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Title:Porosity control of alkali-activated aluminosilicates via functional alkoxysilane additives
Author(s):Glad, Brayden E.
Director of Research:Kriven, Waltraud M.
Doctoral Committee Chair(s):Kriven, Waltraud M.
Doctoral Committee Member(s):Abelson, John R.; Economy, James; Struble, Leslie J.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Geopolymers were synthesized with alkoxysilane and other organo-silane supplements to improve adhesion to organic polymers and to modify porosity. Uncured geopolymer slurry was found to be miscible with various alkoxysilanes, and the resulting cured geopolymer strongly adhered to organic polymers if and only if the alkoxysilane possessed an appropriate coupling agent linking group for the polymer. Issues relating to maximizing reactivity and properly measuring the porosity of these complex systems using gas adsorption porosimetry and mercury intrusion porosimetry (MIP) were addressed. A new pore size distribution calculation using MIP results was tested and applied. Geopolymer mesoporosity could be minimized through the use of alkoxysilanes with acrylic acid or similar functional groups, which Fourier transform infrared spectroscopy identified as transforming to sodium acrylates in the high pH of the geopolymer system. Transmission and scanning electron microscopy identified the self-assembly of 100 nm-1µm sheets and tubes of the acrylate-functional alkoxysilane which bound strongly to geopolymer precipitates. These small phases were able to inhibit the sequestration of surplus water in mesoporous structures through a waterlocking mechanism. Numerous property modifications were observed as a result. Examination with nitrogen adsorption showed an order of magnitude decrease in mesopore volume, while compressive testing found a 48% increase in Weibull modulus for compressive strength and a probable increase in compressive strength. Mercury intrusion porosimetry measurements showed up to a 24% increase in bulk density. Meaningful property modification and mesoporosity reduction required the addition of at least 0.06 mol acrylate/mol geopolymer, and no sheet or tube phases were observed with smaller amounts of acrylate. This is consistent with a saturation model in which about 0.06 mol/mol of alkoxysilane monomer is able to co-precipitate directly with the geopolymer, and any surplus forms the sheet or tube phase. Dilute emulsions of geopolymer reagents with organo-silanes were capable of producing highly porous monolithic solids through templating and surface modification, with or without an additional hydrophobic phase. In the dilute geopolymer emulsion, dimethyldiethoxysilane demonstrated the least phase separation of the several organo-silanes tested and created the most porous monoliths. X-ray diffraction and Fourier transform infrared spectroscopy data illustrated limited geopolymerization in dilute slurries with or without organic additives, but this limitation was reduced through extended mixing of more concentrated slurry, followed by further dilution. Mercury intrusion porosimetry data illustrated the creation of percolating network porous solids with controllable porosity within the range of 60 vol% to 80 vol% or more, and controlled critical percolation pore sizes within the range of approximately 500 nm to the tens of µm. Critical percolation pore size was observed to increase with increasing porosity, and to decrease with increasing emulsion stability. Scanning electron microscopy illustrated the creation of an organic film on the interior surfaces of the pore network. Firing the porous geopolymer at 800 ºC for 4 hours in a nitrogen atmosphere removed the organic film and increased porosity without meaningfully increasing critical percolation pore size. The fired geopolymer sample had X-ray diffraction characteristics of an amorphous glass and demonstrated increased strength. A hard minor phase was identified as ß-SiC.
Issue Date:2013-05-28
Rights Information:Copyright 2013 Brayden E. Glad
Date Available in IDEALS:2013-05-28
Date Deposited:2013-05

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