Rodriguez-Navarro C, Ruiz-Agudo E, Ortega-Huertas M, Hansen E
Department Mineralogia y Petrologia, Universidad de Granada, Fuentenueva s/n, 18002 Granada, Spain, and The Getty Conservation Institute, Los Angeles, California 90049-1684, USA.
Langmuir. 2005 Nov 22;21(24):10948-57. doi: 10.1021/la051338f.
Although Ca(OH)2 is one of the oldest art and building material used by mankind, little is known about its nanostructural and colloidal characteristics that play a crucial role in its ultimate performance as a binder in lime mortars and plasters. In particular, it is unknown why hydrated lime putty behaves as an irreversible colloid once dried. Such effect dramatically affects the reactivity and rheology of hydrated lime dispersions. Here we show that the irreversible colloidal behavior of Ca(OH)2 dispersions is the result of an oriented aggregation mechanism triggered by drying. Kinetic stability and particle size distribution analysis of oven-dried slaked lime or commercial dry hydrate dispersions exhibit a significant increase in settling speed and particle (cluster) size in comparison to slaked lime putty that has never been dried. Drying-related particle aggregation also leads to a significant reduction in surface area. Electron microscopy analyses show porous, randomly oriented, micron-sized clusters that are dominant in the dispersions both before and after drying. However, oriented aggregation of the primary Ca(OH)2 nanocrystals (approximately 60 nm in size) is also observed. Oriented aggregation occurs both before and during drying, and although limited before drying, it is extensive during drying. Nanocrystals self-assemble in a crystallographically oriented manner either along the 100 or equivalent 110 directions, or along the Ca(OH)2 basal planes, i.e., along [001]. While random aggregation appears to be reversible, oriented aggregation is not. The strong coherent bonding among oriented nanoparticles prevents disaggregation upon redispersion in water. The observed irreversible colloidal behavior associated with drying of Ca(OH)2 dispersions has important implications in heritage conservation, particularly considering that nowadays hydrated lime is often the preferred alternative to portland cement in architectural heritage conservation. Finally, our study demonstrates that, fortuitously, hydrated lime could be one of the first nanomaterials used by mankind.
尽管氢氧化钙是人类最早使用的艺术和建筑材料之一，但对于其纳米结构和胶体特性却知之甚少，而这些特性在其作为石灰砂浆和灰泥粘结剂的最终性能中起着关键作用。特别是，尚不清楚熟石灰腻子一旦干燥为何会表现出不可逆胶体的特性。这种效应极大地影响了熟石灰分散体的反应活性和流变学。在此，我们表明氢氧化钙分散体的不可逆胶体行为是干燥引发的定向聚集机制的结果。与从未干燥过的熟石灰腻子相比，烘干的消石灰或市售干水合物分散体的动力学稳定性和粒度分布分析显示沉降速度和颗粒（聚集体）尺寸显著增加。与干燥相关的颗粒聚集还导致表面积大幅减小。电子显微镜分析表明，在干燥前后的分散体中占主导地位的是多孔、随机取向的微米级聚集体。然而，也观察到了初级氢氧化钙纳米晶体（尺寸约为60纳米）的定向聚集。定向聚集在干燥前和干燥过程中都会发生，虽然在干燥前有限，但在干燥过程中广泛存在。纳米晶体以晶体学取向的方式自组装，要么沿着100或等效的110方向，要么沿着氢氧化钙基面，即沿着[001]方向。虽然随机聚集似乎是可逆的，但定向聚集则不然。定向纳米颗粒之间的强相干键合阻止了在水中重新分散时的解聚。观察到的与氢氧化钙分散体干燥相关的不可逆胶体行为在遗产保护方面具有重要意义，特别是考虑到如今在建筑遗产保护中熟石灰常常是波特兰水泥的首选替代品。最后，我们的研究表明，巧合的是，熟石灰可能是人类最早使用的纳米材料之一。
