In many systems, prenucleation clusters (PNC), dense liquids, and solid amorphous phases precede the formation of crystalline phases, which can grow via a nanoparticle aggregation mechanism. Despite intensive efforts, the current understanding of the mechanisms of such nonclassical crystallization processes is far from complete. Here by means of calcium potentiometric titration tests complemented by X-ray diffraction, dynamic light scattering, and electron microscopy analyses, we show that in the case of calcium hydroxide (CH), one of the main components of set Portland cement, PNCs and dense liquid precursors (prenucleation stage), and amorphous CH (ACH) and a metastable nanocrystalline CH (postnucleation stage) precede the formation of stable CH crystals. Such a phase sequence is also observed in the presence of additives commonly used as cement set-retarders and plasticizers (polysaccharides, lignosulfonate, and polyacrylate). We show that the main action of additives occurs during the prenucleation stage via destabilization/stabilization of PNCs, and the promotion/stabilization of dense liquid precursors leading to a significant delay in the onset of ACH nucleation at high supersaturations. Additives also stabilize amorphous and metastable crystalline CH phases and modify the number, size, and morphology of stable CH crystals. In contrast to classical crystallization theory, an inverse relationship between supersaturation at the onset of nucleation and the final number and size of CH crystals is observed. This unexpected result is explained by the fact that CH crystals nucleate after dissolution of ACH, whose solubility marks the maximum supersaturation in the system with respect to primary CH nanoparticles, which subsequently undergo oriented attachment to form large CH particles that further grow via aggregation of ACH nanoparticles. These results help to understand how CH forms, show that nonclassical crystallization can take place in cement systems, and shed light on how cement admixtures work.

中文翻译：

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氢氧化钙通过非晶态前体的非经典结晶及其添加剂的作用

在许多系统中，预核簇（PNC），致密液体和固态无定形相先于结晶相形成，而结晶相可通过纳米粒子聚集机制生长。尽管付出了巨大的努力，目前对这种非经典结晶过程的机理的理解还远远不够。在这里，通过钙电位滴定测试，辅以X射线衍射，动态光散射和电子显微镜分析，我们发现在氢氧化钙（CH）的情况下，硅酸盐水泥，PNC和致密硅酸盐水泥的主要成分之一液态前驱物（成核前阶段），无定形CH（ACH）和亚稳态纳米晶CH（成核后阶段）先于稳定的CH晶体形成。在通常用作水泥缓凝剂和增塑剂的添加剂（多糖，木质素磺酸盐和聚丙烯酸酯）的存在下，也观察到这种相序。我们表明，添加剂的主要作用发生在预成核阶段，通过PNC的失稳/稳定作用，以及致密液体前体的促进/稳定作用，导致高过饱和度的ACH成核发生显着延迟。添加剂还可以稳定无定形和亚稳态的结晶CH相，并改变稳定CH晶体的数量，大小和形态。与经典结晶理论相反，观察到成核开始时的过饱和与CH晶体的最终数量和尺寸之间存在反比关系。CH晶体在ACH溶解后成核，这一事实解释了这一出乎意料的结果，CH晶体的溶解度标志着系统相对于主要CH纳米颗粒的最大过饱和度，随后该CH晶体经历了定向附着，形成了较大的CH颗粒，这些颗粒通过ACH的聚集进一步生长纳米粒子。这些结果有助于了解CH的形成方式，表明在水泥体系中会发生非经典的结晶，并阐明了水泥掺合料的工作方式。