Abstract Calcium silicate hydrate (C-S-H), the most important Portland cement hydration product, determines the mechanical properties and durability of cementitious materials. In marine environment, C-S-H often suffers from sulfate attack – one of the most common and severe degradations for concrete. pH is considered as a critical factor in determining the deterioration behavior of C-S-H during sulfate attack, of which the significance may be overlooked. This study focused on the role of pH on the deterioration of C-S-H in terms of the composition and nano/micro structure under sulfate attack. The results show that lowering pH aggravates the decalcification of C-S-H, whereas a strong alkaline condition is beneficial to increase the resistance to deterioration. An increase in the mean chain length together with the proportion of large pores (>10 nm) is observed when C-S-H is subjected to low-pH sulfate attack (pH = 10–12), resulting in a relatively loose structure. The deep analysis on defective tobermorite model reveals that calcium at the interlayer of C-S-H nanostructure is readily removed under sulfate attack, thereby improving the potential of Si-O-Si groups formation and increases the mean chain length of C-S-H.

中文翻译：

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pH值对硫酸盐侵蚀下水合硅酸钙（CSH）纳米/微米结构的影响

摘要 硅酸钙水合物（CSH）是最重要的硅酸盐水泥水化产物，决定了胶凝材料的力学性能和耐久性。在海洋环境中，CSH 经常遭受硫酸盐侵蚀——这是混凝土最常见和最严重的降解之一。pH被认为是决定硫酸盐侵蚀过程中CSH劣化行为的关键因素，其重要性可能被忽视。本研究的重点是 pH 值对硫酸盐侵蚀下 CSH 的组成和纳米/微结构劣化的作用。结果表明，降低pH会加剧CSH的脱钙，而强碱性条件有利于提高抗劣化性。平均链长的增加以及大孔的比例 (> 当 CSH 受到低 pH 硫酸盐攻击（pH = 10-12）时，观察到 10 nm），导致结构相对松散。对缺陷雪硅钙石模型的深入分析表明，CSH 纳米结构中间层的钙在硫酸盐侵蚀下很容易被去除，从而提高了 Si-O-Si 基团形成的潜力，并增加了 CSH 的平均链长。