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Title:Nucleation seeding in metakaolin geopolymers
Author(s):Sarbapalli, Dipobrato
Advisor(s):Mondal, Paramita
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Metakaolin-sodium hydroxide
salicylic acid methanol extraction
nucleation seeding
zeolite X
Abstract:Geopolymers are a class of amorphous, aluminosilicate-based binders proposed as an alternative to the cement binders used commonly in the construction industry. One of the main advantages of using geopolymer concrete is the fact that waste materials like fly ash are used as precursors for its production. Therefore, minimal CO2 emissions (a greenhouse gas) are associated with the preparation of geopolymer precursors. In comparison, cement production is accompanied with CO2 formation as a by-product, and therefore, geopolymers are believed to be more environment-friendly than cement binders. Despite such benefits, the use of geopolymers in the construction industry is limited due to reasons such as a lack of understanding of geopolymer chemistry. In addition, chemical admixtures used in conventional concrete are not applicable to geopolymers due to differences in chemistry of the two binder materials. Understanding nucleation kinetics in cement binders have led to the development of different admixtures such as accelerators and retarders and it is believed that a clear picture of the nucleation step in geopolymer formation will aid the development of admixtures. Consequently, an increased application of geopolymers in the industry is expected. Therefore, the objective of this research was to prove and study the nucleation step in metakaolin-sodium hydroxide (MK-NaOH) geopolymer formation. The objective was accomplished by adding silica, titania, synthesized zeolite A and zeolite X nanopowders as external seeds in MK-NaOH geopolymers. Isothermal calorimetry revealed that the addition of these nanoparticles accelerated the rate of heat evolution from metakaolin-sodium hydroxide (MK-NaOH) geopolymers at early age (≤24h). This observation provided indirect evidence of a nucleation step existing in geopolymer formation. This hypothesis was subsequently validated by studying the effects of external seeding on a synthesized aluminosilicate-sodium hydroxide (AlSi-NaOH) geopolymer. Fourier transform infrared spectroscopy (FTIR) revealed that the AlSi-NaOH geopolymer forms two products - one similar to MK-NaOH geopolymer product and the other being a silica rich geopolymer product. Adding zeolite X to the AlSi-NaOH binders promoted formation of the MK-NaOH geopolymer product over the silica-rich geopolymer, which proved that zeolite X serves as a nucleation seed for MK-NaOH binders. External seeding in MK-NaOH geopolymer was observed to decrease porosity at early age (≤7d) and consequently, improvements in compressive strength were noted up to 28d for zeolite X seeded mixes, and up to 70d for SiO2 seeded mixes. These observations suggests that external nucleation seeding is a promising method to control and improve the hardened properties of geopolymers. A secondary objective was to understand the dissolution of aluminosilicates in salicylic acid-methanol (SAM) solution. The SAM dissolution is a selective dissolution technique which has been used previously in our laboratory to remove calcium silicate hydrates from sodium aluminosilicate geopolymer product. This approach was taken while assuming that geopolymer-like aluminosilicates are immune to SAM solution, as stated in literature. However, it was revealed that SAM indeed dissolves sodium aluminosilicates. The mechanism of dissolution was revealed to be an acid attack on the oxygen atom in bridged hydroxyl (Si-O-Al-) groups. The acidity of these groups were observed to be controlled by aluminum in the next nearest neighbor positions, with Q4(4Al) Si associated bridged hydroxyl groups being the least acidic. Therefore, salicylic acid selectively attacks and removes aluminum associated with these sites by forming chelates. Since SAM solution can dissolve aluminosilicates based on their structure, it should be used with caution as a selective dissolution technique.
Issue Date:2018-07-18
Rights Information:Copyright 2018 Dipobrato Sarbapalli
Date Available in IDEALS:2018-09-27
Date Deposited:2018-08

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