASH-type gel is formed instead, which has shown to have a nanozeolitic structure. These Ca-poor types of AAMs are often called “geopolymers” [1]
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