Dissolution processes,hydrolysis and condensation reactions during geopolymer synthesis: Part I—Low Si/Al ratio systems |
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Authors: | L Weng K Sagoe-Crentsil |
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Affiliation: | (1) Department of Materials Science, Shenzhen Graduate School, Harbin Institute of Technology, University Town, Xili, Shenzhen, 518055, P.R. China;(2) Manufacturing & Infrastructure Technology, CSIRO, P.O. Box 56, Highett, Victoria, 3190, Australia |
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Abstract: | Initial geopolymeric reaction processes governing dissolution of solid aluminosilicate particles in alkali solutions have
been investigated using conventional experimental techniques, and the data analysed by speciation predictions of the partial
charge model (PCM). For metakaolin powders activated with 5.0 M NaOH, solid-state nuclear magnetic resonance (NMR) spectra
disclose the existence of monomeric Al(OH)4]− species after two hours of dissolution, consistent with PCM predictions. However, no equivalent monomeric silicate species
were observed for 5.0–10.0 M NaOH activator solutions characteristic of systems with nominal Si/Al ≤ 1. The apparent absence
of monomeric silicate species suggest rapid condensation of silicate units with Al(OH)4]− to generate aluminosilicate species, as indicated by the evolution of the shoulder at around −87 ppm in the 29Si NMR spectra. Of the two possible stable silicate species SiO2(OH)2]2− and SiO(OH)3]−, the latter appears most likely to condense with Al(OH)4]− to produce aluminosilicate oligomers, from which larger oligomers subsequently form through further condensation with Al(OH)4]− leading to a gradual build up of aluminosilicate networks and a lowering of system alkalinity. This dissolution and hydrolysis
sequence at the early stages of synthesis suggests a reaction path wholly consistent with predictions of the partial charge
model. |
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