09/06/2024
By Danielle Fretwell
Candidate Name: Arkabrata Sinha
Degree: Doctoral
Defense Date: 11 September, 2024
Time: 2:30 to 4 p.m.
Location: Perry Hall 315
Committee:
Advisor: Jainqiang Wei, Assistant Professor, Civil and Environmental Engineering, University of Massachusetts Lowell
Committee Members*
1. Tzuyang Yu, Professor, Civil and Environmental Engineering, University of Massachusetts Lowell
2. Susan Faraji, Professor, Civil and Environmental Engineering, University of Massachusetts Lowell
3. Zhiyong Gu, Professor, Chemical Engineering, University of Massachusetts Lowell
Brief Abstract:
Alkali-silica reaction (ASR), a major deterioration mechanism that can induce volumetric expansion and cracking in concrete, occurs due to the formation and swelling of gel-like products from the interactions between the silica dissolved from aggregates and the alkalis from cement. The deleteriousness of ASR to concrete is governed by the structure, morphology, and swelling potential of ASR gels. However, the influences of composition, reaction condition, and the use of ASR retarding agents on the phase, molecular structure, and property evolutions of ASR gels remain unclear, which is considered a critical barrier to tailor practical and effective ASR mitigation approaches. This work aims to fill this knowledge gap by investigating ASR gels based on six CaO–SiO2-M2O systems with varying alkali/Si ratios of 0.3 and 1.0, and Ca/Si ratios of 0.1, 0.3, and 0.5. Two reaction conditions (sealed and 97% relative humidity) are studied. A novel swelling test method is introduced for direct measurement of hygroscopic expansion of ASR gels. These efforts enable us to elucidate the evolutions of molecular structure, hygroscopicity, and swelling behavior of ASR gels with different compositions and reaction conditions. The results provide a clear indication of the dependence of ASR gel structures on chemical composition. The low-alkali gels possess a layered silicate structure dominated by Q1 and Q2 sites similar to calcium silicate hydrate (C–S–H), whereas the high-alkali gels exhibited the coexistence of tobermorite-type C–S–H and alkali-silicate hydrates featured with Q3 polymerization. The 29Si nuclear magnetic resonance results showed that both the mean chain length and the degree of polymerization of ASR gels decrease with alkali/Si and Ca/Si ratios but increase with moisture. Under 97 % RH, decreased crystallinity and enhanced formation of ASH and Q3 sites are obtained along with increased polymerization and d-spacing. The moisture uptake and swelling of the high-alkali gels showed a reverse correlation with the Ca/Si ratio, which confirms the role of calcium in suppressing the formation of ASH and the dominant role of Q3 sites in determining the hygroscopicity of ASR products.
The current ASR mitigating methods via the incorporations of supplementary cementitious materials and lithium salts exhibit inconsistent effects and can negatively impact cement hydration and concrete properties. To address this challenge and tailor a cost-effective but robust approach to suppress ASR, a novel magnesium-based chemical admixture was explored in this study with the purpose of converting the hygroscopic and expansive ASR gels into innocuous phases. The influence of magnesium nitrate on the evolutions of phase, molecular structure, hygroscopicity, and mechanical properties of ASR gels was investigated with varying Mg/Si ratios from 0.1 to 1.1. The results indicate that the primary phases of ASR products, tobermorite-type C–S–H and ASH, can be suppressed into brucite and eventually converted into magnesium-silicate-hydrate (M-S-H) in the presence of increasing Mg/Si ratios and the consequent decreasing pH. The formation of Si–O–Si bridging bonds and Si–O symmetric stretching in the Q3 sites of ASR products can be mitigated. These phase and structure modifications resulted in a 93.5% reduction in hydroscopic swelling, a 94.7% decrease in strength, and a 152.3% drop in modulus of elasticity rendering the ASR products less destructive.
My future study plan, as illustrated at the end of this proposal, includes further investigations of ASR gels by understanding the roles of carbonation, lithium, and aluminum-based chemical-based admixtures on their phase evolutions, molecular structures, and hygroscopicity. It is expected that the results from this study will provide an in-depth understanding of the behavior of ASR products and pave a solid pathway for the development of novel and robust ASR mitigation methods in future concrete.