Salt damage in brick and stone: Direct observation of a salt crystallization sequence.

仔细观察一座古老的砖或石头建筑，你会经常注意到它的表面正在坍塌。在靠近大海或暴露在污染大气中的建筑物中，这是一个特别的问题。因此，随着时间的推移，石雕的许多细节和成品，尤其是雕刻和雕像，都丢失了。一个常见的原因是，从地面或雨水中进入石头的水含有溶解的离子。当水蒸发时，它们在石头的孔隙中形成盐晶体。随着晶体的生长，它们挤压孔隙壁，导致裂缝在石头中蔓延。造成的损害程度与形成的盐的类型密切相关。硫酸钠是在这种损伤中经常观察到的一种盐。它是一种复杂的盐，因为它可以形成不同形状和性质的晶体。迄今为止，科学家们试图通过考虑硫酸钠可以形成的两种最常见晶体的行为和特性来了解硫酸钠是如何破坏石头的，即无水芒硝和水合芒硝。然而，佩尔和他在荷兰的同事们测量了当硫酸钠溶液在多孔材料中蒸发时形成的硫酸钠晶体的溶解度，发现了间接证据，表明形成了第三种硫酸钠晶体，称为七水硫酸钠。这是一个最重要但出乎意料的结果，因为它意味着，为了了解这种盐造成的结晶损伤，除了芒硝和芒硝的性质外，可能还需要考虑七水化合物晶体的性质。因为这个来自荷兰的结果是如此重要，我们需要确保它是正确的：也就是说，我们需要这种晶体存在的直接证据来为他们的观察提供独立的验证。我们计划使用高能同步加速器x射线来观察浸泡在硫酸钠溶液中缓慢冷却的砖块和石头的内部，以观察形成什么样的晶体。每个晶体都有独特的x射线指纹。利用我们开发的特殊同步加速器x射线技术，我们希望能观察到整个结晶过程。

Look closely at an old brick or stone building and you will often notice that its surface is crumbling away. This is a particular problem in buildings that are close to the sea or exposed to a polluted atmosphere. As a result much of the detail and the finish of the stonework, particularly in carvings and statues, is lost over time. A common cause is that the water that is drawn into the stone from the ground or from rain contains dissolved ions. These form salt crystals inside the pores of the stone when the water evaporates. As the crystals grow, they press against the pore walls, causing cracks to spread through the stone. The level of damage caused is closely connected to the type of salt that forms.Sodium sulphate is a salt that is often observed in this kind of damage. It is a complex salt, as it can form crystals with different shapes and properties. To date, scientists have tried to understand how sodium sulphate damages stone by considering the behaviour and properties of the two most common crystals it can form, namely anhydrous thenardite and the hydrate mirabilite. However, Pel and co-workers in the Netherlands, measuring the solubility of sodium sulphate crystals formed when sodium sulphate solution evaporates inside a porous material, have found indirect evidence that a third kind of sulphate crystal is formed, called sodium sulphate heptahydrate. This is a most important but unexpected result, as it means that in order to understand crystallization damage caused by this salt, it may be necessary to consider the properties of the heptahydrate crystal, in addition to those of thenardite and mirabilite. Because this result from the Netherlands is so important, we need to be sure that it is correct: that is, we need direct evidence of the presence of this crystal to provide independent verification of their observations. We plan to use high energy synchrotron X-rays to look inside blocks of brick and stone soaked in sodium sulphate solution as they cool slowly order to see what kinds of crystals form. Each crystal has a unique X-ray fingerprint. Using special synchrotron X-ray techniques that we have developed we expect to observe the entire crystallization sequence.