The mechanics of stone decay: relating microscale to macroscale

世界各地的石头建筑和考古材料都在腐烂和退化，保护人员面临着维护文化遗产的持续斗争。腐蚀和损坏的常见原因是多孔石材中的盐结晶。虽然这是一个老问题，但科学家们并不完全了解这种损害是如何造成的。问题的核心有一个悖论。如果我们想象一块石头的孔隙中生长着一个盐晶体，那么我们就会认为当晶体到达孔隙壁并推压它时，就会产生一个向外的应力，但是如果晶体与孔隙壁紧密接触，那么进一步的生长就不可能了，因为离子沿着界面的迁移就不可能发生了。如果盐和孔壁形成粘附接触，那么实际上应该没有应力。根据谢勒的一个重要的新理论，其解释是，在发生盐损害的情况下，在生长的盐晶体和形成孔壁的矿物之间存在短程排斥相互作用，并且在两个固体表面之间存在薄的纳米级液体膜。这层膜允许进一步溶解的盐扩散到晶体表面;随着晶体继续生长，它继续推动孔壁，最终可以产生足够的应力，使石头开裂。在刚刚完成的一个成功的试点项目中，我们使用原子力显微镜直接测量了二氧化硅探针接近硫酸钾晶体的排斥力。这个仪器让我们可以把一个微小的矿物晶体安装在一个微小的柔性悬臂上，并把它带到浸入饱和溶液中的盐晶体的表面。我们现在计划进行更详细的研究，以证实和扩展这些结果，并充分了解排斥的起源。主要感兴趣的矿物晶体是二氧化硅/石英，砂岩的主要成分，和方解石，石灰石的主要成分。我们还将看到，是否有可能通过改变矿物表面，例如用吸附的吸附剂，把排斥力变为粘附力。还有另外两条重要的工作路线。首先，我们将使用原子计算机模拟来了解更多关于固体表面之间的窄缝中的离子分布，从而了解这些高度集中的离子系统的力和热力学。其次，我们将与普林斯顿大学的G谢勒教授合作，将这些微观尺度的测量和概念与在石头中观察到的宏观尺度的结晶损伤联系起来。这将通过精确和定量的梁弯曲测试来完成，该测试检测小样本中的化学机械应力。

Stone buildings and archaeological material throughout the world is subject to decay and degradation, and conservators face a continuing struggle to maintain the cultural heritage. A common cause of decay and damage is the crystallization of salts in porous stone materials. Although this is an old problem, scientists do not fully understand how the damage is caused. There is a paradox at the heart of the problem. If one imagines a salt crystal growing in a pore within a piece of stone, then one thinks that an outward stress develops when the crystal reaches the wall of the pore and pushes against it. But if the crystal is in intimate touching contact with the wall, then further growth should be impossible as ion transport along the interface can no longer occur. If the salt and the wall of the pore make adhesive contact there should in fact be no stress. According to an important new theory by Scherer, the explanation is that in the cases where salt damage occurs, there is a short-range repulsive interaction between the growing salt crystal and the mineral forming the wall of the pore and a thin nanoscale liquid film exists between the two solid surfaces. This film allows further dissolved salt to diffuse to the crystal surface; as the crystal continues to grow it continues to push on the pore wall and eventually sufficient stress can be developed that the stone cracks. Stones are relatively weak and cannot withstand much tensile stress.In a successful pilot project just completed, we used an atomic force microscope to measure this repulsive force directly for a silica probe approaching a potassium sulphate crystal. This instrument allow us to take a tiny mineral crystal mounted on a minute flexible cantilever and bring this up to the surface of a salt crystal immersed in a saturated solution. We now plan to carry out a much more detailed study to confirm and extend these results and to understand fully the origin of the repulsion. Mineral crystals of main interest are silica/quartz, the main constituent of sandstones, and calcite, the main constituent of limestones. We shall also see if it possible to change the repulsion to an adhesive attraction by modifying the mineral surface, for example with adsorbed polyelectrolyte.There are two other important strands of work. First, we shall use atomistic computer simulation to learn more about the distribution of ions in the narrow slot between the solid surfaces; and hence to understand the forces and the thermodynamics of these highly concentrated ionic systems.Second, we shall work with Prof G Scherer at Princeton to relate these micro-scale measurements and concepts to the macro-scale crystallization damage observed in stones. This will be done through precise and quantitative beam-bending tests which detect chemomechanical stress in small specimens.