Multi-scale digital-image-based modelling of cement-based materials

Computer modelling of the properties and performance of cement-based materials is complicated by the large range of relevant size scales. Processes occurring in the nanometersized pores ultimately tiect the performance of these materials at the structural level of meters and larger. One approach to alleviating thk complication is the development of a suiteof models, consisting of individual digjtal-image-hazed structural models for the calcium silicate hydrate gel at the nanometer level, the hydrated cement paste at the micrometer level, and a mortar or concrete at the millimeter level. Computations performed at one level provide input properties to be used in simulations of performance at the next higher level. This methodology is demonstrated for the property of ionic diffusivity in saturated concrete. The more complicated problem of drying shrinkage is also addressed

Microstructure and Transport Properties of Porous Building Materials. II: Three-Dimensional X-ray Tomographic Studies

International audienceThree-dimensional X-ray microtomography is used to obtain three-dimensional images of the microstructure of two types of brick. The images are processed to remove the noise (random and circular pattern) and then thresholded to match the porosity determined experimentally. The 3-D binary images are then analyzed to estimate their vapor diffusivity and air permeability to compare to experimental data published in part one of this report. Care must be taken in obtaining the tomographic images at a resolution that both enables isolation and quantification of the pores of interest and provides a representative elementary volume for the transport property calculations. In general, the agreement between computed and measured properties is reasonable, suggesting that X-ray microtomography can provide valuable information on the characteristics and properties of the pore networks developed in these porous building materials. A preliminary evaluation indicates that the Katz-Thompson relationship between permeability, diffusivity, and pore size is valid for these materials

Phosphorus-bearing molecules in the Galactic Center

Phosphorus (P) is one of the essential elements for life due to its central role in biochemical processes. Recent searches have shown that P-bearing molecules (in particular PN and PO) are present in star-forming regions, although their formation routes remain poorly understood. In this letter, we report observations of PN and PO towards seven molecular clouds located in the Galactic Center, which are characterized by different types of chemistry. PN is detected in five out of seven sources, whose chemistry is thought to be shock-dominated. The two sources with PN non-detections correspond to clouds exposed to intense UV/X-rays/cosmic ray (CR) radiation. PO is detected only towards the cloud G+0.693−0.03, with a PO/PN abundance ratio of ∼1.5. We conclude that P-bearing molecules likely form in shocked gas as a result of dust grain sputtering, while are destroyed by intense UV/X-ray/CR radiation