03/27/2025
By Danielle Fretwell

The Francis College of Engineering, Department of Civil and Environmental Engineering, invites you to attend a Doctoral Dissertation defense by Arkabrata Sinha on: "Effects of Composition and Reaction Condition on the Phase, Structure and Property Evolutions of Alkali-Silica Reaction Gels."

Date: Wednesday, April 9, 2025
Time: 1 - 2:30 p.m.
Location: Perry 415

Committee:
Advisor: Jianqiang Wei, Asst. Professor, Civil and Environmental Engineering, UMass Lowell

Committee Members*
Tzuyang Yu, Professor, Civil and Environmental Engineering, UMass Lowell
Susan FarajiHennessey, Professor, Civil and Environmental Engineering, UMass Lowell
Zhiyong Gu, Professor, Chemical Engineering, UMass Lowell

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 conditions 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 synthetic ASR gels based on CaO–SiO2-M2O systems with varying alkali/Si and Ca/Si ratios. 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. The results indicate a high 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 (ASH) featured with Q3 polymerization. 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 aluminate-containing supplementary cementitious materials and lithium salts exhibit inconsistent effects and can negatively impact cement hydration and concrete properties. This underscores the urgency of exploring the roles of Al and Li in modifying ASR gels, as well as the importance of developing new ASR retarding admixtures. Toward this end, Al at Al/Si ratios of 0.1 to 0.7 and Li at Li/(Na+K) ratios of 0.1 to 0.5, as well as their influences on the structures and properties of ASR gels, are investigated. The results provide an indication that these two traditional materials can reduce the nocuous effect of ASR gels, while they cannot mitigate ASR in the long run. To address this challenge and develop a cost-effective but robust approach to suppress ASR, a novel magnesium-based chemical admixture is explored to convert 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 is investigated at 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, which result in a 93.5% reduction in hygroscopic swelling, a 94.7% decrease in strength, and a 152.3% drop in modulus of elasticity rendering ASR gels less destructive. The impacts of carbonation and freeze-drying conditions on ASR gels are also studied. Under carbonation, ASR gel phases convert into nahcolite, calcite and vaterite along with reduced hygroscopicity. Under freeze-drying, the chemical bonds and mineral components of ASR gels can be maintained during the removal of free and loosely bound water, while slight changes in crystallization and relative contents of ASH and C-S-H phases are observed. The moisture absorption of low-alkali and high-alkali ASR gels are found to be governed by gel pores and mesopores, respectively.

The results from this study provide an in-depth understanding of the behavior of ASR gels and are expected to pave a solid pathway for the development of novel and robust ASR mitigation methods in future concrete.