Use of Sulfur Waste in the Production of Metakaolin-Based Geopolymers

This preliminary study introduces the incorporation and chemical stabilization of sulfur waste into a geopolymer matrix and explores the concept of material production for further environmental and engineering solutions. In this study, a novel synthesis procedure for sulfur-based geopolymers was introduced, and the role of sulfur in geopolymers and its optimal content to obtain a stable structure were explored. Geopolymers were synthesized by dissolving sulfur in an alkaline activator in different proportions. The alkaline solution was then mixed with metakaolin to synthesize the geopolymer matrix. Adding sulfur in amounts from 0 wt.% to 5 wt.%, compared with metakaolin, led to an increase in the compressive strength of the geopolymers from 22.5 MPa to 29.9 MPa. When sulfur was between 5 wt.% and 15 wt.%, a decrease in the compressive strength was observed to 15.7 MPa, which can be explained by defects and voids in the geopolymer’s microstructure due to the solubility of excess sulfur. Because of the incorporation of sulfur into the geopolymers, a compact and dense microstructure was formed, as reported in the SEM analysis. An XRD analysis showed that, besides quartz and analcime, a new phase, Al2·H10·O17·S3, was also formed as a result of sulfur dissolution in the alkaline activator of the geopolymers

Development of functional geopolymers for water purification, and construction purposes

AbstractThis paper deals with the development of functional geopolymers based on local resources such as kaolinitic soil and zeolitic tuff for the construction of water storage containers and water transfer channels. The effect of water content on the mechanical performance and physical properties of synthesized geopolymers was evaluated. The results confirmed that the optimum ratio of water is 28% of clay fraction, which revealed observable improvements of physical, mechanical, and adsorption properties of the geopolymeric products. Such geopolymers showed the highest compressive strength, density, and maximum adsorption capacity toward cadmium among the products and precursors tested. The residual soluble salts in produced geopolymers were markedly reduced by using this optimum water content

Composition and technological properties of geopolymers based on metakaolin and red mud

New geopolymer formulations were designed by sodium silicate/NaOH activation of metakaolin, iron oxide and red mud mixtures. The effects of source materials on the microstructure and mechanical properties were studied. Each formulation induces different degree of geopolymerization reaction as reflected by the phase composition where the amorphous phase is predominant. These vestiges are related to silica provided by sodium silicate more reactive in the geopolymerization than the silica of metakaolin. Moreover, the variation in strength between the geopolymers is attributed to the same factors, with higher porosity and nonreacted phases found in the red mud based geopolymer matrix. In function of curing time, the mechanical strength increased from day 1 to 28 for the samples with a low amount of red mud. In these two cases, longer curing time improves the geopolymerization state resulting in higher compressive load. The metakaolin and metakaolin/red mud products exhibited comparable water absorption and density. (C) 2013 Elsevier Ltd. All rights reserved

Investigation of Calcium Phosphate Catalysts in Sodium Borohydride Methanolysis for Improved Hydrogen Production

In this study, calcium-rich resource minerals such as brushite, tricalcium phosphate (TCP), and hydroxyapatite were tested as catalysts for the methanolysis of alkaline solutions of NaBH4 to generate hydrogen H2. The synthesis of calcium phosphate compounds was characterized by means of X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The hydrogen generation rate with the TCP catalyst (15,214 mL min−1 g−1) was higher than with the hydroxyapatite catalyst (12,437 mL min−1 g−1) and brushite catalyst (6210 mL min−1 g−1) for the methanolysis of 250 mg NaBH4 at 298 K using 25 mg of catalyst. The impact of TCP weight on hydrogen generation was studied. The methanolysis reaction led to a higher hydrogen volume generation over time with an increase in the weight of the TCP catalyst at a temperature of 308 K. The calculated activation energy for NaBH4 hydrolysis with the TCP catalyst was 23.944 kJ mol−1, suggesting the high catalytic activity of TCP. The values of enthalpy (ΔH) and entropy (ΔS) were calculated, and the results showed that ΔH was 21.28 kJ mol−1 and ΔS was −93.096 J·mol−1. ΔH was positive, meaning that the reaction was endothermic, and the negative ΔS meant a decrease in the disorder of the methanolysis reaction. The stability of the catalysis was tested in successive methanolysis tests. The catalyst’s efficiency decreased to 89% after four cycles