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Nonclassical Crystallization of Calcium Hydroxide via Amorphous Precursors and the Role of Additives

机译：无定形前体的氢氧化钙的非化学结晶及添加剂的作用

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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.
机译：在许多系统中，Prenucleation簇（PNC），致密液体和固体无定形相在形成结晶相之前，其可以通过纳米颗粒聚集机制生长。尽管有密集的努力，目前对这种非生物结晶过程机制的理解远非完整。以下通过X射线衍射，动态光散射和电子显微镜分析互补的钙电位滴定试验，我们表明，在氢氧化钙（CH）的情况下，设定波特兰水泥，PNC和密集的主要成分之一液体前体（Prenucleation阶段）和无定形CH（ACH）和稳定的纳米晶CH（后核阶段）在形成稳定的CH晶体之前。在常用用作水泥装置延迟剂和增塑剂（多糖，木质素磺酸盐和聚丙烯酸酯）的添加剂存在下也观察到这种相序。我们表明添加剂的主要作用通过不稳定/稳定化在PNC的稳定/稳定期间发生，以及致密液体前体的促进/稳定，导致高度过饱和度的ACH成核发作的显着延迟。添加剂还稳定无定形和亚稳态结晶CH相，并修饰稳定CH晶体的数量，尺寸和形态。与典型结晶理论相反，观察到核切割发作和CH晶体的最终数量和最终数量和尺寸之间的过饱和之间的反比关系。这种意想不到的结果是通过Ch晶体核肉在溶解后核心的事实来解释，其溶解度标志着系统相对于初级CH纳米颗粒的最大过饱和，其随后经历取向的附着以形成通过ACH的聚集进一步生长的大CH颗粒以形成进一步生长的大CH颗粒纳米粒子。这些结果有助于了解CH形式，表明无菌结晶可以在水泥系统中进行，并阐明水泥混合如何工作。

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