Graduate Thesis Or Dissertation


The Role of Magnesium in the Acid Degradation of Metakaolin-Based Alkali-Activated Cements Public Deposited

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  • Microbial-induced concrete corrosion (MICC) of ordinary portland cement (OPC) concrete sewers is a major infrastructure challenge in the United States (US). Many US municipalities make significant annual financial investments every year for the continual maintenance and replacement of sewer lines—the majority of which have exceeded their design life—to keep them operational. Utilization of alkali-activated cement (AAC), a novel cementitious material that is an alternative to OPC, in sewer applications presents an opportunity to increase the durability of concrete sewer systems. AACs are chemically activated binders made from waste (e.g., fly ash, slag) and natural (e.g., metakaolin) aluminosilicate precursors that often have variable elemental compositions. While in some cases AAC concrete has been shown to exhibit higher acid resistance than its OPC counterparts, the actual mechanism of acid degradation in AACs is not yet fully understood, in part, due to the complex and variable chemistry of the aluminosilicate precursors. This study systematically evaluates the role of a magnesium mineral addition in the form of brucite on the acid degradation of metakaolin-based AACs. The effects of (1) silica content, (2) sodium content, and (3) magnesium mineral addition on the structure (i.e., mineralogy) and properties (i.e., porosity, acid resistance) of AACs are investigated herein. After synthesis, AAC samples were exposed to sulfuric acid solutions with a pH of 2.00 ± 0.05 and allowed to reach equilibrium (defined as a change in pH of less than 0.01 per 4 hours). In order to determine the effect of magnesium mineral addition on the acid durability of AACs, porosity measurements were taken before and after acid exposure. Mineralogy of the samples was determined using X-ray diffraction. Leaching data were collected after the samples reached equilibrium. In addition, this work utilizes a central composite design to develop response surfaces for leaching and porosity to better understand and predict the effect of different alkali and silica contents and mineral additions. Finally, elemental maps produced using electron microprobe analysis (EMPA) were used to visualize the acid corrosion front and diffusion of magnesium ions throughout the AAC samples. These maps are also used to confirm current understanding of how calcium-free AACs respond to exposure to sulfuric acid. Results substantiate that adding unreacted magnesium mineral content to the AACs improves the acid resistance of the material by improving the stability of the aluminosilicate structure as evidenced by a reduction in the amount of leached silicon and aluminum during acid exposure.
Date Issued
  • 2018
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  • 2019-11-18
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