Alkali-silica reaction (ASR), a detrimental process causing volume expansion and cracking in concrete by forming hygroscopic and swellable gel-like products, has been a long-standing challenge faced by concrete structures across the world. Understanding the underlying mechanisms of formation, moisture uptake and swelling of ASR gels requires thorough characterizations, where an appropriate drying method to remove free water and stop the reactions in the samples is essential. This study aims to elucidate the effect of freeze-drying, an emerging drying technique increasingly employed in sample preparations, on the phase, molecular structure, and moisture absorption behavior of ASR gels. Towards this end, six ASR gels with three calcium-to-silica (Ca/Si) ratios (0.1, 0.3, and 0.5) and two alkali-to-silica [(Na + K)/Si] ratios of 0.3 (low-alkali gels) and 1.0 (high-alkali gels) were studied. The results indicate that the chemical bonds and mineral components of ASR gels can be maintained during the removal of free and loosely bound water in freeze drying, while slight changes in crystallization and relative contents of alkali-silicate hydrate and calcium silicate hydrate phases were observed, in particular, for the high-alkali ASR gels. Although slight decreases in Q1, Q2, and Q3 silica polymerization sites and increases in the mean chain length and degree of polymerization were observed from the nuclear magnetic resonance (NMR) spectroscopy, the intrinsic layered silicate structure of ASR gels remains. The moisture absorption of low-alkali and high-alkali ASR gels was found to be governed by gel pores and mesopores, respectively. After freeze-drying, slight increases in moisture sorption (a 6.8 % increase in absorption and a 2.2 % decrease in desorption) and enhanced hysteresis were observed from the low-alkali gels. The high-alkali gels, however, showed a slight decrease in hygroscopicity along with a 16.6 % reduction of mesopores.

中文翻译：

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冷冻干燥下碱-硅胶反应凝胶的相、结构和吸湿性能演变


碱-二氧化硅反应 （ASR） 是一种有害过程，通过形成吸湿性和可膨胀的凝胶状产品，导致混凝土体积膨胀和开裂，一直是世界各地混凝土结构面临的长期挑战。了解 ASR 凝胶形成、水分吸收和溶胀的潜在机制需要彻底表征，其中必须采用适当的干燥方法来去除游离水并终止样品中的反应。本研究旨在阐明冷冻干燥（一种越来越多地用于样品制备的新兴干燥技术）对 ASR 凝胶的相、分子结构和吸湿行为的影响。为此，研究了六种具有三种钙硅 （Ca/Si） 比率（0.1、0.3 和 0.5）和两种碱二氧化硅 [（Na + K）/Si] 比率为 0.3（低碱凝胶）和 1.0（高碱凝胶）的 ASR 凝胶。结果表明，在冷冻干燥过程中去除游离和松散结合的水时，可以保持 ASR 凝胶的化学键和矿物成分，同时观察到碱-硅酸盐水合物和硅酸钙水合物相的结晶和相对含量的微小变化，尤其是高碱 ASR 凝胶。尽管从核磁共振 （NMR） 波谱中观察到 Q1、Q2 和 Q3 二氧化硅聚合位点略有减少，平均链长和聚合度增加，但 ASR 凝胶的本征层状硅酸盐结构仍然存在。发现低碱和高碱 ASR 凝胶的吸湿性分别受凝胶孔和中孔控制。冷冻干燥后，水分吸附略有增加（吸收率增加 6.8%，吸收率增加 2.从低碱凝胶中观察到解吸降低 2%）和增强的滞后。然而，高碱凝胶的吸湿性略有降低，介孔减少 16.6%。