Pore formation during dehydration of a polycrystalline gypsum sample observed and quantified in a time-series synchrotron X-ray micro-tomography experiment
Pore formation during dehydration of a polycrystalline gypsum sample observed and quantified in a time-series synchrotron X-ray micro-tomography experiment
复制标题
DOI：
10.5194/se-3-71-2012
复制
发表时间：
2012-01-01
期刊：
影响因子：
3.4
通讯作者：
Regenauer-Lieb, K.
中科院分区：
文献类型：
--
作者：
Fusseis, F.;Schrank, C.;Regenauer-Lieb, K.
我们在先进光子源（美国）进行了原位X射线微计算机断层扫描加热实验，以加热直径为2.3mm的沃尔泰拉石膏圆柱体。我们使用了一个专门建造的X射线透明炉加热样品到388 K，共310分钟，以获得一个三维的时间序列断层扫描数据集，包括九个时间步。2.2 μ m（3）的体素大小证明足以精确定位反应起始和排水结构在空间和时间上的组织。我们观察到，脱水开始跨越一个狭窄的战线，从边缘传播到中心的样品在四个多小时。这个前沿的推进可以用平方根函数拟合，这意味着样品中反应的开始可以被描述为一个扩散过程，新颖的并行计算机代码允许从非常大的层析数据集（2048（3）体素）中以前所未有的细节量化孔隙度和排水结构的几何形状。我们确定了每个可分辨孔的位置、体积、形状和方向，并在实验持续时间内跟踪这些性质。我们发现，孔径分布遵循幂律。孔隙往往是各向异性的，但很少是裂纹形状的，并且具有优选的取向，可能由样品中预先存在的织物控制。与正在进行的脱水，孔隙合并成一个单一的相互连接的孔隙集群，连接到样品的表面缸，并提供了一个有效的排水pathway.Our的意见可以总结在一个模型中，石膏是稳定的热膨胀应力和局部增加孔隙流体压力，直到脱水前接近约100 μ m。然后，内应力释放，脱水有效地发生，导致新的孔隙空间。压力释放、孔隙的产生和前沿的推进耦合在一个反馈循环中。
We conducted an in-situ X-ray micro-computed tomography heating experiment at the Advanced Photon Source (USA) to dehydrate an unconfined 2.3mm diameter cylinder of Volterra Gypsum. We used a purpose-built X-ray transparent furnace to heat the sample to 388K for a total of 310 min to acquire a three-dimensional time-series tomography dataset comprising nine time steps. The voxel size of 2.2 mu m(3) proved sufficient to pinpoint reaction initiation and the organization of drainage architecture in space and time.We observed that dehydration commences across a narrow front, which propagates from the margins to the centre of the sample in more than four hours. The advance of this front can be fitted with a square-root function, implying that the initiation of the reaction in the sample can be described as a diffusion process.Novel parallelized computer codes allow quantifying the geometry of the porosity and the drainage architecture from the very large tomographic datasets (2048(3) voxels) in unprecedented detail. We determined position, volume, shape and orientation of each resolvable pore and tracked these properties over the duration of the experiment. We found that the pore-size distribution follows a power law. Pores tend to be anisotropic but rarely crack-shaped and have a preferred orientation, likely controlled by a pre-existing fabric in the sample. With on-going dehydration, pores coalesce into a single interconnected pore cluster that is connected to the surface of the sample cylinder and provides an effective drainage pathway.Our observations can be summarized in a model in which gypsum is stabilized by thermal expansion stresses and locally increased pore fluid pressures until the dehydration front approaches to within about 100 mu m. Then, the internal stresses are released and dehydration happens efficiently, resulting in new pore space. Pressure release, the production of pores and the advance of the front are coupled in a feedback loop.