This paper presents the dynamics of confined water and its interplay with alkali cations in disordered sodium aluminosilicate hydrate (N-A-S-H) gel using reactive force field molecular dynamics. N-A-S-H gel is the primary binding phase in geopolymers formed via alkaline activation of fly ash. Despite attractive mechanical properties, geopolymers suffer from durability issues, particularly the alkali leaching problem which has motivated this study. Here, the dynamics of confined water and the mobility of alkali cations in N-A-S-H is evaluated by obtaining the evolution of mean squared displacements and Van Hove correlation function. To evaluate the influence of the composition of N-A-S-H on the water dynamics and diffusion of alkali cations, atomistic structures of N-A-S-H with Si/Al ratio ranging from 1 to 3 are constructed. It is observed that the diffusion of confined water and sodium is significantly influenced by the Si/Al ratio. The confined water molecules in N-A-S-H exhibit a multistage dynamic behavior where they can be classified as mobile and immobile water molecules. While the mobility of water molecules gets progressively restricted with an increase in Si/Al ratio, the diffusion coefficient of sodium also decreases as the Si/Al ratio increases. The diffusion coefficient of water molecules in the N-A-S-H structure exhibits a lower value than those of the calcium-silicate-hydrate (C-S-H) structure. This is mainly due to the random disordered structure of N-A-S-H as compared to the layered C-S-H structure. To further evaluate the influence of water content in N-A-S-H, atomistic structures of N-A-S-H with water contents ranging from 5–20% are constructed. Qⁿ distribution of the structures indicates significant depolymerization of N-A-S-H structure with increasing water content. Increased conversion of Si–O–Na network to Si–O–H and Na–OH components with an increase in water content helps explain the alkali-leaching issue in fly ash-based geopolymers observed macroscopically. Overall, the results in this study can be used as a starting point towards multiscale simulation-based design and development of durable geopolymers.

中文翻译：

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硅铝酸钠水合物凝胶中承压水的动力学及其与碱金属阳离子的相互作用：反应力场分子动力学的见解

本文利用反作用力场分子动力学研究了无水硅铝酸钠水合物（NASH）凝胶中承压水的动力学及其与碱金属阳离子的相互作用。NASH凝胶是通过粉煤灰的碱活化。尽管具有吸引人的机械性能，但地质聚合物仍存在耐久性问题，尤其是碱浸问题，这促使了这项研究。在这里，通过获得均方位移和范霍夫相关函数的演变，来评估纳斯达克州密闭水的动力学和碱金属阳离子的迁移率。为了评估NASH的组成对水动力学和碱性阳离子扩散的影响，构建了Si / Al比为1-3的NASH的原子结构。可以发现，承压水和钠的扩散受Si / Al比的影响很大。NASH中的受限水分子表现出多阶段动态行为，可将其分为流动的和固定的水分子。当水分子的迁移率随着Si / Al比的增加而逐渐受到限制时，钠的扩散系数也随着Si / Al比的增加而降低。水分子在NASH结构中的扩散系数显示出比硅酸钙水合物（CSH）结构低的扩散系数。这主要是由于与分层CSH结构相比，NASH的随机无序结构。为了进一步评估水分对NASH的影响，构建了NASH的原子结构，水分含量在5％至20％之间。水分子在NASH结构中的扩散系数显示出比硅酸钙水合物（CSH）结构低的扩散系数。这主要是由于与分层CSH结构相比，NASH的随机无序结构。为了进一步评估水分对NASH的影响，构建了NASH的原子结构，水分含量在5％至20％之间。水分子在NASH结构中的扩散系数显示出比硅酸钙水合物（CSH）结构低的扩散系数。这主要是由于与分层CSH结构相比，NASH的随机无序结构。为了进一步评估水分对NASH的影响，构建了NASH的原子结构，水分含量在5％至20％之间。结构的Q ⁿ分布表明NASH结构随着水含量的增加而发生明显的解聚。随着水含量的增加，Si–O–Na网络向Si–O–H和Na–OH组分的转化率增加，有助于解释宏观观察到的粉煤灰基地聚合物中的碱浸问题。总体而言，本研究的结果可以用作基于多尺度模拟的耐用土聚合物设计和开发的起点。